Patent Publication Number: US-6665412-B1

Title: Speaker device

Description:
TECHNICAL FIELD 
     The present invention relates to a speaker device capable of stably providing a desired directivity provided by a combination of omnidirectionality and bidirectionality, down to ultra low frequencies and capable of enlarging a listening range providing excellent stereophonic feeling by a comparatively small-sized system constitution and wherein excellence is capable of enlarging a listening range providing surround effect. 
     BACKGROUND OF THE INVENTION 
     Conventionally, there is known a speaker device of a closed type or an open back type. In this case, the directional characteristic at low frequencies constitutes omnidirectionality in a closed type speaker device and bidirectionality in an open-back type speaker device. Further, at middle and high frequencies where a speaker cannot be regarded as an ideal point sound source, radiated sound wave per se of the speaker is provided with directivity. That is, according to the conventional speaker device, as the directional characteristic at a low frequency region, only the omnidirectionality or the bidirectionality can be provided. 
     Further, FIG. 28 shows a state in which a conventional stereophonic speaker device is arranged in a test listening room  100 . The speaker device is constituted by a left channel speaker device  1 L having a sound signal reproducing speaker for reproducing a left channel sound signal and a right channel speaker device  1 R having a sound signal reproducing speaker for reproducing a right channel sound signal. 
     According to such a speaker device, basically, at a listening position on a center line M of the speaker devices  1 L and  1 R, for example, at point a, correct sound image localization is provided between the speaker devices  1 L and  1 R by which inherent sound stage of a 2-channel stereophonic system is reproduced. However, at a listening position deviated from the center line M, for example, point b, there is constituted sound image localization deviated to a direction of the speaker device  1 R owing to sound attenuation in distance of sound wave or the like and then inherent sound stage as the 2-channel stereophonic system is not reproduced. 
     That is, in FIG. 28, when sound signal reproducing speakers of the speaker devices  1 L and  1 R are driven by the same signal, sound image at a listening position on the center line M of the speaker devices  1 L and  1 R, for example, at point a, is localized in a center front direction as shown by an arrow mark. Accordingly, when ordinary stereophonic signals are reproduced, continuous sound stage is reproduced between the speaker devices  1 L and  1 R (illustrated by dot-dash-line La). 
     In this case, in respect of sound image localization of 2-channel stereophonic sound, in view of auditory sensation, the following property is known. When there is a difference in levels of sound pressures of sound waves from the speaker devices  1 L and  1 R, the sound image at a listening position on the center line M of the speaker devices  1 L and  1 R, for example, at point a, is moved in a direction of a speaker device having a higher sound pressure level. Further, when a delay time period of 1 through 30 ms is provided to either of signals for driving sound signal reproducing speakers of the speaker devices  1 L and  1 R, the sound image at a listening position on the center line M of the speaker devices  1 L and  1 R, for example, at point a, is moved in a direction of a speaker from which sound wave arrives earlier in time. This phenomenon is referred to as precedence effect, which is well known. 
     In view of such a property in auditory sensation, consider sound image localization at a listening position, for example, point b, deviated from the center line M of the speaker devices  1 L and  1 R. At point b, distances between a sound receiving point and the speaker devices  1 L and  1 R differ from each other and therefore, in the case where the directivities of the speaker devices  1 L and  1 R are omnidirectional, there causes a difference between arrival times of sound wave from the speaker devices  1 L and  1 R and there causes a difference in levels of sound pressures. That is, the arrival time is earlier in the case of sound wave from the speaker device  1 R and the sound pressure level is higher in the case of sound wave from the speaker device  1 R. 
     Accordingly, sound image at a listening position, for example, at point b, deviated from the center line M of the speaker devices  1 L and  1 R is localized to significantly deviate in a direction of the speaker device  1 R as shown by an arrow mark in FIG. 28 owing to the above-described difference in sound pressure levels in addition to the precedence effect caused by time difference. Accordingly, sound stage in the case of reproducing ordinary stereophonic signals is deviated to a side of the speaker device  1 R (illustrated by two-dot-dash-line Lb). 
     There has been proposed a method of utilizing the directivity of a conventional speaker with regard to a method of reproducing 2-channel stereophonic sound, which intends to improve the problem of deviation of the sound stage, mentioned above, that is, to enlarge the listening position providing excellent stereophonic feeling. In FIG. 29, sound signal reproducing speakers  2 L and  2 R of the speaker devices  1 L and  1 R are attached to closed type cabinets  3 L and  3 R such that respective principal axes (reference axes) are directed inwardly by about 45° in view from a central listening position, which is a method of utilizing directivities provided to speakers per se at middle and high frequencies based on the fact that diaphragms of the speakers  2 L and  2 R are provided with limited areas. Further, in FIG. 30, as sound signal reproducing speakers  4 L and  4 R of the speaker devices  1 L and  1 R in charge of middle and high frequencies, there are used bidirectional (directivity in 8-like shape) speakers and the speakers  4 L and  4 R are attached to baffle plates  6 L and  6 R in cabinets  5 L and  5 R front faces of which are opened such that respective principal axes thereof are directed inwardly by about 450° in view from a central listening position. Further, solid lines a L  and a R  respectively designate the directivities of the speakers  4 L and  4 R. 
     According to the examples shown by FIG.  29  and FIG. 30, by providing directivities to the sound waves radiated from the speaker devices  1 L and  1 R, the sound pressure level of sound wave from the speaker device  1 R is reduced and the sound pressure level of left channel sound wave from the speaker device  1 L is more or less increased at a listening position, for example, at point b, deviated from the center line M (refer to FIG. 28) so that the devices are operated to correct a difference between sound pressure levels based on a difference in sound attenuation in distances from the two left and right channels, whereby a listening position providing excellent stereophonic feeling is enlarged. 
     However, according to the example shown by FIG. 29, when the directivity is intended to provide from lower frequencies, the speakers  2 L and  2 R having larger apertures need to use and the system constitution becomes large-sized. Further, according to the example shown by FIG. 30, when the low frequency limit of the reproduction band is intended to widen, the baffle plates  6 L and  6 R attached to the speakers  4 L and  4 R need to enlarge. Similar to the example shown by FIG. 29, the system constitution becomes large-sized. 
     Further, FIG. 31 shows a state in which conventional front surround speaker devices are arranged in the test listening room  100 . The speaker device is constituted by a left channel speaker device  11 L and a right channel speaker device  11 R. For example, the speaker device  11 L is provided with a sound signal reproducing speaker for reproducing a left sound signal as well as a surround signal reproducing speaker for reproducing a surround signal, and the speaker device  11 R is provided with a sound signal reproducing speaker for reproducing a right sound signal as well as a surround signal reproducing speaker for reproducing a surround signal. 
     In this case, the surround signal reproducing speakers are attached to speaker boxes such that principal axes thereof are directed in front directions of the speaker boxes or outwardly in respect of the front directions by predetermined angles. FIG. 31 shows an example in which principal axes of sound signal reproducing speakers  12 L and  12 R of the speaker devices  11 L and  11 R are directed in front directions F L  and F R  of speaker boxes  14 L and  14 R. Similarly, principal axes SA L  and SA R  of surround signal reproducing speakers  13 L and  13 R are directed in the front directions F L  and F R . Further, FIG. 32 shows an example in which the principal axes of the sound signal reproducing speakers  12 L and  12 R of the speaker devices  11 L and  11 R are respectively directed in the front directions F L  and F R  of the speaker boxes  14 L and  14 R. In contrast thereto, the principal axes SA L  and SA R  of the surround signal reproducing speakers  13 L and  13 R are directed outwardly in respect of the front directions F L  and F R  by predetermined angle δ. 
     According to the front surround speaker device shown by FIG. 31, there is a drawback in which although inherent surround effect is achieved when listening is carried out at a listening position, for example, point e, on the center line M of the speaker devices  11 L and  11 R, the surround effect is significantly deteriorated when listening is carried out at a listening position, for example, at point f, deviated from the center line M. Hereinafter, a description will be given of significantly deteriorating the surround effect at a listening position deviated from the center line M in this way by explaining the principle of a 2-channel front surround stereophonic reproduction system. 
     The front surround stereophonic reproduction system is a system of achieving the surround effect by using two front speaker devices (speaker devices for left and right channels) and is a system in which sound image is produced not only in a front direction but also a transverse direction or a rear direction so that wide-angled sound stage is created in auditory sensation of a listener to provide the auditory sensation in listening to music in a wide theater to the listener. Accordingly, the front surround system is a pseudo-stereophonic sound reproduction system utilizing auditory sensation and is differentiated from an orthodox multiple-channel-stereophonic sound reproduction system, which intends to create a stereophonic actual sound field in an auditory space. 
     First, a 2-channel stereophonic signal is constituted by an M signal component of a monophonic signal and an S signal component providing a direction of sound image localization. That is, an L signal (left sound signal) of a left channel is a sum of two M and S components, that is, L=M+S and an R signal (right sound signal) of a right channel is a difference between the two M and S components, that is, R=M−S. Accordingly, the S signal component is nothing but a signal of a difference between the L signal and the R signal and is a signal the size and the polarity of which differ depending on a direction of sound image localization. 
     That is, at point e on the center line M of the speaker devices  11 L and  11 R of FIG. 31, when sound image is localized in a front central direction, the S signal component is null and only the M signal component is constituted. Further, when sound image is localized on the left side of the front central direction, the polarity of the S signal component becomes positive similar to that of the M signal component and when sound image is localized on the right side of the front central direction, the polarity of the S signal component becomes negative different from that of the M signal component. Accordingly, the S signal component can be expressed as S=±S 0 , when sound image is localized on the left side of the front central direction, L=M+(+S 0 )=M+S 0 , R=M−(+S 0 )=M−S 0 , that is, L&gt;R and when sound image is localized on the right side of the front central direction, L=M+(−S O )=M−S 0 , R=M−(−S 0 )=M+S 0 , that is, L&lt;R. 
     Next, according to the front surround stereophonic reproduction system, there is adopted a method in which in addition to the L and R signals inherent to the 2-channel stereophonic system, a surround signal mainly composed of the S signal component is superposed on said L and R signals in conformity with the polarities of the S signal components of the L and R signals and the superposed signal is reproduced electrically or the surround signal is reproduced by using exclusive speakers thereto. Although the surround signal is basically and mainly composed of the S signal component, to achieve further real surround effect, the S signal component is subjected to a signal processing of time delay, addition of reverberation or emphasizing a specific frequency component or the like. That is, the front surround stereophonic reproduction system is basically nothing but a system in which the S signal component included in the inherent L and R signals is subjected to pertinent signal processing, is further emphasized and is reproduced. By adopting such a reproduction method, intended surround effect is achieved at a listening position, for example, at point e, on the center line M of the speaker devices  11 L and  11 R in FIG.  31 . 
     In this case, a physical sound condition of the reproduced sound reproduced at a listening position, for example, at point e, on the center line M of the speaker devices  11 L and  11 R constitutes a condition in which respective signal sound components of M, S and surround radiated from the speaker devices  11 L and  11 R are provided with equal magnitudes and arrive at the same time. This condition is a condition for reproducing continuous sound stage in auditory sensation between the speaker devices  11 L and  11 R by the two signal sound components M and S and achieving intended surround effect by the surround signal sound components, and is extremely important. However, at a listening position, for example, at point f, deviated from the center line M of the speaker devices  11 L and  11 R in FIG. 31, the level difference and the time difference are caused in reproduced sounds of the speaker devices  11 L and  11 R and then it does not satisfy the above-described condition. 
     Further, when the surround signals are reproduced by the exclusive speakers  13 L and  13 R (refer to FIG.  32  and FIG.  33 ), the principal axes SA L  and SA R  of the speakers  13 L and  13 R are directed in the front direction or the outward directions in view from a listening position, for example, at point e, on the center line M of the speaker devices  11 L and  11 R. As such surround signal reproducing speakers  13 L and  13 R, there are generally used small-sized speakers having an aperture of about 8 cm. In the case of a small-sized speaker having an aperture of about 8 cm, as shown by FIG. 34, a directivity which is dependent on the aperture begins to provide from a frequency of about 1 KHz and the higher the frequency, the sharper the directivity is provided. 
     Accordingly, in respect of the surround signal sound at a listening position, for example, at point f, deviated from the center line M of the speaker devices  11 L and  11 R in FIG. 31, the surround signal sound becomes larger on the right channel side and smaller on the left channel side owing to a difference in the directivities and attenuation of sound waves in distances caused by differences in directions and distances of the speaker devices  11 L and  11 R. Particularly, in the case in which the principal axes of the surround signal reproducing speakers  13 L and  13 R are directed outwardly (refer to FIG.  33 ), the difference in levels caused by the directivities of the surround signal sounds from the left and right channels becomes further significant. Therefore, the surround effect at a listening position, for example, at point f, deviated from the center line M of the speaker devices  11 L and  11 R in FIG. 31 is significantly deteriorated as mentioned above. 
     In view of the above-described points, it is an object of the present invention to provide a speaker device capable of stably providing desired directivity provided by a combination of omnidirectionality and bidirectionality, down to ultra low frequencies. Further, it is another object of the present invention to provide a stereophonic speaker device capable of enlarging a listening range providing excellent stereophonic feeling by a comparatively small-sized system constitution. Further, it is still another object of the present invention to provide a front surround speaker device capable of enlarging a listening range providing excellent surround effect. 
     DISCLOSURE OF THE INVENTION 
     According to an aspect of the present invention, there is provided a stereophonic speaker device comprising a left channel speaker device having a first speaker for reproducing a left channel sound signal and a right speaker device having a second speaker for reproducing a right channel sound signal, characterized in that a first speaker portion constituted by the first speaker of the left channel speaker device and a second speaker portion constituted by the second speaker of the right channel speaker device are provided with directivities each provided by a combination of a bidirectionality and an omnidirectionality having a maximum sensitivity on a front principal axis of the speaker and that the first speaker of the left channel speaker device and the second speaker of the right channel speaker device are attached to the speaker boxes such that principal axes thereof are inclined by a first angle respectively in a counterclockwise direction and a clockwise direction relative to front directions of the speaker boxes. 
     Further, according to still another aspect of the present invention, there is provided a stereophonic speaker device comprising a left channel speaker device having a first and a second speakers for reproducing a left channel sound signal and a right channel speaker device having a third and a fourth speakers for reproducing a right channel sound signal, characterized in that a first speaker portion constituted by the second speaker of the left channel speaker device and a second speaker portion constituted by the fourth speaker of the right channel speaker device are provided with directivities each provided by a combination of a bidirectionality and an omnidirectionality having a maximum sensitivity on a front main axis of each of the speakers, that the first speaker of the left channel speaker device and the third speaker of the right channel speaker device are attached to speaker boxes such that respective axes thereof coincide with front directions of the speaker boxes, and that the second speaker of the left channel speaker device and the fourth speaker of the right channel speaker device are attached to the speaker boxes such that principal axes thereof are inclined by a first angle respectively in a counterclockwise direction and a clockwise direction relative to the front directions of the speaker boxes. 
     The principal axes of the speakers constituting the first and the second speaker portions for reproducing sound signals of the left channel speaker device and the right channel speaker device, are directed inwardly in view from a listening position on the center line of the two speaker devices. Accordingly, by the directivities of the first and the second portions each provided by the combination of the bidirectionality and the omnidirectionality, a difference in sound pressure levels based on a difference in attenuation of sound in distances to the two left and right channels is corrected at a listening position deviated from the center line of the two speaker devices, thereby a listening range providing excellent stereophonic feeling is expanded. 
     According to further aspect of the present invention, there is provided a stereophonic speaker device comprising a left channel speaker device having a first speaker for reproducing a left channel sound signal and a second speaker for reproducing a surround signal and a right channel speaker device having a third speaker for reproducing a right channel sound signal and a fourth speaker for reproducing the surround signal, characterized in that the first speaker of the left channel speaker device and the third speaker of the right channel speaker device are attached to speaker boxes such that principal axes thereof are inclined by a first angle respectively in a counterclockwise direction and a clockwise direction relative to front directions of the speaker boxes, that the second speaker of the left channel speaker device and the fourth speaker of the right channel speaker device are attached to the speaker boxes such that principal axes thereof are inclined by the first angle or by a second angle different from the first angle respectively in the counterclockwise direction and the clockwise direction relative to the front directions of the speaker boxes and that a first speaker portion constituted by the first speaker of the left channel speaker device and a second speaker portion constituted by the third speaker of the right channel speaker device are provided with directivities each provided by a combination of a bidirectionality and an omnidirectionality having a maximum sensitivity on the front principal axis of each of the speakers. 
     Further, according to still further aspect of the present invention, there is provided a stereophonic speaker device comprising a left channel speaker device having a first and a second speakers for reproducing a left channel sound signal and a third speaker for reproducing a surround signal and a right speaker device having a fourth and a fifth speakers for reproducing a right channel sound signal and a sixth speaker for reproducing the surround signal, characterized in that the first speaker of the left channel speaker device and the fourth speaker of the right channel speaker device are attached to speaker boxes such that principal axes thereof coincide with respective front directions of the speaker boxes, that the second speaker of the left channel speaker device and the fifth speaker of the right channel speaker device are attached to the speaker boxes such that principal axes thereof are inclined by a first angle respectively in a counterclockwise direction and a clockwise direction relative to the front directions of the speaker boxes, that the third speaker of the left channel speaker device and the sixth speaker of the right channel speaker device are attached to the speaker boxes such that principal axes thereof are inclined by the first angle or a second angle different from the first angle respectively in the counterclockwise direction and the clockwise direction relative to the front directions of the speaker boxes and that a first speaker portion constituted by the second speaker of the right channel speaker device and a second speaker portion constituted by the fifth speaker of the right channel speaker device are provided with directivities each provided by a combination of a bidirectionality and an omnidirectionality having a maximum sensitivity on a front principal axis of each of the speakers. 
     The principal axes of the speakers constituting the first and the second speaker portions for reproducing sound signals of the left channel speaker device and the right channel speaker devices are directed inwardly in view from a listening position on the center line of the two speaker devices. Accordingly, by the directivities of the first and the second speaker portions each provided by the combination of the bidirectionality and the omnidirectionality, a difference in sound pressure levels based on a difference in attenuation of sound in distances to the two left and right channels is corrected at a listening position deviated from the center line of the two speaker devices, thereby the listening range providing excellent stereophonic feeling is expanded. Further, the principal axes of the third and the sixth speakers for reproducing the surround signal of the left channel and the right channel speaker devices are directed inwardly in view from a listening position on the center line of the two speaker devices. Therefore, at a listening position deviated from the center line of the two speaker devices, by the directivities of the speaker for reproducing the surround signal at middle and high frequencies, a difference in levels based on a difference in attenuation of sound in distances of the surround signal sound of the two left and right channels is corrected. Thus, an effect of enlarging a listening range in respect of the above-described reproduced sound for sound signal (LR signal) is added and the listening range providing excellent surround effect is further enlarged. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a speaker device according to the present invention. 
     FIG. 2 is a circuit connection diagram showing an equivalent circuit of a mechanical system of the speaker device. 
     FIG. 3 is a diagram for theoretical analysis of synthesized sound pressure. 
     FIG. 4 is a diagram showing directivity patterns when α is changed. 
     FIG. 5 is a diagram showing a directivity frequency characteristic by a value of β. 
     FIG. 6 is a diagram showing directivity patterns when β is changed. 
     FIG. 7 is a diagram showing a frequency characteristic of output sound pressure of the speaker device. 
     FIG. 8 is a perspective view showing a stereophonic speaker device according to the present invention. 
     FIG. 9 is a diagram showing directions of principal axes of speakers for reproducing sound signals in the speaker device shown by FIG.  8 . 
     FIG. 10 is a perspective view showing a speaker device used in an auditory sensation test of the speaker device shown by FIG.  8 . 
     FIG. 11 is a diagram for explaining a result or the like of the auditory sensation test of the speaker device shown by FIG.  8 . 
     FIG. 12 is a perspective view showing a stereophonic speaker device according to the present invention. 
     FIG. 13 is a diagram showing directions of principal axes of speakers for reproducing sound signals in the speaker device shown by FIG.  12 . 
     FIG. 14 is a perspective view showing a speaker device used in an auditory sensation test of the speaker device shown by FIG.  12 . 
     FIG. 15 is a diagram for explaining a result or the like of the auditory sensation test of the speaker shown by FIG.  12 . 
     FIG. 16 is a perspective view showing a stereophonic speaker device according to the present invention. 
     FIG. 17 is a view showing directions of principal axes of speakers for reproducing sound signals and a surround signal of the speaker device shown by FIG.  16 . 
     FIG. 18 is a perspective view showing a speaker device used in an auditory sensation test (surround effect) of the speaker device shown by FIG.  16 . 
     FIG. 19 is a diagram for explaining a result or the like of the auditory sensation test (surround effect) of the speaker device shown by FIG.  16 . 
     FIG. 20 is a perspective view showing a stereophonic speaker device according to the present invention. 
     FIG. 21 is a view showing directions of principal axes of speakers for reproducing sound signals and a surround signal of the speaker device shown by FIG.  20 . 
     FIG. 22 is a perspective view showing a front surround speaker device according to the present invention. 
     FIG. 23 is a view showing directions of principal axes of speakers for reproducing a surround signal of the speaker device shown by FIG.  22 . 
     FIG. 24 is a perspective view showing a speaker device used in an auditory sensation test of the speaker device shown by FIG.  22 . 
     FIG. 25 is a diagram showing a result or the like of the auditory sensation test of the speaker device shown by FIG.  22 . 
     FIG. 26 is a perspective view showing a front surround speaker device according to the present invention. 
     FIG. 27 is a view showing directions of principal axes of speakers for reproducing a surround signal of the speaker device shown by FIG.  26 . 
     FIG. 28 is a view for explaining a stereophonic speaker device. 
     FIG. 29 is a view showing an example of enlarging a listening range providing excellent stereophonic feeling. 
     FIG. 30 is a view showing other example for enlarging the listening range for providing excellent stereophonic feeling. 
     FIG. 31 is a view showing a conventional front surround speaker device. 
     FIG. 32 is a diagram showing an example in which principal axes of surround signal reproducing speakers are directed in front directions. 
     FIG. 33 is a view showing an example in which the principal axes of the surround signal reproducing speakers are directed outwardly. 
     FIG. 34 is a diagram showing an example of a directivity frequency characteristic of output sound pressure of a speaker. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     FIG. 1 shows a speaker device  20  as a best mode. A speaker  22  is attached to one end face of a speaker box  21  in a shape of a rectangular parallelepiped and a sound wave radiation port (opening portion)  23  in a circular shape covered with an acoustic resistance material  24  is installed at an end face thereof (upper face in the drawing) different from the one end face. The sound wave radiation port  23  intends to radiate air vibration produced at a rear face of a speaker diaphragm to the outside as sound wave and is formed so that an axis penetrating the center thereof extends in a direction of a principal axis (reference axis) of the speaker  22 . 
     An equivalent circuit of a mechanical system of the speaker device  20  in which an electric system and an acoustic system thereof are equivalently converted into said mechanical system, is represented as shown by FIG. 2 regarding middle and low frequencies. In the drawing, notation F designates vibratory force for moving the diaphragm and is expressed as shown by Equation (1) under constant voltage drive.              F   =     BlE   Rv             (   1   )                         
     E: Drive voltage of voice coil 
     Rv: Electric resistance of voice coil 
     Bl: Force factor (B: Magnetic flux density of magnetic flux gap, l: Effective length of voice coil) 
     Notation s 0  designates equivalent stiffness of an elastic support member of the diaphragm, notation r 0  designates equivalent mechanical resistance including electromagnetic brake resistance and notation m 0  designates effective mass of the diaphragm. Notation s 1  designates equivalent stiffness that elasticity of air in a speaker box is converted into an area of the diaphragm, and the s 1  is represented by Equation (2) in which notation V designates volume of air in the speaker box and notation A designates an effective area of the diaphragm.                s   1     =       ρ                   c   2          A   2       V             (   2   )                         
     (ρ: Density of air, c: Sound speed) 
     Notation r 1  designates equivalent mechanical resistance that acoustic resistance of the acoustic resistance material  24  of the sound wave radiation port  23  is converted into an area of the diaphragm, and notation ml designates equivalent mass that acoustic mass dependent on the structure of the sound wave radiation port  23  and sound wave radiation is converted into an area of the diaphragm. Notation V 1  designates vibration speed of the diaphragm and notation V 2  designates equivalent vibration speed that vibration speed of air at the sound wave radiation port  23  is converted into an area of the diaphragm. 
     Further, in the above-described speaker device  20 , radiation resistances are respectively present on the side of the diaphragm of the speaker  22  and on the side of the sound wave radiation port  23  and power supplied to the radiation resistances is radiated in the air as acoustic power. However, the radiation resistances are very small values compared with the r 0  and the r 1  and do not influence on the V 1  and the V 2  Accordingly, they are omitted in the equivalent circuit of FIG.  2 . 
     Next, in reference to FIG. 3, a theoretical analysis will be given of synthesized sound pressure of sound waves radiated from two sound sources of a front face of the speaker  22  (front face of speaker diaphragm) and the sound wave radiation port  23 . In this case, notation d designates a spatial distance in the principal axis direction of the speaker  22  between the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23 . Further, attention is paid to one-directional components on a horizontal plane of the sound waves radiated from the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23 , and then direction of the principal axis in the direction of the front face of the speaker is set to 0° and an angle made by the principal axis and the direction on which the attention is paid (direction of sound receiving position) is set to θ in the counterclockwise direction. 
     In the drawings, when d is 8 through 10 cm and a distance r between the sound receiving position and the speaker  22  is equal to or more than 1 m, the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23  can be regarded as point sound sources at middle and low frequencies. Further, the sound wave at the sound receiving position may be regarded as substantially a plane wave. 
     When handled in this way, sound pressure P 1  at the sound receiving position of the sound wave radiated from the front face of the speaker  22  is represented by Equation (3) since the vibration speed of the diaphragm is designated by notation V 1 .                P   1     =         j                 ω                 ρ                   AV   1         4      π                 r                   -   j                       ω                 r     c                   (   3   )                         
     Next, sound pressure P 2  at the sound receiving position of the sound wave radiated from the sound wave radiation port  23  is delayed relative to the P 1  over time in correspondence with a distance of dcos θ and is provided with negative polarity since the sound wave is based on air vibration at the rear face of the diaphragm and accordingly, the sound pressure P 2  is represented by Equation (4). Incidentally, d&lt;&lt;r.                P   2     =         j                 ω                 ρ                   AV   2         4      π                 r                     -   j                       ω                 r     c         ·            -   j                       ω                 d     c        oos                 θ                   (   4   )                         
     Therefore, the synthetic round pressure P(θ) is P 1 +P 2  and accordingly, it is represented by Equation (5).                P        (   θ   )       =         j                 ω                 ρ                 A       4      π                 r                     -   j                       ω                 r     c              (       V   1     -       V   2                 -   j                       ω                 d     c        cos                 θ           )                 (   5   )                         
     Here, it is apparent from the equivalent circuit of FIG. 2, the vibration speed V 2  is represented by Equation (6)                V   2     =       V   1       1   +     j                 ω                     r   1       s   1         +         (     j                 ω     )     2                       m   1       s   1                     (   6   )                         
     Equation (6) shows a characteristic of a secondary low pass filter and generates group delay time having a constant value determined by s 1 , r 1  and m 1  in pass band. This time delay and the time delay changed by θ of dcos θ/c provided in space, result in the directivity having a combination of bidirectionality and omnidirectionality. Hence, Equation (7) is established by substituting Equation (6) for Equation (5) to rearrange Equation (5) with setting ω/c=k.                P        (   θ   )       =         jωρ                   AV   1                   -   j                   kr       ·   j                   kd       4      π                   r        (     1   +     j                 ω                     r   1       s   1         +         (     j                 ω     )     2                       m   1       s   1           )           ×     {         1   -            -   j                   kd                 cos                 θ           j                 kd       +         r   1        c         s   1        d       +     j                   kd   ·       m   1       s   1                (     c   d     )     2         }               (   7   )                         
     In Equation (7), when the first term in {} is expanded in series, Equation (8) is resulted.                  1   -            -   j                   kd                 cos                 θ           j                 kd       =       cos                 θ     -                  j                 kd                   cos   2        θ       2   !       +           (     j                 kd     )     2          cos   3        θ       3   !       -                      (     j                 kd     )     3          cos   4        θ       4   !       +   …             (   8   )                         
     As is apparent from Equation (8), at frequencies of kd&lt;&lt;1, when higher order terms of the primary order and higher of jkd are omitted, Equation (8) becomes cos θ. Therefore, the first term in {} of Equation (7) represents a bidirectional component. In contrast thereto, the second term and the third term in {} of Equation (7) do not depend on θ and accordingly, they represents an omnidirectional component. 
     When Equation (9) is put and P(θ ) is shown by normalizing with the sound pressure P(0° ) in the front direction (θ=0°), Equation (8) is represented by Equation (10) at frequency of kd&lt;&lt;1.                    r   1        c         s   1        d       =   α           (   9   )                         P        (   θ   )         P        (     0      °     )         =         cos                 θ     +   α       1   +   α                   =         cos                 θ       1   +   α       +     α     1   +   α                       (   10   )                         
     As is apparent from Equation (10), the synthesized sound pressure P(θ) is provided with a directivity having a combination of a bidirectional component and an omnidirectional component. Distribution of the two components can be changed by a value of α representing the omnidirectional component. The value a represents a ratio of r 1 /s 1  to d/c as apparent from Equation (9). In FIG. 2, r 1 /s 1  represents the group delay time period in the pass band of the low pass filter constituted by r 1 , s 1 , and m 1 . Notation d/c designates a time period required for sound wave to propagate by the space distance d relating to the principal axis direction of the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23 . 
     FIG. 4 shows a result of calculating directivity patterns from Equation (10) in the cases of the values of α of 0.5, 1.0 and 2.0. 
     In Equation (10), when α≦1, P(θ) becomes null at a certain angle. However, when α&gt;1, P(θ) does not become null at any angle. In the case of α≦1, if an angle θ for nullifying P(θ) is designated by notation θ 1 , as shown by FIG. 4, when α=1, θ 1  is equal to 180° and when α=0.5, θ 1  becomes two directions of 120° and 240°. In this way, P(θ) becomes null at θ=θ 1  since the higher order terms of the primary order term and higher of jkd present in the bidirectional component represented by Equation (8) are omitted. However, actually, there are present the higher order terms and accordingly, the higher the frequency, the more the directivity pattern of the bidirectional component is deviated from 8-like shape of an intrinsic circle. As a result, the synthesized directivity pattern becomes different from the basic directivity pattern shown by Equation (10). That is, the directivity is deteriorated. 
     In order to reduce the degree of deterioration of directivity up to higher frequencies, higher order terms of the primary order term or higher of jkd present in the bidirectional component represented by Equation (8) maybe canceled up to higher orders by using the omnidirectional component at a certain angle. When taking a look at Equation (7) and Equation (8), by the third term in {} of Equation (7), the primary term of jkd in Equation (8) can be canceled at a certain angle. 
     Hence, when in respect of the third term in {} of Equation (7) , Equation (11) is put and the canceling angle θ is designated by notation θ 2 , the condition of canceling is represented by Equation (12). In Equation (12), in respect of cos 2 θ 2 , cos 2 θ 2 ≦1 in a range of θ 2 =0° through 360°. Therefore, the value of β satisfying Equation (12) is β≦0.5.                    m   1       s   1              (     c   d     )     2       =   β           (   11   )                     cos   2          θ   2       2     =   β           (   12   )                         
     FIG. 5 shows a result in which the value of β is changed in 5 stages of 0 through 1 in the case of α=1 (unidirectionality) and a change of P(θ)/P(0°) in frequencies at angles of θ of 90° and 180° is calculated based on Equation (7). In this case, the condition of β=0 signifies m 1 =0. However, actually, m 1  cannot be nullified and accordingly, the condition of β=0 cannot be realized. 
     As apparent from FIG. 5, at low frequencies of kd≦0.4, there is constituted a unidirectional pattern in which the synthesized sound pressure level in the rear face direction (θ=180°) of the speaker  22  becomes lower than that in the front direction (θ=0°) by 20 dB or more and the synthesized sound pressure level in the transverse direction (θ=90°) becomes lower than that in the front direction by 6 dB. However, the directivity is deteriorated by an increase in the frequencies and almost no directivity is provided at a frequency of kd≈π. The degree of deteriorating the directivity differs depending on the value of β. 
     FIG. 6 shows a behavior of changing the directivity pattern at a frequency of kd=1.5 (f=1.2 KHz when d=7 cm) according to the values of β. As is apparent from the diagram, although the value of β cannot be nullified, it is necessary to set the value of β to a value as small as possible in order to provide excellent directivity up to higher frequencies. When the value of β is set to be equal to or smaller than 0.5, the frequency of kd=1.5 is regarded as a substantial high frequency limit of the directivity. 
     The above-described theoretical investigation has been carried out in respect of middle and low frequencies where the front face of the speaker  22  and the sound wave radiation port  23  can be regarded as ideal point sound sources. However, actually, the diaphragm of the speaker  22  and the sound wave radiation port  23  are provided with limited areas and accordingly, the respective radiating sound waves per se are provided with directivities at middle and high frequencies. Accordingly, it is proper to apply provision of directivity utilizing group delay time as mentioned above to middle and low frequencies where radiating sound waves of the speakers per se are not provided with directivities. 
     The directivity of the speaker per se differs depending on dimensions and shape of a speaker box to which the speaker is attached and an aperture of the speaker and so on. However, a frequency at a limit where the speaker starts providing directivity has a relationship of inverse proportion to the aperture of the speaker. The following is established when investigating theoretical analysis, actual measurement result and so on, which have already been known. That is, when notation D designates an effective diameter of a speaker diaphragm, the limit frequency where the speaker starts providing directivity can be regarded as a frequency satisfying Equation (13).                  ω   c                   D     =     π   2             (   13   )                         
     When Equation (13) is represented by wavelength λ of radiating sound wave, from a relationship of ω/c=2π/λ, 4D=λ and accordingly, the wavelength λ of the radiating sound wave may be regarded as a frequency 4 times as much as the effective diameter D of the diaphragm. Hence, it is proper to establish a setting condition shown below in order to apply provision of directivity utilizing group delay time as mentioned above to a frequency range of a frequency at a limit where the speaker starts providing directivity or below. 
     According to the speaker device  20  shown by FIG. 1, as previously described, the substantial high frequency limit of the directivity may be regarded as a frequency satisfying Equation (14).                  ω   c                   d     =     π   2             (   14   )                         
     Hence, it seems to be proper that the frequency is set to a frequency where the speaker  22  per se starts providing directivity in the radiating sound wave thereof, that is, the frequency satisfying Equation (13). Therefore, the setting condition becomes as shown by Equation (15) from Equations (13) and (14). 
     
       
         d=D  (15) 
       
     
     That is, it is proper to set the spatial distance d in the principal axis direction of the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23  equal to the effective diameter D of the speaker diaphragm. Further, this setting condition is established also in respect of the directivity in a range of α=0.5 through 2.0 centering on the unidirectionality of α=1. 
     Next, a description will be given of an embodiment when the directivity is set to be unidirectional. 
     A dynamic speaker is used as the speaker  22 , which has the aperture of 8 cm, the effective diameter of the diaphragm of 7 cm and the effective area of 38.5 cm 2 . The speaker box  21  is in a shape of a rectangular parallelepiped, dimensions of the speaker attaching face is 8.6 cm in height and breadth and 14 cm in depth and volume of air in the speaker box is 600 cm 3 . The sound wave radiation port  23  is circular and is installed at the upper face of the speaker box  21  and the effective area of the sound wave radiation port  23  is 26 cm 2 . Further, the spatial distance d in the principal axis direction of the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23  is set to 7 cm from the condition of Equation (15). 
     The acoustic resistance of the acoustic resistance material  24  of the sound wave radiation port  23  is r 1 =0.745 (kg/sec) in MKS unit when represented by the equivalent mechanical resistance converted into an area of the speaker diaphragm from Equations (2) and (9). Accordingly, the equivalent mechanical resistance r 1 ′ represented by the effective area of the sound wave radiation port  23  is calculated by Equation (16) when the effective area of the sound wave radiation port  23  is designated by notation A 1 . Therefore, an acoustic resistance density (equivalent mechanical resistance per unit area) of the acoustic resistance material  24  of the sound wave radiation port  23  becomes 130.8 (kg/sec·m 2 ) by dividing Equation (16) by A 1 .                r   1   ′     =           r   1          (       A   1     A     )       2     =     0.3              [     kg/sec     ]               (   16   )                         
     A bold line a, a dot-dash line b and a broken line c of FIG. 7 show frequency characteristics of the directivity of the output sound pressure of the speaker device  20  according to the embodiment and it is found that the speaker device  20  is provided with unidirectionality at middle and low frequencies. The frequency of the limit where the speaker per se starts providing directivity becomes about 1.2 KHz from Equation (13) when the effective diameter of the diaphragm is 7 cm. Therefore, by setting d=D=7 cm, at frequencies of about 1.5 KHz or lower, there is provided the unidirectionality which is regarded as practically sufficient. A bold line d, a dot-dash line e and a broken line f of FIG. 7 show frequency characteristics of the directivity of the output sound pressure when the sound wave radiation port  23  is not installed and it is provided with no directivity at middle and low frequencies. 
     Further, although not mentioned above, in the speaker device  20  of FIG. 1, there may be installed opening area variable means for making the area of the sound wave radiation port  23  variable. As the opening area variable means, there can be conceived one like, for example, an iris mechanism of a camera. When the area of the sound wave radiation port  23  is changed, the acoustic resistance and the acoustic mass of the sound wave radiation port  23  are changed and then the group delay time in the pass band of the sound wave radiated from the sound wave radiation port  23  via a low pass filter constituted by the acoustic resistance, the acoustic mass and an acoustic capacitance provided by air in the speaker box, is changed. Therefore, by providing the opening area variable means, the group delay time can arbitrarily be adjusted and the user can simply carry out adjustment of the directivity characteristic. 
     Further, the aperture of the sound wave radiation port  23  and the acoustic resistance density of the acoustic resistance material  24  can pertinently be selected without being restricted to numerical values in the above-described explanation. 
     In this way, according to the speaker device  20  shown by FIG. 1, the low pass filter is constituted by the acoustic resistance and the acoustic mass of the sound wave radiation port  23  and the acoustic capacitance provided by air in the speaker box  21 , and accordingly, the directivity is provided by utilizing the group delay time in the pass band of the sound wave radiated from the sound wave radiation port  23  via the low pass filter. Therefore, the desired directivity can stably be provided down to ultra low frequencies by setting the arbitrary group delay time regardless of dimensions or shape of the speaker box  21 . 
     For example, by setting the group delay time to ½ through 2 times as much as the time period required for the sound wave to propagate by the spatial distance in the principal axis direction of the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23 , there can be provided the directivity having a combination of omnidirectionality and bidirectionality such as in a shape of hyper cardioid, cardioid (unidirectionality) or the like. 
     Further, by setting the spatial distance d in the principal axis direction of the two sound sources of the front face of the speaker  22  and the sound wave radiation port  23  to a value of 0.7 through 1.5 times as much as the effective diameter D of the speaker diaphragm, the desired directivity can effectively be provided by being limited to the frequency range of the frequency where the speaker per se starts providing directivity or lower. 
     Next, FIG. 8 shows a stereophonic speaker device  30  as another best mode. The speaker device  30  is constituted by a left channel speaker device  31 L and a right channel speaker device  31 R. 
     A speaker box  32 L constituting the speaker device  31 L is formed with a speaker attaching face  33 L between a front face and a right side face and the speaker attaching face  33 L is attached with a sound signal reproducing speaker  34 L. In this case, as shown by FIG. 9, the speaker  34 L is attached to the speaker box  32 L such that a principal axis MAL thereof is inclined by a predetermined angle γ, for example, 40° through 50°, in the counterclockwise direction relative to a front direction F L  of the speaker box  32 L. 
     A sound wave radiation port (opening portion)  36 L covered with an acoustic resistance material  35 L is installed at an end face (upper face in the drawing) different from the speaker attaching face  33 L of the speaker box  32 L. The sound wave radiation port  36 L intends to radiate air vibration produced from a rear face of a diaphragm of the speaker  34 L to the outside as sound wave and is formed such that an axis penetrating the center thereof is extended in the direction of the principal axis (reference axis) of the speaker  34 L. 
     In this case, an acoustic low pass filter is constituted by acoustic capacitance provided by air in the speaker box  32 L and acoustic resistance and acoustic mass of the sound wave radiation port  36 L. Further, the group delay time in the pass band of the sound wave radiated from the sound wave radiation port  36 L via the low pass filter is utilized and then the speaker device  31 L is provided with a directivity provided by a combination of omnidirectionality and bidirectionality in which synthesized sound pressure of sound waves radiated from two sound sources of the front face of the speaker  34 L and the sound wave radiation port  36 L is provided with a maximum sensitivity on the front principal axis of the speaker  34 L. 
     Further, a speaker box  32 R constituting the speaker device  31 R is formed with a speaker attaching face  33 R between a front face and a left side face thereof and the speaker attaching face  33 R is attached with a sound signal reproducing speaker  34 R. In this case, as shown by FIG. 9, the speaker  34  is attached to the speaker box  32 R such that a principal axis MA R  is inclined by the predetermined angle γ, for example, by 40° through 50° in the clockwise direction relative to a front direction F R  of the speaker box  32 R. 
     A sound radiation port (opening portion)  36 R in a circular shape covered with an acoustic resistance material  35 R is installed at an end face (upper face in the drawing) different from the speaker attaching face  33 R of the speaker box  32 R. The sound wave radiation port  36 R intends to radiate the vibration produced at a rear face of a diaphragm of the speaker  34 R to the outside as sound wave and is formed such that an axis penetrating the center thereof is extended in a direction of the principal axis (reference axis) of the speaker  34 R. Thereby, similar to the above-described speaker device  31 L, the speaker device  31 R is provided with a directivity provided by a combination of omnidirectionality and bidirectionality in which synthetic sound pressure of sound waves radiated from two sound sources of the front face of the speaker  34 R and the sound wave radiation port  36 R is provided with a maximum sensitivity on the front principal axis of the speaker  34 R. 
     In this case, the principle of providing the directionality provided by the combination of omnidirectionality and bidirectionality as mentioned above to the speaker devices  31 L and  31 R is similar to that of the speaker device  20  shown by FIG.  1  and accordingly, an explanation thereof will be omitted. 
     According to the stereophonic speaker device  30  shown by FIG. 8, the principal axes MA L  and MA R  of the sound signal reproducing speakers  34 L and  34 R are respectively inclined by the predetermined angle γ in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  32 L and  32 R and thus the principal axes MA L  and MA R  of the speakers  34 L and  34 R are directed inwardly in view of a listening position on the center line M of the speaker devices  31 L and  31 R (refer to FIG.  9 ). Further, the speaker devices  31 L and  31 R are respectively provided with directivities each provided by the combination of bidirectionality and omnidirectionality having maximum sensitivities on the front principal axes of the speakers  34 L and  34 R. 
     Therefore, at a listening position, for example, at point b (refer to FIG.  28 ), deviated from the center line M of the speaker devices  31 L and  31 R, the sound pressure level of the right channel is reduced and the sound pressure level of the left channel is more or less increased and then a difference in the sound levels based on a difference of attenuation in distances of the two left and right channels is corrected. Therefore, according to the stereophonic speaker device  30  shown by FIG. 8, a listening range providing excellent stereophonic feeling can be enlarged. 
     The applicant has carried out an auditory sensation test in order to confirm that the listening range providing excellent stereophonic feeling as mentioned above is enlarged in the stereophonic speaker device  30  shown by FIG.  8 . FIG. 10 shows a speaker device  41  used as a left channel speaker device as well as a right channel speaker device in the auditory sensation test. The speaker device  41  is provided with unidirectionality. 
     A speaker box  42  is in a shape of a rectangular parallelepiped, and dimensions of a speaker attaching face are 8.6 cm in height and breadth and 4 cm in depth and volume of air in the speaker box is 600 cm 3 . A dynamic speaker is used as a sound signal reproducing speaker  43 , which has an aperture of 8 cm, an effective diameter of a diaphragm of 7 cm and an effective area of 38.5 cm 2 . A sound wave radiation port  44  is circular and is installed at the upper face of a speaker box  42  and an effective area thereof is 26 cm 2 . A spatial distance d in the principal axis direction of two sound sources of the front face of the speakers  43  and the sound wave radiation port  44  is set to 7 cm from Equation (15). 
     Acoustic resistance of an acoustic resistance material  45  of the sound wave radiation port  44  is r 1 =0.475 (kg/sec) in MKS unit when represented by equivalent mechanical resistance converted into an area of the speaker diaphragm from Equations (2) and (9). Therefore, equivalent mechanical resistance r 1 ′ represented by an effective area of the sound wave radiation port  44  is calculated by Equation (16) as mentioned above when the effective area of the sound wave radiation port  44  is designated by notation A 1 . Further, the acoustic resistance density (equivalent mechanical resistance per unit area) of the acoustic resistance material  45  of the sound wave radiation port  44  becomes 130.8 (kg/sec·m 2 ) by dividing Equation (16) by A 1 . 
     The bold line a, the dot-dash line b and the broken line c of FIG. 7, mentioned above, show the frequency characteristics of the directivity of the output pressure of the speaker device  41  and it is found that the unidirectionality is provided at middle and low frequencies. Further, a reproduction frequency band (reduced by 3 dB) in the front direction (0° direction) falls in a range of 300 Hz through 20 KHz. The frequency at the limit where the speaker per se starts providing directivity is about 1.2 KHz from Equation (13) when the effective diameter of the diaphragm is 7 cm. Accordingly, by setting d=D=7 cm, at frequencies equal to or lower than about 1.5 KHz, the sound pressure level in 90° direction is lowered by about 6 dB and the sound pressure level in 180° direction is lowered by 15 dB through 20 dB respectively relative to the sound pressure level in 0° direction and there is provided the unidirectionality, which is regarded practically sufficient. The bold line d, the dot-dash line e and the broken line f of FIG. 7 show the frequency characteristics of the directivity of the output sound pressure when the sound wave radiation port  44  is not installed and it is omnidirectional at middle and low frequencies. 
     An auditory sensation test has been carried out by using the speaker device  41  shown by FIG. 10 as a left channel speaker device and a right channel speaker device and arranging them in the test hearing room  100  having reverberation time of about 0.2 second in a state shown by FIG.  11 . In the auditory sensation test, a listening range providing excellent stereophonic feeling is provided by the following method. 
     That is, when signal sound (for example, vocal or the like) localized in a front central direction at a listening position, for example, at point a, on the central line M of the two speaker devices  41 ,  41  is localized in a direction of a central position (point c) of the two speaker devices  41 ,  41  in view from the listening position, continuous sound stage is reproduced between the two speaker devices  41 ,  41  at the listening position, and excellent stereophonic feeling is provided even at a listening position deviated from the central line M. Hence, in the auditory sensation test, attention is paid to signal sound localized in the front central direction at a listening position, for example, point a, on the center line M of the two speaker devices  41 ,  41 , and then a range in which the signal sound was localized in a direction of a middle position of the two speaker devices  41 ,  41  at a listening position deviated from the center line M is determined. 
     In FIG. 11, broken lines L -1  show boundaries of a listening range providing excellent stereophonic feeling under condition {circle around ( 1 )} in which the sound wave radiation port  44  of the speaker device  41  is closed to constitute a closed type one and the principle axis of the speaker  43  is directed in the front direction (in parallel with side walls of the test listening room  100 ), and excellent stereophonic feeling is provided in a range on the center line M from the broken lines L -1 . Under the condition {circle around ( 1 )}, the listening range providing excellent stereophonic feeling is limited to a narrow range. 
     Further, dot-dash lines L -2  show boundaries of a listening range providing excellent stereophonic feeling under condition {circle around ( 2 )} in which the sound wave radiation port  44  of the speaker device  41  is closed to thereby constitute a closed type one and the principal axis of the speaker  43  is directed inwardly by 45°, and excellent stereophonic feeling is provided in a range on the side of the center line M from the dot-dash lines L -2 . Under the condition {circle around ( 2 )}, compared with the case of the condition {circle around ( 1 )}, the listening range providing excellent stereophonic feeling is considerably enlarged. 
     In contrast thereto, bold line L -3  show boundaries of a listening range providing excellent stereophonic feeling under condition {circle around ( 3 )} in which the sound wave radiation port  44  of the speaker device  41  is opened, the frequency range of about 1.5 KHz or lower is made unidirectional and the principal axis of the speaker  43  is directed inwardly by 45°, and excellent stereophonic feeling is provided in a range on the side of the center line M from the bold lines L -3 . According to the condition {circle around ( 3 )}, compared with the case of the condition {circle around ( 2 )}, the listening range providing excellent stereophonic feeling is further enlarged. The fact shows that the directivity at low frequencies significantly influences on sound image localization at a listening position deviated from the center line M of the two speaker devices  21 ,  21 . 
     The effect of enlarging the listening range providing excellent stereophonic feeling by such a unidirectionality can be considered as follows. For example, in FIG. 11, although a direction of the speaker device  41  for a left channel at a listening position, for example, at point d, considerably deviated from the center line M of the two speaker devices  41 , 41 , is a direction deviated by 10° from the principal axis of the speaker  43  (θ≈10°), the deviation to such a degree may substantially be regarded in a direction of a maximum sensitivity of the speaker device  41 . 
     In contract thereto, a direction of the speaker device  41  for a right channel at the point d is a direction deviated by about 85° from the principal axis of the speaker  43  (θ≈85°). Therefore, the sensitivity of the speaker device  41  for the right channel becomes 0.55, which is lowered by 5.2 dB from the maximum sensitivity (=1), in view from the directivity pattern of the unidirectionality (cardioid) of FIG.  4 . The lowering of the sound pressure level by the directivity is considered to perform excellent sound image localization even at the point d. 
     Considering in this way, in the case of hyper cardioid, the sensitivity at θ≈85° becomes 0.38, which is lowered by 8.4 dB from the maximum sensitivity (refer to FIG.  4 ), and the case of hyper cardioid may be regarded to be more effective in enlarging the listening range than unidirectionality (cardioid). 
     Although a ratio of components of bidirectionality and omnidirectionality in the case of the unidirectionality (cardioid) is 1:1, the above-described component ratio is 1:0.5 in the case of hyper cardioid, and the bidirectional component is twice as much as the omnidirectional component therein. However, in the case of directivity in which the component ratio of the bidirectional component becomes higher than that, the low frequency limit of the reproduction band is narrowed and accordingly, in a practical point of view, the directivity having the directivity pattern in a range from the unidirectionality (cardioid) to hyper cardioid is pertinent. 
     Further, in respect of the angle by which the principal axes of the speakers  43  are directed inward from the two speaker devices  41 ,  41 , when the speakers are excessively directed inwardly, for example, in the case in which listening is carried out at the point d of FIG. 11, the sound level of the speaker device  41  for the right channel is considerably lowered at a high frequency region owing to the directivity which is dependent on the diameter of the speaker diaphragm, and accordingly, there results sound quality deficient of high frequencies. Accordingly, in view from both of the effect of enlarging the listening range and the sound quality, the angle directed inwardly is pertinent in the range of 40° through 50°. 
     Next, FIG. 12 shows a stereophonic speaker device  50  as still another best mode. The speaker device  50  is constituted by a left channel speaker device  51 L and a right channel speaker device  51 R. 
     The speaker device  51 L is of a 2-way system in which a woofer  53 L and a tweeter  54 L constituting sound signal reproducing speakers are attached at a front face of a speaker box  52 L substantially in a shape of a rectangular parallelepiped. The woofer  53 L and the tweeter  54 L are attached to a front face side of the speaker box  52 L such that principal axes of the woofer  53 L and the tweeter  54 L are directed in the front direction of the speaker box  52 L. Further, the speaker box  52 L is formed with a speaker attaching face  55 L by cutting off an upper portion of a corner constituted by a front face and a right side face thereof and the attaching face  55 L is attached with a sound signal reproducing speaker  56 L. In this case, as shown by FIG. 13, the speaker  56 L is attached to the speaker box  52 L such that a principal axis MA L  is inclined by a predetermined angle γ, for example, by 40° through 50°, in the counterclockwise direction relative to a front direction F L  of the speaker box  52 L. 
     Further, a sound wave radiation port (opening portion)  58 L in a circular shape covered with an acoustic resistance material  57 L is installed at an end face (upper face in the drawing) different from the speaker attaching face  55 L of the speaker box  52 L. The sound wave radiation port  58 L intends to radiate air vibration produced by a rear face of a diaphragm of the speaker  56 L and is formed such that an axis penetrating the center is extended in a direction of the principal axis of the speaker  56 L. Although not mentioned above, at inside of the speaker box  52 L, a box portion where the speaker  56 L is attached is partitioned from other box portion. 
     In this case, an acoustic low pass filter is constituted by acoustic capacitance provided by air in the box portion attached with the speaker  56 L of the speaker box  52 L and the acoustic resistance and acoustic mass of the sound wave radiation port  58 L. Further, group delay time in a pass band of sound wave radiated from the sound wave radiation port  58 L via the low pass filter is utilized and a speaker portion constituted by the speaker  56 L of the speaker device  51 L is provided with a directivity provided by a combination of omnidirectionality and bidirectionality such that synthesized sound pressure of sound waves radiated from two sound sources of the front face of the speaker  56 L and the sound wave radiation port  58 L is provided with a maximum sensitivity on the front principal axis of the speaker  56 L. 
     The speaker device  51 R is of a 2-way system attached with a woofer  53 R and a tweeter  54 R constituting sound signal reproducing speakers at a front face of the speaker box  52 R substantially in a shape of a rectangular parallelepiped. The woofer  53 R and the tweeter  54 R are attached to the front face side of the speaker box  52 R such that principal axes thereof are directed in the front direction of the speaker box  52 R. Further, the speaker box  52 R is formed with a speaker attaching face  55 R by cutting off an upper portion of a corner constituted by a front face and a left side face thereof and the attaching face  55 R is attached with a sound signal reproducing speaker  56 R. In this case, as shown by FIG. 13, the speaker  56 R is attached to the speaker box  52 R such that a principal axis MA R  thereof is inclined by the predetermined angle γ, for example, by 40° through 50°, in the clockwise direction relative to a front direction F R  of the speaker box  52 R. 
     Further, a sound wave radiation port (opening portion)  58 R in a circular shape covered with an acoustic resistance material  57 R is installed at an end face (upper face in the drawing)different from the speaker attaching face  55 R of the speaker box  52 R. The sound wave radiation port  58 R intends to radiate air vibration produced by a rear face of a diaphragm of the speaker  56 R to the outside as sound wave and is formed such that an axis penetrating the center thereof is extended in the direction of the principal axis of the speaker  56 R. Although not mentioned above, at inside of the speaker box  52 R, a box portion attached with the speaker  56 R is partitioned by other box portion. 
     In this case, an acoustic low pass filter is constituted by acoustic capacitance produced by air in the box portion attached with the speaker  56 R of the speaker box  52 R and acoustic resistance and acoustic mass of the sound wave radiation port  58 R. Further, group delay time in a pass band of sound wave radiated from the sound wave radiation port  58 R via the low pass filter is utilized and a speaker portion constituted by the speaker  56 R of the speaker device  51 R is provided with a directivity provided by a combination of omnidirectionality and bidirectionality such that synthesized sound pressure of sound waves radiated from two sound sources of the front face of the speaker  56 R and the sound wave radiation port  58 R is provided with a maximum sensitivity on the front principal axis of the speaker  56 R. 
     According to the stereophonic speaker device  50  shown by FIG. 12, the principal axes MA L  and MA R  of the sound signal reproducing speakers  56 L and  56 R are inclined by the predetermined angle γ respectively in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  52 L and  52 R, and then the principal axes MA L  and MA R  of the speakers  56 L and  56 R are respectively directed inwardly in view from a listening position on the center line M of the speaker devices  51 L and  51 R (refer to FIG.  13 ). Further, the speaker portions constituted by the speakers  56 L and  56 R of the speaker devices  51 L and  51 R are provided with directivities each provided by the combination of bidirectionality and omnidirectionality having a maximum sensitivity on the front principal axis of each of the speakers  56 L and  56 R. 
     Therefore, at a listening position, for example, at point b (refer to FIG.  28 ), deviated from the center line M of the speaker devices  51 L and  51 R, a sound pressure level of a right channel is reduced and a sound pressure level of a left channel is more or less increased and then a difference in the sound pressure levels based on a difference in attenuation of sound in distances to the two left and right channels is corrected. Therefore, according to the stereophonic speaker device  50  shown by FIG. 12, a listening range providing excellent stereophonic feeling can be enlarged similar to the stereophonic speaker device  30  shown by FIG.  8 . 
     The applicant has carried out an auditory sensation test in order to confirm that the listening range providing excellent stereophonic feeling is enlarged in the stereophonic speaker device  50  shown by FIG. 12 as mentioned above. FIG. 14 shows a speaker device  60  used as a left channel speaker device and a right channel speaker device in the auditory sensation test. The speaker device  60  is arranged with the speaker device  41  shown by FIG. 10 on a speaker device  61 . The speaker device  61  is of a 2-way system in which a woofer  63  having an aperture of 14 cm and a tweeter  64  having an aperture of 8 cm, for reproducing sound signal are attached to a speaker box  62  of lateral width=20 cm, depth=22 cm and height=30 cm. 
     The auditory sensation test has been carried out by using the speaker device  60  shown by FIG. 14 as the left channel speaker device and as the right channel speaker device and arranging them in the test listening room  100  having reverberation time of about 0.2 second in a state shown by FIG.  15 . In this case, by a method similar to that in the case of FIG. 11, a listening range providing excellent stereophonic feeling is determined. Further, the speaker device  61  of the 2-way system is arranged such that the principal axes of the woofer  63  and the tweeter  64  are directed in the front direction (in parallel with side walls of test listening room  100 ). Further, in a state in which the principal axis of the speakers  43  of the speaker device  41  is directed in the front direction, a sound pressure level at a listening portion, for example, at point a, on the center line M of the two speaker devices  60 ,  60  of the speaker device  41  is set to be lower than a sound pressure level of the speaker device  61  of the 2-way system by 3 dB. 
     In FIG. 15, broken lines L -4  show boundaries of the listening range providing excellent stereophonic feeling under the condition {circle around ( 4 )} in which the sound wave radiation port  44  of the speaker device  41  is closed to thereby constitute a closed type one and the principal axis of the speaker  43  is directed in the front direction, and thus, excellent stereophonic feeling is provided within a range on the side of the center line M from the broken lines L -4 . Under the condition {circle around ( 4 )} the listening range providing excellent stereophonic feeling is restricted to a narrow range similar to the case of the condition {circle around ( 1 )} of FIG.  11 . 
     Further, dot-dash lines L -5  show boundaries of the listening range providing excellent stereophonic feeling under the condition {circle around ( 5 )} in which the sound wave radiation port  44  of the speaker device  41  is closed to thereby constitute a closed type one and the principal axis of the speaker  43  is directed inwardly by 45°, and excellent stereophonic feeling is provided within a range on the side of the center line M from the dot-dash lines L -5 . Although the listening range providing excellent stereophonic feeling is considerably enlarged under the condition {circle around ( 5 )}, as compared with the case of condition {circle around ( 4 )}, the listening range is more or less narrower than that in the case of the condition {circle around ( 2 )} of FIG.  11 . 
     In contrast thereto, bold lines L -6  show boundaries of the listening range providing excellent stereophonic feeling under condition {circle around ( 6 )} in which the sound wave radiation port  44  of the speaker device  41  is opened, unidirectionality is constituted in a frequency range of about 1.5 KHz or lower and the principal axis of the speaker  43  is directed inwardly by 45°, and excellent stereophonic feeling was determined within a range on the side of the center line M from the bold lines L -6 . 
     Under the condition {circle around ( 6 )}, compared with the case of the condition {circle around ( 5 )}, the listening range providing excellent stereophonic feeling is further enlarged. It can say that, although the listening range is more or less narrower than that in the case of the condition {circle around ( 3 )} of FIG. 11, a significant effect is achieved in enlarging the listening range providing preferable stereophonic feeling by adding sound waves radiated from the small-sized unidirectional speaker devices  41  to sound waves radiated from the speaker devices  61  of the 2-way system directed in the front direction under the above-described pressure level condition. 
     In this case, in respect of the directivity of the added speaker device  41 , the directivity having the directivity pattern in a range from the cardioid to hyper cardioid is suitable as mentioned above. Further, in respect of the angle of setting the principal axis of the directivity speaker, the device is suitable to direct inwardly in a range of 40° through 50°. 
     As mentioned above, according to the stereophonic speaker devices  30  and  50  shown by FIG.  8  and FIG. 12, the principal axes of the sound signal reproducing speakers of the speaker devices of the left channel and the right channel are directed inwardly in view from a listening position on the center line of the two speaker devices and the speaker portions constituted by the sound signal reproducing speakers of the two speaker devices are provided with directivities each provided by a combination of bidirectionality and omnidirectionality. Accordingly, at a listening position deviated from the center line of the two speaker devices, a difference in sound pressure levels based on a difference in attenuation of sound in distances to both left and right channels is corrected and the listening range providing excellent stereophonic feeling can be enlarged. Further, the speaker portions constituted by the sound signal reproducing speakers of the two speaker devices are provided with directivities each provided by a combination of bidirectionality and omnidirectionality and then the directivity can be provided by utilizing group delay time of sound wave radiated from, for example, the sound wave radiation port. Accordingly, there is achieved a merit capable of downsizing the system constitution, compared with the conventional case in which the directivity provided by a speaker per se is utilized or a bidirectional speaker is utilized. 
     Next, FIG. 16 shows a stereophonic speaker device  70  as further best mode. The speaker device  70  is constituted by a left channel speaker device  71 L and a right channel speaker device  71 R. 
     The speaker device  71 L is of a 2-way system in which a woofer  73 L and a tweeter  74 L constituting sound signal reproducing speakers are attached to a front face of a speaker box  72 L substantially in a shape of a rectangular parallelepiped. The woofer  73 L and the tweeter  74 L are attached to the front face side of the speaker box  72 L such that principal axes thereof are directed in the front direction of the speaker box  72 L. Further, the speaker box  72 L is formed with a speaker attaching face  75 L by cutting off an upper portion of a corner constituted by a front face and a right side face thereof and the attaching face  75 L is attached with a sound signal reproducing speaker  76 L and a surround signal reproducing speaker  77 L. In this case, as shown by FIG. 17, the speakers  76 L and  77 L are attached to the speaker box  72 L such that principal axes MA L  and SA L  are inclined by a predetermined angle γ, for example, 40° through 50°, in the counterclockwise direction relative to the front direction of the speaker box  72 L. 
     Further, a sound wave radiation port (opening portion)  79 L in a circular shape covered with acoustic resistance material  78 L is installed at an end face (upper face in the drawing) different from the speaker attaching face  75 L of the speaker box  72 L. The sound wave radiation port  79 L intends to radiate air vibration produced by a rear face of a diaphragm of the speaker  76 L to the outside as sound wave and is formed such that an axis penetrating the center thereof is extended in a direction of the principal axis of the speaker  76 L. Although not illustrated, at inside of the speaker box  72 L, a box portion attached with the speaker  76 L is partitioned from other box portion. 
     In this case, an acoustic low pass filter is constituted by acoustic capacitance provided by air in the box portion attached with the speaker  76 L of the speaker box  72 L and acoustic resistance and acoustic mass of the sound wave radiation port  79 L. Further, group delay time in a pass band of sound wave radiated from the sound wave radiation port  79 L via the low pass filter is utilized and the speaker portion constituted by the speaker  76 L of the speaker device  71 L is provided with a directivity provided by a combination of omnidirectionality and bidirectionality in which synthesized sound pressure of sound waves radiated from two sound sources of the front face of the speaker  76 L and the sound radiation port  79 L is provided with a maximum sensitivity on the front principal axis of the speaker  76 L. 
     The speaker device  71 R is of a 2-way system in which a woofer  73 R and a tweeter  74 R constituting sound signal reproducing speakers are attached to a front face of a speaker box  72 R substantially in a shape of a rectangular parallelepiped. The woofer  73 R and the tweeter  74 R are attached to the front face side of the speaker box  72 R such that principal axes thereof are directed in the front direction of the speaker box  72 R. Further, the speaker box  72 R is formed with a speaker attaching face  75 R by cutting off an upper portion of a corner constituted by a front face and a left side face thereof and the attaching face  75 R is attached with a sound signal reproducing speaker  76 R and a surround signal reproducing speaker  77 R. In this case, as shown by FIG. 17, the speaker  76 R is attached to the speaker box  72 R such that principal axes MA R  and SA R  are inclined by the predetermined angle γ, for example, by 40° through 50°, in the clockwise direction relative to the front direction F R  of the speaker box  72 R. 
     Further, a sound wave radiation port (opening portion)  79 R in a circular shape covered with an acoustic resistance material  78 R is installed at an end face(upper face in the drawing) different from the speaker attaching face  75 R of the speaker box  72 R. The sound wave radiation port  79 R intends to radiate air vibration produced by a rear face of a diaphragm of the speaker  76 R to the outside as sound wave and is formed such that an axis penetrating the center thereof is extended in a direction of the principal axis of the speaker  76 R. Although not mentioned above, at inside of the speaker box  72 R, a box portion attached with the speaker  76 R is partitioned from other box portion. 
     In this case, an acoustic low pass filter is constituted by acoustic capacitance provided by air in the box portion attached with the speaker  76 R of the speaker box  72 R and acoustic resistance and acoustic mass of the sound wave radiation port  79 R. Further, group delay time in a pass band of sound wave radiated from the sound wave radiation port  79 R via the low pass filter is utilized and thus, the speaker portion constituted by the speaker  76 R of the speaker device  71 R is provided with a directivity provided by the combination of omnidirectionality and bidirectionality such that synthesized sound pressure of sound waves radiated from two sound sources of the front face of the speaker  76 R and the sound wave radiation port  79 R is provided with a maximum sensitivity on the front principal axis of the speaker  76 R. 
     In this case, in respect of the principle providing the directivity provided by the combination of omnidirectionality and bidirectionality to the speaker portions respectively constituted by the speakers  76 L and  76 R of the speaker devices  71 L and  71 R as mentioned above, the principle is similar to that in the speaker device  20  shown by FIG.  1  and accordingly, an explanation thereof will be omitted. 
     According to the stereophonic speaker device  70  shown by FIG. 16, the principal axes MA L  and MA R  of the sound signal reproducing speakers  76 L and  76 R are inclined by the predetermined angle γ respectively in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  72 L and  72 R, and the principal axes MA L  and MA R  of the speakers  76 L and  76 R are directed inwardly in view from a listening position on the center line of the speaker devices  71 L and  71 R (refer to FIG.  17 ). Further, the speaker portions constituted by the speakers  76 L and  76 R of the speaker devices  71 L and  71 R are provided with the directivities each provided by the combination of bidirectionality and omnidirectionality having a maximum sensitivity on the front principal axes of each of the speakers  76 L and  76 R. 
     Accordingly, at a listening position, for example, at point b (refer to FIG.  28 ), deviated from the center line M of the speaker devices  71 L and  71 R, a sound pressure level of a right channel is reduced, a sound pressure level of a left channel is more or less increased, and then a difference in sound pressure levels based on a difference in attenuation of sound in distances to the two left and right channels is corrected. Therefore, according to the stereophonic device  70  shown in FIG. 16, a listening range providing excellent stereophonic feeling can be enlarged. 
     Further, according to the stereophonic speaker device  70  shown in FIG. 16, the principal axes SA L  and SA R  of the surround signal reproducing speakers  77 L and  77 R are inclined by the predetermined angle γ respectively in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  72 L and  72 R. Therefore, the principal axes MA L  and SA R  of the speakers  77 L and  77 R are directed inwardly in view from a listening position on the center line M of the speaker devices  71 L and  71 R (refer to FIG.  17 ). 
     Generally, an S signal component, which is a major component of the surround signal, is a signal having major components at middle and high frequencies. Further, according to the speakers  77 L and  77 R, the diaphragms are provided with limited areas and accordingly, as shown by FIG. 34, at middle and high frequencies, radiated sound wave per se is provided with a directivity. 
     Therefore, by directing the principal axes SA L  and SA R  of the speakers  77 L and  77 R inwardly as mentioned above, at a listening position, for example, at point f (refer to FIG. 31) deviated from the center line M, a level of the surround signal sound of a right channel is reduced and a level of the surround signal sound of a left channel is more or less increased to perform the correcting of a difference in levels based on a difference in attenuation of sounds in distances of the surround signal sound of the two left and right channels. Accordingly, according to the stereophonic speaker device  70  shown by FIG. 16, a listening range providing excellent surround effect can be enlarged. 
     The applicant has confirmed that the listening range providing excellent stereophonic feeling is enlarged in the stereophonic speaker device  60  shown by FIG. 16 as mentioned above, by an auditory sensation test in which, similar to the stereophonic speaker device  50  shown by FIG. 12, the speaker device  60  shown by FIG. 14 is used as the left channel speaker device and as the right channel speaker device (refer to FIG.  15 ). 
     Further, the applicant has carried out the auditory sensation test in order to confirm to what degree the listening range providing a surround effect can be enlarged in the case in which the surround signal is reproduced by the surround signal reproducing speakers  77 L and  77 R of FIG. 16 in addition to the sound signal (LR signals) reproducing devices (speaker device) enlarging the listening range in such a way. FIG. 18 shows a speaker device  80  used as the left channel speaker device and as the right channel speaker device in the auditory sensation test. The speaker device  80  is arranged with a speaker device  81  between the speaker devices  61  and  41  shown by FIG.  14 . The speaker device  81  is attached with a surround signal reproducing speaker  83  having an aperture of 8 cm in a closed speaker box  82  of lateral width=9 cm, depth=7 cm and height=9 cm, and a reproduction band thereof falls within a range of 300 Hz through 20 KHz. 
     The auditory sensation test has been carried out by using the speaker device  80  shown by FIG. 18 as the left channel speaker device and as the right channel speaker device and arranging them in the test listening room  100  having reverberation time of about 0.2 second in a state shown by FIG.  19 . The test conditions are as follows: Notation {circle around ( 7 )} of FIG. 19 shows a case in which the speaker devices  41  are not used as the conventional system, but the principal axes of the speakers  83  of the surround signal reproducing speakers devices  81  are directed outwardly by 30° with the principal axes of the woofers  63  and tweeters  64  of the speaker devices  61  of two left and right channels being directed in the front direction (in parallel with side walls of the test listening room  100 ); Notation {circle around ( 8 )} of FIG. 19 show a case in which the speaker devices  41  are not used as the conventional system, but the principal axes of the speakers  83  of the surround signal reproducing speakers devices  81  are directed in the front direction with the principal axes of the woofers  63  and the tweeters  64  of the speaker devices  61  of two left and right channels being directed in the front direction; Notation {circle around ( 9 )} of FIG. 19 shows a case in which the speaker devices  41  are used as the system according to the present invention, and the principal axes of the speakers  83  of the surround signal reproducing speaker devices  81  are directed inwardly by 45° in the state that the principal axes of the woofers  63  and the tweeters  64  of the speaker devices  61  of two left and right channels are directed in the front direction and the principal axes of the speakers  43  of the speaker devices  41  are directed inwardly by 45°. 
     Further, in the state in which the principal axes of the speakers  43  of the speaker device  41  are directed in the front direction, a sound pressure level at a listening position, for example, at point e, on the center line M of the two speaker devices  80 ,  80  of the speaker devices  41  is set to be lower than a sound pressure level of the speaker devices  61  of the 2-way system by 3 dB. 
     In FIG. 19, broken lines L -7  show boundaries of a listening range providing excellent surround effect in the case of the above-described test condition {circle around ( 7 )}, and thus an excellent surround effect is achieved within a range on the side of the center line M from the broken line L -7 . Further, dot-dash lines L -7  show boundaries of a listening range providing excellent surround effect in the case of the above-described test condition {circle around ( 8 )}, and thus an excellent surround effect is achieved within a range on the side of the center line M from the dot-dash lines L -8 . Although the listening range providing excellent surround effect is more or less enlarged in this case as compared with the case of {circle around ( 7 )}, the range is limited to a narrow range. 
     In contrast thereto, bold lines L -9  show boundaries of a listening range providing excellent surround effect in the case of the above-described test condition{circle around ( 9 )} according to the present invention, and thus an excellent surround effect is achieved within a range on the side of the center line M from the bold lines L -9 . In this case, compared with the case of {circle around ( 8 )}, the listening range achieving excellent surround effect is significantly enlarged. Thereby, it has been confirmed that the listening range providing excellent surround effect can further be enlarged by the stereophonic speaker device  70  shown by FIG.  16 . 
     Further, although, according to the stereophonic speaker device  70  shown by FIG. 16, the sound signal reproducing speakers  76 L ( 76 R) and the surround signal reproducing speakers  77 L ( 77 R) are respectively attached to the speaker attaching faces  75 L ( 75 R), they can also be combined by single ones of speakers. In that case, a single one of a speaker is driven by a signal produced by adding surround signal to sound signal. 
     Next, FIG. 20 shows a stereophonic speaker device  90  as still further best mode. The speaker device  90  is constituted by a left channel speaker device  91 L and a right channel speaker device  91 R. 
     A speaker attaching face  93 L is formed between a front face and a right side face of a speaker box  92 L constituting the speaker device  91 L and the speaker attaching face  93 L is attached with a sound signal reproducing speaker  94 L and a surround signal reproducing speaker  95 L. In this case, as shown by FIG. 21, the speakers  94 L and  95 L are attached to the speaker box  92 L such that principal axes MA L  and SA L  are inclined by a predetermined angle γ, for example, by 40° through 50°, in the counterclockwise direction relative to a front direction F L  of the speaker box  92 L. 
     Further, a sound wave radiation port (opening portion)  97 L in a circular shape covered with an acoustic resistance material  96 L is installed at an end face (upper face in the drawing) thereof different from the speaker attaching face  93 L of the speaker box  92 L. The sound wave radiation port  97 L intends to radiating air vibration produced by a rear face of a diaphragm of the speaker  94 L to the outside as sound wave and is formed such that an axis penetrating the center thereof is extended in a direction of the principal axis of the speaker  94 L. Although not mentioned above, at inside of the speaker box  92 L, a box portion attached with the speaker  94 L is partitioned from other box portion. 
     In this case, an acoustic low pass filter is constituted by acoustic capacitance provided by air in the box portion attached with the speaker  94 L of the speaker box  92 L and acoustic resistance and acoustic mass of the sound wave radiation port  97 L. Further, group delay time in a pass band of sound wave radiated from the sound wave radiation port  97 L via the low pass filter is utilized and the speaker portion constituted by the speaker  94 L of the speaker device  91 L is provided with a directivity provided by a combination of omnidirectionality and bidirectionality such that synthesized sound pressure of sound waves radiated from two sound sources of the front face of the speaker  94 L and the sound face radiation port  97 L is provided with a maximum sensitivity on the front principal axis of the speaker  94 L. 
     A speaker box  92 R constituting the speaker device  91 R is formed with a speaker attaching face  93 R between a front face and a left side face thereof and the speaker attaching face  93 R is attached with a sound signal reproducing speaker  94 R and a surround signal reproducing speaker  95 R. In this case, as shown by FIG. 21, the speakers  94 R and  95 R are attached to the speaker box  92 R such that principal axes MA L  and SA L  thereof are inclined by the predetermined angle γ, for example, by 40° through 50° in the clockwise direction relative to the front direction FL of the speaker box  92 R. 
     Further, a sound wave radiation port (opening portion)  97 R in a circular shape covered with acoustic resistance material  96 R is installed at an end face (upper face in the drawing) different from the speaker attaching face  93 R of the speaker box  92 R. The sound wave radiation port  97 R intends to radiate air vibration produced by a rear face of a diaphragm of the speaker  94 R to the outside as sound wave and is formed such that an axis penetrating the center thereof is extended in a direction of the principal axis of the speaker  94 R. Although not mentioned above, at inside of the speaker box  92 R, a box portion attached with the speaker  94 R is partitioned from other box portion. 
     In this case, a low pass filter is constituted by acoustic capacitance produced by air in the box portion attached with the speaker  94 R of the speaker box  92 R and acoustic resistance and acoustic mass of the sound wave radiation port  97 R. Further, group delay time in a pass band of sound wave radiated from the sound wave radiation port  97 R via the low pass filter is utilized and the speaker portion constituted by the speaker  94 R of the speaker device  91 R is provided with a directivity provided by a combination of omnidirectionality and bidirectionality such that synthesized sound pressure of sound waves radiated from two sound sources of the front face of the speaker  94 R and the sound wave radiation port  97 R is provided with a maximum sensitivity on the front principal axis of the speaker  94 R. 
     According to the stereophonic speaker device  90  shown by FIG. 20, the principal axes MA L  and MA R  of the sound signal reproducing speakers  94 L and  94 R are inclined by the predetermined angle γ respectively in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  92 L and  92 R and then the principal axes MA L  and MA R  of the speakers  94 L and  94 R are directed inwardly in view from a listening position on the center line M of the speaker devices  91 L and  91 R (refer to FIG.  21 ). Further, the speaker portions constituted by the speakers  94 L and  94 R of the speaker devices  91 L and  91 R are provided with the directivities each provided by the combination of bidirectionality and omnidirectionality having a maximum sensitivity on the front principal axes. 
     Accordingly, at a listening position, for example, at point b (refer to FIG. 28) deviated from the center line M of the speaker device  91 L and  91 R, a sound pressure level of a right channel is reduced, but a sound pressure level of a left channel is more or less increased and then a difference in the sound pressure levels based on a difference in attenuation of sound in distances to the two left and right channels is corrected. Therefore, according to the stereophonic speaker device  90  shown by FIG. 20, the listening range providing excellent stereophonic feeling can be enlarged similar to the stereophonic speaker device  70  shown by FIG.  16 . 
     Further, according to the stereophonic speaker device  90  shown by FIG. 20, the principal axes SA L  and SA R  of the surround signal reproducing speakers  95 L and  95 R are inclined by the predetermined angle γ respectively in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  92 L and  92 R. Therefore, the principal axes SA L  and SA R  of the speakers  95 L and  95 R are directed inwardly in view from a listening position on the center line M of the speaker devices  91 L and  91 R (refer to FIG.  21 ). Therefore, similar to the stereophonic speaker device  70  shown by FIG. 16, at a listening position deviated from the center line M of the speaker device  91 L and  91 R, a difference in levels based on a difference in attenuation of sound in distances of the surround signal sound of left and right channels from the speakers  95 L and  95 R is corrected and accordingly, the listening range providing excellent surround effect can be enlarged. 
     Further, although, according to the stereophonic speaker device  90  shown by FIG. 20, the surround signal reproducing speakers  94 L ( 94 R) and the surround signal reproducing speakers  95 L ( 95 R) are respectively attached to the speaker attaching faces  93 L ( 93 R), they can also be combined by single ones of speakers. In this case, a single one of a speaker is driven by a signal produced by adding a surround signal to a sound signal. 
     In this way, according to the stereophonic speaker devices  70  and  90  shown by FIG.  16  and FIG. 20, the principal axes of the sound signal reproducing speakers of the speaker devices for the left channel and the right channel are directed inwardly in view from a listening position on the center line of the two speaker devices and the speaker portions constituted by the sound signal reproducing speakers of the two speaker devices are provided with the directivities each provided by the combination of bidirectionality and omnidirectionality. Accordingly, at a listening position deviated from the center line of the two speaker devices, a difference in sound pressure levels based on a difference in attenuation of sound in distances to the two left and right channels is corrected and the listening range providing excellent stereophonic feeling can be enlarged. 
     Further, the speaker portions constituted by the sound signal reproducing speakers of the two speaker devices are provided with the directivities each provided by the combination of bidirectionality and omnidirectionality and the directivity can be provided by utilizing group delay time of sound wave radiated from, for example, the sound wave radiation port, so that there is provided a merit capable of downsizing the system constitution as compared with a case in which the directivity provided by a speaker per se is utilized as in the conventional case or a bidirectional speaker is utilized. 
     Further, the principal axes of the speakers for reproducing the surround signal of the speaker devices for the left channel and the right channel are directed inwardly in view from a listening position on the center line of the two speaker devices and accordingly, at a listening position deviated from the center line of two speaker devices, a difference in levels based on a difference in attenuation of sound in distances of surround signal sound from the two left and right channels is corrected by the directivities of the speakers for reproducing the surround signal at middle and high frequencies, an effect of enlarging the listening range in respect of sound signal (LR signal) reproducing sound, mentioned above, is added and accordingly, the listening range providing excellent surround effect can significantly be enlarged. 
     Further, the low pass filter is constituted by the acoustic resistance and the acoustic mass of the sound wave radiation port and the acoustic capacitance provided by air in the speaker box and the directivity is provided by utilizing group delay time in the pass band of sound wave radiated from the sound wave radiation port via the low pass filter, whereby an arbitrary group delay time period is set regardless of dimensions or shape of the speaker box and desired directivity can stably be provided down to ultra low frequencies. 
     Next, FIG. 22 shows a front surround speaker device as still another best mode. The speaker device  110  is constituted by a left channel speaker device  111 L and a right channel speaker device  111 R. 
     The speaker device  111 L is of a 2-way system in which a woofer  113 L and a tweeter  114 L constituting sound signal reproducing speakers are attached to a front face of a speaker box  112 L substantially in a shape of a rectangular parallelepiped. The woofer  113 L and the tweeter  114 L are attached to the front face side of a speaker box  112 L such that principal axes thereof are directed in the front direction of the speaker box  112 L. Further, the speaker box  112 L is formed with a speaker attaching face  115 L by cutting off an upper portion of a corner constituted by a front face and a right side face thereof and the attaching face  115 L is attached with a surround signal reproducing speaker  116 L. In this case, as shown by FIG. 23, the speaker  116 L is attached to the speaker box  112 L such that a principal axis MA L  is inclined by a predetermined angle α, for example, by 40° through 50°, in the counterclockwise direction relative to a front direction F L  of the speaker box  112 L. 
     The speaker device  110 R is of a 2-way system in which a woofer  113 R and a tweeter  114 R constituting sound signal reproducing speakers are attached to the front face of a speaker box  112 R substantially in a shape of a rectangular parallelepiped. The woofer  113 R and the tweeter  114 R are attached to the front face side of the speaker box  112 R such that a principal axis thereof is directed in the front direction of the speaker box  112 R. Further, the speaker box  112 R is formed with a speaker attaching face  115 R by cutting off an upper portion of a corner constituted by a front face and a left side face thereof and the attaching face  115 R is attached with a surround signal reproducing speaker  116 R. In this case, as shown by FIG. 23, the speaker  116 R is attached to the speaker box  112 R such that a principal axis MA R  thereof is inclined by the predetermined angle α, for example, by 40° through 50°, in the clockwise direction relative to a front face direction F R  of the speaker box  112 R. 
     According to the front surround speaker device  110  shown by FIG. 22, the principal axes MA L  and MA R  of the speakers  116 L and  116 R are inclined by the predetermined angle α respectively in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  112 L and  112 R. Therefore, the principal axes MA L  and MA R  of the speakers  116 L and  116 R are directed inwardly in view from a listening position on the center line M of the speaker devices  111 L and  111 R (refer to FIG.  23 ). 
     Generally, an S signal component, which is a major component of a surround signal, is a signal having major components in middle and high frequencies. Further, according to the speakers  116 L and  116 R, diaphragms thereof are provided with limited areas and accordingly, as shown by FIG. 34, radiated sound wave per se is provided with a directivity in middle and high frequencies. 
     Accordingly, as mentioned above, by directing inwardly the principal axes MA L  and MA R  of the speakers  116 L and  116 R, a level of surround signal sound of a right channel is reduced and a level of surround signal sound of a left channel is more or less increased at a listening position deviated from the center line M, for example, at point f (refer to FIG.  31 ), and thus a difference in levels based on a difference in attenuation of sound in distances of surround signal sound of the two left and right channels is corrected. Therefore, according to the front surround speaker device  110  shown by FIG. 22, a listening range providing excellent surround effect can be enlarged. 
     The applicant has carried out an auditory sensation test in order to confirm that the listening range providing excellent surround effect is enlarged in the front surround speaker device  110  shown by FIG. 22 as mentioned above. FIG. 24 shows a speaker device  120  used as a left channel speaker device and as a right channel speaker device in the auditory sensation test. The speaker device  120  is constituted by arranging a speaker device  125  on a speaker device  121 . The speaker device  121  is of a 2-way system in which a woofer  123  having an aperture of 14 cm and a tweeter  124  having an aperture of 8 cm, for reproducing sound signals attached to a speaker box  122  of lateral width =20 cm, depth=22 cm and height=30 cm, and a reproduction band thereof falls in a range of 50 Hz through 20 kHz. Further, the speaker device  125  is attached with a surround signal reproducing speaker  127  having an aperture of 8 cm in a closed speaker box  126  of lateral width=9 cm, depth=7 cm and height=9 cm, and a reproduction band thereof falls in a range of 300 Hz through 20 kHz. 
     An auditory sensation test has been carried out by using the speaker device  120  shown by FIG. 24 as a left channel speaker device and as a right channel speaker device and arranging them in the test listening room  100  having reverberation time of about 0.2 second in a state shown by FIG.  25 . Further, in the auditory sensation test, in a state in which the principal axes (reference axes) of the woofers  123  and the tweeters  124  of the speaker devices  121  constituting the speaker devices  120  for the left channel and for the right channel are directed in the front direction (in parallel with side walls of the test listening chamber  100 ), listening ranges providing excellent surround effect are determined in cases: {circle around ( 1 )} in which the principal axes of the speakers  127  of the speaker devices  125  are directed outwardly by 30°; {circle around ( 2 )} in which the principal axes of the speakers  127  of the speaker devices  125  are directed in the front direction; and {circle around ( 3 )} in which the principal axes of the speakers  127  of the speaker devices  125  are directed inwardly by 45°. 
     In FIG. 25, broken lines L -11  show boundaries of a listening range providing excellent surround effect in the case in which the principal axes of the surround signal reproducing speakers  127  are directed outwardly by 30°, and then an excellent surround effect is achieved within a range on the side of the center line M from the broken lines L -11 . Further, dot-dash lines L -12  show boundaries of a listening range providing excellent surround effect in the case in which the principal axes of the speakers  127  are directed in the front direction, and then an excellent surround effect is achieved within a range on the side of the center line M from the dot-dash lines L -12 . In this case, the listening range achieving the excellent surround effect is more or less enlarged, compared with the case in which the principal axes of the speakers  127  are directed outwardly by 30°. 
     In contrast thereto, bold lines L -13  show boundaries of a listening range providing excellent surround effect in the case in which the principal axes of the speakers  127  are directed inwardly by 45°, and then excellent surround effect is achieved within a range on the side of the center line M from the bold lines L -12 . In this case, compared with the case in which the principal axes of the speakers  127  are directed in the front direction, the listening range providing excellent surround effect is further enlarged. Thereby, it has been confirmed that the listening range providing excellent surround effect can be enlarged by the front surround speaker device  110  shown by FIG.  22 . 
     Next, FIG. 26 shows a front surround speaker device  130  as still further best mode. The speaker device  130  is constituted by a left channel speaker device  131 L and a right channel speaker device  131 R. 
     A speaker box  132 L constituting the speaker device  131 L is formed with a speaker attaching face  133 L between a front face and a right side face thereof. Further, the speaker device  131 L is of a 2-way system in which a woofer  134 L and a tweeter  135 L are attached to the speaker attaching face  133 L. The woofer  134 L and the tweeter  135 L function as sound signal reproducing speakers and function also as sound signal reproducing speakers. In this case, as shown by FIG. 27, the woofer  134 L and the tweeter  135 L are attached to the speaker box  132 L such that a principal axis MA L  thereof is inclined by a predetermined angle β, for example, by 40° through 50°, in the counterclockwise direction relative to a front direction F L  of the speaker box  132 L. 
     Further, a speaker box  132 R constituting the speaker device  131 R is formed with a speaker attaching face  133 R between a front face and a left side face thereof. Further, the speaker device  131 R is of a 2-way system in which a woofer  134 R and a tweeter  135 R are attached to the speaker attaching face  133 R. The woofer  134 R and the tweeter  135 R function as sound signal reproducing speakers and function also as surround signal reproducing speakers. In this case, as shown by FIG. 27, the woofer  134 R and the tweeter  135 R are attached to the speaker box  132 R such that a principal axis MA R  thereof is inclined by the predetermined angle β, for example, by 40° through 50°, in the clockwise direction relative to a front direction F R  of the speaker box  132 R. 
     According to the front surround speaker device  130  shown by FIG. 26, the principal axes MA L  and MA R  of the speakers ( 134 L,  135 L) and ( 134 R,  135 R) reproducing surround signals are inclined by the predetermined angle β respectively in the counterclockwise direction and the clockwise direction relative to the front directions F L  and F R  of the speaker boxes  132 L and  132 R. Accordingly, the principal axes MA L  and MA R  of the speakers ( 134 L,  135 L) and ( 134 R,  135 R) are directed inwardly in view from a listening position on the center line M of the speaker devices  131 L and  131 R (refer to FIG.  27 ). Accordingly, similar to the front surround speaker device  110  shown by FIG. 22, a difference in levels based on a difference in attenuation of sound in distances of surround signal sound of the two left and right channels from the speakers ( 134 L,  135 L) and ( 134 R,  135 R) is corrected at a listening position deviated from the center line M of the speaker devices  131 L and  131 R, and then a listening range providing excellent surround effect can be enlarged. 
     Further, although the speaker devices  110  and  130  show examples of speaker devices of the 2-way system, the present invention is not naturally limited to the system. The gist of embodiments resides in that the speakers for reproducing the surround signal of the speaker devices for the left channel and for the right channel are attached to the speaker boxes such that the principal axes thereof are inclined by the predetermined angle respectively in the counterclockwise direction and the clockwise direction relative to the front directions of the speaker boxes. 
     In this way, according to the front surround speaker devices  110  and  130  shown by FIG.  22  and FIG. 26, the principal axes of the speakers for reproducing the surround signal of the speaker devices for the left channel and for the right channel are directed inwardly in view from a listening position on the center line of the two speaker devices and accordingly, at a listening position deviated from the center line of the two speaker devices, a difference in levels based on a difference in attenuation of sound in distances of surround signal sound of the two left and right channels is corrected by the directivities of the speakers for reproducing the surround signals at middle and high frequencies and accordingly, the listening range providing excellent surround effect can be enlarged. 
     INDUSTRIAL APPLICABILITY 
     As has been described, the speaker device according to the present invention is preferably applied in order to enlarge a listening range providing excellent stereophonic feeling or a listening range providing excellent front surround effect in, for example, a television receiver, an audio system or the like.