WOOFER

Abstract: Passive radiators of the same effective vibration area and the same effective vibration mass disposed in mutual opposition, and driver units of the same effective vibration area and the same effective vibration mass disposed in mutual opposition, are mounted to a bandpass type enclosure. The vibration-reaction forces of the opposing passive radiators and opposing driver units on the enclosure are thereby mutually cancelled, and enclosure vibrations are thus greatly reduced. Powerful bass output can be achieved because the diameter of the passive radiators can be increased at will and the use of two passive radiators achieves an extremely large vibration area.

Description: Field of the invention The present invention relates to a bass speaker designed for dynamic, high fidelity reproduction of low frequency sounds. Description of the prior art Widespread distribution of home entertainment systems for reproducing in the home and other small environments high quality audio sources and audio-visual sources has increased demand for compact speakers capable of reproducing the bass range sounds contained in those sources with power, dynamic presence, and high fidelity. Dynamic reproduction of such bass sounds requires a large-diameter diaphragm and a sound pressure frequency characteristic that is flat to the lowest frequencies. With conventional sealed speaker enclosures and bass reflex enclosures, however, the Q of low band resonance frequencies increases as the size of the speaker opening increases relative to a constant internal speaker volume, thus producing a peak in the sound pressure-frequency characteristic. It has therefore been accepted that the bass speaker (woofer) opening cannot be very large relative to the enclosure size. High fidelity (quality) bass reproduction also means that sounds from enclosure vibration or resonance must be prevented, but it is difficult to suppress these sounds by simply increasing the thickness and weight of the speaker enclosure panels. A bass speaker designed to obtain a flat sound pressure-frequency characteristic to low bass range sounds using a large-diameter vibrator with a small internal enclosure volume is described in "Extreme Low Frequency Sound Reproduction by a Passive Radiator and an Acoustic Transformer" (Yoshii Hiroyuki, Report of the Meeting of the Acoustical Society of Japan, Oct. 1978, p. 281-282). The speaker described in this article is known today as a "bandpass" or "kelton" speakers. The structure of this conventional bass speaker is described below with reference to the simplified cross section thereof shown in Fig. 15. Referring to Fig. 15, the inside of the bandpass-type speaker enclosure 103 is separated by an internal speaker divider 104 into a back cavity 105 and a front cavity 106. The driver unit 101 is mounted on the internal speaker divider 104 and a passive radiator 102 is mounted on the front enclosure panel 103a such that bass sounds are projected from the passive radiator 102. Note that the driver unit 101 and passive radiator 102 create an acoustic transducer in the front cavity 106. The operation of this bass speaker is described next with reference to the electroacoustic equivalent circuit shown in Fig. 16 and Fig. 17 and the frequency characteristic graph shown in Fig. 18. Referring to Fig. 16, drive force Fd is the drive force applied to the vibration system by the voice coil of the driver unit's magnetic circuit, the effective vibration mass Md is the effective vibration mass of the driver unit's vibration system, compliance Cd is the compliance of the driver unit support system (edges and dampers), and resistance Rd is the sum of the mechanical resistance of the driver unit vibration system and the electromagnetic braking resistance resulting from the counter-electromotive force of the magnetic circuit. Also indicated in Fig. 16 and Fig. 17 are the air compliance CB of the back cavity 105, the mechanical resistance RB of the air in the back cavity 105, the air compliance CF of the front cavity 106, the mechanical resistance RF of the air in the front cavity 106, the effective vibration mass Mp of the passive radiator vibration system, the mechanical resistance Rp of the passive radiator vibration system, the compliance Cp of the passive radiator support system (edges and dampers), the driver unit vibration system speed Vd, and the passive radiator vibration system speed Vp. Other values referenced in Fig. 16 and Fig. 17 include the effective vibration area Sd of the driver unit and the effective vibration area Sp of the passive radiator, resulting in an acoustic transducer with a winding ratio of Sd:Sp. If the parameters of the passive radiator are converted from the driver unit side, an electroacoustic equivalent circuit as shown in Fig. 17 is obtained. More specifically, the effective vibration mass Mp is increased (Sd/Sp) 2 times, and the compliance Cp and mechanical resistance Rp are increased (Sp/Sd) 2 times. In this electroacoustic equivalent circuit Vp' is the passive radiator vibration speed, provided that the effective vibration area is assumed to be equivalent to driver unit area. When the frequency is extremely low the impedance of back cavity air compliance CB increases, thereby reducing the speed Vd and resulting in attenuation of speed Vp or Vp'. When the frequency rises, the impedance of front cavity air compliance CF drops. The speed Vd is thus bypassed by the air compliance CF and speed Vp or Vp' attenuates. The attenuation curve is approximately 12 dB/oct at both low and high frequency ranges. In other words, this speaker provides a bandpass characteristic suitable for a bass speaker. In addition, the resonance action between the effective vibration mass Mp, effective vibration mass Md, air compliance CB, and air compliance CF within the bandpass frequency, i.e., the reproducible frequency band, also enable this speaker design to reproduce the bass range with significantly greater efficiency than can sealed-enclosure speakers. For example, resonance between primarily the effective vibration mass Mp and front cavity air compliance CF at frequencies near the lower limit of the reproducible frequency band produce a passive radiator speed Vp' several times greater than the driver unit vibration system speed Vd. The bass reproduction efficiency of bass reflex enclosure speakers is as good as that of bandpass speakers, but at frequency levels below the cutoff frequency (below the antiresonant frequency of bass reflex speakers), the sounds emitted from the speaker unit and sounds emitted from the port are opposite-phase, and sound pressure attenuation below the cutoff frequency is therefore unacceptably great. With such bandpass speakers, however, the back of the driver unit is sealed. Sounds behind the driver unit therefore do not interfere with passive radiator sounds, and sound pressure attenuation below the cutoff frequency is therefore gradual, resembling sound pressure attenuation in sealed enclosure speakers. This gradual attenuation is more effective for the reproduction of bass range sounds. By designing the speaker to optimize the parameters (Md, Mp, CB, CF, and Rd) having the greatest effect on speaker characteristics, a flat bandpass frequency characteristic as shown in Fig. 18 can be achieved. The bandpass width is usually 1 to 2.5 octaves. As shown in Fig. 18, this speaker has two resonance frequencies f1 and f2 and antiresonant frequency fr where resonance frequencies f1 and f2 are the bandpass characteristic cutoff frequency. In other words, the band from f1 to f2 is the reproducible frequency band according to general filter theory. The bandpass characteristics of sealed, bass reflex, and kelton speakers are shown in Fig. 20. An important feature of this speaker is that even if the effective vibration area Sp is increased N times, a similarly flat frequency characteristic can be achieved by increasing the effective vibration mass Mp N 2 times. In other words, because the passive radiator can be sized at will, dynamic bass sounds can be obtained from a large-diameter diaphragm in an enclosure with a small internal volume. Speakers using a speaker balancer as described in Japanese patent laid-open number (kokai) H4-4700 have been designed as speakers capable of sufficiently reducing the sounds of enclosure vibrations and resonance. The construction of this speaker is described below with reference to Fig. 19, a simplified cross sectional diagram of the speaker. As shown in Fig. 19, a speaker unit 111 is mounted to the sealed enclosure 113, and the magnetic circuit 117a of the balancer 117 is mounted to the back of the speaker unit magnetic circuit 111a with a bonding board 118 disposed therebetween. Note that the magnetic circuit 117a of the balancer is identical to the magnetic circuit 111a of the speaker unit. The balancer 117 also comprises a weight 117b of the same mass as the vibration system 111b of the speaker unit 111. The balancer 117 thus generates inertial force equal to the inertial force (reactive force) generated by the speaker unit 111 but in the opposite direction. This speaker operates as follows. The inertial force generated by the balancer 117 has the same magnitude as the inertial force generated by the speaker unit 111 but an opposite vector, thereby causing the two inertial forces to cancel each other out. As a result, the inertial force of the speaker unit 111 does not travel to the sealed enclosure 113, enclosure vibrations are therefore fundamentally reduced. However, because the diameter of the passive radiator is significantly greater (usually 1.3 to 2 times greater) than the diameter of the driver unit in this conventional bass speaker, the effective vibration mass of the passive radiator is also several to twenty times the effective vibration mass of the normal driver unit. As a result, the vibration-reaction force of the passive radiator vibration system on the enclosure via the air inside the front cavity increases significantly. This significantly increases the enclosure vibrations, and the enclosure emits a variety of noises, such as a loose shaking sound, resonance, and echoes. On the other hand, conventional speakers designed to reduce enclosure vibrations have a sealed enclosure with the speaker unit opening limited in size, making it impossible to reproduce dynamic bass range sounds. In addition, half of the electrical input signal is consumed by the speaker balancer, resulting in a 6 dB drop in the output sound pressure level when compared with normal speakers, and extremely poor acoustic conversion efficiency. While the speaker system described in Japanese patent laid-open notification (kokai) H1-140896 (1989-140896) has been proposed as a speaker that sufficiently reduces enclosure vibration and resonance noise, the disclosed speakers are sealed enclosure speakers without a passive radiator and therefore have the same problems as described above. JP-A-573500 discloses a speaker comprising first and second driver units and first and second passive radiators mounted in an enclosure. SUMMARY OF THE INVENTION The object of the present invention is therefore to resolve the above problem and provide a bass speaker for reproducing dynamic bass range sounds using a large-diameter diaphragm that is not limited in size by the internal volume of the enclosure while minimizing enclosure vibrations and not reducing the acoustic conversion efficiency. A further object of the invention is to provide a bass speaker whereby enclosure vibrations are minimized at all reproducible frequencies. A further object of the invention is to provide a bass speaker whereby no sound distortion is produced in the reproducible frequency band no matter how the speaker is positioned or what the listening conditions are. Accordingly, the present invention exists as a bass speaker comprising first and second driver units and first and second passive radiators mounted in a bandpass type enclosure, characterised in that: the first and second passive radiators are mounted on different outside surfaces of the enclosure such that the passive radiator axes are substantially parallel or coaxial to each other; first and second support members are disposed inside the enclosure; and, the first and second driver units are mounted on the first and second support members on or near the axis of the corresponding first and second passive radiators with the driver unit axes being coaxial or substantially coaxial to the axis of the first and second passive radiators; wherein displacement motion of a diaphragm of the first driver unit and the first passive radiator are in opposite direction to the displacement motion of a diaphragm of the second driver unit and the second passive radiator; wherein the first and second driver units have the same effective vibration area and the same effective vibration mass; wherein the first and second passive radiators have the same effective vibration area and the same effective vibration mass; wherein the first resonant frequency in the middle of the enclosure panel connecting the outside enclosure panels in which the first and second passive radiators are mounted is higher than the maximum cutoff frequency of the reproducible frequency band; and wherein the average distance around the enclosure from the acoustic centre of the first passive radiator to the acoustic centre of the second passive radiator is less than half the wavelength of the maximum cutoff frequency of the reproducible frequency band. Because a flat sound pressure frequency characteristic can be achieved even when the size of the passive radiator is great, and because the total effective vibration area is made even greater by using plural passive radiators, extremely powerful, dynamic bass sounds can be reproduced. Furthermore, because the vibration reaction forces of the vibration system of each passive radiator acting on the enclosure are mutually cancelling, enclosure vibrations are reduced. There is also no electrical input signal loss and no drop in acoustic conversion (speaker) efficiency. Enclosure vibrations are also reduced at all reproducible frequencies because the vibration reaction forces of the vibration system of each passive radiator and each driver unit acting on the enclosure are mutually cancelled to the maximum cutoff frequency of the reproducible frequency band. With the bass speaker achieving the object above cancellation of sounds emitted from the plural passive radiators is prevented in the reproducible frequency band irrespective of the speaker orientation and listening conditions, and sound output is therefore not disturbed. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given below and the accompanying figures wherein: Fig. 1 is a simplified cross section of a bass speaker according to the first embodiment of the present invention. Fig. 2 is a graph of the sound pressure-frequency response characteristic of a bass speaker according to the first embodiment of the present invention. Fig. 3 is a graph of the vibration acceleration characteristic of the enclosure of a bass speaker according to the first embodiment of the present invention. Fig. 5 is a simplified cross section of a plan view of a bass speaker according to the third embodiment of the present invention. Fig. 6 is a simplified cross section of a bass speaker according to the fourth embodiment of the present invention. Fig. 7 is a simplified cross section of a bass speaker according to the fifth embodiment of the present invention. Fig. 8 is used to describe the vibration mode of the first resonant frequency of the enclosure in a bass speaker according to the fifth embodiment of the present invention. Fig. 9 is a graph of the vibration acceleration characteristic of the enclosure of a bass speaker according to the fifth embodiment of the present invention. Fig. 10 is a conceptual diagram of the orientation of a bass speaker according to the present invention. Fig. 11 is used to describe the average distance conditions of the present invention. Fig. 12 is used to describe the method of calculating the average distance in the present invention. Fig. 13 is a simplified cross section of a bass speaker according to the sixth embodiment of the present invention. Fig. 14 is a graph of the sound pressure-frequency response characteristic of a bass speaker according to the sixth embodiment of the present invention. Fig. 15 is a simplified cross section of a conventional bass speaker. Fig. 16 is a circuit diagram of the electroacoustic equivalent network of a conventional bass speaker. Fig. 17 is a circuit diagram of the electroacoustic equivalent network of a conventional bass speaker. Fig. 18 is a graph of the sound pressure and impedance frequency characteristics of a conventional bass speaker. Fig. 19 is a simplified cross section of a conventional speaker in which enclosure vibrations are reduced. Fig. 20 is a graph of the bandpass characteristics of sealed, bass reflex, and kelton speakers. Fig. 21 is a graph of the vibration acceleration characteristic of the enclosure of a bass speaker according to the sixth embodiment of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS The preferred embodiments of the present invention are described below with reference to the accompanying figures. Embodiment 1 Fig. 1 is a simplified cross section of a bass speaker according to the first embodiment of the present invention. The speaker of the invention comprises as shown in Fig. 1 a first driver unit 1a and a second driver unit 1b each with a diameter L1 of 22 cm, an effective vibration radius R1 of 85 mm, and an effective vibration mass of 18 g. As a result, both driver units have the same effective vibration area and vibration mass. The minimum resonant frequency of these driver units is 30 Hz, the dc resistance of the voice coil is 12 Ω, and the force factor BL of the magnetic circuit is 15.8 Wb/m. Both first and second driver units 1a and 1b are driven same-phase, and are electrically connected same-phase in parallel. Both the first and second passive radiators 2a and 2b have a diameter L2 of 27 cm, an effective vibration radius R2 of 105 mm, an effective vibration mass of 160 g, and a minimum resonant frequency of 20 Hz. As a result both passive radiators also have the same effective vibration area and vibration mass. The external dimensions of the first and second speaker unit opening panels 3a and 3b, respectively, of the bandpass type enclosure 3 are 39 cm × 31 cm, and the distance between the outside surfaces of speaker unit opening panels 3a and 3b is 76 cm. The enclosure is made from 15 mm thick particle board. The first passive radiator 2a and the second passive radiator 2b are installed at mutually opposing positions in the first and second speaker unit opening panels 3a and 3b, respectively, with each passive radiator facing outside the enclosure. The first and second driver units 1a and 1b are installed at mutually opposing positions in the first and second internal dividers 4a and 4b with the driver units placed back to back. The back cavity 5 formed between the first and second internal dividers 4a and 4b has an internal volume of approximately 60 liters. Both driver units 1a and 1b use this back cavity so that each driver unit has an equivalent back cavity volume of approximately 30 liters. The first and second front cavities 6a and 6b each have an internal volume of approximately 5 liters. The operation of the bass speaker thus comprised according to the present embodiment is described below with reference to Fig. 2 and Fig. 3. Both the first and second driver units 1a and 1b and the first and second passive radiators 2a and 2b operate in the same phase and with the same frequency response. The operation and characteristics of the bass speaker of the invention during bass range reproduction are therefore identical to the operation and characteristics of the conventional bass speaker described above. As shown in Fig. 2, the minimum cutoff frequency f1 (the resonant frequency of impedance) in the present embodiment is 32 Hz, the maximum cutoff frequency f2 (the resonant frequency of impedance) is 180 Hz, and a flat sound pressure frequency response is obtained across the reproducible frequency band from 32 Hz - 180 Hz at -3 dB. By using two passive radiators with a large diameter L2 of 27 cm, the total effective vibration area is equivalent to an extremely large 38 cm diameter radiator, thereby achieving bass output that is significantly greater than would normally be expected from an enclosure of the same size. Because the mutually opposing first and second driver units 1a and 1b and first and second passive radiators 2a and 2b operate with the same frequency response and at the same acoustical phase, i.e., the mutually opposing diaphragms move in opposite directions, the vibration-reaction force of the first passive radiator 2a on the bandpass type enclosure 3 and the vibration-reaction force of the second passive radiator 2b on the bandpass type enclosure 3 have the same magnitude but opposite vectors. In addition, the vibration-reaction forces of the first driver unit 1a on the bandpass type enclosure 3 and the second driver unit 1b on the bandpass type enclosure 3 also have the same magnitude but opposite vectors. This means that the vibration-reaction forces exerted by the first and second passive radiators 2a and 2b on the bandpass type enclosure 3 are mutually cancelling, the vibration-reaction forces exerted by the first and second driver units 1a and 1b are also mutually cancelling, and enclosure vibrations are thereby significantly reduced. The graph shown in Fig. 3 was produced by placing a vibration pickup in front of each speaker unit opening in the enclosure of the bass speaker according to the present embodiment to measure the vibration acceleration. The dotted line was obtained by measuring the vibration acceleration from a conventional bandpass type speaker enclosure, i.e., a speaker enclosure having only one driver unit and matching passive radiator. Note that the conventional bandpass speaker enclosure was also made from 15 mm thick particle board. From Fig. 3 we therefore know that the bass speaker of the invention reduces enclosure vibrations approximately 20 dB on average across the output band when compared with a conventional bass speaker, and by approximately 30 dB at low frequencies. In addition, there is no loss of the electrical input signal with the present embodiment, and there is therefore also no drop in speaker's acoustic conversion efficiency. Using plural large-diameter passive radiators, the present embodiment achieves an extremely large total effective vibration area, thereby enabling the reproduction of very powerful bass sounds. Enclosure vibrations are also significantly reduced, however, because the vibration-reaction forces of the vibration systems of the passive radiators and driver units acting on the enclosure cancel each other out. There is also no drop in speaker's acoustic conversion efficiency. In other words, regardless of how great the effective vibration mass of the passive radiator, the resulting vibration-reaction forces are extinguished and enclosure vibrations can be suppressed to a very low level. As a result, the diameter of the passive radiator is effectively not limited by the vibration mass of the passive radiator. It is also possible to achieve a bass range output with significantly greater power and dynamic presence than can be obtained from bass reflex type speakers and sealed speakers wherein the size of the speaker unit opening is limited. It should be noted that while the first and second driver units 1a and 1b and first and second passive radiators 2a and 2b are coaxially arranged in the above embodiment, the invention shall not be so limited and the same effects can be obtained with other arrangements. For example, the first driver unit 1a and first passive radiator 2a can be placed on a first axis and the second driver unit 1b and second passive radiator 2b placed on a second axis where said first and second axes are parallel but not coaxial. It is also possible to place each of the first and second driver units 1a and 1b and first and second passive radiators 2a and 2b on four discrete but parallel axes. It should be further noted that while the present embodiment has been described using a typical bandpass type enclosure with front and back cavities, it will be obvious that the invention can also be achieved in other variations of the typical bandpass type enclosure. Some of these variations include disposing the port in a side wall to which no driver unit is installed, or disposing the port in a divider placed between the passive radiator and driver unit within the front cavity as in a double bandpass type speaker enclosure. The effects of the present embodiment can also be obtained by assembling two speakers each having one driver unit and one passive radiator in a single enclosure, and then connecting these speakers back to back into a single enclosure. A divider separating the back cavity into two equal-volume chambers can also be alternatively provided. It should also be noted that while the mutually opposing passive radiators and driver units in the above embodiment have been described as having identical specifications, and specifically the same effective vibration area and effective vibration mass, the above-noted effects of the invention can be obtained even if said specifications differ slightly. It should also be noted that while the mutually opposing passive radiators and driver units in the above embodiment are mounted on parallel planes, the same effects can be obtained even if the mounting planes are not perfectly parallel. For example, the same effects can be obtained even if the passive radiator mounting surfaces are not perfectly parallel. It will also be obvious that while only one driver unit and passive radiator each is mounted to one enclosure panel in the above embodiment, plural driver units and passive radiators may be mounted. It is also possible for each passive radiator and driver unit combination to use, for example, one passive radiator and two driver units, or vice versa, insofar as the total effective vibration area and the total effective vibration mass are identically balanced as described above. In addition, the above embodiment has been described using a typical dynamic driver unit, i.e., a moving coil, but other driver unit designs, including electromagnetic and moving magnet, can obviously be alternatively used. The third embodiment of a bass speaker according to the present invention is described next below with reference to Fig. 5. The bass speaker of the third embodiment comprises as shown in Fig. 5 a first driver unit 21a and a second driver unit 21b each with a diameter of 18 cm, an effective vibration radius of 69 mm, and an effective vibration mass of 12 g. As a result, both first and second driver units have the same effective vibration area and vibration mass. The third and fourth driver units 21c and 21d each have a diameter of 14 cm, an effective vibration radius of 52 mm, and an effective vibration mass of 7 g. As a result, the third and fourth driver units have the same effective vibration area and vibration mass. All four driver units 21a, 21b, 21c, and 21d are driven same-phase. Both the first and second passive radiators 22a and 22b have a diameter of 30 cm, an effective vibration radius of 125 mm, an effective vibration mass of 550 g. As a result both first and second passive radiators also have the same effective vibration area and vibration mass. The third and fourth passive radiators 22c and 22d each have a diameter of 25 cm, an effective vibration radius of 106 mm, an effective vibration mass of 150 g. As a result the third and fourth passive radiators also have the same effective vibration area and vibration mass. The bandpass type enclosure 23 of this embodiment has a top panel with external dimensions of 54 cm x 54 cm, an enclosure height of 33 cm, and approximately the same internal volume as the speaker according to the first embodiment. The enclosure is made from 15 mm thick particle board, and comprises first, second, third, and fourth speaker unit opening panels 23a, 23b, 23c, and 23d respectively. The first passive radiator 22a and the second passive radiator 22b are installed at mutually opposing positions in the first and second speaker unit opening panels 23a and 23b, respectively, with each passive radiator facing to the outside of the enclosure. The third and fourth passive radiators 22c and 22d are likewise installed at mutually opposing positions in the third and fourth speaker unit opening panels 23c and 23d, respectively, with each passive radiator facing to the outside of the enclosure. The speaker of this embodiment further comprises first, second, third, and fourth internal dividers 24a, 24b, 24c, and 24d. The first and second driver units 21a and 21b are installed at mutually opposing positions in the first and second internal dividers 24a and 24b with the driver units placed back to back, and the third and fourth driver units 21c and 21d are installed at mutually opposing positions in the first and second internal dividers 24c and 24d, also with the driver units placed back to back. The bass speaker of the present embodiment thus contains a passive radiator and driver unit mounted at every surface of the enclosure other than the enclosure top and bottom with the effective vibration area and the effective vibration mass of the opposing driver units being the same, and the effective vibration area and the effective vibration mass of the opposing passive radiators being the same. The back cavity 26 has an internal volume of approximately 50 liters. The equivalent internal volume occupied by each of the first and second driver units 21a and 21b is approximately 19 liters, and the equivalent internal volume occupied by each of the third and fourth driver units 21c and 21d is approximately 5.9 liters. The front cavity 25 has an internal volume of approximately 20 liters. The first and second passive radiators 22a and 22b therefore occupy an equivalent internal volume of approximately 6.6 liters each, and the third and fourth passive radiators 22c and 22d each occupy an equivalent internal volume of approximately 3.4 liters. Based on the relationship between the effective vibration area and the effective vibration mass described in the discussion of a bass speaker according to the prior art with reference to the electroacoustic equivalent circuit diagram shown in Fig. 12 and Fig. 13, the effective vibration mass of each component in the present embodiment is proportional to the square of the effective vibration area ratio, the driver units 21a, 21b, 21c, and 21d operate at the same frequency response, and the passive radiators 22a, 22b, 22c, and 22d operate at the same frequency response. The equivalent internal volume occupied by each of the driver units and passive radiators is also distributed so that the total internal volume is proportional to the square of the effective vibration area ratio. The operation of the bass speaker thus comprised according to the third embodiment is the same as that of the first embodiment. As a result, the first and second driver units 21a and 11b and the third and fourth driver units 21c and 21d operate in the same phase and with the same frequency response. In addition, the first and second passive radiators 22a and 22b, and the third and fourth passive radiators 22c and 22d, also operate in the same phase and with the same frequency response. The operation and characteristics of the bass speaker of the invention during bass range reproduction are therefore identical to the operation and characteristics of the conventional bass speaker described above. The minimum cutoff frequency f1 in the present embodiment is 43 Hz, the maximum cutoff frequency f2 is 130 Hz, and a flat sound pressure frequency response is obtained across the reproducible frequency band from 43 Hz - 130 Hz at -3 dB. By installing the passive radiators in four side panels of the enclosure, the total effective vibration area is equivalent to a 55 cm diameter. As a result, even more powerful bass sounds can be reproduced. Because the mutually opposing first and second driver units 21a, 21b and 21c, 21d, and the mutually opposing first and second passive radiators 22a, 22b and 22c, 22d operate with the same frequency response and at the same acoustical phase, i.e., the mutually opposing diaphragms move in opposite directions, the vibration-reaction force of the first passive radiator 22a on the bandpass type enclosure 23 and the vibration-reaction force of the second passive radiator 22b on the bandpass type enclosure 23 have the same magnitude but opposite vectors. Furthermore, the vibration-reaction force of the third passive radiator 22c on the bandpass type enclosure 23 and the vibration-reaction force of the fourth passive radiator 22d on the bandpass type enclosure 23 have the same magnitude but opposite vectors. This means that the vibration-reaction forces exerted by the first and second passive radiators 22a and 22b on the bandpass type enclosure 23 are mutually cancelling, and the vibration-reaction forces exerted by the third and fourth passive radiators 22c and 22d on the bandpass type enclosure 23 are mutually cancelling. The vibration-reaction forces of the corresponding driver units 21a, 21b, 21c, and 21d also have the same magnitude but opposite vectors, and therefore are also mutually cancelling. More specifically, enclosure vibrations are greatly suppressed because all vibration-reaction forces are cancelled. As a result, enclosure vibrations are approximately 25 dB less than enclosure vibrations in a conventional bandpass type enclosure averaged across the output band. There is also no electrical input signal loss and no drop in acoustic conversion efficiency. In addition to achieving the effects of the first embodiment described above, the base speaker of the present embodiment is also able to maximize the total effective vibration area achievable in a limited enclosure size by mounting passive radiators to all but the top and bottom surfaces of the enclosure. As a result, an extremely powerful bass range output can be obtained. It will also be obvious that the relationships between the mounting surfaces and positions, the numbers of driver units and passive radiators, the usable enclosure types, and other design parameters can also be varied in this embodiment as in the first embodiment above. It should also be noted that each of the four driver units can have the same effective vibration area and effective vibration mass, and each of the four passive radiators can have the same effective vibration area and effective vibration mass. The fourth embodiment of a bass speaker according to the present invention is described next below with reference to Fig. 6. Referring to Fig. 6, the specifications of the driver units 31a and 31b, the specifications of the passive radiators 32a and 32b, the specifications of the enclosure 33, the relative positions of the speaker unit opening panels 33a and 33b and the internal dividers 34a and 34b, the internal volume of the back cavity 35, and the internal volume of the front cavities 36a and 36b are all identical to those of the first embodiment. In other words, all components used in the bass speaker of this fourth embodiment are identical to those of the first embodiment. This fourth embodiment differs from the first in that the orientation of the second driver unit 31b and the second passive radiator 32b is opposite that of the second driver unit and passive radiator in the first embodiment. Therefore, the two driver units are also electrically connected in opposite phase, and the driver units are thus driven acoustically in the same phase. The acoustical operation and frequency characteristics of the bass speaker according to the present embodiment thus comprised are identical to those of the first embodiment, and the resulting enclosure vibration attenuation operation and effects are therefore also the same. Further description thereof is thus omitted below. Due to the opposite-phase connection of the driver units in the present embodiment, the displacement of the diaphragm of the first driver unit 31a in one direction is accompanied by the displacement of the diaphragm of the second driver unit 31b in the opposite direction, thus compensating for the front-back asymmetry of the vibration system support system, and reducing even-harmonic distortion. This same effect reducing even-harmonic distortion is also achieved with the passive radiators 32a and 32b. As a result, there was an average 5 dB reduction in second harmonic distortion across the reproducible frequency band in the present embodiment as compared with the first embodiment. In addition to achieving the effects of the first embodiment as described above, the bass speaker of the present embodiment is able to cancel out the asymmetry of the diaphragm support system and thereby reduce even-harmonic distortion by reversing the orientation of the passive radiator and driver unit on one side of the speaker according to the first embodiment above. It should be noted that while the orientation of both the driver unit and passive radiator on one side of the speaker system are described as reversed in the present embodiment from the orientation in the first embodiment, the even-harmonic distortion reducing effect of this embodiment will be achieved to some degree even if the orientation of only the driver unit or only the passive radiator is reversed. The fifth embodiment of a bass speaker according to the present invention is described next below with reference to Fig. 7, Fig. 8, and Fig. 9. Referring to Fig. 7, the specifications of the driver units 41a and 41b, the specifications of the passive radiators 42a and 42b, the relative positions of the internal dividers 44a and 44b, the internal volume of the back cavity 45, and the internal volume of the front cavities 46a and 46b are all identical to those of the first embodiment. This fifth embodiment differs from the first in that the enclosure 43 is made from a 25 mm thick, high strength particle board. The enclosure panels are further reinforced by using liberal quantities of adhesive at all joints to bond the enclosure panels together. The outside enclosure dimensions are 41 cm × 33 cm × 80 cm. The acoustical operation and frequency characteristics of the bass speaker according to the present embodiment thus comprised are identical to those of the first embodiment, and the resulting enclosure vibration attenuation operation and effects are therefore also the same. Further description thereof is thus omitted below. With the enclosure constructed as described above the first resonant frequency of the center panel 53c of the enclosure side panels 43c joining the two speaker unit opening panels 43a and 43b is approximately 300 Hz, and is therefore higher than the 180-Hz maximum cutoff frequency of the reproducible frequency band. The first resonant frequency fr1 of a panel that is fixed on all sides is obtained by equation (1) fr1 = kt{E/p(1-u 2 )} 1/2 where t is the material thickness, E is the Young's modulus of the material, u is Poisson's ratio, p is the density of the material, and k is a constant value specific to the shape of the material. The first resonant frequency was raised sufficiently by increasing the material thickness and increasing the Young's modulus of the material based on equation (1). Referring to Fig. 8, the center panels 53c of the enclosure side panels 43c vibrate to the antinodes of the vibration state during the first resonance mode of the center panel 53c when the speaker is driven. As a result, the center panels 53c stop functioning as rigid members, and the mutually cancelling effect of the vibration-reaction forces of the driver units and passive radiators on the enclosure is thereby reduced. This effect is also obtained at the second and higher resonant frequencies whereby plural antinodes are produced in the center panels 53c. Referring again to Fig. 7, however, the first resonant frequency of the enclosure side panels 43c is set higher than the maximum cutoff frequency of the reproducible frequency band as noted above. The enclosure side panel 43c can therefore be treated as a rigid member to the maximum frequency of the reproducible frequency band, thereby retaining the mutually cancelling effect of the vibration-reaction forces of the driver units 41a and 41b and passive radiators 42a and 42b on the enclosure 43. This also assures that the enclosure vibration attenuating effect of the invention is sustained to the maximum reproducible frequency of the speaker. It should be noted that two sizes of enclosure panels are used in the present embodiment, specifically panels measuring 41 cm × 80 cm and panels measuring 33 cm × 80 cm. The first resonant frequency of the larger 41 cm × 80 cm panels is set to approximately 300 Hz, and the first resonant frequency of the smaller 33 cm × 80 cm panels is set to approximately 350 Hz. The frequency used as the first resonant frequency is the lower of these frequencies, i.e., 300 Hz in the case of this embodiment. Referring to the graph in Fig. 9 it is known that a vibration attenuation effect of approximately 20 dB is achieved by the present embodiment even near the maximum cutoff frequency, 180 Hz, of the reproducible frequency band. Note, particularly, that the vibration attenuation effect of this fifth embodiment is greater than that of the first embodiment, shown in Fig. 3, at the high end of the reproducible frequency band. Note, also, that the first resonant frequency at the middle of the enclosure side panels in the first embodiment is approximately 150 Hz. As with the first embodiment above, a bass speaker according to the present embodiment can increase the total effective vibration area by using a large-diameter passive radiator or plural passive radiators, and can thereby reproduce extremely powerful bass range sounds without reducing acoustic conversion efficiency. Enclosure vibrations are also significantly reduced at all reproducible frequencies because the vibration-reaction forces of the vibration system of each passive radiator and each driver unit acting on the enclosure are mutually cancelled to the maximum cutoff frequency of the reproducible frequency band. The thickness of the material used for the enclosure was increased to increase the strength of the enclosure as a means of increasing the first resonant frequency at the middle of the enclosure panels. The same effect can be achieved without greatly increasing the panel thickness, however, by providing a reinforcing member in the same enclosure panels. It should also be noted that the bandpass cutoff frequencies, i.e., the resonant frequencies f1 and f2 of the impedance characteristic, determine the reproducible frequency band of a bass speaker according to the present embodiment based on general filter theory. It is also possible, however, depending upon the design of the bandpass type speaker, for the sound pressure level at resonant frequency f2 to drop greatly from the flat band sound pressure level. In such cases it is appropriate to use the frequency at which the sound pressure level begins to attenuate as the cutoff frequency rather than using resonant frequency f2 of the impedance characteristic. Note, further, that if there is a localized low-strength section in part of the center panel 53c and the resonant frequency of this section is below the maximum cutoff frequency of the reproducible band, this section will produce no noticeable problems insofar as the area of that section is small enough to have no substantial effect on the vibration pattern of the overall center panel 53c. It must also be noted that this localized resonant frequency is distinguished from the first resonant frequency referenced in this specification. The sixth embodiment of a bass speaker according to the present invention is described next below with reference to Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, and Fig. 21. In this sixth embodiment the average distance around the enclosure between the acoustic center of the passive radiator positioned at the front and the acoustic center of the other passive radiator is less than 1/2 the wavelength of the maximum cutoff frequency of the reproducible frequency band. The objective and principle for this design constraint is described first in detail below before proceeding to a description of the embodiment. In the bass speaker of the invention sound is emitted from two or more passive radiators. If the enclosure 63 is placed so that the listening position P is equidistant from the acoustic center P1 of the first passive radiator 62a and the acoustic center P2 of the second passive radiator 62b, there will be no phase difference in the sound emanating from the passive radiators 62a and 62b and reaching the listening position P. It is not always possible to place the enclosure in this ideally oriented position in the listening room, however, and a phase difference thus results between the sounds reaching the listening position P from each of the passive radiators. This phase difference can significantly disrupt the sounds heard at the listening position P, and in certain scenarios the sound pressure level at the listening position P may actually be reduced to zero by this phase difference. The present embodiment specifically addresses this problem and prevents phase-difference induced sound cancellation regardless of where the enclosure is placed. The conditions under which such sound cancellation does not occur are considered below with reference to Fig. 11. The enclosure 73 in this embodiment is cylindrically shaped with a round profile when seen from the listening position P. As a result, there is a constant distance relationship between the listening position P and the acoustic centers P1 and P2 of the first and second passive radiators at all points around the center axis of this cylindrical enclosure 73. The distance difference Ld to the listening position P from the acoustic centers P1 and P2 of the first and second passive radiators is obtained by equation (2). Ld = (P2B + BA + L2) - L1 where the relationship P2B + BA + AP1 + L1 ≥ P2B + BA + L2 is true and the two sides of this equation are equal when L1 = 0. From equations (2) and (3) we derive equation 4. Ld ≤ P2B + BA + AP1 = Lp From equation (4) we know that the distance difference Ld from the acoustic centers P1 and P2 of the first and second passive radiators to the listening position P is less than the distance LP from the acoustic center P1 of the first passive radiator 72a to the acoustic center P2 of the second passive radiator 72b. The relationship between the sound pressure p1 reaching the listening position P from the first passive radiator 72a and the sound pressure p2 reaching the listening position P from the second passive radiator 72b is obtained from equation 5 and equation 6 where p2 = p1 × (L1/P2B + BA + L2) 2 p2 ≤ p1 and equality is achieved when L1 is ∞ (infinity). More specifically, the sound pressure reaching the listening position P from the first passive radiator 72a is less than or equal to the sound pressure reaching the listening position P from the second passive radiator 72b. Sounds emanating from the two passive radiators are cancelled when the sound pressure levels are equal and the phase difference is 180°, i.e., when the distance difference is the half-wave length of the sound. Based on equations (4) and (6), therefore, we know that sound cancellation will not occur at any frequency at which the distance Lp is the half-wave length of the sound or less. Therefore, if the distance Lp (= La + Lb + Lc) around the enclosure sides between the acoustic center P1 of the first passive radiator 72a and the acoustic center P2 of the second passive radiator 72b is less than 1/2 the wavelength of the maximum cutoff frequency of the reproducible frequency band with the cylindrical enclosure 73 of the present embodiment, sound cancellation can be prevented across the reproducible band no matter how the enclosure is oriented. The "average distance" referenced above is also described below. Most speaker enclosures are some type of cuboid as shown in Fig. 12, and virtually no cylindrical enclosures are available. The distance Lp from the acoustic center P1 of the first passive radiator 82a to the acoustic center P2 of the second passive radiator 82b around the enclosure 83 differs according to angle . With such enclosures the average distance used herein can be obtained by obtaining distance Lp averaged around the complete 360° arc around the acoustic center P1. If the distance Lp at angle  is expressed as the function of , f() where Lp = f() the average distance Lp can be obtained from equation (8). Lp={∫ 2π 0 f()d}/2π The value returned by equation (8) is the "average distance" referenced herein. The average distance in this case can be approximated as the distance Lp obtained with a cylindrical enclosure where the area of the circle is the same as the area of the front surface of the cuboid enclosure. Note that the average distance when there are four passive radiators can be obtained by simply expanding the above concepts. More specifically, the required average distance value can be obtained by calculating the average distance from the passive radiator on the front to each of the other three passive radiators using equation (8), adding these three average distances, and then dividing the sum by three. When the sound pressure generated by each of the passive radiators differs, the final average distance can be obtained by weighting each average distance value by the corresponding sound pressure level and obtaining the weighted mean. It is therefore possible with a bass speaker according to this embodiment to completely prevent sound cancellation within the frequency band reproduced by the speaker under all installation and listening conditions by designing the enclosure so that the average distance as calculated by equation (8) from the acoustic center of the passive radiator positioned at the front around the enclosure to the acoustic center of each of the other passive radiators is less than 1/2 the wavelength of the maximum cutoff frequency of the reproducible frequency band. The sixth embodiment of a bass speaker according to the present invention is described next below with reference to Fig. 13. The bass speaker of the sixth embodiment comprises as shown in Fig. 13 a first driver unit 91a and a second driver unit 91b each with a diameter of 14 cm, an effective vibration radius of 56 mm, and an effective vibration mass of 12 g. As a result, both first and second driver units have the same effective vibration area and vibration mass. The minimum resonant frequency of these driver units is 50 Hz, the dc resistance of the voice coil is 10 Ω, and the flux density B1 of the magnetic circuit is 6.0 Wb/m. Both first and second driver units 91a and 91b are driven same-phase, and are electrically connected same-phase in parallel. Both the first and second passive radiators 92a and 92b are flat rectangular radiators with an aperture of 22 cm × 16 cm, an effective vibration radius of 73 mm when converted to a circular diaphragm equivalent, i.e., equivalent to a circular passive radiator with an 18 cm diameter, an effective vibration mass of 42 g, and a minimum resonant frequency of 30 Hz. As a result both passive radiators also have the same effective vibration area and vibration mass. The bandpass type enclosure 93 has a total depth of 27 cm with the external dimensions of the first and second speaker unit opening panels 93a and 93b, respectively, being 22 cm wide by 36.5 cm high. The enclosure 93 is thus very compact. The enclosure is made from 21 mm thick medium density fiber board (MDF). The first passive radiator 92a and the second passive radiator 92b are mounted at opposite positions in the first and second speaker unit opening panels 93a and 93b, respectively, facing toward the outside of the enclosure. The first and second driver units 91a and 91b are installed at mutually opposing positions in the first and second internal dividers 94a and 94b with the driver units facing away from each other toward the outside of the enclosure. Note that the driver unit positions are offset so that the field elements of the opposing driver units do not collide. The back cavity 95 formed between the first and second internal dividers 94a and 94b has an internal volume of approximately 6.4 liters. Both driver units 91a and 91b use this back cavity so that each driver unit has an equivalent back cavity volume of approximately 3.2 liters. The first and second front cavities 96a and 96b each have an internal volume of approximately 2.1 liters. The total internal volume of the enclosure 93 is thus only 10.6 liters. The acoustical operation and the enclosure vibration attenuation effect of the bass speaker thus comprised according to the sixth embodiment are the same as in the first embodiment. As a result, a substantially flat sound pressure frequency response is obtained across the reproducible frequency band from 68 Hz - 185 Hz at -3 dB where the minimum cutoff frequency f1 is 68 Hz and the maximum cutoff frequency f2 is 185 Hz. The total effective vibration area of the passive radiators is equivalent to a 25 cm diameter radiator despite the extremely compact size of the enclosure. As a result, the bass output of the bass speaker according to this embodiment is significantly more powerful than the one which can be obtained from a conventional bass speaker of equivalent volume. Enclosure vibrations are also reduced approximately 20 dB across the output band all the way to the 185-Hz maximum cutoff frequency. The first resonant frequency of the center side panels 93c connecting the speaker unit opening panels 93a and 93b is approximately 600 Hz, and is thus higher than the maximum cutoff frequency. The depth of the enclosure is also greatly reduced by using flat passive radiators 92a and 92b, and mounting the driver units 91a and 91b so that the field magnets do not collide. The resulting average distance from the acoustic center of the first passive radiator 92a on the front of the enclosure to the acoustic center of the second passive radiator 92b around the enclosure is 59 cm, which is less than half the wavelength, i.e., 92 cm, of the 185 Hz maximum cutoff frequency. As a result, it is possible to completely prevent sound cancellation within the frequency band reproduced by the speaker under all installation and listening conditions. In the bass speaker of the first embodiment described above the maximum cutoff frequency is 180 Hz and the half-wave length is thus 94 cm. However, the average distance from the acoustic center of the first passive radiator around the enclosure to the acoustic center of the second passive radiator is 114 cm, which is longer than the half-wave length. As a result, sound cancellation may occur within the reproducible frequency band depending upon the speaker position and listening conditions. As in the preceding embodiments, the bass speaker of the present embodiment can reproduce powerful bass sounds with a flat sound pressure frequency characteristic despite the compact enclosure size by using passive radiators with a large effective diameter, and can operate with no drop in speaker [acoustic conversion] efficiency. Enclosure vibrations are also significantly reduced across the reproducible frequency band of the speaker because the vibration-reaction forces of the vibration systems of the passive radiators and driver units acting on the enclosure cancel each other out. From Fig. 21 we know that the bass speaker of the invention reduces enclosure vibrations approximately 25 dB across the entire output band when compared with a conventional bass speaker. Extremely high fidelity bass reproduction is also achieved regardless of the speaker position or listening conditions because there is no sound cancellation within the reproducible frequency band of the speaker. It should also be noted that the bandpass cutoff frequencies, i.e., the resonant frequencies f1 and f2 of the impedance characteristic, determine the reproducible frequency band of a bass speaker according to the present embodiment based on general filter theory. As in the fifth embodiment, however, it is appropriate to use the frequency at which the sound pressure level begins to attenuate as the cutoff frequency rather than using resonant frequency f2 of the impedance characteristic depending upon the sound pressure - frequency characteristics.

Claims: A bass speaker comprising first and second driver units (1a,1b) and first and second passive radiators (2a, 2b) mounted in a bandpass type enclosure (3), characterised in that : the first and second passive radiators (2a,2b) are mounted on different outside surfaces (3a,3b) of the enclosure (3) such that the passive radiator axes are substantially parallel or coaxial to each other; first and second support members (4a,4b) are disposed inside the enclosure; and, the first and second driver units (1a, 1b) are mounted .on the first and second support members (4a, 4b) on or near the axis of the corresponding first and second passive radiators (2a,2b) with the driver unit axes being coaxial or substantially coaxial to the axis of the first and second passive radiators (2a,2b); wherein displacement motion of a diaphragm of the first driver unit (1a) and the first passive radiator (2a) are in opposite direction to the displacement motion of a diaphragm of the second driver unit (1b) and the second passive radiator (2a); wherein the first and second driver units (1a,1b) have the same effective vibration area and the same effective vibration mass; wherein the first and second passive radiators (2a,2b) have the same effective vibration area and the same effective vibration mass; wherein the first resonant frequency in the middle of the enclosure panel connecting the outside enclosure panels (3a,3b) in which the first and second passive radiators (2a,2b) are mounted is higher than the maximum cutoff frequency of the reproducible frequency band; and, wherein the average distance around the enclosure (3) from the acoustic centre of the first passive radiator (2a) to the acoustic centre of the second passive radiator (2b) is less than half the wavelength of the maximum cutoff frequency of the reproducible frequency band. A bass speaker according to claim 1, wherein the first and second passive radiators (2a,2b) are mounted facing outward from the enclosure (3), and the first and second driver units (1a,1b) are mounted back to back. A bass speaker according to claim 1, wherein the first passive radiator (32a) is mounted facing outward from the enclosure (33), the second passive radiator (32b) is mounted facing inside the enclosure (33), the first driver unit (31a) is mounted facing the first passive radiator (2a), and the second driver unit (1b) is mounted facing away from the second passive radiator (32b). A bass speaker according to claim 1 or 2, further comprising third and fourth passive radiators (22c,22d) that are mounted on different outside surfaces of the enclosure (3), the third and fourth passive radiators (22c,22d) being mounted on different surfaces from those on which the first and second passive radiators are mounted, such that the axes of the third and fourth passive radiators (22c, 22d) are essentially parallel or coaxial to each other, third and fourth support members (24c,24d) disposed inside the enclosure (23), and third and fourth driver units (21c,21d) that are mounted on the third and fourth support members are near the axis of the corresponding third and fourth passive radiators (22c, 22d) with the axes of the third and fourth driver units (21c,21d) being coaxial or substantially coaxial respectively to the axis of said third and fourth passive radiators (22c, 22d).

Inventors: KAGEYAMA SATOSHI, TAMURA KAZUAKI, TANAKA SHOJI, KAGEYAMA, SATOSHI, TAMURA, KAZUAKI, TANAKA, SHOJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: H04R1
IPC codes: H04R1