SOUND REPRODUCING APPARATUS FOR USE IN VEHICLE

Abstract: A sound reproducing apparatus for use in a vehicle having a speaker system constituted by at least one acoustic duct and a speaker unit disposed at a throat of the acoustic duct, an opening formed at the other end of the acoustic duct facing the passenger compartment of a vehicle. The apparatus is arranged such that, F0 and Fs are substantially equal to each other. Where Fs is the lowest frequency in a frequency range within which the imaginary part of the acoustic impedance at the sound radiating surface of the speaker system in the direction of the space within the passenger compartment is zero and F0 is the low-range resonance frequency of the speaker system itself in the free space.

Description: Field of the Invention This invention relates to a sound reproducing apparatus for use in a closed space, such as, for typical example the passenger compartment of a vehicle. Description of the Prior Art Sound reproducing apparatuses such as compact disk players have been lately improved in performance, and there is an increasing demand for expansion of the sound range to reproduce super-bass sound with respect to low-range sound reproduction in the passenger compartment of a vehicle. With this background, a sound reproducing apparatus for use in the passenger compartment of a vehicle such as that shown in Fig. 14 has been known. In Fig. 14, reference numeral 16 denotes a speaker unit and reference numeral 17 denotes a rear trunk. In this example, the rear trunk 17 is used as an enclosed cavity on the rear side of the speaker unit 16 so as to expand the low frequency range of sound pressure frequency response in the passenger compartment of the vehicle. Fig. 15 shows another example of a conventional apparatus which has a speaker system 18 of closed type having a small capacity or a system of bass-reflex type and an electric compensation means 19 such as a tone control or graphic equalizer. In this example, the falling characteristic of the speaker system 18 in the low range is adjusted or improved by the electric compensation means 19 so as to expand the low frequency range of sound pressure frequency response in the passenger compartment. Fig. 16 shows an example of the frequency characteristic (a) of the speaker system and an example (b) of the electric compensation means. However, the conventional arrangement shown in Fig. 14 cannot be applied to a type of vehicle such as a so-called hatch-back car or station wagon which is constructed without any closed rear trunk. Even when it is applied to the type of vehicle that has a rear trunk, the only feasible method of designing the interior sound pressure frequency characteristics is one wherein the constants of the speaker unit itself are changed. The degree of design freedom for this arrangement is thus very restricted. The conventional arrangement shown in Fig. 15 is free from this problem, but it requires large input power for the speaker unit in the low frequency range because of the use of electric compensation means 19, thereby necessitating the provision of a power amplifier of high-power. There is also a problem of distortion due to increased signal amplitude affecting the tone quality. From DE-A-3 404 655 is known a speaker apparatus for use in a space like a living room or another large room which includes a speaker system having sidewalls for forming at least one duct, a speaker unit of a first, low-range resonance frequency when in a free-field space, and a sound radiation surface. For improving the acoustical transmission there either is provided one single duct of which the effective length is one quarter of the wavelength of the lowest frequency to be transmitted, or there are provided two ducts, of which the length of the second duct is three times the length of the first duct, i.e. three quarters of the wavelength corresponding to the lowest frequency to be transmitted, wherein the length of the longer duct is equal to the half wavelength of the resonance frequency in free space of the speaker unit. SUMMARY OF THE INVENTION The object of the present invention is to provide a sound reproducing apparatus for use in a closed space of a relatively small volume, such as the passenger compartment of a motor vehicle. According to a first embodiment of the invention there is provided a speaker apparatus for use in a closed space, comprising at least one speaker system having sidewalls, a speaker unit having a first, low-range resonance frequency FO when in a free-field space, and a sound radiation surface, said sound radiation surface being disposed at a predetermined location in said closed space, said predetermined location having a given acoustic impedance within said closed space, which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, wherein said first low-range resonance frequency FO of said speaker unit is substantially equal to said frequency Fs, so that said speaker unit is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. According to another embodiment of the invention, there is provided a speaker apparatus for use in a closed space, comprising at least one speaker system having sidewalls, a speaker unit, a sound radiation surface and duct means, said sound radiation surface being disposed at a predetermined location in said closed space, said predetermined location having a given acoustic impedance within said closed space, which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, wherein said duct means has a length L1 along a centerline thereof, wherein L1 satisfies a condition C/(4Fs)≦L1≦C/(2Fs) where C is the acoustic compliance of said closed space, so that said speaker unit is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. According to still another embodiment of the present invention there is provided a speaker apparatus for use in a closed space, comprising at least one speaker system having sidewalls, a speaker unit, duct means, a first duct being disposed in front of said speaker unit, and a second duct being disposed at a rear of said speaker unit, and a sound radiation surface, said sound radiation surface being disposed in said closed space at a predetermined location, said predetermined location having a given acoustic impedance within said closed space which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, wherein, said first and second ducts have respective lengths along respective centerlines thereof which differ by a length L2 which satisfies a condition C/(4Fs)≦L2≦C/(2Fs) where C is the acoustic compliance of said closed space, so that said speaker unit is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. Finally the present invention provides for a method of reducing sound distortion of a speaker system for use in a closed space, comprising the steps of selecting a location within the closed space for mounting a speaker unit having a first, low-range resonance frequency F0 when in a free-field space, determining an acoustical impedance at said location within said closed space, which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, including selecting the sidewalls, sound producing means and a sound radiation surface of said speaker unit such that, when disposed in said closed space at said location, said first low-range resonance frequency F0 of said speaker unit is substantially equal to said frequency Fs so that said speaker unit is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. Accordingly, the present invention provides a sound reproducing apparatus for use in a vehicle which is capable of making the sound pressure frequency response curve flat without increasing the level of distortion in the low-frequency range even when applied to a type of vehicle having no closed space and which can be designed most suitably in accordance with the type of vehicle and its mounted position. To this end, the inventive sound reproducing apparatus for use in a vehicle is arranged such that, if the lowest frequency in a frequency range within which the imaginary part of the acoustic impedance at the sound radiating surface of a speaker system in the direction of the space within passenger compartment is zero, is Fs and if the low-range resonance frequency of the speaker system itself in the free space is F0, F0 and Fs are substantially equal to each other. The frequency Fs is obtained by simultation or observation effected at the position at which the speaker system is mounted in the closed space or the passenger compartment. The present inventor has found that the speaker unit operates in a stiffness-control region at frequencies lower than Fs, as in the case of a so-called headphone, so that the sound pressure increases under the driving condition of a constant acceleration. It is possible to design the speaker system by estimating a sound pressure frequency characteristic in the low-frequency range in the passenger compartment as mentioned above. If the speaker system is constructed so as to have an acoustic duct, the level of vibration of the speaker unit and, hence, distortion caused by the vibration are greatly reduced in a low frequency range in the vicinity of Fs because of the phenomenon of resonance of the acoustic duct. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a sound reproducing apparatus for use in a vehicle which represents a first embodiment of the present invention and of the position at which the apparatus is mounted in the passenger compartment; Fig. 2 is a side view of the apparatus shown in Fig. 1; Fig. 3 is a top view of the apparatus shown in Fig. 1; Fig. 4 is a schematic cross-sectional view of the structure of a speaker system in the first embodiment of the present invention; Fig. 5 is a characteristic diagram in which the imaginary part of the interior acoustic impedance of the speaker system shown in Fig. 4; Fig. 6 is a graph of a sound pressure frequency characteristic displayed when a sound radiating surface or a diaphragm of the same speaker system is driven at a constant acceleration relative to frequencies; Fig. 7 is a graph of sound pressure frequency characteristics of the same speaker system in the free space; Fig. 8 is a graph of sound pressure frequency characteristics of the same speaker system in the passenger compartment; Fig. 9 is a schematic cross-sectional view of the structure of a speaker system in a second embodiment of the present invention; Fig. 10 is a side view of the structure of a speaker system in a third embodiment of the present invention and the position at which the speaker system is mounted in the passenger compartment; Fig. 11a is a cross-sectional view of the structure of a speaker system in a fourth embodiment of the present invention; Fig. 11b is a side view showing the position at which the speaker system shown in Fig. 11a is mounted in the passenger compartment; Figs. 12a to 12c and Figs. 13a to 13d are perspective views of sound reproducing apparatuses for use in a vehicle each having acoustic ducts formed by a part of the structure of the vehicle; Fig. 14 is a side view of a conventional interior sound reproducing apparatus utilizing a rear trunk; Fig. 15 is an example of the conventional apparatus using an electric compensation means; and Fig. 16 is a characteristic diagram of the frequency characteristic of the electric compensation means and the sound pressure frequency characteristic in the passenger compartment in the case of the conventional apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with respect to preferred embodiments thereof with reference to the accompanying drawings. Fig. 1 is a perspective view in which a sound reproducing apparatus for use in a vehicle which represents an embodiment of the present invention and the mounting position of this apparatus are illustrated. Fig. 2 is a schematic side view and Fig. 3 is a top view of this apparatus. As shown in Fig. 3, a speaker system 1 is provided with a speaker unit 2 and an acoustic duct 3 which is reflexed by a plurality of partition plates 6 and which has an opening 4, and a rear tray 5 is formed in the passenger compartment. The operation of the thus constituted sound reproducing apparatus for use in a vehicle will be described below. The difference between the acoustic characteristics displayed when the speaker system is mounted in a closed space such as the passenger compartment of a vehicle and the acoustic characteristics in a free space represented by an anechoic room is first explained before the description of this embodiment. For the explanation, the influence on the speaker system when the speaker system is brought into the closed space in the passenger compartment is examined by obtaining the acoustic impedance at the sound radiating surface of the speaker system in the direction of the internal space of the passenger compartment. Fig. 5 shows the imaginary part of the acoustic impedance in the passenger compartment when the sound radiating surface of the speaker system 1 shown in Fig. 1, namely, the front surface of the speaker unit 2 and the radiating surface of the opening 4 of the acoustic duct are driven at the same time. The ordinate in Fig. 5 has been normalized by the impedance of a plane wave in the air. As shown in Fig. 5, the value of the impedance is negative in a hatched region below about 60Hz. From 1/jωC = j (-1/ωC), this is considered to be an indication of a capacitive state. If this transition frequency is Fs, the sound pressure increases as the frequency is reduced in the region below Fs, as shown in Fig. 6, when the sound radiating surface is driven at a constant acceleration relative to frequencies. Therefore, it is possible to flatten the characteristic curve of a frequency range below Fs by setting the sound pressure frequency characteristic of the speaker system in the free space to correspond to the characteristic reverse to that shown in Fig. 6. Ordinarily, it can be readily effected by designing the speaker system so as to adjust its low-range resonance frequency in the free space, namely, so-called F0 to Fs. The operation of the speaker system 1 in accordance with the first embodiment will be described below with reference to Fig. 4. As shown in Fig. 4, the partition plates 6 constitute the acoustic duct 3, as shown in Fig. 3. A reference numeral 7 denotes the center line of the acoustic duct 3 which connects the center of the speaker unit 2 and the center of the opening 4 to each other. The sound output which is supplied from the reverse side of the speaker unit 2 passes through the acoustic duct 3 and is thereafter output from the opening face of the opening 4 of the acoustic duct. If the length of the center line is L1 and if L1 is set to be a half of the wave length corresponding to Fs, that is, C/(2 · Fs) , the sound pressure frequency characteristic of the output power of sound radiated from the front surface of the speaker unit 2 is observed as indicated by the chain line 8 in Fig. 7. The sound pressure frequency characteristic of the output power of sound radiated from the opening face of the opening 4 of the acoustic duct is as indicated by the broken line 9, and the combined sound pressure frequency characteristic of these characteristics is indicated by the solid line l0. If the above-described speaker system is placed on the rear tray provided in the passenger compartment, the sound pressure rises as the frequency is reduced below Fs, as shown in Fig. 6, resulting in the characteristic shown in Fig. 7. In this state, the radiated sound output of the speaker unit 2 itself is small as represented by the characteristic curve indicated by the chain line 8 in Fig. 7 and, therefore, the distortion caused in the speaker unit 2 can be limited to a low level, so that there is no possibility of deterioration in the sound quality due to distortion even when the sound pressure increases at frequencies below Fs. Thus, it is possible to form a generally flat curve of the sound pressure frequency characteristic below Fs in the passenger compartment by designing the speaker system shown in Fig. 4 so as to set the length of the acoustic duct 3 to L1 = C/(2 · Fs). The apparatus can be used as, for example, a super woofer in a multichannel range-dividing system. Next, a second embodiment of the present invention will be described with reference to Fig. 9. As shown in Fig. 9, the reproducing apparatus is provided with a speaker system 20 having two openings 24 and 29, and partition plates 25, 26a, 26b, 26c which constitute two acoustic ducts 28 and 23. The acoustic duct 28 provides communication between the front side of the speaker unit 2 and the opening 29, and the acoustic duct 23 provides communication between the rear side of the speaker unit 2 and the opening 24. In Fig. 9, a reference numeral 30 denotes the center line of the acoustic duct 28, a reference numeral 27 denotes the center line of the acoustic duct 23. In this embodiment, it is possible to effect the same operation as that of the first embodiment by setting difference L2 between the center lines 30 and 27 to L2 = C/(2 · Fs). A third embodiment of the present invention will be described with reference to Fig. 10. Fig. 10 shows a speaker system 12 which is placed on a rear seat 11 disposed in the passenger compartment, a speaker unit 13, an acoustic duct 14 and an opening 15 of the acoustic duct. In this embodiment also, Fs is obtained from the change in the acoustic impedance in the passenger compartment observed over the rear seat 11, thereby calculating the length of the acoustic duct 14. Further, a fourth embodiment of the present invention will be described with reference to Fig. 11. Fig. 11 (a) is a cross-sectional view of a speaker system 31, and Fig. 11(b) is a side view of the speaker system 31 placed on the backrest of the rear seat. Fig. 11(a) shows speaker units 32 and 33, acoustic ducts 36 and 37, partition plates 34 and 35 and an opening 38, 39 of the acoustic duct. This embodiment is arranged by using a pairs of assemblies each provided in accordance with the first embodiment so as to be adapted for two-channel stereo reproduction and to be capable of being integrated with or incorporated in the rear seat. The same design as in the case of the first embodiment is performed with respect to the length of the acoustic ducts 36 and 37. In the above-described three embodiments, some part of the above-described effect is also realized under the following conditions of L1 and L2 because of phase margin: C/(4 · Fs) ≦ L1 ≦ C(2 · Fs) or C/(4 · Fs) ≦ L2 ≦ C(2 · Fs) As described above, the present invention realizes a superior sound reproducing apparatus for use in a vehicle in which a speaker system disposed in a passenger compartment is designed such that the frequency Fs at which the imaginary part of the acoustic impedance from the speaker system in the mounted position toward the interior space of the passenger compartment becomes zero and the low-range resonance frequency F0 of the speaker system itself in the free space are set to be substantially equal to each other, thereby enabling the sound pressure frequency characteristic below these frequencies to be substantially flattened in accordance with the position at which the speaker system is mounted in the passenger compartment while limiting the distortion thereof to a low level. Moreover, these effects can be realized independently of the type of vehicle since the present invention does not utilize a specific type of vehicle structure such as a rear trunk. In the above described embodiments, the acoustic ducts are constructed by using partition plates, but they may be formed by a part of the structure of the vehicle, as shown in Figs. 12a to 12c and Figs. 13a to 13d. In this case, the acoustic ducts may include a pair of parallel acoustic ducts equal or not equal to each other in length, or a system using only one acoustic duct may be applicable. The sectional area of each acoustic duct may be increased as it approaches the opening so as to form a horn and enable an increased sound output by the horn effect. In the case where the acoustic ducts are formed by the structure of the vehicle, the speaker units may be disposed in the trunk room, the rear doors or rear parcels. It goes without saying that various types of duct arrangement are possible by the combination of the number of acoustic ducts and the arrangement in which an acoustic duct is formed on the front side or the rear side of each speaker unit.

Claims: A speaker apparatus for use in a closed space, comprising: at least one speaker system (1) having sidewalls, a speaker unit (2) having a first, low-range resonance frequency FO when in a free-field space, and a sound radiation surface (4), said sound radiation surface (4) being disposed at a predetermined location in said closed space, said predetermined location having a given acoustic impedance within said closed space, which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, characterized in that said first low-range resonance frequency FO of said speaker unit (2) is substantially equal to said frequency Fs, so that said speaker unit (1) is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. A speaker apparatus as set forth in claim 1, wherein said speaker system (1) includes duct means (3), said speaker unit (2) being disposed at one end of said duct means (3), another end of said duct means (3) being open to said closed space. A speaker apparatus as set forth in claim 1 or 2, wherein said closed space is a passenger compartment of a vehicle. A speaker apparatus as set forth in claim 2 or 3, wherein said duct means (3) has a plurality of partitions disposed therein. A speaker apparatus as set forth in claim 3 or 4, wherein said duct means (3) is a portion of a body structure of said vehicle. A speaker apparatus as set forth in claim 3, 4 or 5, wherein one end of said duct means (3) is opened to a door portion of said vehicle. A speaker system as set forth in one of claims 3 to 6, wherein said speaker unit is disposed in one of (a) a trunk portion and (b) a rear parcel compartment portion of said vehicle. A speaker apparatus for use in a closed space, comprising: at least one speaker system (1) having sidewalls, a speaker unit (2), a sound radiation surface (4) and duct means (3), said sound radiation surface (4) being disposed at a predetermined location in said closed space, said predetermined location having a given acoustic impedance within said closed space, which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, characterized in that said duct means (3) has a length LI along a centerline thereof, wherein L1 satisfies a condition C/(4Fs)≦L1≦C/(2Fs) where c is the acoustic compliance of said closed space, so that said speaker unit (2) is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. A speaker apparatus as set forth in claim 8, wherein said duct means (3) has a plurality of partitions disposed therein. A speaker apparatus as set forth in claim 8 or 9, wherein said closed space is a passenger compartment of a vehicle. A speaker apparatus as set forth in claim 10, wherein said duct means (3) is a portion of a body structure of said vehicle. A speaker apparatus as set forth in claim 10 or 11, wherein one end of said duct means (3) is open to a door portion of said vehicle. A speaker apparatus as set forth in claim 10, 11 or 12, wherein said speaker unit (2) is disposed in one of (a) a trunk portion and (b) a rear parcel compartment portion of said vehicle. A speaker apparatus for use in a closed space, comprising: at least one speaker system (1) having sidewalls, a speaker unit (2), duct means (23,28), a first duct (23) being disposed in front of said speaker unit (2), and a second duct (28) being disposed at a rear of said speaker unit (2), and a sound radiation surface (4), said sound radiation surface (4) being disposed in said closed space at a predetermined location, said predetermined location having a given acoustic impedance within said closed space which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, characterized in that, said first and second ducts have respective lengths along respective centerlines thereof which differ by a length L2 which satisfies a condition C/(4Fs)≦L2≦C/(2Fs) where C is the acoustic compliance of said closed space, so that said speaker unit (2) is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. A speaker apparatus as set forth in claim 14, wherein each of said first and second ducts (23,28) have a plurality of partitions (25,26a,26b,26c) disposed therein. A speaker apparatus as set forth in claim 14 or 15, wherein said closed space is a passenger compartment of a vehicle. A speaker apparatus as set forth in claim 16, wherein said first and second ducts (23,28) are each formed by a portion of a body structure of a vehicle. A speaker apparatus as set forth in claim 16 or 17, wherein one end of each of said first and second ducts (23,28) is open to a door portion of said vehicle. A speaker apparatus as set forth in claim 16, 17 or 18, wherein said speaker unit is disposed in one of (a) a trunk portion and (b) a rear parcel compartment portion of said vehicle. A method of reducing sound distortion of a speaker system for use in a closed space, comprising the steps of selecting a location within the closed space for mounting a speaker unit having a first, low-range resonance frequency F0 when in a free-field space, determining an acoustical impedance at said location within said closed space, which has an imaginary component equal to zero at a frequency Fs which is a lowest frequency at which the imaginary component of said acoustic impedance is zero, characterized by selecting the sidewalls, sound producing means and sound radiation surface of said speaker unit such that, when disposed in said closed space at said location, said first low-range resonance frequency F0 of said speaker unit is substantially equal to said frequency Fs so that said speaker unit is operating in said closed space with a relatively flat sound pressure frequency response in a low-frequency range below said frequency Fs. A method as set forth in claim 20, further comprising the following steps: providing at least one acoustic duct, and disposing said speaker unit at one end of said duct, another end of said duct being in communication with said closed space. A method as set forth in claim 20 or 21, wherein said closed space is a passenger compartment of a vehicle. A method as set forth in claim 21, further comprising the step of disposing a plurality of partitions in said duct.

Inventors: KIMURA, YOICHI, MURATA, KOSAKU, NAKAMA, YASUTOSHI, OBATA, SHUICHI, OGAWA, MICHIKO, SAIKI, SHUJI, SATOH, KATSUAKI, TERAI, KENICHI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: H04R1
IPC codes: H04R1