CN1250042C - Loudspeaker - Google Patents

Abstract

A loudspeaker has a bending wave panel (11) and an exciter (13) mounted on the panel to excite bendingg wave modes in the panel. A rear box (15) defines a cavity (17) in cooperation with the panel (11). A resonance of cavity and panel at a coupled mode resonance frequency may be controlled by providing a duct (19) coupled to the cavity to selectively reduce sound pressure at the coupled mode resonance frequency. Damping (21) may be provided in the duct (19).

Description

speaker

technical field

The present invention relates to loudspeakers and more particularly to loudspeakers incorporating resonant panel acoustic emitters.

Background technique

In some applications, it is preferable to mount the loudspeaker within a shallow enclosure.

Especially for diffuse mode loudspeakers mounted on the wall. If the loudspeaker has an open back, the adjacent walls will uncontrollably affect the sound output because the environment is not constant from one loudspeaker position to another. The provision of a shallow shroud can alleviate this problem.

However, shallow hoods have some disadvantages. Specifically, strong coupling occurs between the panel and the enclosure. The air inside the shallow enclosure acts like a spring, and the panel can oscillate on the spring at a frequency, usually in the audible range, like a solid body moving back and forth. This resonance produces an uneven frequency response at the output of the loudspeaker when used. This response produces large audible peaks in the frequency response.

For low quality applications, the electrical circuit balances the peaks properly. However, this balance is not suitable for high-quality products, for example, air temperature can affect the electrical circuit with different sound characteristics.

Damping is another way to reduce this coupled mode peak. Also better suited for low quality applications. Additionally, damping increases the width of the peaks, so the system sounds worse with damping than without.

It would be beneficial if a conventional pistonic loudspeaker could fit into a shallow cavity. Usually such loudspeakers are mounted inside a shroud, because without the shroud behind the loudspeaker, the sound output from the back of the speaker is out of phase from the sound output from the front and tends to cancel at low frequencies.

However, in such pistonic loudspeakers, the overall main mode resonance is almost always at the low frequency end of the loudspeaker's response. In this case, the increased sound output due to resonance compensates for the sound drop at low frequencies and extends the speaker's bass response. This means that the enclosure cannot be too shallow and a large volume is required after the speaker. To make the speaker shallow and within the audio frequency resonances that prevent a uniform frequency response are shifted from the low frequencies used to the higher frequencies would be different than traditionally taught.

Contents of the invention

According to the present invention, there is provided a loudspeaker comprising a panel member for emitting sound, an exciter for exciting the panel member to emit sound, a sound enclosure behind a panel member co-defining a shallow cavity with the panel member, in A coupled resonant mode of the panel member and cavity is generated at a coupled mode frequency, and a conduit is acoustically coupled to the cavity to selectively reduce the sound pressure in the cavity at the coupled mode frequency.

The catheter acts as a pressure relief tool to absorb pressure waves at selected frequencies. While it is believed that simply adding an acoustic absorber to the cavity would still work, using only one absorber would increase absorption over a broad frequency range. Sufficient volume damping for resonance control will attenuate acoustic power below or above resonance due to lack of selective absorption. Also, the depth of the shallow enclosure is not deep enough for the large thickness of the absorber.

By shallow cavity is meant a cavity, which does not function as a volume behind the panel, but whose limited thickness produces effects such as coupling bulk modes in the acoustic range. Generally, the cavity must be less than half the planar dimension of the smaller panel, preferably less than a quarter or within 10% of the smaller dimension, before this effect becomes severe. The shallower the cavity, the thinner the speaker, which is generally desired.

Even though sound can be selectively absorbed at the coupled-mode frequency, it is not necessary to tune the absorption precisely to a specific frequency. For example, the bulk mode resonance can be quite broad and the absorption force can be suitably broad.

In a specific embodiment, the proposed solution has many advantages over alternative methods, firstly, the cost of the catheter can be absorbed by the cost of providing the original tool to form the cavity. The method is also tolerant of temperature changes as these parallel the AV ducts with the air in the cavity. The method is also permanent.

The coupled mode can be a bulk mode, since this mode is usually the mainstream mode in shallow cavities and in modes that need to be attenuated.

One end of the catheter may be open to the cavity. The other end is closed. Slots may be provided in the catheter to adjust the characteristics of the cavity.

The conduit is tunable as a quarter wavelength conduit to a frequency within 10% of the frequency of a coupled mode. Although the starting point of the tube length at the desired frequency is a quarter wavelength of the sound wave, the exact length is determined by the end correction and whether the tube is bent or not. Such modifications are well known to those skilled in the art. Thus, "quarter wavelength pipe" means a pipe that is properly tuned and not a pipe that actually has that wavelength. As noted above, the coupled mode absorption is fairly broad over the frequency range, so the length need not be exactly one quarter of the wavelength of the coupled mode frequency.

Tuning more precisely than the above 10% amplitude is preferable; therefore, the frequency of the catheter is preferably tuned to within 5% of the frequency of the coupled mode.

While the full acoustic impedance cannot be obtained, a quarter wavelength turns the actual impedance into a virtual one and vice versa. Thus, the quarter wave duct acts as an effective acoustic absorber for sound at frequencies with a length of one quarter wave.

The catheter need not be straight. A slight bend will affect the conduit slightly and a large bend will increase the acoustic mass, making it tuned down. As is known, this effect can be counteracted by proper adjustment of catheter length.

Catheters can be placed inside or outside the main cavity as desired.

Experiments have shown that when the sound-absorbing material is added to the duct, there will be a greater improvement. The sound-absorbing material can be acoustic fiber and/or foam plastic.

In another arrangement, the catheter may be coupled to the cavity and separated from the cavity by a membrane such that the catheter effectively forms an auxiliary cavity. The coupled system of catheter and membrane can be arranged so as to resonate within the cavity at a frequency close to the coupled/bulk mode. This cavity can be placed in a different plane than the main cavity. Additionally, absorbent material within the catheter is useful. The membrane itself may absorb by providing damping within the membrane, and/or by providing a damping suspension of the membrane.

Alternatively, the conduit may be an aperture connecting the cavity to the surroundings. Whereas for a near quarter wavelength conduit, the length determines the tuning of the conduit, in this arrangement the width and area of the conduit provide some tuning. This is because the area may be too small to pass lower frequencies, while at higher frequencies the area has a bass launching effect.

In particular embodiments, a strip may be provided around the edge of the panel to support the panel on the enclosure, wherein the strip is omitted from a portion of the edge such that the panel, enclosure and strip define the conduit; In this case, the conduit may exit the cavity and be open to the air. This arrangement is simpler than the solution of providing separate conduits. The elongated sheet may be malleable to resiliently support the panel. In this method, the panels can be installed freely, ie no longer clipped to the edge.

In specific embodiments, the conduit may be a hole in the front of the panel or in the enclosure. The hole is preferably in the central area behind the panel or speaker. Ducts are less frequency selective than quarter sonic panels, but sufficient for some applications.

Practically, multiple conduits as described above can be used to reduce the sound pressure in coupled mode.

It is also possible to provide multiple ducts to absorb sound with multiple frequencies.

A particularly useful method using multiple catheters will now be described. In this method, multiple quarter acoustic waveguides tuned to different frequencies are provided to acoustically couple the cavities. This alters the resonant modes within the cavity, and in particular the conduits can be selected to reduce the fundamental frequency of the cavity and increase the density of frequency-coupled resonant modes.

The panel may be a distributed mode panel operating in different resonant modes spread over different frequencies. This increases the number of modes so that the coupled enclosure system can greatly improve the characteristics of diffuse mode loudspeakers with this multiple selective frequency control.

A quarter acoustic waveguide may be provided along one side of the cavity, in which case that side of the cavity may be considered a sound absorber.

Even though all versions of the invention are applicable to resonant bending wave loudspeakers, this technique can be used to remove unwanted resonances or resonances within any panel that couples to shallow cavities. This can be used to make shallow box piston speakers.

Description of drawings

Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which

FIG. 1 is a schematic diagram of a first embodiment of the present invention,

Figure 2 is a schematic illustration of a second embodiment of the present invention,

Fig. 3 is the experimental result with and without catheter in Fig. 2,

Fig. 4 is the third specific embodiment of the present invention having a membranous object,

Fig. 5 and Fig. 6 are the fourth embodiment of the present invention with slots,

Figures 7 to 9 are details of the calculated acoustic responses as the slot length varies with the arrangement of Figure 5, and

Figure 10 is a fifth embodiment of the present invention having catheters of different lengths.

Detailed ways

In Fig. 1 a diffuse mode panel 11 is produced using the teaching of WO97/09842. The panel has a preferred scatter pattern aspect ratio of 1:882 or 1:0.707. A transducer 13 is installed at a preferred transducer location behind the panel. A shallow speaker 15 defines a shallow cavity 17 together with the panel 11 . A conduit 19 is provided at the end of the cavity. The conduit is a quarter wavelength of the coupled bulk mode of the panel 11 on springs formed by the air in the cavity 17 and any elastomer in the panel 11 support. A sound-absorbing material 21 is provided in the conduit 19 .

Reference is made to Figure 2 which is a second specific embodiment illustrated at the rear of cavity 17 measuring 260mm by 210mm. The panel 11 forms the upper surface of the cavity and the transducer is shown at 13 . The four conduits 19 provided are respectively on the four sides of the cavity. Each duct is 120mm long; therefore, all four ducts are tuned to the same resonance, here the overall mode resonance.

Figure 3 shows the results. The original frequency response is shown as a dotted line, and the response to the catheter is shown as a dashed line.

This result indicates a severe attenuation of the resonant dimension by the conduit at about 740 Hz. A 4dB attenuation is severe for acoustics. With damping, resonance attenuation can be further improved.

FIG. 4 shows an arrangement similar to that of the embodiment of FIG. 1 , except that a membrane 23 is provided to divide the conduit 19 from the cavity 17 . The catheter 19 is tuned to resonate at the bulk mode frequency. This duct 19 may lie in a different plane than the main cavity. A damper 21 is provided inside the conduit; this damper can be acoustic fiber or foam traditionally used for acoustic damping.

The membrane has a density selected to match the elastic support of the membrane so that it vibrates at a resonant frequency to determine the selected resonant frequency of the catheter. The resilient support may be disposed within the conduit and comprise foam, air in a cavity, or a combination of both.

5 and 6 show a fourth embodiment of the present invention. The panel 11 is supported on the sound box 15 by elastic elongated pieces 29 surrounding the edges of the panel. The panel is supported on the sound box at a distance of 5 mm from the frame edge 31 . The speaker 15 and the panel 11 are separated from each other at intervals of 2 mm along the circumference of the panel. The elongated piece defines a slot 33 lengthwise along one side of the panel. The slot functions as a conduit but not as a quarter wave conduit - these conduits are specifically used as vents for the release of sound pressure at bulk mode frequencies.

The damper 21 is disposed in the cavity and fixed to the panel 11 .

Calculations have been carried out by finite element analysis to show the effect of varying the slot length and the results are shown in Figures 7-9. Each figure relates to a different grooving length, shown in the figure in mm. The slot length of 144.44mm provides an exceptionally smooth response. As can be seen, the strong peak around 760 Hz can be eliminated using notches of different lengths. In addition, the length of the slots can be fine-tuned to produce a loudspeaker with a desired acoustic response.

There are some differences in effect between a single continuous slot and a set of slots (Fig. 9) resulting in the same overall effect, but these differences are not serious.

Figure 10 shows six quarter wave guides 19 of different lengths between 80 and 180 mm in length in steps of 20 mm. These catheters provide multiple frequency selective acoustic terminations.

The function of these ducts 19 is to render the boundaries acoustically invisible in a frequency range. Calculations show that the number of useful resonant modes of the coupled system can be greatly increased; these modes are qualitatively different from those without the catheter. The first three columns of Table 1 list the resonant modes with the quarter waveguide and the last three columns list the resonant modes without the guide. It can be clearly seen that with the conduit there is a higher mode number than without the conduit.

Table 1 with catheter no catheter 388.12 1309 2223.9 1320.3 2238.8 479.63 1420.1 2355.7 1492.5 2334.1 568.32 1512.1 2411.3 1516.6 2479.9 646.05 1679.6 2556.8 660.16 1632 2599.2 749.62 1801.4 2624.6 746.26 1980.5 2640.7 837.16 1931.6 2686.6 1992.7 2744.1 962.92 2002.1 2816.2 996.35 1141.6 2081.8 2844.9 1236.2 2190.9 2937.4 2116.4

In addition, the cavity is rendered less rigid at the bulk mode frequency by controlling the sound pressure so that the lowest coupled mode is at a favorable lower frequency. In the example, the lowest mode is at 388 Hz instead of 660 Hz, without changing the air volume or external dimensions of the speaker. The model density of the system is also increased by about 50% into the tabulated 3000 Hz range.

Claims (20)

1. A loudspeaker comprising a panel part for emitting sound, an exciter for exciting the panel part to emit sound, a sound box disposed at the rear of the panel member, together defining a shallow cavity with the panel member, generating a coupled resonant mode of the panel member and the cavity at a coupling mode frequency, It is characterized in that the panel part is a resonant bending wave part, which is acoustically coupled to the cavity through a conduit to selectively reduce the sound pressure of the cavity at the frequency of the coupling mode. 2. The loudspeaker of claim 1, wherein the coupled mode is a bulk mode in which the panel member oscillates on its suspension. 3. The loudspeaker according to claim 1 or 2, wherein a sound-absorbing material is disposed in the duct. 4. A loudspeaker as claimed in claim 1 or 2, wherein the conduit is tuned to a quarter-wavelength conduit with a frequency within 10% of the frequency of the coupled mode. 5. The loudspeaker of claim 4, wherein the frequency of the conduit is tuned to within 5% of the frequency of the coupled mode. 6. The loudspeaker of claim 1 or 2, wherein the conduit is coupled to and isolated from the cavity by a membrane, wherein the frequency of the coupling system comprising the membrane and the conduit is tuned to the frequency of the coupled mode in the cavity 5% range. 7. The loudspeaker as claimed in claim 1 or 2, wherein the duct is a hole disposed in the cavity, located at the center of the panel part or behind the sound box facing the center of the panel part. 8. A loudspeaker as claimed in claim 1 or 2, wherein a plurality of ducts are arranged to absorb sound at the frequency of the coupling mode. 9. A loudspeaker as claimed in claim 1 or 2, wherein a plurality of ducts are arranged to absorb sound of a plurality of frequencies. 10. A loudspeaker as claimed in claim 1 or 2, wherein the panel is a dispersion mode panel. 11. The loudspeaker of claim 1, wherein the conduit is a hole provided at an edge of the cavity. 12. The loudspeaker of claim 11, wherein the coupled mode is a bulk mode in which the panel member oscillates on its suspension. 13. The speaker according to claim 11 or 12, wherein a sound-absorbing material is disposed in the duct. 14. A loudspeaker as claimed in claim 11 or 12, wherein the conduit is tuned to a quarter wavelength conduit with a frequency within 10% of the coupled mode frequency. 15. The loudspeaker of claim 14, wherein the frequency of the conduit is tuned to within 5% of the coupled mode frequency. 16. The loudspeaker as claimed in claim 11 or 12, wherein an elongated piece is provided on the enclosure along the edge of the panel to support the panel, wherein the elongated piece is omitted at part of the edge so that the panel, enclosure and thin The long pieces define the conduit. 17. The loudspeaker of claim 16, wherein the elongated piece is elastic. 18. A loudspeaker as claimed in claim 11 or 12, wherein a plurality of ducts are arranged to absorb sound at the frequency of the coupling mode. 19. A loudspeaker as claimed in claim 11 or 12, wherein a plurality of ducts are arranged to absorb sound of a plurality of frequencies. 20. A loudspeaker as claimed in claim 11 or 12, wherein the panel is a dispersion mode panel.

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