Flat panel loudspeaker with broad-range frequency response

Abstract

A flat panel loudspeaker, according to the present invention, has a housing with a sound panel, opposing sides, and a back panel at least partially enclosing a hollow chamber therebetween. An exciter is rigidly attached to the sound panel within the chamber to selectively induce vibrations in the sound panel. The sound panel has a density of between 465 kg / m3 and 700 kg / m3.

Description

FLAT PANEL LOUDSPEAKER WITH BROAD-RANGE FREQUENCY RESPONSEField of the Invention

[0001] The present invention relates to loudspeakers and, in particular, to distributed mode loudspeakers (DMLs).Background

[0002] A distributed mode loudspeaker (DML) is an alternative type of loudspeaker to the traditional loudspeaker that produces sound by inducing pistonic motion in a diaphragm. By contrast, DMLs produce sound by inducing uniformly distributed vibrational modes in a panel. This makes it possible to construct DMLs of various sizes, including flat panels with very thin profiles, compared to traditional loudspeakers, which usually must accommodate relatively deeper cone-shaped diaphragms of the “woofer”, which produce low-frequency sound waves.

[0003] Flat panel DMLs have existed for a number of years, but generally suffer from problems producing low-frequency sound waves. Attempts have been made to address the frequency range limitations of DMLs, including by increasing the size of the sound panel, which does allow the production of lower frequency sound. A typical DML will have a low-end cutoff frequency of about 200 Hz, while some DMLs may offer low-end response down to as low as 80 Hz. However, it is not always possible or convenient to increase the size of the sound panel to enable the production of low-frequency sound waves.

[0004] Larger sound panels require more sound panel material and will generally require larger, more powerful, or numerous exciters to vibrate the larger sound panel. This increases the cost of the DML and can lead to structural failure, as a result of the higher mechanical stress caused by the stronger vibrations. These limitations are particularly problematic because the loudspeaker field generally teaches the use of very light-weight materials for the diaphragm of both conventional cone type loudspeakers and panel type DMLs.

[0005] However, it is also recognized that high stiffness is also desirable for loudspeaker performance, in particular, for high-frequency response. Accordingly, diaphragms are frequently produced by applying thin, rigid skins of paper laminates, high-stiffness plastic films, metal sheet or foil layers on either side of a supporting honeycomb structure, to provide the desired combination of overall lightness and stiffness, which is thought to be well-suited for loudspeaker diaphragms. A typical paper or plastic loudspeaker diaphragm has a density of about 9 kg / m3, while relatively heavier composite material (i.e. carbon fibre) diaphragms may have a density of about 170 kg / m3and aluminum alloy diaphragms may have density of about 270 kg / m3.

[0006] As discussed above, the mechanical stress caused by the high-powered exciters needed to induce vibrations in larger diaphragms require stiffer and more resilient materials in order to still minimize the weight of the diaphragm. These materials, such as carbon fibre, Nomex™, aerogel metals, and other high performance cellular core materials are expensive and may have other undesirable acoustic or mechanical characteristics for use in loudspeakers. Accordingly, there is a need for alternative materials and configurations of DMLs that permit the use of more widely available materials, while also providing excellent acoustic performance.Summary of the Invention

[0007] A flat panel loudspeaker, according to the present invention, has a housing with a sound panel, opposing sides, and a back panel at least partially enclosing a hollow chamber therebetween. An exciter is rigidly attached to the sound panel within the chamber to selectively induce vibrations in the sound panel. The sound panel has a density of between 465 kg / m3and 700 kg / m3.

[0008] In another embodiment, the sound panel has a density of between 465 kg / m3and 595 kg / m3. In another embodiment, the sound panel has a density of between 575 kg / m3and 595 kg / m3.

[0009] In another embodiment, the sound panel is made of a natural wood material. The natural wood may be from trees of the Yellow Pine and Larch varieties. In another embodiment, the natural wood material is selected from the group consisting of: Tamarack wood, European Larch wood, Jack Pine wood, Lodgepole Pine wood, and Red Pine wood. Preferably, the sound panel is made of Tamarack wood or European Larch wood.

[0010] In another embodiment, the chamber contains acoustic insulation. One or more braces may be each attached, independently, to the one of the sound panel, the back panel, or the opposing sides.

[0011] In another embodiment, the sound panel has a density of between 465 kg / m3and 1 ,100 kg / m3and is made of a composite material.Brief Description of the Drawings

[0012] In order that the invention may be more clearly understood, a preferred embodiment thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

[0013] Figure 1 is a perspective view of a flat panel loudspeaker, according to the present invention.

[0014] Figure 2 is a perspective view of a frame for placing over the flat panel loudspeaker of Fig. 1.

[0015] Figure 3 is a rear view of the flat panel loudspeaker of Fig. 1 , showing the mounting supports on the rear panel and the frame positioned over the housing.

[0016] Figure 4 is a rear view of the flat panel loudspeaker of Fig. 1 , with the rear panel removed to show the interior of the chamber.

[0017] Figure 5 is a detail cross-sectional view of a side of the housing of the flat panel loudspeaker of Fig. 1.

[0018] Figure 6 is a detail cross-sectional view of the exciter, shown attached to the sound panel, of the flat panel loudspeaker of Fig. 1 .

[0019] Figure 7 is another detail cross-sectional view of the exciter, shown attached to the sound panel and a brace, of the flat panel loudspeaker of Fig. 1.

[0020] Figure 8 is another rear view of the flat panel loudspeaker of Fig. 1 , with the rear panel removed, showing multiple exciters and braces attached to the sound panel.

[0021] Figure 9 is a detail cross-sectional view of a side of the housing of the flat panel loudspeaker of Fig. 1 , showing braces attached to the side, sound panel, and back panel.

[0022] Figure 10 is an exploded perspective view of the flat panel loudspeaker of Fig. 1.

[0023] Figure 11 is a stacked curve plot of frequency responses to a sine wave frequency sweep of the flat panel loudspeaker of Fig. 1 , having sound panels made from various materials.

[0024] Figure 12 is a series of graphs showing impulse response to a short sine wave sweep of the flat panel loudspeaker of Fig. 1 , with a sound panel made of Black Spruce, Hard Maple, Jack Pine, and European Larch woods.

[0025] Figure 13 is a curve plot of frequency response to a sine wave frequency sweep of the flat panel loudspeaker of Fig. 1 , with a sound panel made of European Larch wood with and without a polyurethane finish.

[0026] Figure 14 is a series of graphs showing impulse response to a short sine wave sweep of the flat panel loudspeaker of Fig. 1 , with a sound panel made of European Larch wood with and without a polyurethane finish.

[0027] Figure 15 is a curve plot of frequency response to a sine wave frequency sweep of the flat panel loudspeaker of Fig. 1 , with a sound panel made of Lodgepole Pine wood without a poster, with a paper-based poster, and with a latex-based poster.

[0028] Figure 16 is a series of graphs showing impulse response to a short sine wave sweep of the flat panel loudspeaker of Fig. 1 , with a sound panel made of Lodgepole Pine wood without a poster, with a paper-based poster, and with a latex-based poster.

[0029] Figure 17 is a stacked curve plot of frequency responses to a sine wave frequency sweep of the flat panel loudspeaker of Fig. 1 , comparing the frequency responses sound panels made of European Larch wood with the sound panel alone, attached to sides of a housing, and attached to sides and a back panel with insulation in the cavity therebetween.

[0030] Figure 18 is a series of graphs showing impulse response to a short sine wave sweep of the three sound panel configurations of Fig. 17.Description of the Preferred Embodiments

[0031] The flat panel loudspeaker, according to the present invention, breaks from the established approach to building loudspeakers with high stiffness and light-weight diaphragms and instead uses a relatively heavier material having a density between 465 kg / m3and 700 kg / m3. The inventors have found that a sound panel made of such heavier materials as Tamarack or European Larch wood (595 kg / m3and 575 kg / m3, respectively) have significantly improved acoustic performance, compared to similar materials, such as Black Spruce or Hard Maple wood (450 kg / m3and 705 kg / m3, respectively). Acoustic performance has also been found to be improved by the particular configuration of the flat panel loudspeaker, described herein, in combination with a higher density material sound panel. Such materials are less susceptible to damage from the mechanical stress induced by powerful exciters, compared to traditional paper or light-weight composite or metal foil and honeycomb loudspeaker diaphragms. As a result, preferred embodiments of the present invention demonstrate anumber of favourable acoustic properties, compared to other flat panel loudspeaker configurations, including:• Low-end cutoff frequencies as low as 40 Hz;• Broad frequency response with relatively small sound panels (i.e. 45 cm x 60 cm);• Flat frequency response curves;• Rapidly decaying impulse response;• Reduced rear-wall firing and other wave interference affecting acoustic performance; and• Acoustically transparent to lightweight or thin coverings and surface finishings.

[0032] As shown in Figures 1 , 3, and 4, the flat panel loudspeaker, according to the present invention, has a housing 1 made up of opposing sides 3, a sound panel 5, and an opposing back panel 7. There are preferably four sides 3, being two pairs of opposing straight sides of equal length, forming a generally rectangular shape of the housing 1. The sound panel 5 is attached to the sides 3 on a front face of the housing 1 , while the back panel 7 is attached to the sides 3 on the opposing back face of the housing 1. Together, the sides 3, sound panel 5, and back panel 7 at least partially enclose a hollow chamber 9 within the housing 1.

[0033] The dimensions of the housing 1 may vary depending on the desired application.Preferred embodiments of the present invention use a housing with a sound panel 5 having face dimensions of 45 cm x 60 cm or 60 cm x 60 cm, a thickness of about 3 mm, and depth (i.e. distance between the sound panel 5 and the back panel 7) of above about 2 cm, with a wall offset (i.e. distance from the back panel 7 to a supporting wall) of between 1 mm and 50 mm or more than about 75 mm. Certain embodiments with a sound panel 5 having a thickness between 1.5 mm and 1 cm, and face dimensions as small as 13 cm x 19 cm or as large as 60 cm x 92 cm produced desirable acoustic performance. In one particularly preferred embodiment, the housing 1 has face dimensions of 45 cm x 60 cm, a sound panel 5 thickness of 3 mm, a depth of 3.5 cm, and a wall offset of 5 mm. The minimum dimension of the depth is primarily limited by the width of the exciter 11. It is possible that narrower exciters11 may enable shallower dimensions for the depth of the housing 1 , while providing similar acoustic performance. Acoustic testing indicates that the preferred wall offset varies depending on the depth of the housing 1. The flat panel loudspeaker may also be used without a wall offset or more substantially spaced apart from the nearest adjacent wall (i.e. not wall-mounted), such that the panel is more than about 75 mm from the nearest wall surface behind the back panel 7.

[0034] At least the sound panel 5 of the housing 1 is made of a material having a density of between 465 kg / m3and 700 kg / m3. Preferably, the sides 3 or the back panel 7, or both, are also made of a material having a density between 465 kg / m3and 700 kg / m3. Preferred materials include natural wood materials from trees of the Yellow Pine and Larch varieties. Tamarack wood (or Eastern Larch wood) of the species Larix laricina, having a density of 595kg / m3and a stiffness (elastic modulus) of about 11.3 GPa is the most preferred material for the sound panel 5 and, optionally, the sides 3 or back panel 7, or both. Another highly preferred material is European Larch wood of the species Larix decidua with a density of about 575 kg / m3and a stiffness (elastic modulus) of about 11.7 GPa. Without wishing to be limited by theory, it is believed that preferred materials, in particular the natural woods described herein, possesses a favourable combination of density, stiffness, and resin content for use in flat panel loudspeakers according to the present invention. The resin content provides a degree of acoustic dampening in the sound panel 5 that results in favourable overall acoustic performance.

[0035] The densities referred to in the present application for natural wood materials are the average dried weight measurements (in kg / m3) of the wood types referenced herein. It was found that other natural wood materials outside of the density range between 465 kg / m3and 700 kg / m3did not produce desirable acoustic characteristics. For example, Engelmann Spruce wood (385 kg / m3), Black Spruce wood (450 kg / m3), and Hard maple wood (705 kg / m3) were tested and none produced desirable acoustic characteristics. On the other hand, Jack Pine wood and Lodgepole Pine wood (500kg / m3and 465 kg / m3) were tested and producedgood results. Red Pine wood (545 kg / m3) was also tested and produced acceptable acoustic characteristics.

[0036] Alternatively, the sound panel 5 of the housing 1 may be made of a composite material, such as particle board, chipboard, plywood veneer sheet, polymer matrix composite, or other manufactured material having a density of between 465 kg / m3and 1 ,100 kg / m3. It has been found that relatively higher density manufactured materials, compared to the preferred natural wood materials described above, such as low-density fiberboard (LDF), medium-density fiberboard (MDF) or high-density fiberboard (HDF), demonstrate desirable acoustic performance when used to construct the sound panel 5 of a flat panel loudspeaker, according to the present invention. Generally, LDF has a density of between 550 kg / m3and 650 kg / m3, MDF has a density between about 600 kg / m3and 800 kg / m3, and HDF has a density between about 800 kg / m3and 1 ,100 kg / m3. Additionally, plywood veneer sheets made of preferred natural wood materials described above may have densities as low as 465 kg / m3. Suitable types of composites may include wood-based composites (i.e. plywood, particleboard, and fiberboard), polymer matrix composites (i.e. polypropylene or polyethylene), or wood-plastic composites. Polypropylene composite materials may have a density of between 900 kg / m3and 940 kg / m3, while polyethylene composites, such as low-density polyethylene (LDPE), high- density polyethylene (HDPE), or ultra-high molecular weight polyethylene (UHMWPE) may have densities between 910 kg / m3and 970 kg / m3, and wood-plastic composites, such as a wood-polyurethane composite, may have a density between 800 kg / m3and 1 ,100 kg / m3.Without wishing to be bound by theory, it is believed that composites such as particle board have similarly favourable material properties, including density, elasticity, and resin content, compared to natural wood materials, and that the binding agent used in such composites has minimal impact on acoustic performance. Composite materials are thereby able to provide desirable acoustic properties within a generally higher (although overlapping) density range compared to natural wood. Higher density composite materials may require stronger exciters11 , due to the increased overall weight of the sound panel 5, in order to produce the same volume of sound as less dense natural wood materials.

[0037] As shown in Figures 4, 6, and 8, an exciter 11 is attached to the sound panel 5 inside the chamber 9. The exciter 11 may be any a loudspeaker voice coil suitable for use with DMLs. Preferred examples of suitable exciters include: BillionSound™ 14K13-BS, Dayton Audio™ DAEX25SHF-4, DAEX25FHE-4, DAEX25, TT25-16, DAEX30HESF-4, DAEX32QMB- 4, Visaton™ 243-EX80S-80HM, Tectonic™ TEAX32C30-4B, and TAEX25C05-8. Preferably, as shown in Figure 6, the exciter 11 is “floating” inside the chamber 9, in that it is not attached to any part of the housing 1 other than the sound panel 5. This may eliminate the possibility of the movement of the exciter 11 being impeded by any misalignment of a rigid mounting or seating structure, which could be caused by expansion or contraction of the materials of the housing 1 , due to environmental changes, such as temperature, moisture, etc. fluctuations.

[0038] As shown in Figure 4, a single exciter 11 may be attached to the sound panel 5 and positioned roughly centrally inside the chamber 9, between the opposing sides 3. Unlike other DML systems, the exciter 11 is not limited to a specific positioning offset to avoid undesirable resonance and the exciter 11 may positioned at the exact centre of the sound panel 5, without creating undesirable resonance in the sound panel 5. It is believed that the undesirable resonance this creates in other DML systems is avoided by the dampening effect of directly attaching the sound panel 5 to the sides 3 of the housing 1.

[0039] Alternatively, as shown in Figure 8, multiple exciters 11 may be attached to the sound panel 5. This may be desirable with larger sized sound panels 5 or in applications where higher volume sound production is required. Preferably, where more than one exciter 11 is used, the plurality of exciters 11 are offset from one another so as not to be equidistant from any of the sides 3 of the housing 1. Additionally, where one or more braces 19 are attached to the sound panel 5 and a plurality of exciters 11 are used, preferably, none of the braces 19 are positioned between any of the exciters 11 , as shown in Figure 8. This configuration ispreferred to avoid the sections of the sound panel 5 with exciters 11 on opposite sides of a brace 19 from acting as independent, but connected DMLs, rather than the entire sound panel 5 acting as a single DML. The use of multiple exciters 11 significantly increases the complexity of the design of the flat panel loudspeaker according to the present invention and, for that reason, a single exciter 11 is preferred.

[0040] In order to provide the desired wall offset, the housing 1 is configured to be mounted on a supporting wall surface by way of one or more mounting apertures 13 in the back panel 7. Preferably, four mounting apertures 13 are positioned between the centre and the outside corners of the back panel 7, as shown in Figure 10. One or more mounting supports 15 having hooks 17 shaped to engage with the mounting apertures 13 may be attached to the supporting wall surface by any convenient means of secure attachment. Screws may be driven through the mounting supports 15 into the supporting wall or a strong adhesive may be used between the mounting supports 15 and the supporting wall surface. In any case, the hooks 17 on the one or more mounting supports 15 are positioned to align with the mounting apertures 13, so that the housing may be positioned over the hooks 17 and suspended from the mounting supports 15 on the supporting wall surface.

[0041] Preferably, the mounting supports 15 are sized with a thickness suitable for maintaining the desired wall offset between the back panel 7 and the supporting wall surface when the back panel is placed directly against the mounting supports 15. The surface of the mounting supports 15 facing the back panel 7 may be covered in a mineral wool, felt, or other vibration dampening material to reduce any vibration or “buzzing” sounds from being produced by the movement of the back panel 7 against the mounting supports 15.

[0042] In order to provide additional stiffness or resilience, one or more braces 19 may be attached to the sound panel 5 or back panel 7, or both, as shown in Figure 8. One or more braces 19 may also be attached to the sides 3, but the same brace 19 is preferably not also attached to the sound panel 5 or back panel 7, as shown in Figure 9. An air gap betweensound panel 5 and any brace 19 that is attached to the sides 2 is provided to permit vibration of the sound panel 5, without any interference from the brace 19, which could cause a “buzzing” sound, over-dampening, and otherwise impair the acoustic performance of the flat panel loudspeaker. As shown in Figure 8, the braces 19 may be oriented parallel to an adjacent edge 3 or may be oriented at an angle to an adjacent edge 3.

[0043] As shown in Figure 9, a brace 19 on the sound panel 5 or the back panel 7 may have a tapered edge 19a and preferably is not also attached to the sides 3. A separate brace 19 may be attached to one or more of the sides 3 and is preferably spaced apart from both the sound panel 5 and the back panel 7 (and any brace 19 attached to either). As shown in Figure 8, the braces 19 attached to the sides 3 preferably extend between two opposing sides 3 to provide additional structural stability to the housing 1. Preferably, where such braces 19 are used, they are positioned off-centre so as not to interfere with the optimal placement of the exciter(s) 11. Alternatively, as shown in Figure 7, the braces 19 attached to the sides 3 may also provide a mounting structure for the exciter 11. In such configurations, the exciter 11 may be directly attached to the brace 19 or may be attached via an elastic cradle (not shown) to permit a desired degree of relative movement in one or more dimensions, which can help accommodate expansion and contraction of the materials of the housing 1 , without impairing the movement and operation of the exciter 11.

[0044] As shown in Figure 10, the chamber 9 contains acoustic insulation 21 to dampen the reverberations that would otherwise be created between the sound panel 5 and the back panel 7 inside the chamber 9. Preferably, the insulation 21 is shaped to substantially fill the available space inside the chamber 9, while not interfering with the exciter(s) 11 and permitting space for any braces 19. The insulation may also contain a cutout area to fit around an amplifier mounting cavity 23, inside the chamber 9. The insulation 21 is preferably a mineral wool acoustic insulation, but other suitable types of insulation may be used, including: cellulose fiber, fiberglass batting, polyurethane foam, loose polyester batting, mass loaded vinyl, neoprene, or soundproofing wood fiber board (e.g. SONOpan™).

[0045] As shown in Figures 3 and 4, the housing 1 may include an amplifier mounting cavity 23 within the chamber 9. The amplifier mounting cavity 23 is preferably formed by one or more braces 19 attached to one of the sides 3 to define a space within the chamber 9 for an amplifier 25 and provide sufficient structural support to mount the amplifier 25 within the amplifier mounting cavity 23. The amplifier 25 is preferably positioned adjacent one of the sides 3 to permit accessible placement of inputs and controls for the flat panel loudspeaker on one of the sides 3, rather than on the sound panel 5, which could interfere with acoustic performance. Where no amplifier 25 is provided inside the chamber 9, audio cables or inputs are preferably also provided on one of the sides 3. Alternatively, the inputs and controls may be located on the back panel 7, with or without an amplifier 25.

[0046] As shown in Figures 2, 3, and 10, a frame 27 may be sized and configured to be removably positioned over the housing 1 to permit mounting of artwork, such as a poster 31 , over the sound panel 5. Preferably, one or more magnets 29 are positioned about the perimeter of the sound panel 5 and align with magnets 29 on the inside of the frame 27 to facilitate easy attachment of the frame 27 over the housing 1. Alternatively, other more permanent means of attachment, such as clips, snap fasteners, or screws may be used to fix the frame 27 in position over the housing 1 with a poster 31 therebetween. Felt or another vibration dampening material may be used between the inside of the frame 27 and the sound panel 5 or the sides 3, or both, to prevent buzzing or unwanted interaction between the sound panel 5 or sides 3 and the frame 27.

[0047] As shown in Figures 15 and 16, it has been found that the placement of a lightweight poster 31 or similar piece of artwork between the frame 27 and sound panel 5 does not significantly impair the acoustic performance of the flat panel loudspeaker, according to the present invention. The artwork may, optionally, be adhered to the sound panel 5 to maintain contact therebetween. Alternatively, the artwork may be painted, printed, burned, or otherwise created directly on the surface of the sound panel 5. It has been found that artwork on a heavy, dense medium, such as a stretched canvas that is not adhered to the sound panel 5,impairs the acoustic performance of the flat panel speaker. Without wishing to be limited by theory, it is believed that the high weight and density of the sound panel 5 (relative to traditional loudspeaker diaphragm materials) is able to maintain consistent acoustic performance despite coming into contact with a relatively lighter poster 31 or despite having a slightly heavier piece of artwork adhered to the sound panel 5.Acoustic Testing

[0048] As shown in Figures 11A, B, and C, and 12A, B, and C, various embodiments of the present invention with different dimensions and parameters were subjected to acoustic testing to measure their relative performance. The results shown in the Figures and described below were obtained using a flat panel loudspeaker, according to the present invention, positioned at least 30 cm from a rear wall, having a housing 1 (including the sound panel 5) the made entirely of European Larch wood (except where the material is indicated otherwise). A calibrated microphone was positioned approximately 1 m away from the sound panel 5, aiming toward the centre of the sound panel 5. A short sine wave sweep was played through the flat panel loudspeaker to determine the frequency response and impulse response of various configurations of the flat panel loudspeaker.

[0049] As shown in Figure 11 , shows a stacked curve plot (meaning the curves are offset to avoid overlap and their position is not indicative of absolute decibel level) with sound panels 5 made of different materials having dimensions of 60 cm by 60 cm. The different materials had similar responses, but Black Spruce wood and Red Pine wood showed noticeable valleys in the 2-5 kHz range. Figure 12 shows the impulse response of the same sound panels 5 shown in Figure 11 . The impulse response is significantly noisier, especially after about 5 ms, in Black Spruce wood and Hard Maple wood, compared to Jack Pine wood and European Larch wood. Although not shown, significantly noisier impulse responses were observed in materials such as 6061-T6 aluminum and glass.

[0050] As shown in Figure 13, acoustic testing was performed to show the frequency response to a short sine wave sweep of a flat panel loudspeaker, according to the present invention, witha sound panel 5 made of European Larch wood and having dimensions of 45 cm by 60 cm. The frequency plots follow very similar patterns and produce very similar sound pressure level volumes. This indicates that a polyurethane finish does not have any significant acoustic impacts to the frequency response. Figure 14 shows the impulse response of the same panels tested in Figure 13. Both plots show a similar decay, indicating that a polyurethane coating does not have a significant effect on the impulse response. This indicates that surface treatments, such as polyurethane coatings, do not negatively impact the acoustic performance of the flat panel loudspeaker.

[0051] As shown in Figures 15 and 16, show the frequency response and impulse response, respectively, of a flat panel loudspeaker, according to the present invention, with a Lodgepole Pine sound panel 5, without any artwork and with a paper-based poster and latex-based poster positioned over the sound panel 5. The frequency responses are similar without artwork and with both posters, with slightly reduced sound pressure level. The impulse responses were also similar in all three embodiments, suggesting the presence of the posters does not significantly impact the acoustic performance of the flat panel loudspeaker.

[0052] As shown in Figures 17 and 18, show a stacked curve plot of the frequency response to a short sine wave sweep of a sound panel 5 made of European Larch wood with dimensions of 13 cm by 19 cm, in three configurations: (A) free (i.e. unmounted), (B) attached to sides 3 made of 3.5 cm thick European Larch wood, and (C) as part of a full housing 1 , having 3.5 cm thick sides 3 and a back panel 7 made of European Larch wood with mineral wool insulation 21 in the chamber 9. Configuration B had a significantly improved frequency response, compared to configuration A. Configuration C had a further improved, “flatter” frequency response than either configuration A or B. The impulse response of configuration B was worse than configuration A. Configuration C, again, had a significantly better impulse response than either configuration A or B. Auditory testing indicated that configuration B produced an undesirable echo or reverberation and both configurations A and B produced undesirable rear wall reflections (sometimes called “rear wall firing”) when positioned near a rear wall.

[0053] The present invention has been described and illustrated with reference to an exemplary embodiment, however, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as set out in the following claims. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed herein.

Claims

What is Claimed is:1 . A flat panel loudspeaker, comprising: a housing with a sound panel, opposing sides, and a back panel at least partially enclosing a hollow chamber therebetween; and an exciter rigidly attached to the sound panel within the chamber to selectively induce vibrations in the sound panel; wherein the sound panel is rigidly attached to the opposing sides of the housing; and wherein the sound panel has a density of between 465 kg / m3and 700 kg / m3.

2. The flat panel loudspeaker of claim 1 , wherein the sound panel has a density of between 465 kg / m3and 595 kg / m3.

3. The flat panel loudspeaker of claim 2, wherein the sound panel has a density of between 575 kg / m3and 595 kg / m3.

4. The flat panel loudspeaker of claim 1 , wherein the sound panel is made of a natural wood material.

5. The flat panel loudspeaker of claim 4, wherein the natural wood material is from trees of the Yellow Pine and Larch varieties.

6. The flat panel loudspeaker of claim 5, wherein the natural wood material is selected from the group consisting of: Tamarack wood, European Larch wood, Jack Pine wood, Lodgepole Pine wood, and Red Pine wood.

7. The flat panel loudspeaker of claim 6, wherein the sound panel is made of Tamarack wood.

8. The flat panel loudspeaker of claim 6, wherein the sound panel is made of European Larch wood.

9. The flat panel loudspeaker of claim 1 , wherein the exciter is only attached to the housing by attachment to the sound panel.

10. The flat panel loudspeaker of claim 1 , wherein the exciter is positioned centrally on the sound panel, between the opposing sides.11 . The flat panel loudspeaker of claim 1 , comprising one or more additional exciters each offset from the exciter and one another so as not to be equidistant from any of the opposing sides.

12. The flat panel loudspeaker of claim 1 , wherein the chamber contains acoustic insulation.

13. The flat panel loudspeaker of claim 12, wherein one or more braces are each attached, independently, to one of the sound panel, the back panel, or the opposing sides.

14. The flat panel loudspeaker of claim 13, wherein an amplifier mounting cavity is located inside the chamber.

15. The flat panel loudspeaker of claim 1 , comprising a frame sized to be removably positioned over the sound panel.

16. The flat panel loudspeaker of claim 1 , wherein the sound panel has face dimensions of between 13 cm by 19 cm and 60 cm by 92 cm.

17. The flat panel loudspeaker of claim 16, wherein the distance between the sound panel and the back panel is above 2 cm.

18. A flat panel loudspeaker, comprising: a housing with a sound panel, opposing sides, and a back panel at least partially enclosing a hollow chamber therebetween; and an exciter rigidly attached to the sound panel within the chamber to selectively induce vibrations in the sound panel; wherein the sound panel is rigidly attached to the opposing sides of the housing; and wherein the sound panel is made of a composite material having a density of between 465 kg / m3and 1 ,100 kg / m3.

19. The flat panel loudspeaker of claim 18, wherein the sound panel is made of particle board, chipboard, or plywood veneer sheet.

20. The flat panel loudspeaker of claim 18, wherein the sound panel is made of mediumdensity fiberboard or high-density fiberboard.

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