AU2013100833A4 - A loudspeaker and a loudspeaker damping apparatus - Google Patents

Abstract

A loudspeaker damping apparatus configured to attenuate oscillations of an audio loudspeaker substantially along a fore and aft direction of the audio loudspeaker caused by oscillations of at least one audio frequency transducer of the audio loudspeaker, the damping apparatus including: a counter mass; and coupling means configured to couple the counter mass to the loudspeaker while allowing substantial oscillations of the counter mass relative to the loudspeaker along the fore and aft direction so as to attenuate the fore and aft oscillations of the loudspeaker.

Description

- 1 A LOUDSPEAKER AND A LOUDSPEAKER DAMPING APPARATUS TECHNICAL FIELD The present invention relates to high-fidelity sound reproduction, and in particular to a loudspeaker and a loudspeaker damping apparatus for attachment to a loudspeaker to 5 attenuate oscillations of the loudspeaker. BACKGROUND High-fidelity sound reproduction continues to be an area of active research and development. In particular, the design of audio frequency transducers and loudspeakers in 10 general continues to evolve, producing continuing improvements in the perceived quality of sound reproduction. Many recent developments in loudspeaker design and technology have been in the areas of diffraction and vibration control, often using advanced materials. However, these and similar issues continue to limit the performance of loudspeakers in general. 15 It is desired to provide a loudspeaker and a loudspeaker damping apparatus that alleviate one or more difficulties of the prior art, or that at least provide a useful alternative. SUMMARY In accordance with some embodiments of the present invention, there is provided a loudspeaker, including: an audio transducer support structure; at least one audio transducer mounted to the support structure and configured to generate audio frequency sound waves from a corresponding electrical input signal by corresponding oscillations of the at least one audio transducer relative to the audio transducer support structure, wherein the oscillations of the at least one audio transducer cause oscillations of the audio transducer support structure substantially along a fore and aft direction of the loudspeaker and at a corresponding natural frequency of the loudspeaker; a counter mass; and -2 coupling elements coupling the counter mass to the audio transducer support structure and configured to allow substantial oscillations of the counter mass relative to the support structure and along said fore and aft direction; wherein the counter mass and coupling means are configured so that the fore and aft oscillations of the audio transducer support structure generate corresponding oscillations of the counter mass relative to the support structure so as to attenuate the fore and aft oscillations of the audio transducer support structure. In some embodiments, the oscillations of the counter mass relative to the audio transducer support structure along the fore and aft direction have a frequency that is substantially equal to or at least close to said natural frequency of the loudspeaker. In some embodiments, the oscillations of the audio transducer support structure are substantially out of phase relative to the oscillations of the counter mass relative to the audio transducer support structure. In accordance with some embodiments of the present invention, there is provided a loudspeaker damping apparatus configured to attenuate oscillations of an audio loudspeaker substantially along a fore and aft direction of the audio loudspeaker caused by oscillations of at least one audio frequency transducer of the audio loudspeaker, the damping apparatus including: a counter mass; and coupling means configured to couple the counter mass to the loudspeaker while allowing substantial oscillations of the counter mass relative to the loudspeaker along the fore and aft direction so as to attenuate the fore and aft oscillations of the loudspeaker. In some embodiments, the oscillations of the loudspeaker substantially along said fore and aft direction have a first natural frequency, and the oscillations of the counter mass relative to the support structure along said fore and aft direction have a second natural frequency that is substantially equal or at least close to the first natural frequency. The oscillations of -3 the loudspeaker may be substantially out of phase relative to the oscillations of the counter mass relative to the loudspeaker. In some embodiments, the loudspeaker damping apparatus included a base to support the counter mass and the coupling means, and to couple the counter mass and the coupling means to the audio loudspeaker. In some embodiments, the base includes a recess configured to receive the counter mass. In some embodiments, the coupling elements are disposed between the counter mass and the base. In some embodiments, the coupling elements include springs and damping elements. In other embodiments, the coupling elements are composed of an elastomer. In some embodiments, the elastomer is Dermasol. In some embodiments, the loudspeaker is a stand mount loudspeaker including a stand. In some embodiments, the loudspeaker is a floor-standing loudspeaker. Also described herein is loudspeaker damping apparatus, including a tuned mass damper configured to attenuate fore-and-aft oscillations of a loudspeaker caused by movements of at least one audio transducer of the loudspeaker. The loudspeaker damping apparatus may be configured for removable attachment to the loudspeaker. Alternatively, the loudspeaker damping apparatus may be incorporated into the loudspeaker. The fore-and-aft oscillations of the loudspeaker may be at a natural or resonant frequency of a fore-and-aft vibration mode of the loudspeaker, and the tuned mass damper may have a fore-and-aft vibration mode having a natural or resonant frequency equal or least close to the natural or resonant frequency of the fore-and-aft vibration mode of the loudspeaker.

-4 BRIEF DESCRIPTION OF THE DRAWINGS Some embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a schematic diagram illustrating a typical prior art stand mount 5 loudspeaker; Figure 2 is a schematic diagram illustrating a fore-and-aft mode of oscillation of oscillation of the loudspeaker of Figure 1; Figures 3 are 4 are plan and side views, respectively, of a loudspeaker damping apparatus in accordance with a first embodiment of the present invention; 10 Figures 5 are 6 are plan and side views, respectively, of a loudspeaker damping apparatus in accordance with a second embodiment of the present invention; and Figure 7 is a schematic diagram illustrating the operation of the loudspeaker damping apparatus of Figures 3 and 4 on the loudspeaker of Figures 1 and 2. 15 DETAILED DESCRIPTION As shown in Figure 1, a typical loudspeaker 100 includes at least two audio transducers referred to in the art as 'drivers' 102, 104 mounted to a common and typically generally planar baffle of a support structure 106. Each of the drivers 102, 104 generates audio frequency sound waves having frequencies in a corresponding frequency range from a 20 corresponding electrical input signal by corresponding vibrations or oscillations of a moving portion of the driver 102, 104. Although some loudspeakers have only one driver to generate sound waves of all frequencies, referred to as a 'full range' driver, most loudspeakers have multiple drivers of different sizes and/or types so that each driver can be optimised to reproduce frequencies in a relatively narrow band of frequencies. 25 Other than the drivers themselves, the support structure 106 can take a variety of different forms. The most common of these is the conventional box-type loudspeaker being in the form of a generally cuboid hollow enclosure or box, as shown in Figure 1, which may be sealed or may have an opening or 'port' to generate sound waves in a low frequency band -5 and to increase the efficiency of the loudspeaker. Other types of loudspeaker structure include unipolar open baffle in which the support structure is essentially just a planar baffle with openings through which the drivers are mounted, planar or bipolar loudspeakers, and horn loudspeakers. 5 Irrespective of the number and type of drivers and the type of supporting structure, vibrations and resonances of the drivers 102, 104 and also the supporting structure 106 degrade the quality of sound reproduction. These vibrations or resonances of the cabinet/enclosure (or other form of support structure) itself can generally be described as 10 mechanical resonances of the exterior and interior structural panels of the loudspeaker, and depend on such factors as the shape and dimensions of each panel and its stiffness or rigidity. Consequently, it has been found that a mediocre loudspeaker can be transformed into an outstanding loudspeaker by improving the rigidity and damping of the supporting structure itself 106, without changing anything else. For example, the inventor has 15 developed a range of extremely high quality two-way loudspeakers without using very expensive drivers 102, 104, but principally by using very high quality parts in the filtering components used to divide the input signal into respective frequency bands for the low and high frequency drivers, and by extensively bracing the supporting structure, in this case being in the form of a cabinet or enclosure, including lining the internal surfaces of the 20 enclosure with steel plates. Despite continuing developments in loudspeaker damping, the inventor has determined that a particularly important form of vibration or oscillation that degrades the perceived sound quality of a loudspeaker and has not been previously identified or addressed by the 25 prior art is the oscillation of the entire loudspeaker along a forward and aft direction relative to the speaker direction, that is, normal to the baffle to which the drivers 102, 104 are mounted, as shown by the arrow 202 in Figure 2. Oscillatory motion of the loudspeaker along this direction is directly driven by the piston-like motion of the low frequency driver(s) 102 in particular, since these drivers have a relatively large mass and relatively 30 large excursions back and forth along the same axis.

-6 An ideal speaker would be infinitely rigid and would have an infinite mass, so that the rigid supporting components of the drivers that are directly coupled to the baffle would be fixed in space. However, this of course in never the case in practice, and despite the designer's best efforts, some unwanted vibrations/oscillations of the supporting structure 5 will always be present. The inventor has determined that this mode of oscillation is particularly deleterious to sound quality, causing negative group delay, Doppler distortion, and other forms of audible distortion. The type of loudspeaker shown in Figure 1 is referred to as a stand mounted loudspeaker, 10 and includes an enclosure or supporting structure 106 mounted on a stand, where the stand includes a vertical elongate post or pillar 108 of square or circular cross section, and top and bottom plates 110. The stand is typically made of steel, and the post 108 may be hollow and filled with a high mass material such as sand or lead shot. The stand itself may include spiked feet 112 to support the bottom plate in a spaced arrangement relative to the 15 floor 114. Application of an audio signal to the low frequency driver (or 'woofer') 102 causes the driver 102 to reciprocate or oscillate at the frequencies of the audio signal. This movement and the resulting forces applied to the air in front of the driver and the overall asymmetry 20 of this arrangement result in corresponding forces on the entire supporting structure 106. Because the aggregate structure consisting of the enclosure 106 and stand is not perfectly rigid, these forces cause the post 108 to flex and the enclosure 106 to oscillate back and forth, generally along the forward and aft direction of the loudspeaker, being the principal direction of sound wave emission, but more accurately in an arc, as shown schematically 25 by arrow 202. Moreover, this mode of vibration or oscillation of the aggregate structure has a natural or resonant frequency that will be excited by the oscillatory movements of the driver. Although this frequency depends on the precise composition and configuration of all of the parts of the enclosure and stand, the inventor has determined that this natural frequency is in the range of about 15 to 22 Hz for his range of loudspeakers. As will be 30 apparent to those skilled in the art, for an arbitrary speaker, the relevant frequency can be determined using an accelerometer attached to the enclosure 106 and a frequency generator -7 electrically coupled to the audio transducers 102, 104 of the loudspeaker 100 via a standard audio amplifier. As shown in plan and side views, respectively, in Figures 3 and 4, a loudspeaker damping 5 apparatus or assembly includes an inertial mass 302 supported by intermediate coupling elements 304. As shown in Figures 5 and 6, in some embodiments the loudspeaker damping apparatus includes a base 502 in the form of an open topped receptacle having an open cavity into which the inertial mass 302 and intermediate coupling elements 304 are disposed to facilitate handling of the loudspeaker damping apparatus as a unit and to 10 reduce the likelihood of damage to or loss of the coupling elements 304, or damage caused by impact of the inertial mass 302 on an object or person. In order to reliably locate the coupling elements 304 at desired locations relative to the base 502 and/or the inertial mass 302, either or both of the base 502 and the inertial mass 302 may shallow recessed portions (not shown) whose lateral dimensions match those of the coupling elements 304 so that a 15 small portion of the coupling elements 304 can be placed within those recesses in order to preserve the lateral spacings between the various components 302, 304, 502. The coupling elements 304 are configured to allow the inertial mass 302 (which in the described embodiments has a rectangular cuboid form) to oscillate in a horizontal plane 20 horizontally relative to the generally planar base 502, for example if the base is moved in a horizontal plane, and also to apply a restoring force to restore the inertial mass 302 back to an equilibrium or central position. The coupling elements 304 can therefore be considered to be springs from a functional point of view, but also include damping to prevent prolonged oscillation of the inertial mass 302 in the absence of a driving force or 25 excitation. The coupling elements 304 can therefore be described as damped springs. In some embodiments, the coupling elements 304 are damped springs, including springs (which may be helical) with damping elements or shock absorbers, which may be hydraulic. The springs may be used to support the inertial mass 302. In some 30 embodiments, the inertial mass 302 is suspended from above. Many possible alternative configurations will be apparent to those skilled in the art in the light of this disclosure.

-8 However, in the described embodiments, the coupling elements 304 are pieces of an elastomeric material that support the inertial mass 302 and experience shear when the inertial mass 302 moves relative to the base 502 (or other supporting surface). This shear 5 results in the coupling elements 304 applying an increasing restoring spring force to return the inertial mass 302 to its equilibrium or central position. Due to the substantial mass of the inertial mass 302 (being, in the described embodiment, about 3 kg), the inertial mass 302 travels past the equilibrium or central position, resulting in the coupling elements 304 experiencing shear in the opposite direction, and hence resulting in an oscillatory motion. 10 Due to losses in the coupling elements 304, the oscillatory motion is damped. However, this mode of oscillation has its own natural or resonant frequency that can be selected or 'tuned' by modifying the mass of the inertial mass 302 and the configuration of the coupling elements 304, namely their number, dimensions and composition. In particular, the natural frequency can be tuned to be equal to or at least substantially close to, the 15 natural frequency of the loudspeaker oscillation mode described above. One elastomer that has been found particularly suitable is a thermoplastic elastomer developed by California Medical Innovations and sold under the name DermasolTM. As will be understood by those skilled in the art, the coupling of the loudspeaker damping 20 apparatus to the loudspeaker will itself modify the natural or resonant frequency of the fore-and-aft oscillation of the loudspeaker (or, more accurately, the natural or resonant frequency of the assembly formed by the mutually coupled loudspeaker and loudspeaker damping apparatus), and this may need to be taken into account when tuning the resonant frequency of the loudspeaker damping apparatus. 25 The loudspeaker damping apparatus can be attached to the top surface of the loudspeaker enclosure. This can be done in a removable manner by attaching self-adhesive polymer 'bumpers' 602 or the like to the underside of the base 206, as shown in Figure 6, and then simply placing the loudspeaker damping apparatus onto the top surface of the loudspeaker 30 enclosure, since the bumpers grip the top surface and do not slip, particularly as they are supporting the considerable mass of the inertial mass 302. Similarly, if the base 502 is not -9 used, the coupling elements 304 can be placed directly onto the speaker, and the inertial mass 302 placed onto the coupling elements 304. With this arrangement, oscillatory forward and aft motion of the loudspeaker 100 causes 5 the base 502 (if present) attached to the loudspeaker enclosure, to move correspondingly. Due to the considerable inertial mass of the inertial mass 302, it lags behind the base 502, but is pulled along in the same direction by the coupling elements 304. However, by the time the inertial mass 302 is catching up with the base 502, the loudspeaker enclosure 106, and hence the attached base 502, is now moving in the opposite direction back towards 10 (and subsequently past) the equilibrium position. Where the loudspeaker damping apparatus is tuned to a frequency that is substantially equal to the frequency of the forward and aft loudspeaker oscillation (preferably taking into account the presence of the loudspeaker damping apparatus as described above), the loudspeaker enclosure 106 and the inertial mass 302 are oscillating at the same frequency, but phase shifted by 180 15 degrees. Thus the coupling elements 304, while applying a restoring force to the inertial mass 302, are simultaneously applying an equal and opposite force to the loudspeaker enclosure 106, thereby attenuating the oscillatory movement of the loudspeaker enclosure. The loudspeaker damping apparatus can therefore be regarded as a form of tuned mass damper, and the inertial mass 302 can be referred to as a counter mass. The result is a 20 substantial improvement in the perceived sound quality produced by the loudspeaker. Moreover, and perhaps surprisingly, the inventor has also determined that the loudspeaker damping apparatus has a similar beneficial effect on some floorstanding speakers. 25 In an alternative embodiment, a loudspeaker includes an integrated receptacle to receive the coupling elements 208 and inertial mass 204. In a further alternative embodiment, the coupling elements 208 and inertial mass 204 are completely integrated or internal to a loudspeaker. 30 Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims (5)

1. A loudspeaker, including: an audio transducer support structure; at least one audio transducer mounted to the support structure and configured to generate audio frequency sound waves from a corresponding electrical input signal by corresponding oscillations of the at least one audio transducer relative to the audio transducer support structure, wherein the oscillations of the at least one audio transducer cause oscillations of the audio transducer support structure substantially along a fore and aft direction of the loudspeaker and at a corresponding natural frequency of the loudspeaker; a counter mass; and coupling elements coupling the counter mass to the audio transducer support structure and configured to allow substantial oscillations of the counter mass relative to the support structure and along said fore and aft direction; wherein the counter mass and coupling means are configured so that the fore and aft oscillations of the audio transducer support structure generate corresponding fore and aft oscillations of the counter mass relative to the support structure so as to attenuate the fore and aft oscillations of the audio transducer support structure. - 11

2. A loudspeaker damping apparatus configured to attenuate oscillations of an audio loudspeaker substantially along a fore and aft direction of the audio loudspeaker caused by oscillations of at least one audio frequency transducer of the audio loudspeaker, the damping apparatus including: a counter mass; and coupling means configured to couple the counter mass to the loudspeaker while allowing substantial oscillations of the counter mass relative to the loudspeaker along the fore and aft direction so as to attenuate the fore and aft oscillations of the loudspeaker.

3. The loudspeaker damping apparatus of claim 2, wherein the fore and aft oscillations of the loudspeaker have a first natural frequency, and the oscillations of the counter mass relative to the support structure have a second natural frequency that is substantially equal or at least close to the first natural frequency.

4. The loudspeaker damping apparatus of any one of claims 1 to 3, wherein the fore and aft oscillations of the counter mass are substantially out of phase relative to the fore and aft oscillations of the loudspeaker or audio transducer support structure.

5. The loudspeaker damping apparatus or loudspeaker of any one of claims 1 to 4, including a base to support the counter mass and the coupling means, and to couple the counter mass and the coupling means to the loudspeaker or audio transducer support structure.