Source: https://patents.google.com/patent/EP1988740A1/en
Timestamp: 2020-02-17 09:17:07
Document Index: 205419367

Matched Legal Cases: ['art 6', 'art 6', 'arts 6', 'arts 6', 'art 6', 'art 6', 'art 6', 'art 6', 'art 6', 'arts 9', 'art 9', 'art 9', 'art 9', 'art 9']

EP1988740A1 - Sound generator - Google Patents
Sound generator Download PDF
EP1988740A1
EP1988740A1 EP07107454A EP07107454A EP1988740A1 EP 1988740 A1 EP1988740 A1 EP 1988740A1 EP 07107454 A EP07107454 A EP 07107454A EP 07107454 A EP07107454 A EP 07107454A EP 1988740 A1 EP1988740 A1 EP 1988740A1
EP07107454A
Arthur Perry Berkhoff
2007-05-03 Application filed by Nederlandse Organisatie voor Toegepast-Natuurwetenschappelijk Onderzoek (TNO) filed Critical Nederlandse Organisatie voor Toegepast-Natuurwetenschappelijk Onderzoek (TNO)
2007-05-03 Priority to EP07107454A priority Critical patent/EP1988740A1/en
2008-11-05 Publication of EP1988740A1 publication Critical patent/EP1988740A1/en
229910052717 sulfur Inorganic materials 0 description 6
A sound generator, particularly a loudspeaker, configured to emit sound, comprising a rigid element (2) enclosing a plurality of air compartments (3), wherein the rigid element (2) has a back side (B) comprising apertures (4), and a front side (F) that is closed, wherein the generator is provided with at least one actuator (6), for instance one or more electromagnetic actuators and/or piezoelectric elements, configured to actuate the rigid element (2) for the generation of the sound.
The invention relates to a sound generator, particularly a loudspeaker, configured to emit sound.
Loudspeakers are known in many different variants. A conventional type of loudspeaker is the piston type loudspeaker, having a vibrating cone to drive the air, so that a beam of sound can be generated. In many applications, the conventional loudspeaker comprises a relatively large cabinet extending behind the vibrating cone, to prevent that backwardly radiating sound can enter the same space as an emitted beam of sound.
Another type of loudspeaker is the so called "flat panel" speaker, see for example US 6,481,173 B1 . The known flat panel speaker comprises a radiating panel, and an exciter hooked up to the panel to cause the panel to vibrate. Use is made of a complex random ripple of wave forms on the panel surface, leading to an omni-directional sound generation. Compared to piston-like movement of the conventional cone-type loudspeaker, the motion of the flat panel speaker is determined by many different modes spread over the radiator surface, possibly leading to incoherent sound radiation. A common disadvantage of known flat panel speaker systems is that they are not good in producing low frequency sound.
The present invention aims to provide an improved sound generator. Particularly, the invention aims to provide an efficient, preferably compact, sound generator that can provide sound in a relatively large frequency range, particularly at or including relatively low frequencies.
According to an embodiment of the invention, this is achieved by the features of claim 1.
Advantageously, a sound generator, particularly a loudspeaker, comprises a rigid element enclosing a plurality of air compartments, wherein the rigid element has a back side comprising apertures, and a front side that is closed, wherein the generator is provided with at least one actuator, for instance one or more electromagnetic actuators and/or piezoelectric elements, configured to actuate the rigid element for the generation of the sound.
In this way, an improved, relatively efficient sound generator can be provided, that can be made relatively compact and that can provide sound in a relatively large frequency range. Particularly, the present sound generator can be used to produce both low frequency sound (for example in the frequency range below about 100 Hz) and medium and high frequency sound (for example above about 100 Hz).
Moreover, since the rigid element has a back side comprising apertures, a space that is enclosed by the rigid elements (i.e. a cumulative internal space of all the air compartments that are provided in the rigid element) can preferably be available as an acoustic load in order to improve efficiency, that can preferably provide a substantially linear air spring. In a non-limiting embodiment, the sound generator can be designed to generate directional sound, contrary to above-described omni-directional flat panel type systems. For example, according to an embodiment, the sound generator can be designed to provide piston-type of actuation of the rigid element.
According to a further embodiment, the apertures of said back side of the rigid element are in air connection (i.e. fluid communication) with an air space extending at least behind the rigid element, particularly such that air in that air space can communicate directly with air in the air compartments of the rigid elements.
In an embodiment, the sound generator can be used in an infinite-baffle type configuration.
Preferably, the sound generator can comprise an internal air chamber extending at least behind the back side of the rigid element, the apertures of said back side being in air connection with that internal air chamber. For example, the volume of the internal air chamber can be smaller than an overall internal volume of the rigid element, so that a relatively compact sound generator can be provided, comprising relatively large air compartments in the rigid element. For example, the rigid element can be made relatively thick compared to a thickness of the mentioned air chamber. For example, there can be provided a housing or cabinet enclosing the mentioned air chamber and being provided with the rigid element.
Good results can be obtained in case the apertures are arranged to provide air connections to all of the air compartments of the rigid element.
According to a further embodiment, the sound generator comprises a, preferably resilient, support structure to hold the rigid element, such that the rigid element can be actuated with respect to the support structure to generate the sound.
A relatively compact arrangement can be provided when the at least one actuator at least partly extends in the rigid element.
The rigid element can be constructed in various ways and of various materials. For example, the rigid element can be made of metal, plastic, paper, glass, an alloy, wood, a composite material, reinforced material, for example carbon fiber or glass fiber reinforced material, and/or a combination thereof.
Preferably, the rigid element has a sandwich structure. For example, the rigid element can be constructed from a rigid, solid (i.e., not interrupted, continuous) front plate or sheet, providing the closed front surface thereof, and a rigid core material that can provide the air compartments. A back side of the core can provide a mentioned back side of the rigid element. Alternatively, a core comprising the air compartments can be provided with a perforated back plate or sheet, to further improve rigidity of the rigid element. Various parts or layers of the rigid element can be made of the same material(s), however, this is not necessary.
Also, according to an embodiment, good rigidity can be provided in the case that the rigid element has a honeycomb cell structure. For example, a core part of the rigid element can be a core consisting of honeycomb cells (the cells having honeycomb cross-sections when viewed in longitudinal panel cross-section, perpendicular to a direction of sound emission).
The front and back side of the rigid element are preferably substantially parallel. Also, the rigid element can be substantially flat, having a thickness that is significantly smaller than longitudinal dimensions of the element. The rigid element can have various forms and shapes, for example having a substantially flat, even shape, or being curved, convex, concave, cone or dome shaped, or formed differently if desired.
Besides, an other aspect of the patent application provides a sound generator, particularly a loudspeaker, configured to emit sound, comprising a rigid element having a front side that is closed, wherein the generator is provided with at least one actuator, for instance one or more electromagnetic actuators and/or piezoelectric elements, configured to actuate the rigid element for the generation of the sound, wherein the rigid element further comprises one or more piezo-electric elements that can be actuated to adjust frequency response characteristics of the rigid element.
Further elaborations of the invention are described in the subclaims. The invention will now be elucidated on the basis of exemplary embodiments and with reference to the drawing, in which:
Fig. 1 shows a front view of an exemplary first embodiment of the invention;
Fig. 2 shows a transversal cross-section over line II-II of the front view shown in Fig. 1;
Fig. 3 shows a detail Q of Fig. 2;
Fig. 4 shows a similar detail as Fig. 3, of an alternative embodiment;
Fig. 5 shows cross-section similar to Fig. 2, of an exemplary second embodiment of the invention;
Fig. 6 schematically shows an undesired tilting of an embodiment;
Fig. 7 shows a front view of a further embodiment; and
Fig. 8 is a cross-section over line XIII-XIII of fig. 7.
Similar or corresponding features are denoted by similar or corresponding reference signs in the present patent application.
Figures 1-3 show a first embodiment of a sound generator 1, for example a loudspeaker, configured to emit sound. The sound generator 1 can be used in many different applications, for example to reproduce music, to generate anti-sound that is to cancel or reduce external noise of other sound sources, and other applications.
Preferably, the sound generator 1 is provided with a rigid, stiff element 2. For example, the rigid element 2 can be a panel or plate, and can have various shapes and dimensions. In the present embodiment, a panel shaped rigid element 2 is provided, having a rectangular front face F, however, other shapes can also be implemented, for example an element having a triangular, square, circular, elliptical, polygonal, curved or and/or differently shaped front side.
The rigid element 2 comprises a large number of internal hollow air compartments or cells 3. In the present embodiment, the rigid element 2 is provided with a rigid internal wall structure 3a defining the air compartments 3. Preferably, such internal walls 3a extend substantially transversally through the rigid element 2, from a front side F to a back side B, as in the present embodiment. For example, the internal air compartments 3 can be distributed homogeneously along the rigid element 2, viewed in longitudinal directions, and the internal air compartment walls 3a can be spaced-apart from each other at substantially equal distances (in longitudinal directions). However, the rigid element 2 can also comprise inhomogeneously distributed internal air compartments.
In the present embodiment, the rigid element 2 has a longitudinal back side B comprising apertures 4, and a longitudinal front side F that is closed (i.e., the front side F as such does not comprise any apertures). Preferably, the back side B has the same outer contour (for example rectangular, or a different contour) as the front side F. Sound that can be emitted by the sound generator is schematically indicated by arrows s in Fig. 2. Particularly, the sound generator 1 is designed to emit the sound s in a direction that is substantially away from the front side F, towards a space R (for example a listening room, or sound cancellation area) extending in front of the front side F.
As is described above, the rigid element 2 can have a sandwich structure. For example, various layers or parts F, B, 3a of the sandwich structure can be integrally connected to or joined to each other, for example using suitable adhesive means, glue, and/or other joining means. Preferably, the rigid element 2 has a honeycomb cell structure providing air cells 3 with honeycomb cross-sections, however, the internal air compartments 3 can also be provided using a different internal structure, for example cells 3 having circular cross-sections, rectangular or square cross-sections and/or differently shaped cells 3.
According to an embodiment, a thickness M (see Fig. 3) of each air compartment 3 of the rigid element 2, measured from the front side F to the back side B, is larger than approximately 1 mm, for example in the range of about 1-50 mm. For example, the thickness M of each air compartment 3 can be larger than 1 cm, if desired, particularly to provide a relatively large active internal air volume of the element 2, and, at the same time, a relatively large stiffness of the rigid element 2. Similarly, the thickness of the rigid element 2 as such can be about the same or slightly larger than that of the air compartments 3, depending for example on the thicknesses of the front and back sides F, B. According to a further embodiment, the front side F of the rigid element 2 is a relatively rigid thin plate or layer, having a preferred thickness smaller than 1 mm, for example 0.5 mm or smaller. Similarly, the back side B of the rigid element 2 can be a thin plate or layer, having a preferred thickness smaller than 1 mm, for example 0.5 mm or smaller.
Besides, in the present embodiment, the front and back side of the rigid element 2 are substantially parallel, which can provide good results. As follows from the drawing, the rigid element 2 can be substantially flat, having a maximum thickness that is significantly smaller than longitudinal dimensions of the element 2, as in the drawing. By providing a rigid, very stiff element 2, a cutoff frequency of the sound generator can be relatively high, to provide a large frequency operating range.
The generator 1 is provided with at least one actuator 6, configured to actuate (vibrate) the stiff element 2 to generate the sound. Particularly, the one or more actuators 6 (two, in the embodiment of Figures 1-3) are configured to drive the element 2 to perform a piston type of movement during operation, in transversal directions X with respect to the front and back sides F, B. The actuators 6 can have various configurations, as will be appreciated by the skilled person, and -according to a further embodiment- can be driven by suitable driving signals that can be applied to the actuators 6 via suitable wiring 17. For example, the actuators 6 can include electromagnetic actuators (for example comprising electromagnetic drivers), piezoelectric elements 6 and/or other actuators, see Figures 3-4.
In Fig. 3, each actuator comprises a first part 6a and a second part 6b, which parts 6a, 6b can move with respect to each other by electromagnetic interaction (such movement is schematically indicated by arrow v in Fig. 3). For example, one of the actuator parts can be an electromagnet, and the other actuator part can be made of magnetic material (for example in case the actuator part is or comprises a permanent magnet), as will be appreciated by the skilled person. Also, one of the actuator parts 6a is attached to the moveable rigid panel 2, and the other part 6b to a stationary construction or element 9. For example, an actuator part 6a can extend at least partly in the rigid element 2. Advantageously in view of manufacturability, the actuator part 6a extends or reaches in an air compartment of the rigid element 2, and can be attached to an interior wall of that compartment. For example, at least part 6a of the actuator can be integrally connected to or joined with the rigid element, for example using suitable adhesive means, glue, and/or other attachment means. Besides, in an embodiment, at least part 6a of the actuator can be made in one piece with the rigid element 2. As an example only, the rigid element 2 can be made of magnetic material that can be actuated using one or more electromagnet actuator parts.
Figure 4 shows an alternative embodiment, wherein the rigid element 2 is provided with the one or more actuators 6'. In this case, for example, each actuator 6' can be a piezoelectric element, wherein vibrations v of the piezo-element can cause a desired actuation of the rigid element 2 to produce the sound s.
There can be provided a support structure 8 to hold the rigid element 2, such that the rigid element 2 can be actuated with respect to the sound receiving space R to generate the sound. The support structure can be provided by a guiding mechanism to guide the rigid element 2. Also, the support structure can be a resilient support structure 8, which can be configured in various ways, for example comprising flexible means, for instance spring means and/or elastic material. Such a resilient support 8 can counteract the actuated movement of the rigid element 2 using resiliency or spring forces, to move the rigid element 2 towards a non-actuated initial position. Thus, the rigid, stiff element 2 has a certain freedom of movement with respect to stationary parts 9, 9' of the sound generator. Preferably, the support structure 8 is configured to allow an above-mentioned piston type movement, in the transversal direction X. Besides, the support structure 8 can be configured to provide a seal around the rigid element, for example to provide an air-tight sealing to a holding structure 9, 9'.
Preferably, an air space S, S' extending behind the back surface B of the actuated element 2 is not in (direct) fluid communication with the front space R that is to receive the sound s. For example, the closed front part F of the rigid element 2 can provide a fluid-tight separation between the sound receiving area R and the rear or posterior space S, S' . In the present embodiment, the apertures 4 of the back side of the rigid element 2 are arranged to provide air connections from the rear space S, S' to substantially all of the air compartments 6 of the rigid element 2.
According to an embodiment, the mentioned rear space can be an internal air chamber S that extends at least behind the back side B of the rigid element 2 (i.e., the back side B extends between that rear space and the internal air compartments 3), such as in figures 1-4. All apertures 4 of said back side B are preferably in air connection with that internal air chamber S, such that air can freely move between the rear air chamber S and air compartments 3 via those apertures 4. As is mentioned above, the volume of the internal air chamber S can be smaller than an overall internal volume of the rigid element 2, which is a preferred embodiment of the invention in view of device compactness and device efficiency. For example, according to a preferred embodiment, the volume of the internal air chamber S is smaller than 50% of an overall internal volume (i.e. the cumulative volume of all air cells 3) of the rigid element 2, more preferably smaller than 25% of the overall internal volume of the rigid element 2. For example, the volume of the internal air chamber S can be in the range of about 5-20 %, for example about 10%, of an overall internal volume of the sound generator 1 (which total internal volume consists of the volume of that internal air chamber S plus the cumulative volume of all air cells 3 of the element 2).
Besides, preferably, the sound generator 1 comprises a rigid housing or cabinet 9, functioning as a holder to hold the rigid element 2, for example via the mentioned support means 8. The housing 9 can be configured in various ways and can be made of various materials.
The housing can be integrally provided with the resilient support structure 8. In the non-limiting fig. 1-4 embodiments, the housing or cabinet 9 is relatively flat, and provides a rigid back part 9a extending opposite the back side B of the rigid element 2, spaced-apart from the rigid element. A front face K of the rigid back part 9a and the back side B of the rigid element preferably extend in parallel with one another. A rigid side wall 9b of the housing can be provided, protruding upwardly from the back part 9a and including the support structure 8 to hold the rigid panel 2. The height of the side wall 9b is preferably relatively small, for example about twice the thickness M of the rigid element 2 or smaller. Also, the housing (cabinet) 9 is preferably configured to enclose the above-mentioned internal air space with the rigid element 2, and support structure 8 in the present embodiment. Preferably, the longitudinal dimensions of the back part 9a (measured in the directions orthogonal with respect of a element movement directions X) of the housing are substantially the same as or only slightly larger than the dimensions of the rigid element 2.
During operation of the embodiments shown in Figures 1-4, the rigid element 2 can be actuated by the respective actuators 6, 6', using suitable electric signals, to emit the sound s into the sound receiving space R. As a result of the above-described configuration, the rigid element can perform a substantially piston-like movement as a result of the actuation (in above-mentioned transversal directions X). The rigid air-compartment-enclosing type of construction of the rigid element 2 (which is preferably a sandwich structure, as mentioned above) can suppress undesired random wave form ripples on the front part F thereof, and the motion of the front part F can be substantially coherent. Besides, the air compartments of the rigid element 2 can significantly boost acoustic efficiency in communicating with the enclosed air space S extending there-behind within the housing of the sound generator 1. For example, these air compartments and the remaining enclosed space S can communicate to provide a compound air spring, consisting of both the air within the rigid element 2 as air extending in the enclosed air space S. Thus, a relatively flat sound generator can be provided, having an actuated rigid element 2 wherein the cell structure of the element can both provide a desired enhanced rigidity as well as improved acoustic efficiency.
Figure 5 shows a further embodiment, where a sound generator comprising a rigid element 2 (as described above) is mounted in an aperture in a wall 9' that separates the sound receiving area R from an area S' extending behind the back face B (i.e. to provide a so called infinite baffle configuration, as in Fig. 5). Again, one or more actuators are provided (not shown in Fig. 5) to actuate the element 2 to emit the sound s. The operation of the Fig. 5 embodiment is substantially the same as the operation of the Fig. 1-4 embodiments. For example, the wall 9 can comprise above-mentioned support structure 8' and/or holding means 9b' for supporting and/or holding the rigid element 2. In this case, the air compartments of the rigid element 2 can communicate with the air space S' extending there-behind, to provide improved device efficiency.
To further improve the sound generator 1, a further embodiment of the invention (see Fig. 6) provides for the application of one or more detectors 15 to detect a position or orientation of the rigid element, and preferably comprising means to counteract undesired displacements or misalignments of the rigid element 2. The mentioned detectors 15 can be configured in various ways, and may include one or more of: optical detectors, strain sensors, electrical sensors, alignment detectors, acceleration detectors, positioning means and/or other sensor types. A mentioned undesired displacement or misalignment can be detected using results provided by such detectors 15, and can be an undesired tilting (over an angle α) as has been schematically indicated in Fig. 6, and/or undesired rotation(s) in other directions. A means to counteract a detected undesired displacement or misalignment can simply comprise one or more of the above-mentioned actuators 6. For example, there can be provided a suitable controller or signal processor, that is configured to control the actuators 6 to actuate the rigid element 2 to emit sound s using a first actuation signal, and additionally to actuate the rigid element 2 to counteract an undesired movement (such as the tilting) using an additional second actuation signal (for example being modulated onto the first signal).
Figures 7-8 depict a further embodiment 101 of the invention, which differs from the embodiments shown in Figures 1-6 that it comprises a relatively large number of actuators 6 (more than 2, for example at least 4) that are provided at various locations of the rigid element 102. Also, a number of detectors 15 are provided (for example being integrated within the element 2, or being arranged differently) to detect the position or orientation of the rigid element 102 (particularly with respect to an initial, non-actuated position).
Also, according to an embodiment, a sound generator can comprise one or more frequency response adjusters 19 to adjust frequency response characteristics of the rigid element 102. A non-limiting example of a frequency response adjuster 19 is shown in Fig. 7-8, and can comprise a piezo-electric element that is fixed to the front part F of a rigid element 102 and that can be controlled by a frequency response adjusting signal provider. For example, the frequency response adjusters 19 can be controlled to provide a substantially flat frequency response over a relatively large frequency operating range. For example, the frequency response adjusters 19 can be integrated within the rigid element 102, or be located on top of the front surface F (as in fig. 8). A preferred location of such a frequency response adjuster 19 is at a position of relatively high strain (in the rigid element) for the vibration mode shape of interest. For example, in case of a 1st mode and a symmetric geometry (as in the present embodiment), the adjuster is preferably located at the centre of the rigid element 102.
A sound detector and/or calibration means (not shown) can be provided to calibrate the operation of the frequency response adjuster 15, to provide a desired frequency response. For example, in this case, preferably, the rigid element 102 can be provided with air compartments 3 as in the embodiments described above, however, this is not necessary. Thus, advantageously, there can be provided a sound generator, particularly a loudspeaker, configured to emit sound, comprising a rigid element 102 having a front side F that is closed, wherein the generator is provided with at least one actuator 6, for instance one or more electromagnetic actuators and/or piezoelectric elements, configured to actuate the rigid element 102 for the generation of the sound, wherein the rigid element 102 further comprises one or more piezo-electric elements 19 that can be actuated to adjust frequency response characteristics of the rigid element 102.
Although the illustrative embodiments of the present invention have been described in greater detail with reference to the accompanying drawings, it will be understood that the invention is not limited to those embodiments. Various changes or modifications may be effected by one skilled in the art without departing from the scope or the spirit of the invention as defined in the claims.
It is to be understood that in the present application, the term "comprising" does not exclude other elements or steps. Also, each of the terms "a" and "an" does not exclude a plurality. Also, a single processor or other unit may fulfil functions of several means recited in the claims. Any reference sign(s) in the claims shall not be construed as limiting the scope of the claims.
A sound generator according to claim 1, wherein the apertures (4) of said back side (B) are in air connection with an air space (S) extending at least behind the rigid element (2).
A sound generator according to claim 2, wherein the air space is an internal air chamber (S) having a volume that is smaller than an overall internal volume of the rigid element (2).
A sound generator according to any of the preceding claims, wherein the apertures (4) are arranged to provide air connections to all of the air compartments of the rigid element (2).
A sound generator according to any of the preceding claims, comprising a support structure (8) to hold the rigid element (2), such that the rigid element (2) can be actuated with respect to the support structure to generate the sound.
A sound generator according to any of the preceding claims, comprising a rigid back part (9) extending opposite the back side (B) of the rigid element (2), spaced-apart from the rigid element, wherein a front face (K) of the rigid back part (9) and the back side (B) of the rigid element preferably extend in parallel with one another.
A sound generator according to any one of the preceding claims, wherein the at least one actuator (6) at least partly extends in the rigid element (2).
A sound generator according to any of the preceding claims, wherein the rigid element (2) has a sandwich structure.
A sound generator according to any of the preceding claims, wherein the rigid element (2) has a honeycomb cell structure.
A sound generator according to any one of the preceding claims, wherein the thickness (M) of each air compartment of the rigid element, measured from the front side to the back side, is larger than approximately 1 mm, for example in the range of about 1-50 mm.
A sound generator according to any of the preceding claims, wherein the thickness (M) of each air compartment of the rigid element, measured from the front side to the back side, is larger than 1 cm.
A sound generator according to any one of the preceding claims, wherein the front and back side of the rigid element (2) are substantially parallel.
A sound generator according to any of the preceding claims, wherein the rigid element (2) is substantially flat, having a thickness that is significantly smaller than longitudinal dimensions of the element (2).
A sound generator according to any of the preceding claims, comprising one or more detectors (15) to detect a position or orientation of the rigid element, and preferably comprising means to counteract undesired displacements or misalignments of the rigid element (2).
A sound generator according to any of the preceding claims, comprising one or more frequency response adjusters to adjust frequency response characteristics of the rigid element (2).
A sound generator, particularly a loudspeaker, configured to emit sound, comprising a rigid element (2; 102) having a front side (F) that is closed, wherein the generator is provided with at least one actuator (6), for instance one or more electromagnetic actuators and/or piezoelectric elements, configured to actuate the rigid element (2; 102) for the generation of the sound, wherein the rigid element (2; 102) further comprises one or more piezo-electric elements that can be actuated to adjust frequency response characteristics of the rigid element (2; 102).
EP07107454A 2007-05-03 2007-05-03 Sound generator Withdrawn EP1988740A1 (en)
EP07107454A EP1988740A1 (en) 2007-05-03 2007-05-03 Sound generator
PCT/NL2008/050266 WO2008136668A1 (en) 2007-05-03 2008-04-29 Sound generator
EP08741687.1A EP2153690B1 (en) 2007-05-03 2008-04-29 Sound generator
US12/598,006 US9154882B2 (en) 2007-05-03 2008-04-29 Sound generator
EP1988740A1 true EP1988740A1 (en) 2008-11-05
ID=38069135
EP07107454A Withdrawn EP1988740A1 (en) 2007-05-03 2007-05-03 Sound generator
EP08741687.1A Active EP2153690B1 (en) 2007-05-03 2008-04-29 Sound generator
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