WO2015008306A1

WO2015008306A1 - Wave-guide acoustic transformer

Info

Publication number: WO2015008306A1
Application number: PCT/IT2014/000128
Authority: WO
Inventors: Claudio Gandolfi
Original assignee: Robin S.R.L.
Priority date: 2013-07-15
Filing date: 2014-05-13
Publication date: 2015-01-22
Also published as: ITSS20130007A1; EP3022945A1

Abstract

A wave-guide acoustic transformer is described, comprising: a membrane (4) having a cylindrical shape open at its sides, used as sound wave guide and omnidirectional acoustic passive radiator, a loudspeaker (3) adapted to send sound waves (7) inside the cylindrical membrane (4), an acoustically reflecting circular plug (5), a gasket (6) with the housings of the screws to be used for supporting the loudspeaker; the cylindrical membrane (4) receives energy only from the sound wave fronts traveling in the internal air volume and has a strongly anisotropic modulus of elasticity in parallel with the cylinder axis and along the circumference of the cylinder itself, the internal sound waves (7) not modifying the length of the cylinder since, along the axis, the elastic constant is very high, the internal sound waves (7) themselves being adapted to modify the circumference of the cylindrical membrane (4) since, along the circumference, the elastic constant is much lower.

Description

WAVE-GUIDE ACOUSTIC TRANSFORMER

The present invention refers to a wave-guide acoustic transformer.

The wave-guide acoustic transformer is an acoustic box for loudspeakers. With this system, it is possible to make single-way or multi-way diffusers with one or two anisotropic wave guides for every loudspeaker.

Normally, the acoustic box is used as support for loudspeakers. These are heavy transducers: therefore, in order to stably support them, it is necessary to use suitably sturdy structures which prevent the use of very light-weight materials. Using an acoustically inert structure to support the loudspeaker, it is possible to design extremely light-weight systems with original acoustic solutions. In the acoustic transformer, the loudspeaker sends the sound waves inside wave guides made of cylindrical membranes. The elastic constant of the side wall of the cylinders is anisotropic and characterized by high values along the cylinder axis and by much lower values for the circumference of the cross section. In their path, the wave fronts apply a perpendicular pressure on the side surface of the cylinders, deform their section and generate a sound wave front with radial symmetry which is diffuse in the listening environment .

These features make them a class of diffusers which is different from all those that have already been made. As for closed boxes with pneumatic suspension having a cylindrical shape, there is a low distortion for a uniform distribution of the load on the loudspeaker membrane and reduced internal reflections. As for rear loaded system with trumpet, the acoustic pressure generated on the rear loudspeaker side is sent to the listening environment by optimally distributing it on the surfaces of the side diaphragm. As for dipoles, the counter-phase emission of the rear loudspeaker wall in a medium range is sent to the listening environment; in this case, however, the radial symmetry is perfectly observed with respect to the loudspeaker axis, facilitating the space reconstruction of the sound image. High frequencies are not directly sent along the listening point direction, but through the side wall of a cylindrical passive acoustic radiator, improving the reproduction quality.

Object of the present invention is solving the above prior art problems by providing a wave-guide acoustic transformer that allows making single-way or multi-way diffusers with one or two anisotropic wave guides for every loudspeaker, providing an acoustically inert structure for supporting the loudspeaker, so that it is possible to design extremely light-weight systems with original acoustic solutions.

The above and other objects and advantages of the invention, as will appear from the following description, are obtained with a wave-guide acoustic transformer as claimed in Claim 1. Preferred embodiments and non-trivial variations of the present invention are the subject matter of the dependent claims.

It is intended that all enclosed claims are an integral part of the present description.

It will be immediately obvious that numerous variations and modifications (for example related to shape, sizes, arrangements and parts with equivalent functionality) could be made to what is described, without departing from the scope of the invention, as appears from the enclosed claims.

The present invention will be better described by some preferred embodiments thereof, provided as a non-limiting example, with reference to the enclosed drawings, in which:

Figure 1 shows a preferred, but not limiting, embodiment of the acoustic transformer according to the present invention;

Figure 2 shows a wave guide made of a diaphragm with a cylindrical coil;

Figure 3 shows a wave guide made of a very light-weight diaphragm with a single sheet;

Figure 4 shows a wave guide whose length is different for right and left channels in order to make an asymmetrical stereophony wherein the negative interference of a channel is partially compensated by the other channel;

Figure 5 shows an upper guide adapted to create a positive resonance which brings about a partial compensation;

Figure 6 shows a panel (10) of the sizes of the loudspeaker fastened at 45 degrees; and

Figure 7 shows a loudspeaker (3) with the wave guide (4) fastened thereover. With reference to the Figures, a preferred embodiment of the wave-guide acoustic transformer according to the present invention is shown and described .

Regarding the building of a prototype of the wave-guide acoustic transformer of the invention, the prototype of the wave-guide acoustic transformer has a single wide-band loudspeaker and uses two cylindrical anisotropic deformable membranes with sound waves.

A plastic gasket (6) is fastened through screws to the bracket of a supporting rod (2) and operates as support for the loudspeaker (3) . Below the gasket, the wave guide for medium and low frequencies is fastened, made of a spirally-wound sheet of paper (Figure 2) . The side wall of the cylinder (diameter 10 cm, height about 80 cm) is the diaphragm (4) with suitable stiffness to operate as sound wave guide, but yieldable enough to allow small distortions causes by internal wave fronts (7). The closed base of the cylinder (5) has the same loudspeaker surfaces and reflects the internal wave fronts upwards, preventing the acoustic pressure from directly propagating towards the listening environment. The sound emitted by the upper side (8) of the loudspeaker (3) is sent partially to the listening environment and partially inside the upper wave guide (9) obtained by folding a sheet of paper as shown in Figure 3 and open on both sides. The bigger fold (3.1) is used for fastening the guide to the supporting rod. The smaller fold (3.2) makes the cylinder sufficiently yieldable to be deformed along the circumference also at high frequencies. The wave guide is suitably spaced to allow the direct emission of parte of medium frequencies.

As regards the impulse responses, with a sound speed of 340 meters/second, in 2.5 milliseconds about 80 cm are traveled. If a pulse is sent whose length is 25 microseconds (a half-period at 20 kHz) the below described events occur in the loudspeaker .

The loudspeaker membrane is accelerated upwards, creating a pressure increase above and simultaneously a pressure decrease below, inside the sound wave guide.

After 100 microseconds, the internal wave front directed downwards has traveled for about 3.5 cm below the loudspeaker membrane and interacts in a complex way with the side wall of the wave guide and the structure of the loudspeaker itself. The result of these interactions at high frequency is not relevant for listening.

After 200 microseconds, the wave front has traveled for about 7 cm and passed the loudspeaker magnet. From that time on, the wave front orthogonal to the wave guide moves downwards, interacting only with the side diaphragm on which the pressure, previously generated by the loudspeaker membrane, is applied.

After about 2.5 milliseconds, the front arrives at the closed base of the wave guide which it reflects upwards. The result of this reflection can interact with the loudspeaker membrane only after having traveled the wave guide along an opposite direction in other 2.5 milliseconds. Not all the reflected energy arrives at the membrane for two reasons: the front is partially reflected downwards by the loudspeaker magnet and is dampened by interacting with the side diaphragm.

The box geometry makes that, for about 5 milliseconds, the loudspeaker membrane does not interact with the internal reflections generated by the box itself and afterwards interacts only with dampened wave fronts, obtaining a strong reduction of the distortion. Also outside, the box interacts in a very reduced way with the reproduced sound. Acoustically, an exceptionally clear response to transients is obtained.

The acoustic behavior changes when the frequency changes,; in particular, 10,000, 1,000, 100 Hz allow explaining the operation for high, medium and low frequencies.

At 10 kHz the wavelength is 3.4 cm, lower than the diameter of about 10 cm of the loudspeaker which behaves in a directive way, mainly sending the acoustic waves upwards. This frequency, already attenuated by the presence of the loudspeaker, deforms in a neglected way the side diaphragm of the lower wave guide. The sound waves (8) sent upwards apply a described pressure from half-waves of about 1.7 cm which, for about 50 microseconds, internally or externally deform the side surface of the diaphragm (9) creating undulations which slide upwards. With this mechanism, the passive radiator (9) re-addresses high frequencies at 360 degrees in the listening environment.

At 1 kHz the wavelength is 34 cm, greater than the loudspeaker diameter whose emission can be deemed spherical and with reduced effects on the upper wave guide. Inside the lower wave guide, there are areas of about 17 cm in which the pressure is directed inwards or outwards for times long enough to modify the side surfaces of the diaphragm, generating an undulation which slides vertically and is able to create a series of secondary fronts. For the geometric features of the box, the secondary fronts have a perfectly radial symmetry with respect to the cylinder axis and facilitate the space localization of the origin of sounds in the sound image. This effect has a relevance increasing when the frequency decreases.

As regards basses, at 100 Hz the wavelength is 340 cm, greater than the cylinder height (80 cm) of the lower wave guide. The difference in acoustic load makes that, already at few cm from the loudspeaker membrane, the external sound front has a pressure lower than the internal sound front. The internal pressure for the majority of time exerts on the side surface of the wave guide an outward or inward directed pressure on the whole length of the diaphragm. For listening, the acoustic signal coming from the side diaphragm of the wave guide is more important than the upward directed signal from the loudspeaker. The loudspeaker membrane is the primary of an acoustic transformer, while the side diaphragm of the wave guide is the secondary. As analogy with an electric transformer, one is intuitively taken to think that the pressure exerted by the loudspeaker membrane is uniformly distributed on the whole side diaphragm, strongly decreasing due to the relationship between the surfaces, but it is not so; in the acoustic field, there are no transfers at the speed of light for signals between primary and secondary: in the various points of the side diaphragm, a pressure is exerted which is generated by the loudspeaker membrane, delayed by the speed of sound in air along the wave guide.

The pressure of the wave front generated by a stationary sinusoidal signal applied to the loudspeaker can be considered uniform in a disk whose thickness is infinitesimal and which moves downwards. The pressure can be represented with a sinusoidal function:

P(x, t) = a sin (cot - kx) )

x distance from the loudspeaker center along the cylinder axis,

t tempo, ω pulsation, k wave number of the sound in air. The total force applied to the whole side surface of the diaphragm is the integral of the function extended to the surface itself at instant t :

F (t ) = A cos (cot - cp)

A and φ constant depending on the geometry of the wave guide and on the speed of sound.

The total force F(t) is a sinusoidal function which changes in time and is applied orthogonally on the surface of the side diaphragm of the wave guide. The system is linear within the linearity of the speed of sound and of the box geometry. The force is not uniform on the whole diaphragm since there is a gradient which is modified in time. This gradient does not disturb the listening, since the human ear is accustomed to listen to sounds emitted by bodies on which stationary and non-stationary sound vibrations pass.

Acoustically, basses are more powerful and defined with respect to the loudspeaker itself assembled in a box with rigid walls.

The behaviors described separately for highs, mediums and basses are present at all frequencies, but with different relevance on listening.

For mediums and basses, a wave guide is suitable, which is made of a diaphragm with cylindrical coil, as shown in Figure 2, sturdy enough to resist to strong stresses generated by low frequencies. It is better that it is closed on both sides to minimize the distortion generated by the deformation of the side wall.

For highs, a wave guide is suitable, which is made of a very lightweight diaphragm with single sheet, as shown in Figure 3. It is better that it is open on both sides in order to also send upwards part of the sound energy.

With reference to Figure 3, the shape is an indication: the passive radiator operates provided that there is at least one inflection which allows the rest of the circumference to be dilated or compressed.

In particular cases with loudspeakers with high resonance frequency, like those used on consumer electronics, it is better to remove the lower plug (5) . With the lower side open, air inside the wave guide stops operating mainly as elastic means which contrasts the movement of the loudspeaker membrane and becomes more important than the additional mass which acts on the moving part of the loudspeaker and lowers the resonance frequency of the system. In this case, a greater amount of basses is sent to the environment, with a resulting increase of the distortion, improving however the perception of the music message with respect to the closed lowed plug (5) .

With the lower wave guide closed, it is possible to have an excessive negative interference for the wavelength equal to double the length of the guide due to the wave reflected by the lower plug, which, by moving upwards, exerts a pressure in counter-phase with respect to the half-wave which is moving downwards. This phenomenon can be compensated in three ways:

1 - increasing the amount of energy emitted by the passive radiator for unit length and reducing the amplitude of the wave reflected by the lower plug (5)

2 - making wave guides with different length for right and left channels in order to make an asymmetrical stereophony in which the negative interference of a channel is partially compensated by the other channel (Figure 4)

3 - regulating the length of the upper guide in order to create a positive resonance which brings about a partial compensation (Figure 5) . Differently from other systems used in acoustic boxes, it must be pointed out that the wave-guide acoustic transformer:

can be easily made also with common and inexpensive materials

- does not require a high structure stiffness does not require a high construction accuracy.

The geometry of the wave-guide acoustic transformer has as further advantages a strong reduction of internal reflections and a uniform load distribution on the loudspeaker membrane. If correctly made, it is suitable as optimum acoustic box for systems with single-way and multi-way sound reproduction.

A very inexpensive version, to be coupled with a PC and consumer electronics, can be made by removing the supporting rod and using the lower plug (5) as base, the side diaphragm (4) as support for gasket (6) and loudspeaker (3). Highs can be re-addressed towards the listener with a panel (10) with the same sizes of the loudspeaker fastened at 45 degrees, as shown in Figure 6.

If the wave guide is able of irradiating all frequencies emitted by the loudspeaker, as in a subwoofer, the loudspeaker (3) can be fastened as base, and itself as base, and the wave guide (4) can be fastened over it, as shown in Figure 7.

The acoustic transformer has the majority of its surface which can be made of paper, transparent films or other material which can be easily molded and can be used as sound photograph-carrier, to send information or a furniture complement.

Summarizing, according to the invention, the wave-guide acoustic transformer comprises:

- a membrane (4) having a cylindrical shape open at its sides, used as sound wave guide and omnidirectional acoustic passive radiator,

a loudspeaker (3) adapted to send sound waves (7) inside the cylindrical membrane (4),

an acoustically reflecting circular plug

(5) ,

- a gasket (6) with the housings of the screws to be used for supporting the loudspeaker.

The cylindrical membrane (4) receives energy only from the sound wave fronts traveling in the internal air volume and has a strongly anisotropic modulus of elasticity in parallel with the cylinder axis and along the circumference of the cylinder itself, the internal sound waves (7) not modifying the length of the cylinder since, along the axis, the elastic constant is very high, the internal sound waves (7) themselves being adapted to modify the circumference of the cylindrical membrane (4) since, along the circumference, the elastic constant is much lower.

The cylindrical membrane (4) is made by winding in a cylindrical coil a single sheet of paper tightened with a force of few grams, in order to leave a layer of air between the overlapped surfaces of the turns, the layer of air remaining in communication both with the internal volume, and with the external volume of the cylindrical membrane ( 4 ) .

In the cylindrical coil, the layer of air present between the overlapped surfaces has a medium thickness on the order of the tenth of a millimeter, the layer of air operating as viscous and elastic fluid which uniformly distributes the pressure on the overlapped surfaces, the layer of air keeping low the friction between the turns, decreasing the number of contact points, and facilitating the sliding of the overlapped surfaces one over the other in the direction of the circumference inside the cylindrical membrane (4). The plane and circular wave front (7) which propagates itself inside the wave guide is adapted to exert on the single annular section of the cylindrical membrane (4) a uniform radially oriented pressure which modifies the diameter of the affected section, the sum of the effect of the single sections generating outside the cylindrical membrane (4) a wave front with radial symmetry with respect to the cylinder axis.

The gasket (6) is made of a plastic material with the housings for the screws for fastening to an external bracket and with an edge able to be overlapped to the flange of the loudspeaker (3) for fastening the gasket (6) to the flange of the loudspeaker (3) with an airtight seal.

An edge of the gasket (6) is fastened to an edge of the cylindrical membrane (4) with an airtight seal.

The circular plug (5) is fastened to an edge of the cylindrical membrane (4) with an airtight seal and reflects the internal sound waves (7) .

The fraction of reflected sound energy of the circular plug (5) is decreased by applying sound- absorbing material on the internal side of the circular plug (5) or by making the circular plug (5) with material with a higher transmission point or by completely removing it.

The cylindrical coil is made by winding sheets of materials different from paper with surfaces able to slide one over the other.

The cylindrical coil is replaced by a single layer of film with a strongly anisotropic modulus of elasticity, so that such modulus along the cylinder axis is much greater with respect to the modulus along the circumference.

The film is fastened to a support shaped as a cylindrical sector and emits sound waves on a circular sector lower than 360 degrees.

Increasing the thickness of the paper and the number of turns, the cylindrical membrane ( 4 ) is adapted to support the weight of the loudspeaker (3) .

The circular plug (5) is adapted to perform the function of bearing base for placing the diffuser on the floor or on the shelf of a piece of furniture .

An anisotropic cylindrical membrane (9) open on both sides is adapted to be assembled separate with suitable spacers on the front side of the loudspeaker (3) . The cylindrical membranes (4, 9) are adapted to be pre-molded and/or decorated before being assembled in the acoustic transformer.

Claims

1. Wave-guide acoustic transformer comprising:

an acoustically reflecting circular plug

(5) ,

- a gasket (6) with the housings of the screws to be used to support the loudspeaker,

characterized in that the cylindrical membrane (4) receives energy only from the sound wave fronts travelling in the internal air volume and has a strongly anisotropic modulus of elasticity in parallel to the cylinder axis and along the circumference of the cylinder itself, the internal sound waves (7) not modifying the length of the cylinder since, along the axis, the elastic constant is very high, the internal sound waves (7) being adapted to modify the circumference of the cylindrical membrane (4) since, along the circumference, the elastic constant is much lower.

2. Acoustic transformer according to claim 1, characterized in that the cylindrical membrane (4) is made by winding in a cylindrical coil a single sheet of paper tightened with a force of few grams, in order to leave a layer of air between the overlapped surfaces of the turns, the layer of air remaining in communication both with the internal volume, and with the external volume of the cylindrical membrane (4).

3. Acoustic transformer according to claim 2, characterized in that, in the cylindrical coil, the layer of air present between the overlapped surfaces has a medium thickness on the order of the tenth of a millimeter, the layer of air operating as viscous and elastic fluid which uniformly distributes the pressure on the overlapped surfaces, the layer of air keeping low the friction between the turns, decreasing the number of contact points, and facilitating the sliding of the overlapped surfaces one over the other in the direction of the circumference inside of the cylindrical membrane (4).

4. Acoustic transformer according to claim 1, characterized in that the plane and circular wave front (7) which propagates itself inside the wave guide is adapted to exert on the single annular section of the cylindrical membrane (4) a uniform radially oriented pressure which modifies the diameter of the affected section, the sum of the effect of the single sections generating outside the cylindrical membrane (4) a wave front with radial symmetry with respect to the cylinder axis.

5. Acoustic transformer according to claim 1, characterized in that the gasket (6) is made of plastic material with the housings for the screws for fastening to an external bracket and with an edge able to be overlapped to the flange of the loudspeaker (3) for fastening the gasket (6) to the flange of the loudspeaker (3) with an airtight seal .

6. Acoustic transformer according to claim 1, characterized in that an edge of the gasket (6) is fastened to an edge of the cylindrical membrane (4) with an airtight seal.

7. Acoustic transformer according to claim 1, characterized in that the circular plug (5) is fastened to an edge of the cylindrical membrane (4) with an airtight seal and reflects the internal sound waves ( 7 ) .

8. Acoustic transformer according to claim 7, characterized in that the fraction of reflected sound energy of the circular plug (5) is decreased by applying sound-absorbing material on the internal side of the circular plug (5) or by making the circular plug (5) with material with a higher transmission point or by completely removing it.

9. Acoustic transformer according to claim 2, characterized in that the cylindrical coil is made by winding sheets of materials different from paper with surfaces able to slide one over the other.

10. Acoustic transformer according to claim 2, characterized in that the cylindrical coil is replaced by a single layer of film with a strongly anisotropic modulus of elasticity, so that such modulus along the cylinder axis is much greater with respect to the modulus along the circumference.

11. Acoustic transformer according to claim 10, characterized in that the film is fastened to a support shaped as a cylindrical sector and emits sound waves on a circular sector lower than 360 degrees.

12. Acoustic transformer according to claim 2, characterized in that, by increasing the thickness of the paper and the number of turns, the cylindrical membrane (4) is adapted to support the weight of the loudspeaker (3) .

13. Acoustic transformer according to claim 12, characterized in that the circular plug (5) is adapted to perform the function of bearing base to place the diffuser on the floor or on the shelf of a piece of furniture.

14. Acoustic transformer according to claim 6, characterized in that an anisotropic cylindrical membrane (9) open on both sides is adapted to be assembled separate with suitable spacers on the front side of the loudspeaker (3).

15. Acoustic transformer according to claim 1, characterized in that the cylindrical membranes (4, 9) are adapted to be pre-molded and/or decorated before being assembled in the acoustic transformer.