EP3243334A1

EP3243334A1 - Waveguide acoustic diffracting device

Info

Publication number: EP3243334A1
Application number: EP15848137.4A
Authority: EP
Inventors: Claudio GANDOLFI
Original assignee: Robin Srl
Current assignee: Robin Srl
Priority date: 2015-01-08
Filing date: 2015-12-15
Publication date: 2017-11-15
Also published as: WO2016110876A1

Abstract

A waveguide acoustic diffracting device is described, comprising: a waveguide (6), of an elongated shape and octagonal section, open at its sides and made of a sheet of bent cardstock, a loudspeaker (5) adapted to generate sound waves (8) sent inside the waveguide (6) and to support the waveguide (6) fastened to the flange of the loudspeaker (5), a bearing base composed of a double cylindrical ring (22, 23) which keeps the loudspeaker (5) in position, acoustically insulating it from the floor, wherein the waveguide (6) has many series of holes (7) on the side wall and simultaneously performs the following functions: a) waveguide (6), b) omni-directional passive acoustic radiator, c) acoustic delay, d) container, e) screen, f) non-elastic dissipater of acoustic energy, g) self-bearing and lightweight structural element.

Description

WAVEGUIDE ACOUSTIC DIFFRACTING DEVICE

The present invention deals with a waveguide acoustic diffracting device. It is an acoustic diffuser with omni-directional emission horizontally on the whole audio band, and capable of generating a realistic sound field.

The structure is extremely lightweight, can be made with economic materials which can be easily found and worked. When designing acoustic boxes, the diffraction produced by the edges is an undesired effect which tends to be minimized: on the contrary, the present invention makes use of a geometry which maximizes its effects .

When reproducing a recorded musical piece, the positioning of the acoustic diffusers in the listening environment is critical. Sound waves which directly arrive from the transducers to the listening point precede the reflections generated by walls and objects present in the environment. Too many reflections make listening confused, too few reflections make sound unnatural since hearing does not allow mentally reconstructing realistic environment features .

A first object of the present invention is facilitating the positioning of the acoustic diffuser in the environment and making hearing realistic. The first wave front (18) perceived by the listener is related to the sound wave moving along the shortest distance between loudspeaker (5) and the listening point (15). Afterwards, secondary sound waves (38, 48) are emitted by diffraction with wave fronts delayed by the path inside the waveguide (6) which reach the listening point before the reflections generated by wall and objects present in the listening environment. Intensity and delays of the secondary waves (38, 48) are identical for the two stereo channels, are controllable at design level and prevail at psycho-acoustic level on the reflections of the listening environment, making realistic the reproduced sound field, The omnidirectional horizontal emission avoids that the sound energy concentrated on a single object or wall in the environment generates reflections which result predominant or excessive in a part of the audio spectrum.

The sound of a music instrument derives from the combination of vibration modes of its parts, and at the same time sound waves are generated in different points of its structure. The sizes of the instruments change in the majority of cases from 50 cm to a couple od meters, so that sound waves simultaneously emitted by different parts can arrive at a listener with wave fronts offset by a few milliseconds. The offset between wave fronts generated by sound waves emitted at the same time is further increased by the possible paths of the reflections in the listening environment. The delays introduced by offsets contribute to the perception of the characteristics both of the sound emitted by the instrument and of the environment in which one listens.

A second object of the present invention is simulating the size of the music instrument by emitting fractions of sound energy of the same wave many times, with increasing delays and in different positions .

The above and other objects and advantages of the invention, as will result from the following description, are obtained with a waveguide acoustic diffracting device 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 the enclosed claims are an integral part of the present description.

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 is a schematic sectional detailed view of a loudspeaker used in the diffracting device according to a first preferred embodiment of the present invention;

- Figure 2 is a front schematic sectional view of the diffracting device of Figure 1;

- Figure 3 is a front view of the diffracting device of Figure 2;

Figure 4 schematically shows an arrangement with two diffracting devices per channel;

Figure 5 schematically shows another arrangement with two diffracting devices;

Figure 6 shows the diffracting device of the present invention applied to conventional loudspeakers;

Figure 7 is a front view of the diffracting device according to a second preferred embodiment of the present invention; Figure 8 is a front schematic sectional view of the diffracting device of Figure 7;

Figure 9 is a top schematic view of the diffracting device of Figure 7;

Figure 10 shows in section a detail of the base of the loudspeaker of the diffracting device of Figure 7;

Figure 11 shows the path of the sound waves emitted by the rear side of the loudspeaker and the path related to secondary sound waves emitted by diffraction from an hole in the centre of the waveguide and an hole at the end of the waveguide;

Figure 12 shows the behaviour of the acoustic pressure in the listening point for an acoustic signal, formed by only one 1- kHz positive half-wave, sent to the loudspeaker considering only the rear emission and the one from the holes; - Figure 13 shows a virtual configuration capable of generating the behavior of the acoustic pressure shown in Figure 12;

- Figure 14 shows the omnidirectional distribution of the secondary sound waves emitted by diffraction and the main forces generated by the diffraction itself;

- Figure 15 shows how the holes can be replaced by a thin slit;

- Figure 16 shows how the acoustic diffracting device can be used in a 2-way system;

- Figure 17 shows how to decrease the turbulences generated by the rear emissions of the loudspeaker; and

- Figure 18 shows the section of a plug, which covers the upper opening of the waveguide of Figure 7.

According to the first preferred embodiment of the waveguide acoustic diffracting device of the present invention, the sound energy (8) emitted by the loudspeaker (5) is sent inside a waveguide

(6) from which it is extracted by diffraction by a series of suitably spaced and sized holes (7) .

The energy of the single wave front (8) emitted by the loudspeaker (5) membrane is divided into a series of spherical fronts emitted with increasing delays from mutually aligned and spaced holes

(7) .

The sound diffraction depends on the speed of sound in air and on geometric parameters of the acoustic diffracting device: therefore, the wave fronts emitted in different points and in different times are propagated with predetermined phase delays also in the listening point.

At psycho-acoustic level, the sum of the spherical fronts prevails on the effect of the reflections of the listening environment, facilitating the recognition of single sounds, making the reproduction more realistic and reducing the position criticality both of the diffusers, and of the listener.

A waveguide (6), preferably 1 meter long, fastened to the loudspeaker (5), allows generating delays from 0 to about 3 milliseconds. The waveguide (6) has also a function of screen to reproduce low frequencies, since it prevents the acoustic short circuit between front and rear emissions of the loudspeaker (5) .

A series of holes (7) a few millimeters long, aligned at a distance of 1 cm on the side part of the waveguide (6), are used for the delayed diffraction of the wave fronts. The holes (7) have sizes lower than the lengths of the sound waves to be reproduced, and therefore they behave as omni-directional acoustic radiators.

The waveguide (6) keeps the phase coherence of internal wave fronts for its whole length, and allows positioning the loudspeakers (5) at the base of the acoustic box, facilitating the removal of undesired vibrations between floor and loudspeakers.

In their path, the internal wave fronts (8) apply a perpendicular pressure on the internal side surface of the waveguide (6) . By obtaining suitably sized holes (7), it is possible to extract and transfer outside a fraction of the sound energy which travels inside the waveguide (6) without meaningfully perturbing the coherence of the internal wave front (8).

By aligning the holes (7) on the side surface of the waveguide (6), due to the Huygens principle, a wave front is obtained which is the sum of the contribution of the single holes. The internal wave front (8) moves in parallel to the line of the holes (7) and applies thereon the pressure with increasing delays, modulating it in the space. The same pressure modulation propagates itself in air till it reaches the listening point.

The acoustic transformer of the waveguide type is a technology which allows obtaining the same type of sound field, as disclosed in documents SS2013A0000007 and PCT/2014/000128 of the same Applicant of the present invention. In the acoustic transformer, time and space offsets of the wave fronts are obtained by exploiting the waveguide deformation with the limit of linearity of the deformation itself, which appears on low frequencies at high power. In the waveguide acoustic diffracting device, there are no structure parts which can be deformed by the sound waves, thereby the whole power of the loudspeakers can be used also at low frequencies.

The Applicant has made a prototype of the inventive diffracting device in the following way. On a wooden base (1) a metallic bit (2) has been fastened, which is a fulcrum to support the magnet (4) of the loudspeaker (5) kept suspended in a balance at about 1 millimetre from the base (1) . The bit (2) acoustically uncouples the base (1) on the floor and the loudspeaker (5) .

Some wedges (3) made of sound-dampening material keep the magnet

(4) in position. With forces of few grams, one can compensate for inclinations due to: floor, building tolerances, loudspeaker connecting wires, vibrations generated when operating.

On the front side of the loudspeaker (5), the waveguide (6) is fastened with a cylinder 1 meter high and with a diameter of about 8 cm and its top side open. Along the side wall the 2-mm holes (7) are aligned at a distance of 1 cm oriented towards the listening point.

As regards the acoustic behaviour of the inventive diffracting device, the sound waves (8) emitted from the front side of the loudspeaker (5) are send inside the waveguide (6) and, by moving along it, interact with the holes (7) which become the originating points of the spherical wave fronts. At the end, the sound waves (8) go out of the upper side of the cylinder, which is a further generating point of diffracted spherical waves for medium and low frequencies. The highest, directive frequencies continue upwards and afterwards are reflected by the room ceiling.

The sound waves emitted from the rear side of the loudspeaker

(5) , at a height of a few centimetres with respect to the floor, are directly irradiated in the listening environment without meeting further obstacles.

As regards the pulse response, the envelope of sounds generated by instruments is divided into the phases of attack, decay, holding and release. The phases of attack, decay and release are transients with very quick amplitude variations of the sound pressure which can be assimilated to pulses.

The pulse response of the waveguide acoustic diffracting device is optimized by the reduced reflections added to the diffuser structure. The wave front (8) emitted upwards is diffused in the environment by crossing the waveguide. The wave front emitted downwards is diffused in the environment only from the floor.

Vibrations transmitted to the loudspeaker (5) basket from the movement of the membrane are mainly directed vertically and neglectably interact with the wave fronts generated by diffraction which propagate horizontally.

The waveguide acoustic diffracting device generates a series of wave fronts in different times and positions, using fractions of energy of the same pulse. The coherence of the phase delays is greater with respect to the one normally provided by acoustic boxes with direct radiation with the reflection of the listening environment and the reconstruction of a model of the sound source is more realistic.

The phase of holding can be assimilated, as a first approximation, to a stationary regime. In the holding phase, the real instrument emits energy from different structure areas which simultaneously irradiate towards the listening point. The sound energy is distributed in space on many simultaneously emitted fronts.

In the acoustic diffracting device, the sum of spherical fronts generated by the aligned holes (7) can be described as a cylindrical surface which horizontally propagates towards the listening point, in which the pressure is vertically modulated along the direction of the sound waves (8) inside the waveguide. The cylindrical surfaces generated in succession have pressure profiles translated along the direction of the cylinder axis. While the sound waves interact with the pinna, they transport information related both to the horizontal movement, and to the vertical displacement of the pressure profile, facilitating the sound decoding. The pressure maxima generated by the loudspeaker in different times are simultaneously present next to the holes (7) at a distance equal to the wavelength. The sound waves propagate by complying with the Huygens principle and reach the listening point, transferring onto the surface which contains it the same pressure maxima. The re production realism derives from the simulation of different wave fronts generated by the instrument at the same time with wave fronts generated by the loudspeaker in different times.

The shape of the waveguide (6) can be changed, provided that it keeps the phase coherence between the frequencies of the wave trains (8) which cross it.

The distribution of holes on the side surface of the waveguide can be changed depending on design needs. By modifying density and alignment of the holes, the emission can be kept at 360 degrees of the sound energy related to some frequencies can be concentrated in pre-determined angular sectors. The distribution of holes can be used to constructively or destructively interact with some frequencies present inside the waveguide.

Many holes can be joined in a slit to increase the sound energy transmitted outside. The same open terminal section of the waveguide generates diffraction phenomena.

In general, by increasing the energy emitted by diffraction, the acoustic screen effect of the waveguide is reduced, together with the response at low frequencies.

Some types of loudspeakers are designed to work in closed volumes, so that in these cases it is convenient to use waveguides with the opposite end to the loudspeaker closed and the diffraction reduced.

The function of the waveguide acoustic diffracting device as omni-directional acoustic radiator for high frequencies can also be applied to conventional acoustic boxes (11), as can be seen in Figure 6. It is enough to orient the loudspeaker (5) upwards and abut thereon the cylinder-shaped waveguide (12) with a slit on the side wall .

The function of the waveguide acoustic diffracting device as acoustic impedance adaptor for low frequencies requires sizes on the order of a meter, and therefore resonances can be triggered inside the waveguide (6). These are resonances whose frequency is related with the sizes. Two diffusers in parallel for every channel, which suitably diversified acoustic characteristics, avoid that there are frequencies reproduced at an insufficient or too high level. Δ similar result can be obtained with a DSP which allows compensating also possible resonances generated by the listening environment.

Figure 4 shows a possible configuration with a double waveguide. For every channel, there are two loudspeakers and two waveguides. The waveguide (9) maximized the emission by diffraction with a continuous slit some millimetres long. The waveguide (7) has a series of 2-mm holes and a reduced diffraction, and the upper side is open in order to use air inside as additional mass which lowers the resonance frequency of the diffuser.

Figure 5 shows the acoustic diffracting device (9) coupled with a subwoofer ( 10 ) .

In order to be able to perceive the realistic reconstruction of sounds, it is necessary that the whole reproduction chain is of an adequate level. Distortions and/or background noises can cover information which are driven by minimum phase variations of the wave fronts, and also the interaction between diffuser and floor can degrade the reproduction quality.

According to the second preferred embodiment of the acoustic diffracting device of the present invention, as shown in Figures 7 to

18, a sheet of 350-g/mq cardstock bent with an octagonal section is the waveguide open at its sides used in the waveguide acoustic diffracting device.

The position near the floor for the loudspeaker has two advantages: it simplifies the construction of the diffuser and facilitates the removal of undesired vibrations.

Above the loudspeaker there remains only the waveguide (6), fastened to the flange of the loudspeaker (5), which can also be made of very lightweight materials like cardstock. The barycentre of the diffuser is near the floor and coincides with the loudspeaker: this makes the acoustic diffuser stable.

Being loudspeaker and floor nearby, separated by a non-elastic material like paper, avoids that undesired vibrations are generated, which would propagate themselves to the structure.

In the inventive waveguide acoustic diffracting device, the sound wave (18) emitted from the rear side of the loudspeaker (5) is directly sent in the listening environment. The sound wave (28) emitted by the front side of the loudspeaker (5) is sent inside a waveguide (6). The waveguide has no transverse projections so that the wave emitted at floor level for the loudspeaker (5) travels up to the ears of a listener (15) without generating reflections and finally goes out of the open side (9) of the waveguide. Along the path, the internal sound wave generates by diffraction, next to the holes (7), secondary sound waves (38, 48) with increasing delays with respect to the sound wave emitted from the rear side of the loudspeaker (18) and with spherical wave fronts (Fig. 14) with a centre next to the holes (7) . The sound diffraction depends on the speed of sound in air and on geometric parameters of the acoustic diffracting device: therefore, the secondary sound waves emitted by diffraction in different points and at different times propagate themselves with predetermined intensity and delays also in the listening point. Intensity and delays are identical for the two stereo channels. The secondary sound waves reach the listening point before the reflections generated by the walls of the listening room. At psycho-acoustic level, the effect of the secondary sound waves (38, 48) diffracted by the diffuser prevails on the effect of the reflections of the listening environment, facilitating the recognition of single sounds, making the reproduction more realistic and reducing the criticality of the position both of the diffusers, and of the listener.

Every hole (7) is coplanar with other three holes symmetrically distributed at the same distance of the loudspeaker. An omnidirectional horizontal emission is obtained, on the whole audio band from 20 Hz to 20 KHz, efficient on an angle of 360 degrees as pointed out in Figure 14. The holes symmetry makes null the resultant of the forces deriving from diffractive phenomena, reducing to a minimum the perturbation of the internal sound wave (8) . The omni-directional horizontal emission avoids that the sound energy is concentrated in some angular sectors in which there are objects or walls capable of generating excessive reflections in part of the audio spectrum.

The waveguide (6) 1 metre long, fastened to the loudspeaker (5), allows gene rating delays from 0 to about 3 milliseconds. The sound field produced by the waveguide acoustic diffracting device is equivalent to the one produced by a virtual loudspeaker (5') placed next to the diffuser and at the height of the ears of the listener (15), surrounded by a series of acoustically reflecting panels (37', 47') on the back with respect to the virtual loudspeaker (5') if seen from the listening point (15) . The distance between virtual panels (37', 47') and virtual loudspeaker (5') is about half the distance between real loudspeaker (5) and diffraction holes (37, 47) . The horizontal emission is omni-directional, so that if the listener (15) moves, also the virtual panels (37', 47') move behind the virtual loudspeaker (5') which remains fixed. The listener can freely move in front of the diffusers, always having an optimum stereophonic image. Also in case of many people sitting in front of the diffusers, all will listen under optimum acoustic conditions.

The air volume contained inside the waveguide (6) open on the upper side (9) has a mass which is added to the moving mass of the loudspeaker (5) reducing the resonance frequency and improving the response of the acoustic diffracting device at low frequencies.

From empirical test performer on the prototypes, it has been verified that the efficiency of the acoustic screen composed of the waveguide (6) can also be obtained with materials capable of being deformed by the sound waves, like the cardstock. The density of a 350-g/mq cardstock is enough to confine the sound waves emitted inside the waveguide (6). The non-elastic deformations of the cardstock are used to dissipate acoustic energy and dampen the low frequencies inside the waveguide (6). Medium and high frequencies are not attenuated, since inertia due to the cardstock mass prevents appreciable deformations from being generated. The maximum linear deformation of the section of the waveguide due to the internal acoustic pressure limits the maximum acoustic pressure which can be used by the acoustic diffracting device. The presence of the octagon edges increases the maximum linear deformation. The octagonal section has been chosen for its easy making and can be replaced by other shapes which keep the symmetry with respect to the axis of the waveguide (6) with bends which facilitate its deformation.

The bends compose ribs which increase the mechanical sturdiness, making it suitable to obtain self-carrying waveguides even one meter long .

In the acoustic transformer, the time and space offsets of the sound waves are obtained by exploiting the deformation of the waveguide to emit secondary sound waves. On the low frequencies at high power there occurs the linearity limit of the deformation which reduces the maximum sound which can be reproduced. Another limit of the acoustic transformer is at highest frequencies, where the density of the waveguide membrane becomes important. The waveguide acoustic diffracting device does not exploit the deformation of the structure for emitting the secondary sound waves, so that the whole power of the loudspeakers at low frequencies can be used, and high frequencies can also be optimally reproduced.

The waveguide (6) has no transverse projections which could generate reflections or perturb the sound waves (8) which cross it.

The waveguide (6) made of rigid materials like wood or plastics makes the acoustic diffracting device adapted for mobile installations and/or for outside.

The Applicant has also made a prototype of the diffracting device of the above described second embodiment. A double cylinder (22, 23) made of undulated cardstock to be placed next to the floor of the listening room supports and keeps in position the loudspeaker (5) . Paper is a non-elastic material which prevents the formation of undesired vibrations between loudspeaker (5) and floor which, being propagated to the structure of the diffuser, would mask the effect produced by the secondary waves (38, 48) .

On the front side of the loudspeaker (5), the waveguide (6) is fastened and made with a 1 meter high tube, the section of an octagon with a 4-cm apothem and the upper side open, Along the side wall the 3-mm holes (7) are aligned at a distance of 1 cm arranged on four symmetrical rows with respect to the axis of the waveguide.

The sound waves (28) emitted from the front side of the loudspeaker (5) are sent inside the waveguide (6) and, moving along it, interact with the holes (37, 47) which become points of origin of diffracted secondary sound waves with spherical wave fronts (7). At the end, the sound waves go out of the upper side of the cylinder (9), which is a further point of generation of diffracted secondary sound waves with spherical wave fronts for medium and low frequencies. The highest frequencies, directive, go on upwards and are afterwards reflected by the room ceiling.

The sound waves emitted from the rear side of the loudspeaker (18), at a height of few centimetres with respect to the floor, are directly irradiated in the listening environment without meeting further obstacles.

The envelope of sounds generated by instruments is divided into the phases of attack, decay, keeping and release. The phases of attack, decay and release are transients with very quick amplitude variations of the sound pressure which can be assimilated to pulses. The phase of keeping can be assimilated as first approximation to a stationary regime.

The impulse response of the waveguide acoustic diffracting device is optimized by the lack of reflections added to the structure of the diffuser. The sound wave (28) emitted upwards is diffused in the environment by crossing the waveguide (6) . The sound wave emitted downwards is diffused in the environment limited only by the floor.

The waveguide acoustic diffracting device generates a series of secondary sound waves in different times and positions, using fractions of energy of the same pulse, The phase delays between the secondary sound waves are predetermined by the geometry of the acoustic diffracting device, are identical for the two channels and allow an accurate reconstruction of the stereophonic image.

In the phase of keeping, the real instrument emits energy from different areas of the structure which are simultaneously irradiated towards the listening point. The sound energy is distributed in the space on many simultaneously emitted waves. In the acoustic diffracting device, the sum of the secondary sound waves with spherical wave fronts generated by the holes ( 7 ) creates a complex dynamical system which can be assimilated to a cylinder with the same axis of the acoustic diffracting device and which horizontally expands towards the listening point. On this surface, the modulation profiles of the acoustic pressure translate upwards due to the delays generated inside the waveguide (6). While the secondary sound waves interact with the pinna, they transport information related both to the horizontal movement, and to the vertical movement of the pressure profile, facilitating the sound decoding. The re production realism can be attributed to the fact that in the listening point many secondary sound waves generated by the loudspeaker at different times simulate the sound waves generated by the instruments at the same time in different points of its structure.

The distribution of holes on the side surface of the waveguide can be changed depending on design needs. Many holes can be joined in a slit to increase the sound energy transmitted outside, The same upper open section of the waveguide originates diffraction phenomena. In general, by increasing the energy emitted by diffraction, the acoustic screen effect of the waveguide is reduced, together with the response at low frequencies.

Making a mobile plug (10) of materials with suitable shape, transmitting and reflecting power, it is possible to modify the frequency response of the acoustic diffracting device. A mobile plug (10) made of acoustically reflecting material allows using loudspeakers designed to work in closed volumes. A plug (10) made of a thin sheet of paper leaves unchanged the acoustic characteristics and protects from dust the acoustic diffracting device interior.

The term "acoustic box" is the most suitable for classifying the waveguide acoustic diffracting device made of cardstock.

The cardstock is not suitable for being used in the open, in humid environments or in mobile installations subjected to impacts during transport. In these cases, in order to make the waveguide (6), a material can be used which is more resistant to impacts and to humidity, like plastics or wood. The acoustic diffracting device becomes heavier and there is a slight loss of quality due to the fact that materials like plastics or wood have more elasticity with respect to cardstock and facilitate the formation of spurious vibrations in the structure.

The function of the waveguide acoustic diffracting device as omni-directional acoustic radiator for high frequencies can also be applied to multi-way acoustic boxes, as shown in Figure 16. The tweeter (5) is oriented upwards, and on it the waveguide (12) is fastened, shaped as an octagon with four slits (7) on the side walls.

The function of the waveguide acoustic diffracting device as acoustic impedance adapter for low frequencies requires sizes on the order of the metre, and therefore resonances can be triggered inside the waveguide (6) . These are resonances whose frequency is related to the sizes. The problem can be soled with a DSP which allows compensating also possible resonances generated by the listening environment. In order to perceive the realistic sound reconstruction, it is necessary that the whole reproduction chain is of an adequate level. Distortions and/or background noises can cover information which are sent by minimum phase variations of the secondary sound waves and also the interaction between diffuser and floor can degrade the reproduction quality.

The latter prototype has a total weight of 675 grams only, including electric connections. The loudspeaker by itself weights 525 grams. With the technology of the waveguide acoustic diffracting device, adding only 150 grams of cardstock and glue to a common electro-dynamical, wide band, 3-inch loudspeaker, an acoustic diffuser is obtained with an audiophile quality, capable of generating over 90 dB of acoustic pressure in a house environment.

Claims

1. Waveguide acoustic diffracting device, characterized in that it comprises :

- a waveguide (6), of an elongated shape with an octagonal section open at its sides made of a sheet of bent cardstock,

- a loudspeaker (5) adapted to generate sound waves (8) sent inside the waveguide (6) and to support the waveguide (6) fastened to a flange of said loudspeaker (5) ,

- a bearing base composed of a double cylindrical ring (22, 23) which keep in position the loudspeaker (4.5) acoustically insulating it from the floor,

wherein the waveguide (6) has a plurality of series of holes (7) on the side wall and simultaneously performs the following functions: a) waveguide (6) to transfer the sound energy emitted at floor level from the loudspeaker (5) up to a height of ears of the listener (15) without generating reflections of the sound waves (28) which cross it,

b) omni-directional passive acoustic radiator on the horizontal plane on the whole audio band, preferably from 20 Hz to 20 KHz, with sound waves emitted by diffraction from the holes (7) on the side part of the waveguide (6),

c) acoustic delay which simulates the presence of acoustically reflecting panels (37', 47') on the back with respect to a virtual loudspeaker (5') coinciding with the acoustic diffracting device seen by the listening point (15), in which, by changing the listening point (15), the virtual panels (37', 47') change position,

d) container which constrains the air mass inside the waveguide (6) open only upwards, which reduces the lower resonance frequency of the acoustic diffuser,

e) screen which acoustically separates the front emission (28) of the loudspeaker (5) from the rear emission (18) directly irradiated in the listening environment for reproducing the low frequencies, f) non-elastic dissipater of acoustic energy through the deformation of a section of the bent cardstock (6) caused by the low frequency sound waves (28) present inside the waveguide (6),

g) self-bearing and lightweight structural element, to make an acoustic diffuser with a total reduced weight with respect to the sum of three elements only: base (22, 23), loudspeaker (5) and waveguide (6) .

2. Acoustic diffracting device according to claim 1, characterized in that said double cylindrical ring (22, 23) of said bearing base is made of cardstock bands.

3. Acoustic diffracting device according to claim 1 or 2, characterized in that a loudspeaker (5) actuated by an electrical signal with acoustic frequency, preferably from 20Hz to 20KHz, is used to send sound waves (28) inside the waveguide (6), while a positive electrical signal generates a positive acoustic pressure above the loudspeaker (5) and simultaneously a negative acoustic pressure below the loudspeaker (5), and vice versa for a negative electrical signal, the sound waves (28) generated by the loudspeaker (5) crossing the waveguide (6) with plane wave fronts having the same shape as the section of the waveguide (6) .

4. Acoustic diffracting device according to claim 1, characterized in that the holes (7) on the side wall of the waveguide (6) have a size of some millimetres, lower than the wavelength of the sound waves (28) generated by the loudspeaker (5) which propagate themselves with plane wave fronts inside the waveguide (6) and generate outside by acoustic diffraction sound waves (38, 48) with wave fronts with spherical symmetry (7), the sound wave emitted by the rear side (18) of the loudspeaker (5) being the first one to reach the listening point (15), afterwards reached by the sound waves (38, 48) emitted by diffraction from the holes (37, 47), in the listening point (15) the emission (38, 48) of the holes being perceived as coming from acoustically reflecting virtual panels (37', 47') on the back with respect to the virtual loudspeaker (5') if seen from the listening point ( 15 ) .

5. Acoustic diffracting device according to claim 4, characterized in that tens of diffraction holes (7) placed at increasing distances from the loudspeaker (5) simulate as many virtual panels (37', 47') on the back and at increasing distances with respect to the virtual loudspeaker (5¹) if seen from the listening point (15) .

6. Acoustic diffracting device according to claim 5, characterized in that the diffraction holes (7) coplanar and at the same distance from the loudspeaker (5) are symmetrically arranged with respect to the axis of the waveguide (6) and make efficient the diffusion of the sound waves on a horizontal angle of 360 degrees, and further cancel the resultant of the forces generate by diffraction phenomena.

7. Acoustic diffracting device according to claim 1, characterized in that a slit (7), whose width is preferably equal to 2 - 5 millimetres, lower than the wavelengths of the sound waves generated by the loudspeaker (5), replaces many aligned diffraction holes.

8. Acoustic diffracting device according to claim 7, characterized in that the waveguide (12) with four symmetrical slits (7) coupled with a tweeter (5) makes a unit for high frequencies with omni-directional horizontal emission at 360 degrees for a multi-way system.

9. Acoustic diffracting device according to claim 1, characterized in that the cardstock used for making the waveguide (6) is a non-elastic material adapted to dissipate acoustic energy to attenuate the internal sound waves (8) at low frequencies, the inertia deriving from the cardstock mass preventing medium and high frequencies from generating meaningful deformations and therefore they are not attenuated.

10. Acoustic diffracting device according to claim 1, characterized in that the longitudinal bends of the cardstock increase the maximum linear deformation of the section of the waveguide (6) produces by the pressure of the sound waves (28) which cross it, increasing the maximum acoustic pressure which can be reproduced.

11. Acoustic diffracting device according to claim 10, characterized in that the octagonal section is modified both as shape and as sizes along a longitudinal direction, using sections with symmetrical shape with respect to the axis of the waveguide (6) in order to keep null the resultant of the forces applied on the perimeter by diffraction phenomena .

12. Acoustic diffracting device according to claim 1, characterized in that a drilled trunk of cone (24) made of wood or other plastic material replaces the cylinder (22) in order to decrease the turbulences generated by the rear emission (18) of the loudspeaker (5) .

13. Acoustic diffracting device according to claim 1, characterized in that the upper opening (9) of the waveguide (6) is totally or partially closed with a mobile plug (10) made of different materials to optimize the frequency response of the system and protect the interior from dust.

14. Waveguide acoustic diffracting device comprising:

- a waveguide (6), preferably of a cylindrical shape and open at its side, having a series of holes (7) aligned on the side wall,

- a loudspeaker (5) adapted to emit and send sound waves (8) inside the waveguide (6),

characterized in that it further comprises: - a metallic bit (2) on which the loudspeaker (5) rests, adapted to support the weight of the loudspeaker (5),

- a base (1) on which the metallic bit (2) is fastened,

- wedges (3) made of acoustically insulating material inserted between a magnet (4) of the loudspeaker (5) and the base (1) adapted to balance the loudspeaker (5),

wherein said holes (7) are adapted to diffuse spherical sound waves generated by diffraction by the sound wave fronts which travel in the internal air volume.

15. Acoustic diffracting device according to claim 14, characterized in that the holes (7) aligned on the side wall of the waveguide (6) have a size of a few millimetres lower than the wavelength of the sound waves diffused from the holes (7) .

16. Acoustic diffracting device according to claim 15, characterized in that many holes (9 - 12) are joined in a slit to increase the sound energy transmitted outside, the same end section of the open waveguide (6) originating diffraction phenomena.

17. Acoustic diffracting device according to claim 14, characterized in that the waveguide (12) is adapted to be applied to an acoustic box (11) to improve the dispersion of high frequencies.