FR2759526A1 - Transmission line loudspeaker enclosure for use with tunable audio reproduction equipment - Google Patents

Abstract

The enclosure (10) has concentric cylindrical structures (20,30) each defining an acoustic space (21,31). The spaces are in communication with each other via an end reflector (32) to establish an acoustic coupling between a rear-travelling sound wave (18) from a speaker driver cone (12), and a surrounding air mass. The sound wave eventually passes along the outer enclosure wall (26) leaving the bottom opening (27) of the enclosure. An audio amplifier (218,220,230,232) tunable to the resonant frequency of a listening room (21) removes very low narrow frequency band components of an audio signal produced by the speaker enclosures (10a,10b) and the listening room cavity resonance is measured by injecting a frequency-varying sound wave into the listening room while detecting peak sound energy within the room. A filter (220) eliminates frequencies associated with the measured resonance.

Description

 The invention relates generally to a sound reproduction apparatus, and in particular to loudspeakers for loudspeakers comprising an acoustic coupling by transmission line and an acoustic reproduction material tuned to a selective listening environment.

 Acoustic reproduction systems continue to evolve towards increasingly high quality sound reproduction. Progress with regard to the natural defect of linearity, that is to say the variation of acoustic energy as a function of the wavelength of sound, is obtained following research and development. From the recording of sounds to their reproduction, a significant improvement in the quality of the material benefits the demanding listener. Unfortunately, there are still issues to be resolved regarding distortion and frequency response for most hearing aids, particularly at low frequencies or bass wavelengths. Even very sophisticated equipment lacks fidelity in reproducing a linear presentation of sounds at extremely low frequencies.

 A high quality musical sound wave emerges from a speaker membrane in acoustic coupling with a listening room, and a corresponding reverse phase sound wave emerges from the back of the speaker membrane. This sound wave moving backwards, when it eventually returns by coupling into the ambient air mass, introduces a linearity defect in the sound, otherwise of high quality, produced in the room by the sound wave moving forward. Solutions have been developed, but without always improving the quality of the sounds in proportion to the cost.

 A conventional conical diaphragm speaker pushes air forward outside the enclosure, producing sound waves in a listening room. The sound waves emanating from the front and back of the speaker diaphragm are complementary, that is, in a phase relationship of 1800. Consequently, the coupling of the sound waves moving towards the 'forward and moving backwards inside a common listening room can introduce a lack of linearity in the presentation of sounds due to interference and cancellation of sound waves. Ideally, the backward traveling sound waves are introduced by coupling into a separate listening chamber, so as to avoid interference and cancellation of the sound waves. For example, mounting speaker membranes in walls sends the waves moving forward to a first listening room and the waves moving backwards to a second listening room. Unfortunately, advanced speaker systems, mounted in walls, are impossible for most listeners to implement.

 The classic mechanism delivering a presentation of sounds in a listening room is a speaker inside an enclosure. The diaphragm of the loudspeaker is in direct coupling by its front surface with the listening room, and by its rear surface with the interior of the enclosure. Unfortunately, high quality sound reproduction requires that the rearward moving sound waves are ventilated or released from the enclosure, i.e. the rearward traveling sound waves must ultimately exit the enclosure . It is preferable that the sound waves moving towards the rear, when they come out of the enclosure, generate little or no interference or cancellation of the sound waves compared to the sound waves moving towards the before.

 Speakers with an acoustic transmission line manage sound waves traveling backwards inside a loudspeaker enclosure. In general, a transmission line loudspeaker enclosure performs acoustic coupling from the rear surface of the loudspeaker membrane to the listening room via a transmission line or chamber of a given length and an area in cross section. The length of an acoustic transmission line is a function of the wavelength of a particular sound frequency, for example the resonance of the loudspeaker. The cross section corresponds to the useful area of the surface of the sound source, for example the useful area of the surface of the diaphragm of the loudspeaker.

 Various speakers with acoustic transmission line are known and commercially available.

Unfortunately, due to the large length required for the room in most loudspeakers with an acoustic transmission line, i.e. those designed to manage very low frequency sound waves, line loudspeakers of acoustic transmission have resulted in large and massive structures. The acoustic transmission line can be "folded" or routed inside the enclosure in a labyrinth so that the required length is obtained inside an overall shape similar to a box. Panels, usually of wood, inside the enclosure, form the requested acoustic transmission line or chamber having an appropriate cross section along its length. To resist deformation of the panels under the effect of sound pressure inside the acoustic transmission line, these panels must be of sufficient structural integrity, that is to say of sufficient thickness, to remain rigid against the pressure of sound waves. The combination of thick panel structures forming the acoustic transmission line in the form of a labyrinth folded back inside the loudspeaker enclosure results in massive loudspeaker enclosures having a large overall volume.

 The object of the present invention is to approach this aspect of loudspeakers with transmission line by proposing a loudspeaker enclosure with transmission line having an acoustic transmission line of an appropriate length and cross section. , but without the need for a massive, high-volume speaker enclosure structure.

 A reverberation sound wave, established by the surrounding walls, floor and ceiling, also causes interference with other sound waves in the listening room. This interference introduces a linearity defect in the otherwise high quality sound produced by the loudspeaker. Acoustic absorption material in the listening room and elaborate tuning processes try to minimize this lack of linearity, but these methods and devices do not always improve the quality of sounds in proportion to the amount of expenditure required.

 The cavity resonance of a listening chamber is an important source of reverberation interference degrading a presentation of high quality sounds. The cavity resonance of a chamber acts at a given fundamental frequency and associated harmonic frequencies. In a range of usual chamber dimensions, the fundamental resonant frequency falls within a range of audible frequencies. Due to the cavity resonance, the acoustic energy at the fundamental frequency does not dissipate as is the case with other acoustic frequencies. The sound pressure, developed at the fundamental frequency and at the harmonic frequencies, tends to accumulate. The listener perceives a relatively stronger sound at the resonant frequency and the harmonic frequencies. In other words, the sound pressure tends to accumulate excessively at the fundamental frequency and at the harmonic frequencies in a given listening room and, for an attentive listener, it ends up deviating in an annoying way from the linear presentation. sounds.

 Unfortunately, the cavity resonance for a given listening room varies depending on the density of the air, the furnishing of the room or the barometric conditions. Providing a narrow band cavity resonance in a given listening chamber becomes impossible. The width of the cavity resonance can go down to one hertz (Hz) in some listening rooms. Consequently, the narrow and unpredictable nature of the fundamental frequency and of the harmonic resonance frequencies leads to the failure any anticipation of the cavity resonance and the elimination by filtering of these narrow bands of fundamental frequency.

 The object of the invention is to tackle this aspect of acoustic reproduction by opposing an apparatus and a method for eliminating cavity resonance from a presentation of sounds.

 A speaker enclosure with an acoustic transmission line according to an embodiment of the invention comprises a site for mounting a speaker defining front and rear directions. A first cylinder is positioned relative to the speaker mounting site to receive, at a first end, a sound wave moving backwards and to deliver, at a second end, a sound wave moving backwards . A second cylinder, concentric with the first cylinder and relatively larger than the latter, surrounds the first cylinder.

A cap at the second end of the second cylinder directs the sound wave moving backward from the first cylinder into a space between the first and second cylinders.

 Other cylinders can be added in a concentric arrangement. Each cylinder establishes an acoustic space between itself and a neighboring inner cylinder and having a cross-sectional area equal to the cross-sectional area of the central cylinder, the desired cross-sectional area of the speaker with acoustic transmission line . The lengths of the cylinders vary to establish a desired length of acoustic transmission line.

 More generally, a loudspeaker enclosure with transmission line according to the invention comprises several sleeves arranged concentrically. One, central, of the sleeves, defines, inside, an associated acoustic space having a given cross-sectional area. Each remaining sleeve defines an associated acoustic space between itself and one, smaller and adjacent, of the sleeves. All acoustic spaces have a cross section equal to the given cross section. Caps connect the edges of some, alternating, sleeves to establish, through the acoustic spaces, a line of acoustic transmission inside the enclosure.

 According to another aspect of the invention, a component for tuning a listening room and an acoustic reproduction system receives an audio signal and produces a filtered audio signal. The tuning component includes a variable frequency sound source which can be applied to the listening room and includes a frequency indicator.

An input transducer for its measurement and indicates the acoustic energy in the listening room. A filter receives the audio signal and produces the filtered audio signal. The filter comprises at least one command imposing a filtered frequency band and calibrated with respect to the frequency indicator. Under the effect of injecting a range of frequencies including resonant chamber cavity frequencies, a peak value of the acoustic energy indicates cavity resonant frequencies to be applied as filter control.

 A method for connecting an acoustic system to a listening chamber according to the invention begins with the detection of a cavity resonance frequency of the listening chamber and then by adjusting a filter at the detected resonance frequency to filter an audio signal at the resonant frequency. The method then applies the filtered audio signal to sound transducers inside the chamber.

The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting examples and in which
Figure 1 schematically illustrates a speaker enclosure with multiple and concentric transmission line according to the invention
Figure 2 is a sectional view along line 2-2 of Figure 1 illustrating the cross sections of the acoustic transmission line presented by the enclosure of Figure 1
Figure 3 is a sectional view of the enclosure of Figure 1, along line 2-2 of this same figure
FIG. 4 illustrates a second embodiment of a loudspeaker with a multiple and concentric transmission line according to the invention
Figure 5 is a sectional view along line 5-5 of Figure 4 of the loudspeaker enclosure of Figure 4
Figure 6 is a cross-sectional view showing other details of the structure of the loudspeaker enclosure of Figure 1
Figure 7 is a simplified block diagram of an analog audio amplifier according to a first embodiment of the invention, which can be connected to a given listening room; and
Figure 8 is a simplified block diagram of a digital audio amplifier according to a second embodiment of the invention, which can be connected to a given listening room.

 FIG. 1 schematically illustrates a loudspeaker enclosure 10 with a three-cylinder acoustic transmission line. Externally, the enclosure 10 has an upper wall 14, side walls 26 and a lower opening 27. A loudspeaker 12 is mounted in the upper wall 14. The loudspeaker 12 emits a sound wave before 16, this is that is to say outwards and upwards with respect to the enclosure 10, and a rear sound wave 18. The rear sound wave 18 moves inside the enclosure 10 and ends up leaving by lower opening 27. A central cylinder 20 rests concentrically with respect to the central axis 22 of the enclosure 10. An upper end 20a of the central cylinder 20 opens in the form of a rim 24 extending radially towards the outside to the side walls 26 of the enclosure 10. The rear sound wave 18 enters a space 21 inside the central cylinder 20 through an end 20a passing through the upper surface 24a of the rim 24. L the lower end 20b of the central cylinder 20 remains open.

 A second cylinder 30, with a radius greater than that of the central cylinder 20, also extends concentrically with the central axis 22. The central cylinder 20 therefore extends generally inside the second cylinder 30 and concentrically with that -this. The upper end 20a and the rim 24 extend beyond an upper end 30a of the second cylinder 30 and towards the loudspeaker 12. The perimeter of the upper end 30a of the cylinder 30 remains open. A cap 32 covers the lower end 30b of the second cylinder 30, but at a given distance from the open lower end 20b of the central cylinder 20. While the rear sound wave 18 descends into the space 21 of the cylinder 20 and leaves at the lower end 20a, this sound wave 18 ends up meeting the inner surface 32a of the cap 32.

The cap 32 directs the sound wave 18 from the interior of the interior cylinder 20 to the space 31 of the second cylinder 30.

In particular, the cap 32 directs the sound wave 18 into the space 32 and makes it rise along the space 31 formed between the interior cylinder 20 and the exterior cylinder 30.

As illustrated in FIG. 1, the interior surface 32a comprises a convex central part and a concave peripheral part. This contour can be refined mathematically as a function of a desired guide path for the sound wave 18 from the central cylinder 20 to the second cylinder 30. In addition, the cap 32 must be placed at an appropriate distance from the end 20b of the cylinder 20 to maintain the desired cross section in the transition from the space 21 to the space 31.

 The sound or acoustic wave 18 then travels through the space 31 going up along the perimeter of the cylinder 30 and it finally reaches the upper end 30a of the cylinder 30. The outer lateral walls 26 can be produced in the form of a cylinder which is also concentric with the axis 22. In the particular embodiment shown here, the exterior structure used is not a cylinder, but it establishes a space 29 between the exterior of the second cylinder 30 and the side walls 26.

The assembly consisting of the central cylinder 20 and the second cylinder 30 rests concentrically inside the enclosure 10, that is to say by being centered inside the side walls 26 of the enclosure 10 In the particular embodiment shown here, the side walls 26 define in cross-section a "half-square" shape as described more fully below.

 In each case, the side walls 26 define a space 29 between the outside surface of the second cylinder 30 and the inside surface of the side walls 26.

The space 29 is open towards the listening chamber via the lower opening 27 of the enclosure 10.

When the sound wave 18 passes through the upper end 30a of the second cylinder 30, it meets the lower surface 24b of the rim 24. The lower surface 24b returns the sound wave 18 downward along the inner surface of the side walls 26, that is to say in the space 29 between the second cylinder 30 and the side walls 26. The sound wave 18 finally emerges from the lower opening 27 of the enclosure 10. Feet 40 are connected to the side walls 26 and establish a space between the lower opening 27 and the ground 42 on which the enclosure 10 rests.

 Figure 2 shows in cross section the spaces 21, 31 and 29 inside the enclosure 10, which establish an acoustic transmission line of uniform cross section. The cross section of the space 21 is circular and corresponds to the useful area of movement of the loudspeaker 12. The area of the cross section of the space 31, that is to say that between the cylinder 20 and the cylinder 30, is annular and equal to the cross-sectional area of the space 21. The space 29 has a cross-sectional area which is also equal to that of the spaces 21 and 31. Although the use of a cylinder section to form the side walls 26 would give an annular cross section to the space 29, this particular embodiment of the invention uses a structure having a "half-square" shape.

 FIG. 3 schematically illustrates the structure of the enclosure 10 as seen along line 2-2 of FIG. 2, but detailing the "half-square" shape established by the side walls 26. In FIG. 3, the shape of the “half-square” cross section for the outer walls 26 begins with a cylinder 60. A cylinder “half-square” shape is given by taking four sectors 62, each parallel to the central axis 22. Each sector thus defines a wall 64 with a flat panel connected to the remaining adjacent parts 63 of the cylinder 60. A curved plate 66, having sufficient flexural strength, is fixed to the interior surface of each wall 64 with a flat panel. In this way, the curved plates 66 provide sufficient flexural strength to the otherwise flat walls 64. FIG. 3 also shows that the curved plates 66 are fixed to the outside of the cylinder 30 at support points 70 to help support the assembly formed by the cylinders 20 and 30. In addition, support arms 72 connect the outer surface of the cylinder 20 and the inner surface of the cylinder 30 to further strengthen the support and the rigidity of the structure.

Thus, the enclosure 10 has an acoustic transmission line returning by coupling the sound wave 18 to the chamber filled with air outside the enclosure 10. The following formula makes it possible to calculate the length (L) of the acoustic transmission line as a function of the length (A) of the sound wave N
L =
4
The minimum length requested for the acoustic transmission line of the speaker 10 must be calculated at the lowest frequency of the sounds to be reproduced by the speaker 12. For example, to widen the smooth response to a wave sound 18 of 30 Hz, the minimum length of the transmission line is L = 2,886 meters.

As can be appreciated, the concentric multiple-cylinder architecture of the enclosure 10 allows the length of the acoustic transmission line to be modified in a simple manner by simply varying the length of the various cylindrical structures.

 In addition to the length, the acoustic transmission line must also have, along each part of its travel, a cross-sectional area equal to that of the sound wave which it carries, that is to say substantially equal to the cross-sectional area of the speaker 12.

Speaker manufacturers usually provide, as a specification, the usable area of displacement produced by a given speaker. By appropriately choosing the radius of each cylindrical structure, a cross section is obtained having a uniform area over the entire length of the acoustic transmission line.

The area of the interior cross section of the central cylinder 20 corresponds to the area of displacement of the loudspeaker 12, designated here by Al. The following formula makes it possible to calculate the radius for the interior surface of the central cylinder 20 with respect to l central axis 22

 The thickness of the wall of the central cylinder 20, that is to say the difference between the radii of the inner surface and the outer surface relative to the central axis 22, takes account of the material used and the high resistance to flexion desired. This thickness varies according to design criteria and manufacturing cost but, in accordance with the invention, it is generally minimized due to the high flexural strength naturally presented by a cylindrical body such as the central cylinder 20. More particularly , the high flexural strength of the cylindrical structure as used in the speaker enclosure with a cylindrical, concentric and multiple transmission line according to the invention, makes it possible to use very thin cylinder walls.

 The diameter of a woofer can range from fifteen to thirty centimeters. For such speakers, the thickness of the wall of the cylinders 20 and 30 can drop to 0.5 to 1.5 mm in the case of a material of the aluminum type. Such a structure, although extremely thin, is robust enough to withstand deformation due to the vibration induced by the impact of the acoustic pressure in this structure. A similar result, i.e. a very small wall thickness, can be obtained using plastics.

 The use of aluminum and plastics to form a multiple and concentric sound transmission line simplifies manufacturing compared to the use of another, conventional material, such as wood. In addition, aluminum and plastics can be recycled, which provides favorable characteristics for the protection of the environment and nature. For example, an aluminum cylinder can be compared to a duct formed from a wooden panel.

For an internal diameter of 300 mm and a wall thickness of 0.5 mm, an aluminum cylinder deforms radially by approximately 0.14 mm in response to an internal pressure of 200 kPa. A duct formed of a wood panel having the same interior cross section, for example a square interior section of 266 mm side, requires a wall thickness of approximately 12 mm to support a displacement of 0.18 mm in response to an internal pressure of 200 kPa. Therefore, with approximately the same resistance to deformation under the effect of air pressure, the cylindrical structure allows for significantly thinner walls, i.e. a duct formed of a wooden panel has walls about 24 times thicker than those of the aluminum cylinder.

The outside radius of the cylinder 20, i.e. the inside radius increased by the wall thickness of the cylinder 20, can be designated by R1 and the inside radius of the second cylinder 30 can be calculated as follows

The thickness of the wall of the second cylinder 30 establishes a desired resistance to bending, taking into account the material used. The outside radius of the second cylinder 30 can be designated by R2 and the inside radius of the next concentric cylinder can be calculated as follows

 Any number of additional cylinders having an appropriate internal radius relative to the external radius of the preceding cylinder may be added to maintain, in the space between them, a cross-sectional area equal to the useful area of the surface of the top speaker 12. An appropriate number of cylinders and appropriate lengths of cylinders establish an acoustic transmission line of a desired length within a speaker enclosure.

 The use of a cap 32 and a flange 24 to direct a sound wave from one cylinder to a next cylinder must maintain the desired area of the cross section. Consequently, the specific dimensions and shape of these structures, for example the cap 32 and the flange 24, can be calculated to maintain such an area in cross section in the path traveled by the sound wave 18.

 FIG. 4 schematically illustrates a second embodiment of the present invention comprising concentric cylinders connected together to form an enclosure 100 of loudspeakers with acoustic transmission line. In FIG. 4, the enclosure 100 comprises a tank 114 supported at a distance from a floor 142 by means of feet 140.

The tank 114 serves as a mounting site for a speaker 112.

A sound wave 116 moving forward leaves the speaker 112 and passes between the tank 114 and the ground 142.

The enclosure 100 has a central upper opening 127. The speaker 112 produces a sound wave 118 moving towards the rear. The sound wave 118 travels inside the enclosure 100 and ends up leaving the enclosure 100 at the upper central opening 127, that is to say that it travels from the outer cylinders to a central cylinder defining the opening 127.

 A central cylinder 120 rests directly above the loudspeaker 112 and defines, at its upper end, the upper central opening 127. A second cylinder 122 with a radius greater than that of the cylinder 120 rests concentrically with the cylinder 120. A third cylinder 124 with a radius greater than that of the cylinder 122 rests concentrically with the cylinders 120 and 122. A fourth cylinder 126 with a diameter greater than that of the cylinder 124 rests concentrically with the cylinders 120, 122 and 124. An outer cylinder 128 forming a wall lateral, with a radius greater than that of the cylinder 126, rests concentrically with respect to the cylinder 126, 124, 122 and 120. The cylinder 128 forming the external lateral wall is directly connected to, and is supported directly at its lower edges by, the tank 114. The assembly formed by the concentric cylinders 120, 122, 124, 126 and 128 is maintained in a fixed arrangement by means of elements 130 of sup port and link, which is best seen in Figure 5.

 The interior of the cylinder 120 defines a space 121. The interior of the cylinder 122, outside of the cylinder 120, defines a space 123. The interior of the cylinder 124, outside of the cylinder 122, defines a space 125 The interior of the cylinder 128, outside the cylinder 126, defines the cylinder 128, outside the cylinder 126, defines a space 129. The cylinders 120, 124 and 128 rise above the cylinders 122 and 126.

 An annular cap 150 extends between the upper edges of the cylinders 126 and 128. Similarly, an annular cap 152 extends between the upper edges of the cylinders 120 and 124. As explained more fully below, the cap 150 directs a sound wave 118 coming from space 127 in space 129. Similarly, the cap 152 directs the sound wave 118 from space 125 into space 123.

A cap 154, comprising a convex central part and a concave peripheral part, closes the lower end of the cylinder 122. The concave-convex contour of the inner surface of the cap 154 directs the sound wave 118 from the space 123 into the space 121. As can be appreciated, the cap 154 must be placed at a sufficient distance from the cylinder 120 to maintain a desired cross-sectional area for the path traveled by the sound wave 118. An annular cap 156 s extends between the lower edges of the cylinders 126 and 122, thus directing the sound wave 118 from the space 127 in the cap 154 which directs it into the space 121 of the cylinder 120.

The sound wave 118 then rises and leaves the enclosure 100 through the upper central opening 127.

 As described above, the length of the transmission line established in the enclosure 100 can be adjusted to suit a particular wavelength, by acting on the overall dimension in length of the cylinders 120, 122, 124, 126 and 128, in association with the spacing with respect to the caps 150, 152, 154 and 156. The relative spacing between the caps 150, 152, 154 and 156 and the associated cylinders 120, 122, 124, 126 and 128 must take account maintaining a desired cross-sectional area along the acoustic transmission line formed by the enclosure 100. In addition, the relative dimension, that is to say the radius, of the cylinders 120, 122, 124, 126 and 128 is calculated as described above to maintain a cross-sectional area of equal amplitude for spaces 121, 123, 125, 127 and 129.

 FIG. 6 illustrates in more detail the structure of a loudspeaker enclosure according to the embodiment of FIGS. 1 to 3. In FIG. 6, a loudspeaker enclosure 10 ′ is illustrated in cross section, similar to the view in cross section of Figure 3. The enclosure 10 'receives a 20 cm speaker (not shown in Figure 6). The enclosure 10 'takes the form of a "half-square" described above. The width, in both vertical and horizontal directions of the view of Figure 6, is 280 mm. The height of the enclosure 10 ′ is imposed by the length chosen for the transmission line, that is to say that it is a function of a specific wavelength optimally coupled to the mass of surrounding air. The outer side walls 26, having the "half-square" cross-sectional shape described above, have a thickness of 1.5 mm. The internal wall structures, i.e. cylinder 20 and cylinder 30, are only 0.5mm thick. The cylinder 20 has a radius of 87.5 mm and the cylinder 30 has a radius of 125.00 mm. The cylinder 60, forming the base of the "half-square" shape of the side walls 26, has a radius of 165.00 mm. The curved plates 66 have a thickness of 1.5 mm and their curved part extends along an arc 150 of 51.39 degrees, with a radius of 116.00 mm. The rounded corners of the enclosure 10 ', that is to say the remaining parts of the cylinder 60, extend over an arc 152 of 26.09 degrees.

 The enclosure 10 ′ also includes support arms 72 extending radially outward at 4 locations distributed at equal angles to each other. More particularly, a support arm 72a connects the cylinder 20 to the cylinder 30, while a support arm 72b connects the cylinder 30 and one of the rounded corners of the side wall 26. Similarly, support arms 72c and 72d extend radially outwards in the direction of a rounded angle close to the enclosure 10 ′, the support arm 72c connecting the cylinder 20 and the cylinder 30 and the support arm 72d connecting the cylinder 30 and the side wall 26. Support arms 72e and 72f are placed similarly with respect to a third of the rounded corners of the enclosure 10 '. Finally, support arms 72g and 72h extend radially outward in a similar manner towards the last of the rounded corners of the enclosure 10 '.

 Therefore, a speaker enclosure with an improved acoustic transmission line has been shown and described. The loudspeaker enclosure according to the invention uses the natural rigidity and the high natural resistance to bending of cylindrical structures to form, by concentrically arranging such structures, an acoustic transmission line with a length and an area in cross section chosen. The multiple concentric cylindrical architecture allows a simple design strategy to obtain a desired length and specification, in cross-section, and does not impose any limit on the position of the speaker, the number of cylinders requested or the orientation in which sounds are made. The invention provides an enclosure for speakers of low weight, saving space, using recyclable materials. The backward movement of the sound wave can be set to go from the outer cylinder to the central cylinder or from the central cylinder to the outer cylinder.

 Although cylinders are illustrated here, other sleeve-like structures have high natural flexural strength and can be used in place of cylinders for the more ideal sleeve structure, i.e., a sleeve cylindrical in shape. For example, the outer walls 126 of the embodiment of Figure 1 form a sleeve structure having, in cross section, a "half-square" shape.

 The speaker enclosures shown here have the ability to produce extremely equal, low frequency sound waves. Conventional speakers usually cannot reproduce such equal low-frequency sound waves. Consequently, the use of such loudspeaker enclosures with multiple concentric cylinders introduces a new range of sound reproduction, that is, an ability to produce very low frequencies very evenly. The production of these equal low frequency sound waves is a desirable feature for the attentive listener, but such very equal low frequency sound waves can produce a resonance effect in a listening room. In other words, the speaker enclosures illustrated here faithfully reproduce sound waves at frequencies low enough to cause cavity resonance in a typical listening room.

 FIG. 7 represents a simplified functional diagram of a system 210 for reproducing sounds placed in a given listening chamber or cavity 212. As can be appreciated, the chamber or cavity 212 has a given cavity resonance comprising a frequency fundamental and associated harmonic frequencies. System 210 includes a sound source 214 having right and left audio channels 216a and 216b leading to a buffer amplifier 218. Buffer amplifier 218 amplifies the signals of audio channels 216 and presents the signals 216 to a series of filters narrow band notch 220, individually designated herein as filters 220a, 220b and 220c. The notch filters 220 are, for example, variable or tunable notch filters each having, for characteristics, a narrow frequency band and a high rejection ratio. For example, at around 30 Hz, each filter 220 has a "notch" or filter band with a width of 1 to 1.5 Hz. Each filter 220 includes three variable adjustment resistors synchronized to form a notch filter variable that can be tuned over a very narrow frequency band.

 Each of the notch filters 220 receives channels 216a and 216b, filters a wavelength corresponding to a low and very narrow frequency which they contain, and delivers the output signals of channels 216a and 216b to a following component. The notch filter 220a receives the signals from channels 216a and 216b from the buffer amplifier 218 and transmits the signals from channels 216a and 216b to the notch filter 220b. The notch filter 220b transmits the signals from channels 216a and 216b to the notch filter 220c, and the notch filter 220c transmits the signals from channels 216a and 216b to an equalizer filter 230. Each of the notch filters 220a 220c includes a corresponding command 222a-222c, respectively, imposing the filtered wavelength of the signals of channels 216a and 216b.

 The equalizer filter 230 is a conventional equalizer filter producing a modification in several relatively wide frequency bands. The equalizer filter 230 transmits the signals from channels 216a and 216b to an output driver 232. The output driver 232 applies the signal from channel 216a to a loudspeaker enclosure 10a, illustrated diagrammatically on the Figure 7, and the signal from channel 216b to a speaker enclosure 10b, which is also shown diagrammatically. The speakers 10a and 10b correspond to the speakers of speakers with an acoustic transmission line, with multiple concentric cylinders described above. Each speaker enclosure 10a and 10b includes a speaker producing a sound or acoustic wave 240a and 240b, respectively, inside the cavity 212. As described above, the speaker enclosures 10a and 10b faithfully reproduce very low frequency sound waves, low enough to produce a resonance effect inside the cavity 212. The output driver 232 comprises several controls 232a which correspond to conventional audio control parameters, for example the tone, balance and volume.

 The sound waves 240 enter the cavity 212 and produce the desired presentation of the sounds conforming to the source 214 of sounds. However, due to the resonance of the cavity 212, certain parts of the sound waves 240 tend to accumulate and have a perception of a volume relatively greater than that of a desired presentation of the audio source 214. In particular, certain very low frequency sound waves tend to accumulate inside the cavity 212.

 System 210 therefore generally works in the manner of a conventional sound reproduction system, but it includes a series of notch filters with a very narrow frequency band, so that selected narrow bands of low frequency, passing in channels 216a and 216b, are eliminated by manipulation of the controls 222.

 According to the present invention, the system 210 further comprises a sine wave signal generator 250 from 20 Hz to 20 kHz, producing an input sine wave 252 for the output driver 232. The signal generator 250 includes a command 250a imposing the frequency of the signal 252. A frequency display 254 coupled to the signal generator 250 provides a visual indication of the frequency of the signal 252. Thus, by manipulating the command 250a, a user of the system 250 injects into the cavity 212 sound waves 240 having a given frequency.

 The system 210 further comprises a transducer or microphone 260 coupled to an amplifier 262. The amplifier 262 drives a display 264 of acoustic energy. By monitoring the acoustic energy display 264 while operating the control 250a, the user determines the resonance of the cavity 212. More particularly, while the user is moving the control 250a, a range of frequencies of the sound wave 240 appears in the cavity 212.

When a frequency coinciding with the fundamental frequency of the cavity 212 enters the cavity 212, the acoustic energy present in the chamber 212 is of a relatively greater amplitude. Consequently, at this fundamental frequency, the acoustic energy display 264 reaches a maximum value. In this way, the user of the system 210 determines the actual fundamental cavity resonance for the chamber 212.

 Once the control 250a has been set to develop sound waves 240 at the fundamental frequency, the user observes the frequency display 254. The frequency display 254 then represents the fundamental frequency for the cavity 212. The user then adjusts one of the notch filters 220, that is to say that he adjusts a command 222, to make it correspond to the value presented by the frequency display 254. As can be appreciated, the calibration performed on control 250 and controls 222 can be coordinated in a manner allowing the user to adapt a setting of control 222 to a setting of control 250a. Alternatively, the controls 222 can be calibrated against the information presented by the frequency display 254. In all cases, one of the notch filters 220 is adjusted to a given frequency band adjusted on the basis of the frequency of the sine wave injected into the cavity 212 and establishing therein an acoustic energy of relatively greater amplitude. big. In this way, the user eliminates a narrow frequency band coming from the audio source 214.

 Other frequency bands, namely harmonic frequencies, can also introduce undesirable linearity faults in the presentation of sounds. These harmonic frequencies can also be detected by an additional operation of the command 250a and an observation of the acoustic energy display 264. If the user observes the additional peak frequencies, that is, the peak values indicated by the acoustic energy display 264, several notch filters 220 are used to filter corresponding narrow bands of frequency. As can be appreciated, more or less than three notch filters 220 can be used in a given embodiment of the invention.

 Frequency suppression, that is, filtering by band-cut filters 220, is usually done below 250 Hz. At frequencies above 250 Hz, the reverberation interference band is much wider and the equalizer filter 230 can be used to attenuate all of these wideband interference frequencies. However, the conventional equalizer filter cannot adequately eliminate the cavity resonance due to the narrow, extremely low frequency bands associated with the resonance of the cavity.

 FIG. 8 illustrates a second embodiment of the invention, namely a digital system 310 providing a more automated method of tuning a resonance of a given cavity 312. In FIG. 8, a digital sound source or digital audio source 314 produces a digital audio signal 316, comprising signals of right and left stereophonic channels, and applies them to a block 318 for conditioning digital signals. The digital signal conditioning block 318 drives a digital filter 320 with variable parameters. As can be appreciated, the digital filter reacts to parameters applied to establish the functions of one or more selected frequency filters. The output signal of the digital filter 320 drives an analog and drive digital converter circuit 332. The drive circuit 332 produces an amplified analog version of the signal 316, designated 316a, which it applies to output transducers 340, c ' that is, to concentric multi-cylinder speaker enclosures, as previously described, which receive signals 316a from the right and left channels.

 As described so far, the system 310 generally works in conventional digital sound reproduction, but it includes a digital filter 320 with variable parameters in series between the block 318 for conditioning digital signals and the driver circuit 332 .

 A parameter adjustment and control block 350 controls the operation of the digital filter 320.

The parameter setting control block 350 receives a frequency signal 352 and a sound level signal 354. The frequency signal 352 comes from a sine wave generator 356 and arrives via a frequency counter and a display block 358. In addition, the output signal from the sine wave generator 356 is applied to the digital signal conditioning block 318 as another audio source. In this way, the system 310 injects a sound wave into the cavity 312, at a chosen frequency.

 An input transducer, that is to say a microphone, 360 picks up the sound waves inside the cavity 312 and drives an amplifier 362. The amplifier 362 drives a block 364 of sound level. The sound level block 364 delivers the acoustic energy signal 354 to the parameter adjustment and control block 350. The microphone 360 can be placed at a chosen point, namely an optimal listening point, inside the chamber 312 to establish ideal listening conditions at this chosen listening point.

 The system 310 is initialized with respect to a given cavity resonance, i.e. a given set of conditions for the chamber 312, by first injecting a sine wave signal with a frequency varying slowly in the cavity 312 The transducer 360 receives the sound wave and produces, via the amplifier 362, a representation of this wave which it transmits to the sound level block 364. The parameter setting and control block 350 controls the signal 354, representing the amplitude of the acoustic energy detected inside the chamber 312, and detects a peak amplitude in the signal 354.

 The parameter setting and control block 350 associates a given frequency present in a frequency signal 352 with an indication of peak amplitude in the signal 354, thus detecting the resonance of the cavity for the chamber 312. The block 350 of parameter and control setting then establishes within the digital filter 320 a frequency parameter corresponding to the detected cavity resonance of the chamber 312. This process can be repeated to detect other sound levels having peak amplitudes read in room 312 and associated frequency values. In this way, one or more frequency parameters are applied to the digital filter 320 to eliminate from the signal 316 narrow frequency bands associated with the resonance of the cavity 312.

 After initialization, the system 310 operates the digital audio source 314 in a normal manner, but eliminates, at the digital filter 320 with variable parameters, the detected narrow frequency bands associated with the cavity resonance of the chamber 312. The audio reproduction system 310 is thus tuned to a specific cavity resonance for the chamber 312. As can be appreciated, this tuning can also be done manually by a user who follows the variations of condition inside the cavity 312.

 We have therefore represented and described an improved system for reproducing sounds having the possibility of agreeing on a specific cavity resonance. According to the invention, advanced speaker enclosures can produce sound waves of very low and equal frequency, including very narrow low frequency bands associated with resonance of the cavity. These frequencies are eliminated by filtering an audio signal before application to the advanced speakers. In this way, the audio signal is "pre-damped" at the frequencies corresponding to the resonant frequencies of the cavity, thus eliminating the accumulation of sounds inside the cavity as a function of the resonance of the cavity.

The attentive listener thus enjoys a more faithful, that is to say more linear, reproduction of the presentation of the sounds as provided for in the original recording.

 It goes without saying that numerous modifications can be made to the enclosure and to the system described and shown without departing from the scope of the invention.

Claims (6)

 1. Component for achieving the agreement between a sound reproduction system and a listening chamber (212) and receiving an audio signal and producing a filtered audio signal, the tuning component being characterized in that it comprises a variable frequency sound source (214) connectable to the listening room and having a frequency indicator (254); a sound input transducer (260) measuring and indicating the acoustic energy in the listening chamber; and a filter (220) receiving the audio signal and producing said filtered audio signal, the filter comprising at least one control (222a-222c) imposing a frequency band filtered by the filter and calibrated with respect to the frequency indicator.  2. Component according to claim 1, characterized in that the filter is a narrow band notch filter.  3. Component according to claim 1, characterized in that the filter is a narrow band, tunable band-cut filter.  4. Component according to claim 2, characterized in that the filter is a digital filter (320) with variable parameters.  5. Component according to claim 1, characterized in that the filter comprises several controls (222a222c) each imposing a filtered frequency band, each of the controls being calibrated with respect to the frequency indicator.  6. Component according to claim 1, characterized in that the variable frequency sound source is a sound source (250) with variable frequency sine wave.