EP0270394B1 - Method of dividing the acoustical reproduction of modulated signals in transducers mounted in parallel, and set of transducers therefor - Google Patents

Abstract

The invention relates to a method for selectively restoring a sound signal delivered by an electric source in several parallel mounted transducers, each transducer being specialized for restoring a frequency range of the spectrum of the signal. The method consists more precisely in permanently fractionating the sound signal into frequencies so as to assign each component to the transducer optimized for the corresponding frequency range, without using any additional selection circuit such as filtering circuits.

Description

The subject of the invention is a method making it possible to selectively restore a sound signal supplied by an electrical source in several transducers mounted in parallel, each transducer being specialized for the reproduction of a frequency interval of the spectrum of the signal. The method consists more precisely in frequency splitting at any time the sound signal to assign each component to the transducer optimized for the corresponding frequency interval, without using an additional selection circuit such as filtering circuits or the like.

The invention also relates to all sets of associated transducers allowing the implementation of the method.

In terms of the reproduction of audible signals by electro-acoustic transducers, in particular transducers formed from at least one loudspeaker coupled to an acoustic load, for example a resonant enclosure, it is always advantageous to use several loudspeakers, specialized each for the restitution of a band of the sound spectrum. Indeed, the reproduction of an audible sound signal in the maximum range from 20 Hz to 20,000 Hz for example, implies that the loudspeaker satisfies contradictory characteristics of response time and movement of the mobile part, which vary from across the sound spectrum. Decomposing the spectrum into several bands avoids a weakening of the spectrum components at the extremes.

Generally speaking, in high fidelity systems, three parallel voices are provided corresponding to the bass, mid-range and treble.

• simplement exploiter les effets de limitation des fréquences résultant des caractéristiques intrinsèques des modes de construction particuliers des transducteurs (exemple : certains des haut-parleurs dits "boomers") ;
• l'adjonction de circuits spéciaux en amont du transducteur, tels que les circuits de filtrage, actifs ou passifs ;
• l'association aux haut-parleurs de charges acoustiques introduisant un facteur de sélectivité.

The techniques implemented so far for obtaining frequency fractionation are based on the design and production of the transducers on the one hand, and the processing of the electrical signal upstream of the transducer. The techniques consisting in:

• simply exploit the frequency limiting effects resulting from the intrinsic characteristics of the particular construction modes of the transducers (example: some of the so-called "boomers"speakers);
• the addition of special circuits upstream of the transducer, such as the filtering circuits, active or passive;
• the association with loudspeakers of acoustic loads introducing a selectivity factor.

However, these known methods include in particular three types of major drawbacks which adversely affect the performance of the electro-acoustic transducers and, consequently, the quality of reproduction of the sound signal.

First of all, the use of filter circuits leads to losses in signal power which must be compensated by an increase in amplification. This therefore harms the overall performance of the installation, and involves the use of components working at higher power, with increased quality requirements to avoid distortion. In addition, the filters themselves are a signal distortion factor, due to the impossibility of giving them a perfectly linear transfer function in their bandwidth. Filters are also subject to variations in their response characteristic depending on the ambient temperature, or even their aging.

Secondly, the implicit rules adopted so far with regard to signal restitution in high fidelity sound have led to orient the design of electro-acoustic speakers in a direction that increasingly impairs their performance.

   avec B induction dans la bobine,
   M masse dynamique totale du système mobile,
   S section du piston,
   Z_e impédance électrique de la bobine mobile,
   l longueur totale du fil constituant la bobine mobile,
   v vitesse de déplacement de la membrane,
   ρ masse volumique de la membrane,
   R_i impédance interne de la source.The efficiency of an electrodynamic loudspeaker is expressed in the form:

with B induction in the coil,
M total dynamic mass of the mobile system,
S piston section,
Z _e electrical impedance of the voice coil,
l total length of the wire constituting the voice coil,
v displacement speed of the membrane,
ρ density of the membrane,
R _i internal impedance of the source.

The interpretation of this theoretical result is always difficult, because certain conditions remain contradictory. We can say to simplify that we have so far almost unanimously made the choice to maximize the product B1, taking the reasoning that the value of this product varies in the same direction as the yield, and that its increase is reflected by a high damping favorable to a good response in transient regime.

However, the maximization of product B1 is not neutral with respect to the electrical impedance Z _e of the voice coil, which in turn adversely affects the yield. This impedance is indeed related to the length of the wire 1.

In addition, particularly in the low-frequency range, the conditions of use of the loudspeakers lead to the provision that for certain frequencies the mobile assembly of the loudspeaker moves over a relatively large distance, which increases the length of the voice coil, and therefore indirectly its impedance Z _e .

The third type of drawbacks inherent in existing systems relates to the response curve of the "speaker + speaker" assemblies currently produced. The typical shape of this response curve is represented in FIG. 1, curve A. It can be seen that from 50 Hz the level of frequency reproduction is naturally damaged before stabilizing approximately above 100 Hz. Now, the correction means used to linearize this response curve generally consist in playing on the characteristics of the vent or other, to strongly dampen the parasitic acoustic phenomena of the enclosure, for example by stuffing of its internal walls at the using a "choking" material (glass wool), or even playing on the parameters of the loudspeaker (notably QTS and VAS according to the methods of Thiele and Small). However, it can be seen from the response curve B of the transducer obtained that these known techniques lower, at least for low frequencies, the level of sound power restored by the transducer: this attenuation reaches more than 5 dB, which is considerable.

The linearization of the response curve of the loudspeaker, according to these techniques, therefore leads to greatly reducing the efficiency of the loudspeaker and enclosure assembly, with the corresponding drawbacks already mentioned above.

Already known in particular a "loudspeaker system for high quality sound reproduction" as described in German patent DE-35 06 139 issued on June 5, 1986 to Mr. LUDENDORFF. This document describes an assembly with several speakers mounted on the same very narrow flat cabinet, presented as avoiding the use of an enclosure and the drawbacks which result therefrom, but with the same advantages. The loudspeakers, adapted to operate in different frequency bands belonging to the audible range, are each associated in series with a capacitor for damping the mechanical vibrations of the membrane; being mounted on the same flat cabinet of small width, they operate practically in dipoles; all assemblies consisting of a loudspeaker and a capacitor are connected in parallel; thus, each loudspeaker is operational in the frequency interval for which it has the best response, the different intervals being juxtaposed; however, as is known, the operation of the dipole loudspeakers and the damping are particularly disadvantageous for the performance.

A system is also known, described in the patent in the United States of America No. 3,845,246, in the name of Mr. OUVRIER, in which a double acoustic enclosure consists of two closed enclosures associated in order to reduce the distortion over any the extent of the distortion / frequency curve; more precisely, the two loudspeakers are arranged in two contiguous speakers which resonate respectively at 30 and 40 Hz, instead of being arranged in a single enclosure having a lower resonant frequency, the distortion being supposed to be lower, and the high -speakers are connected either in parallel or in series (cf. col. 1, lines 46 and 47) without interposition of filters; as, at the resonant frequency, the impedance of a loudspeaker mounted in a closed enclosure passes through a maximum, the loudspeakers in parallel or in series without interposition of filters simultaneously emit at a substantially constant sound level over the entire frequency range, but the efficiency remains low while the displacements of the diaphragms of the loudspeakers maintain a large amplitude, which prevents applying to the coil a strong electrical power and thus obtain a high acoustic power.

There is also known a system of loudspeakers as described in French patent application 77 06 227 filed on March 3, 1977 in the name of the MERCURIALE SPECIFIQUE ACOUSTIQUE Company. The purpose of the system described in this prior document is to regularize the response curve of a loudspeaker mounted in an acoustic enclosure, in particular by means of mechanical means for decompressing the diaphragm of the loudspeaker combined with an electric resonator. Here again, the proposed configuration consists in damping the response of the transducer at the level of the resonant frequency, and therefore in reducing its efficiency.

The objective of the present invention is to overcome these various drawbacks of existing devices.

More specifically, a first object of the invention is to provide a method of splitting an acoustic signal between several transducers, without any circuit or any additional load, and therefore at a reduced cost and increased reliability.

A second object of the invention is to provide such a method which makes it possible to optimize the use of each transducer by making it restore only the components of the sound signal located within a given frequency range. where it dissipates minimal power.

This optimal operating zone also corresponds in particular to a minimum displacement of the mobile assembly of each loudspeaker, and in a relatively homogeneous manner over its entire working frequency range (FIG. 3).

A complementary object of the invention is to provide a method for choosing the different transducers mounted in parallel with a given set of sound signal reproduction, so that each of them actually works optimally.

These objectives and others which will appear later are achieved with the help of a method of sound reproduction of electrical signals by means of electro-acoustic transducers intended to be connected to the output of an electrical source so as to each work selectively within a given frequency interval, method characterized in that loudspeakers are selected speakers having a resonance for a different frequency, we delimit, on each side of the respective frequencies corresponding to this resonance, frequency intervals juxtaposed so as to cover the total range of frequencies of the sound signal to be reproduced, we associate each high -speaker to an acoustic enclosure which has been constituted in the form of a Helmholtz enclosure resonating at substantially the same frequency as said speaker, each speaker is mounted in its enclosure and the assembly is adjusted so forming a transducer having a working frequency range in which the transducer presents te a minimum impedance, and juxtaposed at the working frequency intervals of the other transducers, and the transducers are connected in parallel to the electrical source without interposition of filter (s) for selection or additional correction (s).

The invention also relates to a set of electro-acoustic transducers for the sound reproduction of electrical signals, comprising several electro-acoustic transducers intended to be connected at the output of an electrical source so as to each work selectively in a given frequency interval, and each comprising a loudspeaker mounted in an enclosure, set of transducers characterized in that the transducers each consist of a loudspeaker having a resonance for a different frequency, and of an acoustic enclosure of the Helmholtz type tuned substantially to frequency speaker resonance, the speakers are mounted in their enclosure and the transducers thus formed each have a different working frequency interval in which they have a minimum impedance while the working frequency intervals of all the transducers are juxtaposed so as to cover the total frequency range of the signals to be restored, and the transducers are mounted in parallel on the electrical source without the interposition of filter (s) for selection or additional correction (s).

The invention also relates, more particularly advantageously, to a method of sound reproduction of electrical signals of frequencies below about 150 Hz by means of electro-acoustic transducers intended to be connected at the output of an electrical source so as to each work selectively in a given frequency interval, method characterized in that speakers having a resonance for a different frequency are selected, juxtaposed frequency intervals are delimited on either side corresponding to this resonance so as to cover the total range of the sound signal to be reproduced, each loudspeaker is associated with an acoustic enclosure which has been constituted in the form of a Helmholtz enclosure resonating at substantially the same frequency as said speaker, we mount each speaker in its enclosure and adjust the assembly so as to form a transducer having a working frequency interval in which the transducer has a minimum kinetic impedance, and being juxtaposed with the working frequency intervals of the other transducers, and the transducers are connected in parallel to the electrical source without interposition of additional selection or correction filter (s).

Similarly, it relates to a set of electro-acoustic transducers for the sound reproduction of electrical signals of frequency less than approximately 150 Hz, comprising several electro-acoustic transducers intended to be connected at the output of an electrical source so as to each work selectively in a given frequency interval, and each comprising a loudspeaker mounted in an enclosure, set of transducers characterized in that the transducers each consist of a loudspeaker having a resonance for a different frequency, and of an enclosure Helmholtz type acoustic tuned substantially to the resonant frequency of the loudspeaker, the loudspeakers are mounted in their enclosure and the transducers thus formed each have a different working frequency range in which they have a minimum kinetic impedance while the working frequency intervals of all l he transducers are juxtaposed so as to cover the total frequency range of the signals to be restored, and the transducers are mounted in parallel on the electrical source without the interposition of filter (s) for selection or additional correction (s).

Advantageously, the loudspeaker and the resonant enclosure are chosen for each transducer so that the working frequency interval assigned to this transducer corresponds to a zone of minimum impedance for this transducer relative to the impedance offered. by the other transducers over the same interval, the complete set of transducers connected in parallel to the source consisting of at least two transducers whose respective zones of minimum impedance are juxtaposed in frequency.

• la figure 1 représente la courbe de réponse typique d'une enceinte résonnante munie d'un haut-parleur, sans correction (courbe A) et avec correction par techniques connues (courbe B);
• la figure 2 représente la variation de l'impédance cinétique d'un transducteur (HP + enceinte) pour illustrer le phénomène de surcouplage entre le HP et l'enceinte (zone A) exploité par l'invention,
• la figure 3 illustre le déplacement de l'équipage mobile du haut-parleur monté dans son enceinte en fonction de la fréquence, en correspondance avec la figure 2;
• la figure 4 schématise un jeu de deux transducteurs montés en parallèle, pour la reproduction de la voie basse d'un signal sonore;
• la figure 5 illustre le choix des différents transducteurs composant un jeu complet de restitution d'un spectre sonore suivant l'invention, en fonction de leurs zones optimales de fonctionnement respectives;
• la figure 6 illustre les niveaux de restitution du signal sonore en sortie de chacun des transducteurs d'un jeu complet de transducteurs suivant l'invention.

Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment of the invention given without limitation and the accompanying drawings in which:

• FIG. 1 represents the typical response curve of a resonant enclosure fitted with a loudspeaker, without correction (curve A) and with correction by known techniques (curve B);
• FIG. 2 represents the variation of the kinetic impedance of a transducer (HP + enclosure) to illustrate the phenomenon of over-coupling between the HP and the enclosure (zone A) exploited by the invention,
• FIG. 3 illustrates the movement of the mobile assembly of the loudspeaker mounted in its enclosure as a function of the frequency, in correspondence with FIG. 2;
• Figure 4 shows schematically a set of two transducers mounted in parallel, for the reproduction of the low channel of an audible signal;
• FIG. 5 illustrates the choice of the different transducers making up a complete set of restitution of a sound spectrum according to the invention, as a function of their respective optimal operating zones;
• FIG. 6 illustrates the levels of restitution of the sound signal at the output of each of the transducers of a complete set of transducers according to the invention.

The principle of fractionation of the invention is based on the new idea of specializing the operation of each transducer in a limited range of the frequency spectrum, for which on the one hand the fidelity of sound reproduction is the highest in the transducer considered, and on the other hand, the frequencies are routed automatically to each transducer.

• tout d'abord, chaque transducteur doit présenter une zone d'impédance minimale sur l'étendue B du spectre total du signal sonore à restituer.

This process is achieved when two conditions are met:

• first of all, each transducer must have a minimum impedance zone over the extent B of the total spectrum of the sound signal to be reproduced.

est l'impédance cinétique

em. Cette impédance cinétique est liée au déplacement de l'équipage mobile du haut-parleur monté dans son enceinte.In low frequencies, the significant component of impedance

is the kinetic impedance

em. This kinetic impedance is linked to the movement of the moving part of the loudspeaker mounted in its enclosure.

em, (figure 2) pour une gamme de fréquence dans laquelle le déplacement de l'équipage mobile du HP monté dans son enceinte est minimal (figure 3). Du fait de la prépondérance de

em, la zone d'impédance

minimale du transducteur correspond approximativement à la zone A de la figure 2. Selon l'invention cette zone A sera choisie pour la définition de la gamme de fréquence affectée au transducteur correspondant.The invention consists in choosing the loudspeaker and its enclosure so as to cause a strong acoustic over-coupling between the loudspeaker and the enclosure. This is notably achieved by coupling a loudspeaker and an enclosure of identical or similar resonant frequencies. We then observe an area A of strong decrease in kinetic impedance

em, (Figure 2) for a frequency range in which the displacement of the moving part of the HP mounted in its enclosure is minimal (Figure 3). Due to the preponderance of

em, the impedance zone

minimum of the transducer corresponds approximately to zone A of FIG. 2. According to the invention, this zone A will be chosen for the definition of the frequency range assigned to the corresponding transducer.

em. Mais il est clair que tout moyen permettant d'obtenir une zone d'impédance

minimale autour d'une fréquence quelconque reste dans le cadre de l'invention. Cette condition ne s'accompagne en outre d'aucune autre exigence en ce qui concerne la régularité de la courbe de réponse du transducteur (haut-parleur + enceinte) pourvu que la zone A d'impédance minimale se distingue clairement.

• la seconde condition est que les différents transducteurs connectés en parallèle en sortie de la source électrique de modulation présentent des zones A d'impédance minimale qui soient compatibles. Cette condition est illustrée en figure 5, où l'on a représenté la façon dont s'effectue le fractionnement du spectre de fréquence compris entre F₁ et F₄ en trois plages de fractionnement de trois transducteurs interconnectés, en fonction de leur zone d'impédance
  
  minimale.

The method according to the invention preferably applies to low frequencies, because of the particular role played by

em. But it is clear that any means making it possible to obtain an impedance zone

minimum around any frequency remains within the scope of the invention. This condition is not accompanied in addition by any other requirement as regards the regularity of the response curve of the transducer (loudspeaker + enclosure) provided that the zone A of minimum impedance is clearly distinguished.

• the second condition is that the various transducers connected in parallel at the output of the electrical modulation source have zones A of minimum impedance which are compatible. This condition is illustrated in FIG. 5, in which the way in which the frequency spectrum between F₁ and F₄ is split into three ranges of fractionation of three interconnected transducers, as a function of their impedance zone
  
  minimal.

avec :

<P>

puissance moyenne consommée,

Re

(...) partie réelle de ...

T

période du signal

Indeed, the fact that, for each frequency component f of the signal to be restored, there is a loudspeaker whose impedance Z at this frequency is minimal compared to the other loudspeakers mounted directly in parallel, has the consequence that this component will preferably be restored in this loudspeaker. More specifically, the transducer considered will draw a preponderant fraction of current I, and therefore consume a preponderant fraction of the power P supplied by the electrical source:

with:

<P>

average power consumed,

Re

(...) real part of ...

T

signal period

• les plages F₁ -F₂, F₂-F₃; F₃-F₄ de fréquence de travail des transducteurs parallélisés T₁,₂, T₂,₃, T₃,₄ respectivement, doivent être juxtaposées pour couvrir l'ensemble du spectre F₁ - F₄ du signal sonore à restituer, sans affaiblissement préjudiciable d'une fréquence quelconque du signal;
• pour permettre la sélectivité, il faut en outre que le transducteur affecté à chaque intervalle de fréquence présente une impédance
  
  plus faible que l'impédance offerte dans cette même gamme par tous les autres transducteurs du jeu complet de transducteurs suivant l'invention.

In order for this automatic selection of frequencies to be carried out over the whole extent of the sound spectrum to be reproduced, it is necessary again that two conditions are fulfilled:

• the ranges F₁ -F₂, F₂-F₃; F₃-F₄ of the working frequency of the parallelized transducers T₁, ₂, T₂, ₃, T₃, ₄ respectively, must be juxtaposed to cover the whole spectrum F₁ - F₄ of the sound signal to be reproduced, without detrimental attenuation of any frequency of the signal;
• to allow selectivity, the transducer assigned to each frequency interval must also have an impedance
  
  weaker than the impedance offered in this same range by all the other transducers of the complete set of transducers according to the invention.

minimale.Advantageously, the frequency interval F₁ -F₂, F₂-F₃, F₃-F₄ assigned to each transducer T₁, ₂; T₂, ₃; T₃, ₄ is approximately centered around the frequency for which it has an impedance

minimal.

The invention was carried out on a prototype in which four speakers each provided with a loudspeaker were mounted directly in parallel at the output of a single modulation source.

The transducers were chosen so as to selectively reproduce the following respective frequency bands: 31 to 40 Hz; 40 to 60 Hz; 60 to 85 Hz; 85 to 120 Hz.

Each of the transducers consisted of a loudspeaker of the electrodynamic type, coupled to a resonant Helmholtz cavity, so that for each set, the resonant frequency of the loudspeaker and that of the resonant cavity are identical or similar.

The value of this resonance frequency was chosen, for each transducer, in the central zone of the frequency band which was assigned to it.

The following table shows the characteristics of each speaker and the corresponding cavity:

Of course, the number of transducers mounted in parallel does not constitute a limitation of the invention.

Advantageously, however, the invention finds a preferential application for the reproduction of low frequencies, less than 150 Hz. Indeed, it is in this frequency range that we encounter the most problems in high fidelity , particularly for the reproduction of a high power signal. In this respect, the method according to the invention which improves the efficiency of electro-acoustic systems, and specializes each transducer in a frequency range where it has a minimum impedance, provides an optimal solution to these problems.

The invention has the advantage of no longer requiring the use of additional filtering or correction devices for the frequency fractionation of the signal to be restored in the various output transducers. However, it is clear that in certain applications, the method of the invention may be used in cooperation with electrical or acoustic correction filtering devices. It is also possible to use several amplifiers power in cooperation with the same set of transducers according to the invention, the amplifiers working either over the entire frequency range of the signals to be reproduced, or selectively over only part of this range. The same amplifier can supply a single transducer or even several transducers of the set of transducers considered. In addition, the additional filtering or correction devices can be either inserted into, or constituted by, the power amplifier (s) placed directly upstream of the transducers.

The invention also has the advantage of regularizing the impedance curve of the complete transducer system, by improving the amplifier / transducer system interface. Indeed, while, in the conventional system, the charge of the amplifier can vary in a ratio of 1 to 10 for the reproduction of frequencies around the resonant frequency, the coupling of the transducers according to the invention makes it possible to limit load variations in a ratio of approximately 1 to 2 for the embodiment described.

FIG. 6 schematically illustrates the output levels of the three transducers T₁, ₂; T₂, ₃; T₃, ₄ associated, according to the frequency.

Note that the output sound levels correspond to a maximum, for each transducer, in the frequency zone which is assigned to it. In addition, the output sound levels must be defined so that they take account of the cumulative levels. More precisely, each frequency component f of the sound signal is essentially reproduced by the transducer of the corresponding frequency band. However, each transducer also restores, at a weakened level, the components of the signal whose frequencies lie outside its working interval. The total level of restitution of each component results therefore the accumulation of its restitution by all the transducers.

Consequently, in order to take account of this accumulation, the maximum output level P₁ of the transducer T₁, ₂ is greater than the output level P₂ of the transducers T₂, ₃; T₃, ₄. It is also possible to carry out an even more precise adjustment, by associating a specific and different maximum level for each transducer T₁, ₂; T₂, ₃; T₃, ₄, the differences in adjustment of each transducer being notably linked to the asymmetry of each response curve. In all cases, the objective is to adjust these maximum sound output levels of each transducer so as to obtain a cumulative level of restitution of the sound signal which is substantially constant over the entire frequency range of this signal.

• déplacement minimal de l'équipage mobile, et donc impédance cinétique faible;
• rendement maximal, car la puissance perdue est minimale du fait que chaque transducteur fonctionne en surtension sous faible impédance (proche de ce qu'on appelle sa "valeur nominale d'impédance", relevée généralement à 400 Hz ou à 1000 Hz), dans la zone de fréquence de travail;
• effet de sélectivité naturelle, par affectation automatique de chaque fréquence, pour l'essentiel, au transducteur correspondant.

In conclusion, the method according to the invention has the essential advantage of optimally using each transducer in a working frequency zone where it combines several very positive properties:

• minimum displacement of the moving assembly, and therefore low kinetic impedance;
• maximum efficiency, because the power lost is minimal because each transducer operates in overvoltage with low impedance (close to what is called its "nominal impedance value", generally recorded at 400 Hz or 1000 Hz), in the working frequency zone;
• natural selectivity effect, by automatic assignment of each frequency, essentially, to the corresponding transducer.

Claims (8)

1. Method for the acoustic restoration of electrical signals by means of electro-acoustic transducers (T_1,2;T_2,3), intended to be connected to the output end of an electrical source in such a way as to each work selectively within a given interval of frequencies (F₁-F₂;F₂-F₃), such method being characterised by:
      selecting loudspeakers having different resonant frequencies, intervals of frequencies being defined on each side of the respective resonant frequencies and being juxtaposed in such a way as to cover the entire range of frequencies (F₁-F₃) of the sound signal to be restored, each loudspeaker being associated with an acoustic chamber made in the form of a Helmholtz housing resonating at substantially the same frequency as said loudspeaker, each loudspeaker being mounted in its housing, and the assembly being adjusted in such a way as to form a transducer (T_1,2;T_2,3) having an interval of operating frequencies in which the transducer has a minimum impedance (Z), and being juxtaposed at the intervals of operating frequencies of the other transducers, and the transducers (T_1,2;T_2,3) being connected in parallel to the electrical source without the interposition of any additional selection or correction filter(s).
2. Method for the acoustic restoration of electrical signals of frequencies lower than about 150 Hz by means of electro-acoustic transducers (T_1,2;T_2,3), intended to be connected to the output end of an electrical source in such a way as to each work selectively within a given interval of frequencies (F₁-F₂;F₂-F₃), such method being characterised by selecting loudspeakers having different resonant frequencies, intervals of frequencies are defined on each side of the respective resonant frequencies and being juxtaposed in such a way as to cover the entire range (F₁-F₃) of the sound signal to be restored, each loudspeaker being associated with an acoustic chamber made in the form of a Helmholtz housing resonating at substantially the same frequency as said loudspeaker, each loudspeaker being mounted in its housing, and the assembly being adjusted in such a way as to form a transducer (T_1,2;T_2,3) having an interval of operating frequencies, in which the transducer has a minimum kinetic impedance (Z_em), and being juxtaposed at the intervals of operating frequencies of the other transducers, and the transducers (T_1,2;T_2,3) being connected in parallel to the electrical source without the interposition of any additional selection or correction filter(s).
3. Method according to any of claims 1 or 2, characterised in that the interval of operating frequencies of each transducer is centred around the frequency for which this transducer has, respectively, a minimum impedance (Z) and a minimum kinetic impedance (Z_em).
4. Method according to any of claims 1 to 3, characterised in that the transducer (T_1,2;T_2,3) assigned to each interval (F₁-F₂;F₂-F₃) has, respectively, a weaker impedance (Z) and a weaker kinetic impedance (Z _em) than the impedance offered in this same interval by all of the other transducers in the complete set of transducers connected in parallel to the electrical source.
5. Method according to any of claims 1 to 4, characterised in that the maximum sound output level of each transducer (T_1,2;T_2,3) is adjusted in such a way as to obtain a cumulated level of restoration of the sound signal which is substantially constant over the entire frequency range (F₁-F₃).
6. Set of electro-acoustic transducers for the acoustic restoration of electrical signals, comprising a plurality of electro-acoustic transducers intended to be connected to the output end of an electrical source in such a way as to each operate selectively within a given interval of frequencies (F₁-F₂;F₂-F₃), and each comprising a loudspeaker mounted in a chamber, such set of transducers being characterised in that the transducers each comprise a loudspeaker having a different resonant frequency, and an acoustic housing of the Helmholts type adapted substantially to the resonant frequency of the loudspeaker, the loudspeakers being mounted in their housings, and the transducers thus formed each have a different interval of operating frequencies in which they have a minimum impedance (Z), while the intervals of operating frequencies of all of the transducers are juxtaposed in such a way as to cover the entire range of frequencies (F₁-F₃) of the signals to be restored, and the transducers (T_1,2;T_2,3) being connected in parallel to the electrical source without the interposition of additional selection or correction filter(s).
7. Set of electro-acoustic transducers for the acoustic restoration of electrical signals of a frequency lower than about 150 Hz, comprising a plurality of electro-acoustic transducers intended to be connected to the output end of an electrical source in such a way as to each operate selectively within a given interval of frequencies (F₁-F₂;F₂-F₃), and each comprising a loudspeaker mounted in a chamber, such set of transducers being characterised in that the transducers each comprise a loudspeaker having a different resonant frequency, and an acoustic housing of the Helmholtz type adapted substantially to the resonant frequency of the loudspeaker, the loudspeakers being mounted in their housings, and the transducers thus formed each have a different interval of operating frequencies in which they have a minimum kinetic impedance (Z_em), while the intervals of operating frequencies of all of the transducers are juxtaposed in such a way as to cover the entire range of frequencies (F₁-F₃) of the signals to be restored, and the transducers (T_1,2;T_2,3) being connected in parallel to the electrical source without the interposition of additional selection or correction filter(s).
8. Set of transducers according to any of claims 6 and 7, characterised in that the loudspeaker or loudspeakers and the corresponding housing are chosen in such a way that the movable parts of the loudspeaker have a minimum displacement in the interval of frequencies which is assigned to the transducer.

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