FR2663181A1 - Method of sound reproduction and enclosure for the purpose of implementing it for the purpose of covering a large passband and of having a low rate of distortion - Google Patents

Abstract

The invention relates to a method of sound reproduction and to the enclosure for the purpose of implementing it, especially for the purpose of covering a large passband and of having a low rate of distortion. This method is characterised in that, around the natural attenuation frequency of the external loudspeaker in the low frequencies, called coupling frequency, an acoustic phase shift is automatically produced, by which the external (4) and internal (9) loudspeakers are made to operate: - in acoustic phase opposition in the low frequencies, which substantially doubles the pressure of the enclosure and, - in acoustic phase in the higher frequencies, which substantially doubles the acoustic speed of the loudspeakers.

Description

 The invention relates to a method of reproducing sounds and to the enclosure with a view to its implementation in particular with a view to covering a large bandwidth and having a low rate of distortion.

 To reproduce with great fidelity all of the audible frequencies which it is commonly accepted that the frequency spectrum is between twenty and twenty thousand hertz, an acoustic enclosure must be able to transmit an acoustic energy as constant as possible for an electrical power. given regardless of the frequency of the electrical signal to be transformed into sound waves.

 To reproduce a sound, we generally use at least one speaker.

 This loudspeaker mainly comprises a membrane actuated by a motor formed by a magnet and a coil.

 Manufacturers have long equipped these speakers with a rear housing to prevent the rear wave from neutralizing the front wave.

 Thus were born the speakers, initially, with a single speaker.

 Unfortunately, if it protected the speaker, this box also constituted a resonator disturbing the reproduction of sound at certain frequencies.

 To avoid these drawbacks, large devices were first made, for example with an exponential horn.

 These by their size, however, were not suitable for domestic use and caused significant distortions.

 An enclosure is also known (CARLISLE and PIERSON US-A-1,837,755) comprising a housing which, on its front face, has a loudspeaker and which internally houses at least one partition each delimiting in the housing two cavities communicating with each other by an orifice in which an acoustically resistive element is provided, called an event brake.

 Optionally, another orifice is also provided in the rear face of the housing and in the same way receives a vent brake.

 If this configuration comprising several resonant cavities improved the results, it was not less than this.

 Then appeared enclosures known as bass reflex, closed, resonator or symmetrically charged.

 The bass-reflex enclosure included a housing housing a single cavity but whose front face, in addition to a speaker, was provided with a vent.

 This system is very widespread because of its simplicity. I1 gives a fairly good performance but it remains of a fairly large size with a characteristic body sound, a strong deformation of the membrane due to an elongation and a large clearance where an alteration of the sounds and a strong distortion phase.

 The closed enclosure is similar to the bass reflex but without vent and especially with a smaller size.

 This system has a particularly low yield because of its high damping coefficient.

The resonator enclosure resembles the closed enclosure but has a housing which, like the CARLISLE enclosure and
PIERSON, shelters a partition delimiting two cavities and provided with a vent (FR-A-2.291.675).

 Its advantage is to eliminate the natural frequency of the enclosure by agreement of the annex cavity on this same frequency.

 This adjustment guarantees great neutrality of the timbre.

 The enclosure of the symmetrically charged enclosure (FR-A-1,587,761) also has two cavities separated by a partition but the loudspeaker is mounted on the partition and the front face presents only one vent.

 Called A.S.W (acoustic super woofer) they obtained a resounding sweetness because they were one of the rare systems capable of reproducing very low frequencies with low distortion.

 Unfortunately, these solutions have not made it possible to obtain a satisfactory result over the entire frequency range mentioned above, hence the search for a solution with speakers with two suitable loudspeakers for different frequency ranges and, therefore, allowing both to broaden the bandwidth and to reduce phase distortion.

 Is known (US-A-2,688,373) OLSON, an enclosure which comprises a box with two cavities separated by a partition and which comprises, on the one hand, on its front face, a first so-called front speaker radiating towards the outside the envelope and, on the other hand, on its partition, a second so-called internal speaker radiating in the room common to the two speakers, each speaker receiving, by electrical connections to the same source, the same signal.

 Conventionally, these speakers are suitable, one and in particular the internal speaker for the low frequencies, that is to say the frequencies between twenty hertz and about two hundred hertz, and the other and in particular the external speaker for the higher frequencies, ensuring that, for medium frequencies, these two speakers achieve an acceptable compromise.

 When thus several speakers are connected to the same electrical source, it is usual for the electrical signal to pass through a selective filter which, depending on the frequency of the signal, routes the signal to one and / or the other of the speakers without changing the phase or with a slight constant phase shift to take into account the distance between the speakers.

 If the sound reproduction of medium and high frequencies is relatively easy to obtain, it is not the same for the low frequencies.

 This is due to the fact that, depending on whether the signal to be reproduced belongs to low frequencies or to other frequencies, a loudspeaker works according to two distinct operating modes.

 When the signal to be transmitted is of medium or high frequency, a loudspeaker operates at acoustic speed, that is to say that the diaphragm of the loudspeaker must move quickly but with a movement of small amplitude.

 Therefore, the volume of air displaced by the membrane is low and this does not affect the performance of the speaker.

 If the signal to be transmitted is of low frequency, a loudspeaker operates under acoustic pressure, that is to say that the membrane moves much more slowly than in the medium or high frequencies but with a much greater range of motion.

 The membrane then acts like a piston and thereby displaces a large volume of air which plays a non-negligible role in the attenuation of the efficiency of the loudspeaker.

 The frequency at which this attenuation occurs is inversely proportional to the radiation surface of the membrane.

 Furthermore, the coupling of this air mass with the membrane is never ideal and causes a phase shift.

 Between these two frequency ranges determining different operating modes, the loudspeaker obviously has an intermediate so-called coupling range.

 In these structures with two cavities and two loudspeakers of the aforementioned enclosure, certain manufacturers have sought a technical solution which is simple to implement and which makes it possible to simplify the problems of tuning of the elements.

 For this purpose, some have attempted (FR-A-2.304.237) to arrange the partition so that the cavities are substantially equal in volume and, in addition to the internal speaker, place a vent on the internal partition making communicate between them the two cavities.

 Optionally, on the front, in parallel with the front speaker, some have even placed a second vent to communicate the corresponding cavity with the outside.

 Is also known (FR-A-2.405.608) a two-cavity enclosure whose external and internal speakers are arranged and supplied so as not only to be in acoustic phase but also so that the internal speaker receives a higher power to that received by the external speaker.

 Likewise is known (FR-A-2,338,616) an enclosure comprising at least one sound transmitter, itself made up of at least two co-axial loudspeakers, mounted across the same conduit, turned in the same direction or in opposite direction, but supplied to be always in acoustic phase.

 There is also known (FR-A-2,361,032) an enclosure with two cavities and two loudspeakers, one being arranged on the front in parallel with a vent, provided with a tube of adjustable length and section, both radiating directly outwards, the second loudspeaker being arranged on an internal partition dividing the enclosure of the enclosure into two distinct cavities and radiating towards the cavity where the first loudspeaker is located.

 Finally, there is known (FR-A-2,468,271) an enclosure provided with loudspeakers on the front, rear and lower faces and one of which, using a reversing contactor controlled by the user and depending on the timbre sought by it and the acoustics of the room, can be connected to operate either in acoustic phase, or in acoustic phase opposition with at least one of the other speakers.

 None of these known embodiments has given complete satisfaction, these speakers allowing interference phenomena inducing distortions.

 Furthermore, as indicated above, the technical characteristics of the speakers are generally different, for example, one being suitable for low frequencies, the other for higher frequencies; however, because of this, the acoustic inertia of each of them, which depends on the acoustic mass of the mobile assembly of the loudspeakers used, is generally different and this also causes phase disturbances.

 One of the results which the invention aims to obtain is a method of reproducing sound in an acoustic enclosure of the aforementioned type, the behavior of which over a wide frequency band is improved and which makes it possible to restore an electrical signal with distortion at most negligible.

To this end, the subject of the invention is a method of the aforementioned type, in particular characterized in that, around the natural attenuation frequency of the external loudspeaker in low frequencies, called the coupling frequency, an acoustic phase shift is automatically caused by which we operate the external and internal speakers
- in acoustic phase opposition at low frequencies which substantially doubles the pressure of the enclosure and,
- in the acoustic phase at higher frequencies, which significantly doubles the acoustic speed of the loudspeakers.

 It also relates to the enclosure for the implementation of this process.

The invention will be better understood with the aid of the description below given by way of nonlimiting example with regard to the attached drawing which schematically represents
- Figure 1: an acoustic enclosure,
- Figure 2: an alternative embodiment,
- Figures 3, 4: an embodiment detail,
- Figure 5: another alternative embodiment of the invention,
- Figure 6: individual frequency response of the loudspeakers of an acoustic enclosure.

 Referring to the drawing, it can be seen that the enclosure 1 comprises a box 2 which, on its front face 3, has at least one loudspeaker 4 and which houses at least one partition 5 delimiting two cavities 6, 7.

 The front face 3 which carries the so-called external or front loudspeaker 4 is furthermore provided with a vent 8 opening into a common cavity or front cavity 6.

 On the partition 5 separating the two cavities, at least one second speaker 9 is mounted.

 Each of these external 4 and internal 9 speakers can in fact obviously consist of one or more associated speakers.

 The first speaker 4 says external radiates towards the outside of the envelope while the second speaker says internal radiates in the cavity 6 common to the two speakers and said front cavity, each speaker 4, 9 receiving its electrical signals E1 , E2 by links 10, 11 to a source 12.

 Conventionally, these speakers 4, 9 are suitable for ranges of different frequencies distributed on either side of an intermediate frequency which, for example, is the frequency of natural coupling to the lowest frequencies of one of the speakers .

 For example, one and in particular the second so-called internal speaker 9 is suitable for the lowest frequencies, that is to say, for example, the frequencies between approximately twenty and two hundred hertz and the other, in particular the first speaker 4 says external, for higher frequencies.

 Depending on whether the signal they produce belongs to low frequencies or higher frequencies, these loudspeakers 4, 9 conventionally work in acoustic speed or in acoustic pressure and between these two frequency ranges determining different operating modes, the loudspeaker obviously has an intermediate connection range called coupling.

 This enclosure is also provided with means 13, by which one of the loudspeakers operates either in acoustic phase, or in acoustic phase opposition with the other loudspeaker.

 Each loudspeaker 4, 9 is constituted by a membrane 14 carried by a structure 15, commonly called a salad bowl, and which moves under the action of a motor 16 or electromagnet housed in a cylinder head 17 itself fixed to the 'rear of the structure 15, which structure has cutouts 18 by which the radiation from the rear face of the membrane 14 escapes.

 Preferably, the external 4 and internal 9 speakers have different technical characteristics, for example, one being intended to reproduce more particularly the medium and high frequencies, that is to say the frequencies at least greater than two hundred hertz and l 'other more particularly the low frequencies ie less than two hundred hertz.

 The technical characteristics of the two loudspeakers 4, 9 being different, the acoustic mass of the membrane and of the magnet of each loudspeaker is generally different and with it the equivalent acoustic inertia of each of the loudspeakers 4, 9.

According to an essential characteristic of the invention, around the natural attenuation frequency of the external speaker in low frequencies, called the coupling frequency Fc, we automatically cause an acoustic phase shift by which we operate the external speakers 4 and internal 9
- in acoustic phase opposition (arrows in solid lines) in the low frequencies F b which substantially doubles the pressure in the enclosure and,
- in the acoustic phase (arrows in broken lines) in the higher frequencies Fe, which significantly doubles the acoustic speed of the loudspeakers.

 According to another characteristic of the method of the invention, the two loudspeakers 4, 9 are tuned by controlling their equivalent acoustic mass, the latter being seen from the common cavity and defined for each loudspeaker as the ratio between the weight of the membrane divided by the radiation area of said membrane squared.

 For this purpose, the differences in equivalent acoustic mass between the chosen loudspeakers are determined and a small additional mass is added to the mobile assembly of one of them, rebalancing said masses.

 According to another characteristic of the method of the invention, in combination with mass control, the rear radiation of the external speaker is re-expanded until, despite the partial occlusion of the rear face of this speaker by the structure 15 and the magnet, this rear radiation is equivalent to its front radiation.

With a view to implementing this method, the enclosure is characterized in that it comprises means 13 which cause around the natural attenuation frequency of the external loudspeaker in low frequencies, called the frequency of coupling, an automatic acoustic phase shift of determined magnitude to operate the external 4 and internal 9 speakers
- in acoustic phase opposition (arrows in solid lines) in the low frequencies F b which substantially doubles the pressure in the enclosure and,
- in the acoustic phase (arrows in broken lines) in the higher frequencies Fe, which substantially doubles the acoustic speed of the loudspeakers.

 In addition, it comprises means 20 which tune the two loudspeakers by controlling their equivalent acoustic masses, these being seen from the common cavity and defined for each loudspeaker as the ratio between the weight of the membrane divided by the surface of radiation from said membrane squared.

 These means 20 consist for example of at least one small additional mass 20 added to the mobile assembly of one of the loudspeakers 4, 9 to rebalance their respective equivalent acoustic masses.

 Depending on the case, it will be for example a mass of the order of five to thirty grams.

 In combination with these acoustic mass control means 20, the enclosure also comprises means 21 for re-expansion of the rear radiation of the external loudspeaker until, despite the partial occlusion of the rear face of this loudspeaker by the structure 15 and the magnet, this rear radiation is equivalent to its front radiation.

 The means 13 which, around a frequency Foc, called the coupling frequency, cause a phase shift, can consist of an acoustic line, an acoustic phase-shifting circuit constituted by one or more additional cavities, an electrical circuit with passive components, a electrical circuit with active components, but preferably, they consist of means causing a progressive phase rotation between the electrical signal E1 and the electrical signal E2.

 Thanks to these means, if the frequency of the signal delivered by the source 12 is lower than the coupling frequency Fc, the loudspeakers are electrically supplied so that, when the membrane of the external loudspeaker 4 moves towards the inside of the common cavity 6, that of the internal speaker 9 also moves towards the interior of the common cavity 6 and if the frequency of the signal delivered by the source is greater than the above-mentioned coupling frequency, when the membrane of the external speaker 4 moves towards the outside of the common cavity 6, the membrane of the internal speaker 9 moves towards the inside of the common cavity.

According to the invention, the means 13 causing the progressive phase rotation consist of an electronic filter centered on the coupling frequency Fc
According to the invention, the coupling frequency F c is defined as that of the phase shift which corresponds to a start of attenuation in the low frequencies, of the amplitude of the sound produced by the loudspeaker intended more particularly for the higher frequencies.

 As a first approximation, this frequency can be defined as the ratio 0.3 x C / 4 x X x R where the parameter C represents the speed of the sound in the air, R the radius of the loudspeaker intended to reproduce the medium frequencies and T the constant Pi.

 To re-expand the rear radiation, the means 21 used consists of a deflection surface 21a which, placed around the bowl has an aerodynamic profile adapted to act as in a venturi tube.

 For example, the deflection surface 21a is obtained by milling in the thickness of the envelope on which the external speaker is fixed.

 In another advantageous embodiment, the deflection surface 21a is produced by the internal wall of a half-shell which, provided at its top with an orifice 23, comes to center on the bowl.

 In combination with this internal deflection surface 21a, the enclosure can include a rounded shape 24 on the rear of the engine.

 The enclosure may also include a vent 26 on its rear face 25 and communicating with the rear cavity 7 in order to respect the acoustic phases of the loudspeakers.

 The mathematical modeling of the enclosure leads to the observation that the vents are in acoustic series, which gives a listening impression of great ventilation of the sound message.

 By taking into account these technical particularities and the way of organizing them, we obtain a loudspeaker which has a very good performance and a substantially linear response curve whose distortion rate is less than 1% over the whole bandwidth, ie about three four times less than a conventional speaker.

 The method can be applied both for the production of loudspeakers and loudspeakers.

 Thus were obtained completely satisfactory results in the assembly of two speakers commonly called "tweeters" and coupled together by tuned cavities.

Claims (10)

 1. Method for reproducing sounds using an enclosure (1) comprising  - a housing (2) which, on its front face (3), has a loudspeaker (4) and which houses at least one partition (5) delimiting two cavities (6, 7),  - on the front face (3) which carries the speaker (4) called external or front, a vent (8) opening into a common cavity or front cavity (6),  - on the partition (5) separating two cavities, at least one second speaker (9),  - means (13), by which one of the loudspeakers operates either in acoustic phase, or in acoustic phase opposition with the other loudspeaker,  - Each of these external (4) and internal (9) loudspeakers being itself in fact made up of at least one loudspeaker constituted by a membrane (14) carried by a structure (15) commonly called a salad bowl, and which is moves under the action of a motor (16) or electromagnet housed in a cylinder head (17) itself fixed to the rear of the structure (15), which structure has cutouts (18) through which it radiates coming from the rear face of the membrane (14) escapes,  in which pregnant  - the first so-called external speaker (4) radiates out of the envelope while the second so-called internal speaker radiates into the cavity (6) common to the two speakers and called the front cavity, each speaker (4 , 9) receiving its electrical signals (E1, E2) by connections (10, 71) to a source (12),  - conventionally, these loudspeakers (4, 9) are suitable for ranges of different frequencies distributed on either side of an intermediate frequency which, for example, is the frequency of natural coupling towards the lowest frequencies of a loudspeakers, one and in particular the second loudspeaker (9) said to be internal, for the lowest frequencies and the other, in particular the first loudspeaker (4) said to be external, for the highest frequencies,  - depending on whether the signal they produce belongs to low frequencies or higher frequencies, these loudspeakers (4, 9) work conventionally in acoustic speed or in acoustic pressure and between these two frequency ranges determining different operating modes, the loudspeaker obviously has an intermediate range called coupling,  this process being CHARACTERIZED in that, around the natural attenuation frequency of the external loudspeaker in low frequencies, called coupling frequency (jib), an acoustic phase shift is automatically caused by which the external loudspeakers are operated (4 ) and internal (9)  - in acoustic phase opposition at low frequencies (Fb) which significantly doubles the pressure of the enclosure and,  - in the acoustic phase at higher frequencies (Fe) which significantly doubles the acoustic speed of the loudspeakers.  2. Method according to claim 1 characterized in that the two loudspeakers (4, 9) are tuned by controlling their equivalent acoustic mass, the latter being seen from the common cavity and defined for each loudspeaker as the ratio between the weight of the membrane divided by the radiation area of said membrane raised to the square.  3. Method according to claim 1 or 2 characterized in that one determines the differences in equivalent acoustic mass between the chosen loudspeakers and a small additional mass is added to the mobile assembly of one of them, rebalancing the said masses.  4. Method according to claim 2 or 3 characterized in that in combination with mass control, the rear radiation of the external speaker is re-expanded until, despite the partial occlusion of the rear face of this speaker by the structure (15) and the magnet, this rear radiation is equivalent to its front radiation.  5. Enclosure (1) including  - a housing (2) which, on its front face (3), has a loudspeaker (4) and which houses at least one partition (5) delimiting two cavities (6, 7),  - on the front face (3) which carries the speaker (4) called external or front, a vent (8) opening into a common cavity or front cavity (6),  - on the partition (5) separating two cavities, at least one second speaker (9),  - means (13), by which one of the loudspeakers operates either in acoustic phase, or in acoustic phase opposition with the other loudspeaker,  each of these external (4) and internal (9) loudspeakers being itself in fact made up of at least one loudspeaker constituted by a membrane (14) carried by a structure (15) commonly called a salad bowl, and which is moves under the action of a motor (16) or electromagnet housed in a cylinder head (17) itself fixed to the rear of the structure (15), which has cutouts (18) through which the radiation from from the rear face of the membrane (14) escapes,  in which pregnant  the first so-called external speaker (4) radiates towards the outside of the envelope while the second so-called internal speaker radiates into the cavity (6) common to the two speakers and called the front cavity, each speaker (4, 9) receiving its electrical signals (E1, E2) by connections (10, 11) to a source (12),  - conventionally, these loudspeakers (4, 9) are suitable for ranges of different frequencies distributed on either side of an intermediate frequency which, for example, is the frequency of natural coupling towards the lowest frequencies of a loudspeakers, one and in particular the second loudspeaker (9) said to be internal, for the lowest frequencies and the other, in particular the first loudspeaker (4) said to be external, for the highest frequencies,  - depending on whether the signal they produce belongs to low frequencies or higher frequencies, these loudspeakers (4, 9) work conventionally in acoustic speed or in acoustic pressure and between these two frequency ranges determining different operating modes, the loudspeaker obviously has a so-called intermediate coupling range,  this enclosure being characterized in that it comprises means (13) which cause around the natural attenuation frequency of the external speaker in low frequencies, called the coupling frequency (Fc), an automatic acoustic phase shift of determined magnitude to operate the external (4) and internal (9) speakers:  - in acoustic phase opposition at low frequencies (Fb) which significantly doubles the pressure of the enclosure and,  - in the acoustic phase at higher frequencies (Fe) which significantly doubles the acoustic speed of the loudspeakers.  6. Enclosure according to claim 5 characterized in that the means (13) consist of a phase shifting acoustic circuit constituted by one or more additional cavities.  7. Enclosure according to claim 5 characterized in that the means (13) causing the progressive phase rotation consist of an electronic filter centered on the coupling frequency.  8. Enclosure according to claim 5 characterized in that it comprises means (20) which tune the two loudspeakers by controlling their equivalent acoustic masses, these being seen from the common cavity and defined for each loudspeaker as the ratio between the weight of the membrane divided by the radiation area of the said squared membrane.  9. Enclosure according to claim 8 characterized in that the means (20) consist of at least one small additional mass (20) added to the mobile assembly of one (4) of the speakers (4, 9) to rebalance their respective equivalent acoustic masses.  10. Enclosure according to claim 8 or 9 characterized in that in combination with these acoustic mass control means, it also comprises means (21) for re-expansion of the rear radiation of the external loudspeaker until, despite the partial occlusion of the rear face of this loudspeaker by the structure 15 and the magnet, this rear radiation is equivalent to its front radiation.