GB2297880A

GB2297880A - Loudspeaker

Info

Publication number: GB2297880A
Application number: GB9501551A
Authority: GB
Inventors: John Ronald Watkinson; Richard James Salter
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-01-26
Filing date: 1995-01-26
Publication date: 1996-08-14
Anticipated expiration: 2015-01-26
Also published as: GB9501551D0; GB2297880B

Abstract

An additional transducer 3 is mounted behind a conventional diaphragm type transducer 1 and actively driven in such a way that the acoustic conditions on the rear of the conventional transducer diaphragm are substantially as if it were in an enclosure of infinite volume, e.g. the pressure in the front portion of the enclosure 2 remains substantially constant. The differing acoustic conditions experienced by the two transducers require that their respective drive signals will be substantially different. The driving signal for the rear transducer may be formed by feedforward processing of the signal driving the front transducer and/or feedback processing of a pressure signal. The rear transducer may be replaced by several transducers (figs 2A, 2B). Electrostatic or piezolectric transducers may be used in either front or rear position (figs 3,4), or a hydraulic ram may be used in the rear position (fig 5). A drive unit having dual independently driven diaphragms that is suitable for this application is disclosed (fig 6).

Description

COMPLETE SPECIFICATION LOUDSPEAKER We, John Ronald Watkinson and Richard James Salter, of 2, Hillside Burghfield Common, Reading RG7 3BQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention concerns an improved loudspeaker in the form of a combination of electroacoustic transducers, amplifiers and analog or digital electronic signal processing which achieves low distortion in its acoustic output, which output and quality extend to include exceptionally low frequencies.A further object of the invention is that the stated performance is obtained from a unit which is considerably more compact than hitherto possible yet which does not consume excessive power to obtain considerable sound pressure levels.

A non-restrictive example of an advantageous application of the present invention is in domestic enjoyment of pre-recorded music where previously the presence of very large permanently sited loudspeakers often intruded on other activities. A further example is in the reproduction of sound channels associated with domestic display of moving pictures in which the realistic reproduction of low frequencies is used to increase the involvement of the viewer. In such an application four or five loudspeakers are typically required in addition to the display apparatus and the compact dimensions allowed by the present invention allow a significant saving of space.

A further application of the present invention is in the audio recording industry in which it is required to have monitoring loudspeakers sited adjacent to a mixing console yet without obstructing the view of the associated studio. In outside broadcast vehicles the size and weight of apparatus are both advantageously minimised and the present invention allows monitoring loudspeakers in mobile applications to perform as well as those in fixed locations.

A further application of the present invention is in the generation of sound at high levels for structural research and testing purposes.

Sound reproduction systems are serial in nature in that any unit in an audio chain can impair the overall quality. In seeking to improve the quality of reproduction it is logical to apply effort in any area where quality is impaired the most. In considering recent developments in audio equipment, it is evident that the audible shortcomings of certain microphones, analog to digital and digital to analog convertors, digital recorders, signal processors and power amplifiers have been reduced to the point where the ideal is very nearly reached. The same cannot be said for loudspeakers in which a greater degree of compromise has hitherto been necessary.

In prior loudspeakers the compromise is usually serious because of the conflicting criteria for the reproduction of high and low frequencies. In reproducing midrange and high frequencies, particularly in stereophonic systems, it is advantageous if the transducers and their enclosures are small in order to obtain uniform dispersion of the sound over a wide angle in the horizontal plane and to render insignificant the effects of diffraction at the edges of the enclosure. In reproducing low frequencies it is necessary to consider that long wavelengths are involved, and that unless the antiphase radiation from the rear of the transducing diaphragm is separated from the front radiation substantial cancellation will take place in the far field. It has been common in the prior art to provide an enclosed volume behind the radiating diaphragm to prevent such cancellation.The air in said enclosed volume acts as a stiffness which is mechanically in parallel with the stiffness of any support provided to the said diaphragm. The combined stiffness of said support and said air volume form a mechanically resonant system in conjunction with the mass of said diaphragm. The output of a conventional loudspeaker falls at frequencies below the resonant frequency. There are a number of compromises which are then made. If a small enclosure is required, the enclosed air volume will be stiffer, raising the resonant frequency. Thus the mass of the diaphragm must be increased to compensate, lowering the efficiency. The increased stiffness also increases the force on the diaphragm which then has an increased tendency not to move as a rigid piston, but mechanically to distort and cause audible colouration.The force on the diaphragm may be reduced by making it smaller, but it then requires a longer throw or travel to displace the same volume of air and it is difficult to make a long throw transducer both linear and efficient. Accordingly if low frequency reproduction is required with low distorion and realistic sound pressure level it has in prior loudspeakers been necessary to use an enclosure of physically large volume in order to reduce the stiffness of the enclosed air. The forces on the walls of a large enclosure due to internal pressures are appreciable and so they must have substantial stiffness and mass in order to prevent colouration of the sound due to flexing of the enclosure. It is difficult to marry such a large low frequency transducer with the mid range and high frequency transducers which require a small enclosure.As a result there have hitherto been available moderately sized loudspeakers which have poor low frequency response but good stereophonic imaging, and there are large, heavy loudspeakers which have good low frequency response but poor stereophonic imaging.

The present invention represents a significant improvement over prior loudspeaker design because it is possible to reproduce sound with low distortion yet using a compact enclosure. The reduced size of the enclosure permits improved dispersion and imaging performance. A further advantage is that the small enclosure is more readily rendered sufficiently rigid to eliminate colouration due to structural flexing. This rigidity further enhances the sound quality whilst avoiding excessively heavy apparatus.

The present invention is characterised by the presence of an additional transducer which is mounted behind a conventional diaphragm type transducer and actively driven in such a way that the acoustic conditions on the rear of the conventional transducer diaphragm are substantially as if it were in an enclosure of infinite volume. The differing acoustic conditions experienced by the two transducers require that their respective drive signals will be substantially different. In the context of the present invention, a diaphragm may be taken to include any structure achieving a similar purpose such a cone, a dome, a membrane or a ribbon.

There are known loudspeakers in which two transducers are arranged in tandem. In one such arrangement two identical transducers are driven by identical signals from the same amplifier but are mounted in opposition and connected in antiphase which arrangement does no more than to cancel asymmetries in the transfer characteristic. In other known tandem arrangements one power amplifier is arranged to feed both identical transducers by means of identical passive networks which do no more than to filter out frequencies which are to be reproduced by other transducers. In other known tandem arrangements the transducer crossover network is implemented at signal level and two power amplifiers are used, one for each transducer, but driving them with identical signals.In all such prior apparatus the substantially different acoustic loading experienced by the two transducers is not understood nor is the requirement to provide substantially different drive signals met. Consequently prior apparati with tandem transducers have not been able to reduce the size of the enclosure and such apparati are as large as conventional loudspeakers and may even be heavier because of the presence of the additional transducer.

This invention distinguishes itself from the prior art in recognizing the need to drive the transducers with significantly different signals so that the front transducer acts substantially as if it were in an infinite enclosure over the designed frequency range.

The present invention is described further, by way of example, with reference to the accompanying figures: FIGURE 1 shows an example of the present invention showing the transducers, enclosure and associated signal processes and amplification.

FIGURES 2,3,4 and 5 show further examples of applications of the present invention in which only the enclosure and the transducers have been drawn for simplicity. The amplification and processing required will be substantially as described for Figure 1.

FIGURE 6 shows a further example of an application of the present invention in which two transducers have been incorporated into a single unit. The enclosure, amplification and processing requirements are not shown.

FIGURE 1 is a view of one example of the present invention.

Figure 1 may represent the low frequency section of a multi-way loudspeaker, or it may be a sub-woofer which is intended only to reproduce low frequencies to augment a separate loudspeaker.

Alternatively a full range loudspeaker, or a mid-range or high frequency loudspeaker may be represented.

Whilst Figure 1 shows the use of moving coil type transducers the invention may also be put into practice with other known types of transducer including but not restricted to electrostatic transducers. The transducer 1, hereinafter referred to as the front transducer, is mounted with one face of its diaphragm exposed to a substantially sealed chamber 2, hereinafter referred to as the enclosure, and the other face exposed to the outside of the enclosure. Known means should be employed to substantially eliminate any flexing of the structure of the enclosure. In this example the second transducer 3, hereinafter referred to as the rear transducer, also has a diaphragm which divides the enclosure 2 into two volumes hereinafter referred to as the intermediate chamber and the rear chamber. The two transducers may differ in transduction principle.If the two transducers operate on the same principle they may differ in size or in other parameters. One or the other or both transducers may be replaced by a number of transducers.

The relative disposition of the two transducers may be other than that shown and it is not essential that the transducers are arranged on the same axis. The dimensions of the intermediate chamber between the two transducers must be sufficiently small to preclude the creation of standing waves. If this is not achieved over the whole audio frequency range, is still possible to implement a full range loudspeaker in which the rear transducer only operates up to a certain frequency limit, beyond which the front transducer operates conventionally. In this case the advantages of the present invention will only be obtained at frequencies below said limit.

The front transducer 1 is driven by a suitable power amplifier 4 which in turn is operated by a signal which has passed through a process 5. In the case of the low-frequency section of a multi-way loudspeaker or of a subwoofer, one joint function of the amplifier 4 and the process 5 will be to restrict the frequency range of the input signal to those frequencies which the unit is intended to reproduce. In the case of a full range loudspeaker, said restriction will not occur and the front transducer will be a known full-range type.Another joint function of the amplifier 4 and the process 5 may be to implement a frequency response function which represents an inverted replica of the frequency response function of the transducer 1 in order to cancel or reduce the Q factor of any fundamental resonance of said transducer and/or any other irregularities in its frequency response and to extend said response beyond its normal frequency range.

An advantageous characteristic of the amplifier 4 and process 5 is that over the intended frequency range the velocity of the front transducer diaphragm when in a baffle whose dimensions are large compared to the longest wavelengths in the system shall be substantially proportional to the audio input signal voltage and substantially in phase with it. Known methods of motional feedback may be applied to transducer 1 and its power amplifier to improve this proportionality and reduce the magnitude of any phase error. The known technique of rendering the output impedance of the amplifier 4 negative in order to oppose the transducer coil resistance may also be employed.

The rear transducer 3 is driven by a signal from a suitable power amplifier 6 and a process 7 which may differ considerably from the signal driving the front transducer 1.

The present invention requires that the joint characteristic of the amplifier 6 and the process 7 must be such that over the intended working frequency range the volume of air displaced by the transducers 1 and 3 in the intermediate chamber is always substantially equal and opposite. The polarity of the system is such that when one diaphragm moves into the intermediate chamber the other moves out of it. Consequently the pressure in the intermediate chamber is maintained substantially constant even when the loudspeaker is producing high sound pressure levels.

In order to provide the required characteristic, one joint function of the amplifier 6 and of the process 7 in this application may be to implement a frequency response function which represents an inverted replica of the joint frequency response function of the rear transducer 3 with its associated rear chamber in order substantially to cancel the characteristic fundamental resonance of said transducer with the stiffness of the air in the rear chamber.

When the required characteristic is provided, the operation of the front transducer 1 is substantially unaffected by the presence of the intermediate chamber or the rear transducer which acts as if it were mounted in the wall of an enclosure of infinite volume. As a result the fundamental resonant frequency of the front transducer is not raised by the presence of the enclosure. As the compliance of the air behind the front diaphragm is effectively rendered infinite, motion of said diaphragm is not resisted as it would be in a conventional enclosure. The forces on said diaphragm are much smaller, leading to increased efficiency, reduced phase error and reduced mechanical diaphragm distortion. As it is rendered so efficient, the front drive unit can produce appreciably more sound pressure level than can the same unit in a conventional enclosure.

The operation of the rear transducer is such that in moving in sympathy with the front transducer in order to render the pressure in the intermediate chamber substantially constant it must in consequence change the pressure in the rear chamber.

There will thus be considerable pressure differences across the rear diaphragm, which are not experienced by the front diaphragm.

For a given displacement, the rear diaphragm must exert greater forces than the front diaphragm. The diaphragm will be more prone to mechanical distortion, but any audible effects will be isolated from the listener by the presence of the front diaphragm which is advantageously made from a suitably rigid material. Consequently the rear transducer can be driven harder than would be possible in a conventional loudspeaker. The stiffness of air in the rear chamber will raise the fundamental resonant frequency of the rear transducer. Thus even if the two transducers are identical, the characteristics of the two processes 5 and 7 and their associated amplifiers will need to be substantially different.

As the present invention operates by isolating the front transducer from the finite volume of the rear chamber by keeping the volume of the intermediate chamber constant, the overall response of the loudspeaker is substantially independent of the volume of both chambers. Provided the rear transducer and its associated amplifier and signal process can meet the specified criteria, the chamber volumes are arbitrary. It is an advantageous feature of this invention that the overall size and weight of the loudspeaker can be made much smaller than before even though lower distortion is obtained.

The intermediate chamber may be as small as can be practically achieved without mechanical conflict between the two transducers. When a small rear chamber volume is utilised, the stiffness experienced by the rear diaphragm rises significantly and with it the joint fundamental resonanceof the rear transducer and the rear chamber. Said resonance may be set above the intended frequency range of the loudspeaker such that the rear transducer is operating below resonance in the so-called stiffness controlled mode. It may be advantageous in this case if the rear transducer is current driven rather than voltage driven as the combination of the air spring and a transducer characteristic providing force proportional to drive current produces a combined characteristic in which the transducer displacement is substantially proportional to the amplifier drive voltage.The air spring acts as an energy storage device and when the rear diaphragm is returning to the neutral position it will be assisted by the stored energy. If the transducer is electromagnetic, it will regenerate and return power to thepower amplifier. The power amplifier and the power supply must be designed to function correctly in the presence of transducer regeneration. Correctly applied, regeneration actually reduces the power consumed by the amplifier driving the rear transducer although it will be substantially higher than the power required by the front transducer. The minimum rear chamber volume will primarily be set by the heat dissipating capabilities of the rear transducer.

As an alternative, the rear chamber may be made large enough to set the resonance of the rear transducer within the operating frequency range of the front transducer. Various techniques are then available to counteract the resonance. One known technique is densely to pack the rear chamber with long fibre wool or similar material in order to reduce the Q factor of the resonance. In the case where the rear transducer is electromagnetic, raising the strength of the magnetic field will increase the damping. An amplifier with negative output impedance will also reduce the 9 of the resonance. Final correction of the overall response of the rear transducer can then be undertaken by the process 7.

The operating criteria of the present invention can be met in a number of ways. In a pure feedforward arrangement, the process 7 contains a model of the characteristics of the rear transducer and rear chamber and can predict how to drive the rear transducer from the input signal and need not measure the intermediate chamber pressure. An optional temperature sensor 8 may be incorporated in order to modify the model in the case where transducer parameters are a function of temperature.

In a pure feedback arrangement a pressure transducer 9 is installed in the intermediate chamber and the process 7 drives the rear diaphragm in such a way as to maintain the intermediate pressure constant without reference to the audio input. Any suitable combination of feedback and feedforward can be used in practice, including adaptive arrangements in which the feedback signal is analysed in order to refine the modelling of the feedforward process.

The audio input may be an analog waveform or a pulse code modulated data stream and processes 5 and 7 may be implemented in analog or digital hardware. Although the processes 5 and 7 are quite different they may be implemented in the same digital signal processor on a time shared basis.

The different characteristics required in the processes 5 and 7, the generally different fundamental resonance frequencies of the transducers and the greater requirement to handle regenerated currents in the rear transducer suggest that the present invention is best put into practice in a system in which each transducer has its own power amplifier as shown. However, a single power amplifier might be used in conjunction with entirely passive processes to derive the two quite different drive signals required. It is difficult to achieve optimal damping factors in this way and the overall efficiency might be poor. Nevertheless many of the advantages of the present invention would be obtained at least in some measure in an economical apparatus which could be connected to an existing amplifier of conventional design.

FIGURE 2a) is a further example of an application of the present invention in which two electromagnetic transducers 1 and 3 are arranged in the enclosure 2 in a manner which is more compact and allows operation up to higher frequencies before standing waves can occur. FIGURE 2b) is a further example in which two rear transducers 3 have been used. Such a configuration may be employed to aid in the dissipation of heat or to raise the fundamental resonance of the rear transducers.

FIGURE 3 is a further example of an application of the present invention in which the front transducer 1 has an electrostatically driven diaphragm whereas the rear transducer 3 has an electromagnetically driven diaphragm. The electrostatic diaphragm may be driven by known means to reproduce sound over the entire audible frequency range. The rear transducer 3 need not be used over the entire range but may advantageously be restricted to operate only at low frequencies.

FIGURE 4 is a further example of an application of the present invention in which the rear transducer 3 is a piezo-electric or magnetostrictive element or some other device whose absolute volume can be caused to change under the influence of an applied signal as symbolised by the double headed arrows. Such transducers may not have diaphragms in which case the rear chamber is unnecessary and the enclosure 2 is undivided. The front transducer 1 may be an electromagnetic or electrostatic type.

FIGURE 5 is a further example of an application of the present invention. Extremely high sound pressure levels are possible at low frequencies without excessive distortion for special purposes. The front transducer 1 may be a conventional moving coil device but of considerable size. The enclosure 2 will be of massive construction. The rear transducer diaphragm 3 is driven hydraulically by a ram 4 under the control of an electrohydraulic valve 5 which may use positional feedback. The electrohydraulic valve is supplied with an electrical input signal 6 and pressurised fluid 7.

FIGURE 6 shows a further example of an application of the present invention in which the two transducers have been integrated into one assembly in order to allow the smallest volume between the diaphragms and consequently to raise the maximum frequency of operation. The dual magnetic circuits 1 and 2 are arranged to provide a field in a pair of concentric gaps in which two concentric moving coils 3 and 4 operate. Each coil drives its own independent diaphragm 5 and 6 which in this example are conical. The dual concentric magnetic circuit and moving coil arrangement is known from prior loudspeakers in which the diaphragms are concentric. In such devices a smaller high frequency cone operates within a larger low frequency cone.

In the example of FIGURE 6 this is not the goal. The diaphragms 5 and 6 are not concentric but are acoustically in tandem. The diaphragms are independently provided with centering and restoring compliance using flexible elements 7. The intermediate volume between the cones is substantially sealed by the flexible elements 7 and the chassis 8. The dual transducer is mounted in a suitable enclosure (not shown) and driven using substantially the same techniques as disclosed in connection with Figure 1.

Transducers based on the construction shown in Figure 6 are especially advantageous for the clear reproduction of mid-range and high frequencies. A common drawback of existing transducers is the reflection of the rearward radiation by the magnet and enclosure structure which reflected radiation then passes through the cone and reduces the clarity of the forward radiation. The apparatus of Figure 6 when driven according to the methods disclosed in the present invention allows a substantial reduction of this phenomenon.

Claims

WHAT WE CLAIM IS:

1. A loudspeaker in which a first electro-acoustic transducer having a diaphragm driven by an audio signal to be reproduced is fitted across an aperture in the surface of a substantially sealed enclosure within which enclosure is a second electro-acoustic transducer driven by a different signal to the first transducer and which is thereby caused to displace the air behind the diaphragm of the first transducer in such a way that said diaphragm responds to the audio signal to be reproduced substantially as if it were fitted across an aperture in an enclosure of infinite volume.

2. A loudspeaker according to claim 1 in which the second transducer has a diaphragm dividing the enclosure into two parts.

3. A loudspeaker according to claim 2 in which the volume behind the diaphragm of the second transducer is sufficiently small to raise the fundamental resonant frequency of the second transducer substantially above that of the first transducer and in which signal processing means are provided in the drive to the second transducer to eliminate the effect of said raised resonance.

4. A loudspeaker according to claim 1 in which the second transducer is of a type which is capable of changing its absolute volume as a function of a drive signal.

5. A loudspeaker according to claim 1 in which the drive signal to the second transducer is obtained by the use of feedforward processing from the audio signal to be reproduced.

6. A loudspeaker according to claim 1 in which the drive signal to the second transducer is obtained by the use of feedback from a pressure transducer behind the diaphragm of the first transducer.

7. A loudspeaker according to claim 1 in which any combination of feedforward and feedback with or without adaptation is used to obtain the drive signal for the rear transducer.

8. A loudspeaker according to claim 1 in which the driving circuitry of the rear transducer is arranged to return regenerated current to the power supply.