AU1768700A

AU1768700A - Flat surface loudspeaker and method for operating the same

Info

Publication number: AU1768700A
Application number: AU17687/00A
Authority: AU
Inventors: Robert Boesnecker
Original assignee: Siemens AG; Siemens Corp
Current assignee: Puren GmbH
Priority date: 1999-04-19
Filing date: 1999-10-21
Publication date: 2000-11-02
Anticipated expiration: 2019-10-21
Also published as: ATE243910T1; EP1169884B1; JP2002542745A; EP1169884A1; DE19917584A1; DK1169884T3; DE59906131D1; AU767985B2; WO2000064217A1

Abstract

The invention relates to a method for operating a flat surface loudspeaker (1). According to the method, at least one moving coil (3, 4) is applied to a platform-shaped surface (2) with predetermined material properties. Said surface is excited to vibration by the at least one moving coil, which is electrically excited by a source of sound (7). The acoustic frequency response of the flat surface loudspeaker is measured and the inverse frequency curve to its frequency curve is determined. This inverse frequency curve is simulated in a filtering device (8) as its transfer function. The filtering device is connected between the sound source (7) and the flat surface loudspeaker (1) in the operating arrangement, so that the frequency response of the flat surface speaker is compensated based on its transfer function. The compensation of the frequency response of the flat surface loudspeaker improves its transfer properties even to hi-fi standards.

Description

GR 99 P 1665 -1 Description Flat surface loudspeaker, and a method for its operation 5 The invention relates to a flat surface loudspeaker as claimed in the precharacterizing clause of patent claim 4, and to a method for its operation as claimed in the precharacterizing clause of patent claim 1. 10 Flat surface loudspeakers of said generic type have been known per se for a long time, for example from German Patent 484 872. An oscillating coil is used in a flat surface loudspeaker, operating on the 15 electrodynamic principle and being placed directly on a surface - intrinsically initially of any desired size and thickness and composed of a chosen material -, and being mechanically fixed there. When the oscillating coil is stimulated electrically by a sound transmitter, 20 then its oscillations are transmitted to the surface, which acts as a membrane, so that it is itself used as a sound-emitting surface. There will be a large number of potential applications per se for an electroacoustic transducer of this generic type. Apart from a few 25 exceptions, it has nevertheless not been used to any major extent so far owing to its electroacoustic characteristics, in particular its transfer function. The operation of the sound-emitting surface is 30 primarily governed by its mechanical characteristics. This surface can transmit sounds or tones only by oscillating mechanically. Quite apart from the way in which it is clamped in, that is to say the mechanical mounting and the point at which the oscillating coil is 35 fixed on it, a surface in the form of a plate in which, preferably, bending oscillations are stimulated is intrinsically a relatively complex structure in terms of its oscillation behavior. Whereas with commercially GR 99 P 1665 - la available loudspeakers based on the electrodynamic principle it is still largely possible, even if actually only by making compromises, to optimize the acoustic characteristics GR 99 P 1665 - 2 of the sound-emitting membrane, this is not directly possible with flat surface loudspeakers. This problem can be illustrated by an example: if the glass surface of a shop window on which an oscillating coil is 5 mounted is used as a flat surface loudspeaker, then the material, shape and dimensions of the sound-emitting surface, and the way in which it is clamped in as well, are essentially fixed. In this example, the frequency response of the flat surface loudspeaker is thus 10 essentially predetermined. Typically, the natural resonances of the surface used for sound emission with this material and the dimensions of the shop window have a frequency response which - in simple terms - can be described by enhanced response in the low frequency 15 area and, furthermore, by a tendency to produce a tinkling noise, which is due to the influence of higher-order natural resonances that are still in the audible range. Corresponding characteristic nonlinearities also occur with other materials, such as 20 wood or synthetic materials. As is known, for example, from US-A-3 728 497 as well as US-A-3 636 281 or US-A-3 449 531, efforts have been made to overcome the known disadvantages of a flat 25 surface loudspeaker by means of physical measures. Certain improvements have been possible in this way, but a fundamental solution which would give flat surface loudspeakers a wide range of applications has not yet been obtained from the experiments carried out 30 so far. The invention is thus based on a first partial object of specifying a means, using a method of the type mentioned initially, using which the nonlinearities in 35 the frequency response of flat surface loudspeakers can at least be coped with to such an extent that their sound spectrum is sufficiently natural for the respective application.

GR 99 P 1665 - 2a A second partial object is to use such a method to provide a flat surface loudspeaker of the type GR 99 P 1665 - 3 mentioned initially, whose electroacoustic characteristics are - depending on the application optimized such that predetermined requirements in an individual application relating to the quality of sound 5 produced in this way are thus satisfied. In a method of this generic type for operating a flat surface loudspeaker, the first partial object is achieved by the features described in the 10 characterizing part of patent claim 1. In a flat surface loudspeaker of the type mentioned initially, the second partial object is achieved by the features described in the characterizing part of patent 15 claim 4. In the field of electroacoustics, it has long been known in the development of optimized electroacoustic transducers that the effects of the influencing 20 variables which govern the transmission quality of an electroacoustic transducer often counter one another in a contrary manner. A physical/mechanical solution in which all these influencing variables are optimized in the same way is thus impossible, and every 25 electroacoustic transducer is invariably a compromise solution, due to systematic factors. One relevant example of this is the known loudspeaker box, with a number of individual, specifically designed loudspeakers. The solutions to the two partial objects 30 according to the invention are based on the joint idea that such compromises, which are characterized by physical measures, have far less probability of leading to a satisfactory result in a flat surface loudspeaker. A flat surface loudspeaker is actually not composed of 35 individual, specifically designed loudspeaker units, like a loudspeaker box. The development of flat surface GR 99 P 1665 - 3a loudspeakers so far has shown that solution approaches which attempted to improve flat surface loudspeakers by physical measures did not lead to a satisfactory result.

GR 99 P 1665 - 4 The invention is a departure from the conventional ideas of electro-acousticians and adopts a different approach. The electroacoustic characteristics of flat surface loudspeakers are governed by the total effect 5 of the characteristics of the oscillating coil or coils used, and by the mechanical characteristics of the sound-emitting surface that is used. The electroacoustic transfer function for each arrangement of a flat surface loudspeaker defined in this way is 10 thus defined in the form of its frequency response apart from tolerances. If the corresponding frequency curve is determined by measurement, then the frequency response of the flat surface loudspeaker can be compensated for, and hence linearized, by means of a 15 filter device which is arranged in the operating arrangement of the flat surface loudspeaker between the sound source and the amplifier located upstream of the oscillating coil or oscillating coils, provided the transfer function of the filter device is essentially 20 the inverse of the corresponding function for the combination of an oscillating coil or coils and the sound-emitting surface. According to developments of the invention, the 25 transfer function of the filter device is simulated by means of digital filters, in particular by means of FIR (Finite Impulse Response) filters, whose filter coefficients are derived from the inverse frequency curve of the flat surface loudspeaker. 30 The filter device preferably has a sample and hold element as the input element, which is connected via an analogue/digital converter to the digital filter, whose output is connected to a digital/analogue converter. 35 According to another development of the invention, the filter device is equipped with a digital signal processor.

GR 99 P 1665 - 4a Nowadays, digital signal processors are widely used and, owing to the progress in the development of integrated circuits, are also already available for relatively GR 99 P 1665 - 5 computation-intensive "real time" applications. Digital signal processors are freely programmable, even if this is only to a limited extent due to the available volume for the program memory. It is thus possible to match 5 the operation of the digital signal processor to different sound-emitting surface materials, such as wooden materials, glass, plastics, and, inter alia, polyurethane foam. Furthermore, it is also possible to provide sound-emitting surfaces with different shapes. 10 It is thus clear that the invention has, in particular, overcome the greatest obstruction to the widespread use of flat surface loudspeakers in the past. The shape and material selection for the sound-emitting surface are largely unconstrained, without any need to accept any 15 reduction in the sound emission quality. Although very high quality, which is thus still relatively expensive due to complexity reasons, is not required for every application, it is nevertheless feasible to implement embodiments which even completely satisfy hifi (high 20 fidelity) requirements. Volume and weight savings with flat surface loudspeakers compared to commercially available loudspeaker boxes are a major advantage, and not only in these applications. 25 Further advantages and refinements of the solution according to the invention can be found in the following description of exemplary embodiments. Exemplary embodiments of the invention will be 30 described in more detail in the following text with reference to the drawing, in which: Figure 1 shows a flat surface loudspeaker in conjunction with a measurement arrangement 35 for measuring its frequency response, GR 99 P 1665 - 5a Figure 2 shows a first embodiment of the circuit arrangement for operating the flat surface loudspeaker, and Figure 3 shows a further embodiment of the circuit 5 arrangement as shown in Figure 2.

GR 99 P 1665 - 6 Figure 1 shows, schematically, a flat surface loudspeaker 1 which has a sound-emitting surface 2 in the form of a plate and on which, by way of example, two oscillating coils 3 and 4 are arranged. The 5 oscillating coils 3 and 4 are mechanically fixed on the sound-emitting surface 2 such that, when they are electrically stimulated, the mechanical oscillations which they carry out in this case are transmitted to the sound-emitting surface 2 in order that it is itself 10 caused to oscillate, and hence to emit sound. In a functional operating circuit, the oscillating coils 3, 4 are connected in parallel to the outputs of an amplifier 5 whose input, during normal operation, is coupled to a sound source, which is not shown in 15 Figure 1. For a person skilled in the art of technical acoustics, it is immediately evident that, inter alia, the characteristics of the sound-emitting surface 2, its 20 shape, the size of its surface area, it thickness and, above all, also its mechanical characteristics together with the configuration of the oscillating coil or coils 3 and 4 as well as their local arrangement on the sound-emitting surface 2 govern the acoustic 25 characteristics of the flat surface loudspeaker 1. Since, for example, completely different materials can be used for the sound-emitting surface 2, this itself results in a difficulty in material selection. This is because this depends on whether the flat surface 30 loudspeaker 1 has a high level of attenuation, on the one hand in particular in the higher frequency range, as in the case of wooden materials, or on the other hand in the low-frequency range as, for example, in the case of glass and plastics so that, in the latter case, 35 high frequency components are reproduced excessively, thus resulting in a tendency to tinkling.

GR 99 P 1665 - 6a These problems have resulted in flat surface loudspeakers not so far being used in large numbers in intrinsically feasible applications, even though the principles relating to this have been known for a very 5 long time, since other electroacoustic transducers are known whose frequency response can be corrected more easily.

GR 99 P 1665 - 7 In order to solve this problem, Figure 1 now also shows a measurement arrangement by means of which the transmission characteristics of a flat surface loudspeaker 1 are analyzed acoustically. In order to 5 determine the frequency response of the measurement object, that is to say of a specific type of flat surface loudspeaker 1, a frequency analyzer 6 is provided which emits a defined electrical measurement signal to the amplifier 5 at a predetermined level and 10 at a tunable frequency, and causes the flat surface loudspeaker 1 to emit sound via the oscillating coils 3, 4. A measurement microphone 61, which is connected to the input of the frequency analyzer 6, is arranged at a defined distance from the flat surface loudspeaker 15 1, preferably along its center axis. The frequency response of the measurement object is determined using this measurement arrangement, which is preferably set up in an anechoic room, in order to 20 simulate sound propagation in free space as exactly as possible in measurement conditions. As indicated above, this frequency response of a flat surface loudspeaker 1 is governed by object-typical nonlinearities, for which reason it must be measured individually, at least for 25 each object type. This results in an essential measure for the electroacoustic transmission characteristics of a flat surface loudspeaker 1. The inverse function of the frequency curve obtained in this way is formed in order to compensate for the nonlinearities of the 30 frequency response. Figure 2 uses an operating circuit for the flat surface loudspeaker 1 to illustrate, schematically, how the described measurement result is used in order to 35 correct for the distortion in the transmission characteristics of a specific electroacoustic transducer. By way of example, the sound source is illustrated in Figure 2 in the form of a magnetic tape recorder 7, whose output is connected to the amplifier GR 99 P 1665 - 7a 5 for the flat surface loudspeaker 1, via a filter device 8. As is indicated schematically in Figure 2, a transfer function is implemented in the filter device 8 which is essentially the inverse of the characteristic 5 frequency curve GR 99 P 1665 - 8 measured for this type of flat surface loudspeaker 1. The profile of the transfer function of the filter device 8 must be approximated more closely to the inverse frequency curve of the flat surface loudspeaker 5 1 the more stringent the requirements to which the resultant transmission quality of the flat surface loudspeaker 1 is subject in the respective application. In the filter device 8, the electrical sound signals supplied from the magnetic tape recorder 8 are 10 subjected to preemphasis in such a way that this just counteracts the frequency response of the flat surface loudspeaker 1. This sound signal, with preemphasis, is supplied via the amplifier 5 to the oscillating coils 3, 4 of the flat surface loudspeaker 1. The transfer 15 function of the conversion to acoustic signals in the flat surface loudspeaker 1 counteracts this preemphasis once again. The resultant frequency response of the flat surface loudspeaker 1 [lacuna] better linearized the more accurately the transfer function of the filter 20 device 8 approximates to the inverse frequency curve of the flat surface loudspeaker 1. As is known, electrical filters can also be formed from discrete elements, but complex transfer functions for a 25 bandpass filter in the audible range, such as those which are used in this field of application in conjunction with flat surface loudspeakers 1, can be provided using discrete components only with great complexity, and then only to a first approximation. 30 Implementations of the filter device 8 using discrete components are therefore suitable in conjunction with a flat surface loudspeaker 1 only when its transmission quality is subject only to restricted requirements in a particular application. 35 Figure 3 thus shows a further embodiment for the operating circuit of a flat surface loudspeaker 1, by GR 99 P 1665 - 8a means of which even hifi (high fidelity) requirements can be satisfied. The embodiment shown in Figure 3 differs from the embodiment shown in Figure 2 in the further refinement of the filter device 8. Figure 3 5 shows the filter device 8 as a digital filter. Its GR 99 P 1665 - 9 input circuit, which is connected to the magnetic tape recorder 7 (which is indicated once again as an example of a sound source) is in the form of a sample and hold element 9 - frequently also referred to as a sample and 5 hold circuit. The electrical sound signal supplied as an analogue signal from the magnetic tape recorder 8 is thus sampled using a predetermined sampling theorem, and the respectively sampled instantaneous value is buffer-stored and is supplied to an analogue/digital 10 converter 10 which is connected to it and which converts the successive instantaneous values to digital signals expressed in binary form. The signals are supplied in this form to a digital signal processor 11. On the output side, the digital signal processor 11 is 15 connected to a digital/analogue converter 12, by means of which its binary output signal is converted back to an analogue electrical signal, which is supplied via the amplifier 5 to the flat surface loudspeaker 1. 20 This refinement of the filter device 8 advantageously makes use of the progress in the development of digital signal processing. Nowadays, the semiconductor industry offers the user powerful signal processors, which are already in widespread use, even for real-time 25 applications. Application options for digital signal processors as well as refinements by means of appropriate programs can therefore be assumed to be known in this case. The circuit design of the digital signal processor is therefore not shown in detail in 30 the schematic illustration in Figure 3. Normally, in addition to a microcontroller, the actual control unit, has a signal processor a program memory, a data memory and an input/output memory, which are connected to one another via a bus system with parallel address, control 35 and data lines. The capability to store a specific program relating to the respective application in the program memory makes the digital signal processor GR 99 P 1665 - 9a suitable for an electronic circuit which can be used universally and, in the present field of application, is used to simulate the transfer function of the filter device 8.

GR 99 P 1665 - 10 It is advantageous in this case for the filter or filters to be in the form of FIR (Finite Impulse Response) filters, by means of which even complex transfer functions for real-time requirements can be 5 provided in a known manner. If the transmission quality of the flat surface loudspeaker is subject to very stringent requirements, such as those for hifi quality, in a specific application, then, owing to the signal processing required in real-time conditions, it may be 10 necessary to carry out this signal processing by parallel operation of a number of signal processors, without in the process needing to depart from the fundamental solution approach. 15 The embodiments described above open up a wide range of applications for flat surface loudspeakers. The capability to program the digital signal processor 11 freely allows the complexity for the measurement of the frequency response of the respective type of flat 20 surface loudspeaker 1 and the conversion of the measured frequency curve to an inverse transfer function (which is a greater or lesser approximation of this) of the filter device 8 to be optimized for the respective application. Both physically small and large 25 format flat surface loudspeakers can be produced. Since the choice of materials for a flat surface loudspeaker designed according to the invention is no longer to a major extent subject to the conventional restriction, even materials with a very low relative density, for 30 example, can also be chosen for the sound-emitting surface. Particularly in mobile applications, in which transport capabilities invariably play a substantial role, it is a major advantage to move a light flat surface loudspeaker composed of polyurethane foam 35 instead of a heavy, voluminous conventional loudspeaker box. Flat surface loudspeakers according to the invention can thus be used not only for commercial GR 99 P 1665 - 10a purposes, such as public sound-emission facilities and advertising hoardings, but also as high-quality loudspeaker device in the personal field, which are at the same time very flat and, for example, are 5 integrated in furniture.

Claims

1. A method for operation of a flat surface loudspeaker (1), in which at least one oscillating 5 coil (3, 4) is mounted on a surface (2) in the form of a plate and having predetermined material characteristics, via which sound is emitted by a coil or coils stimulated electrically by means of a sound source (7), stimulated to oscillate, 10 characterized in that the acoustic frequently response of this flat surface loudspeaker is measured and its frequency curve is determined, in that the inverse frequency curve to this frequency curve is determined, in that this inverse 15 frequency curve is simulated in a filter device (8) as its transfer function, and in that the frequency response of the flat surface loudspeaker is compensated for by means of the filter device, which is connected between the sound source and 20 the flat surface loudspeaker in the operating state, on the basis of its transfer function.

2. The method as claimed in claim 1, characterized in that the transfer function of the filter device 25 (8) is simulated by digital filters.

3. The method as claimed in claim 2, characterized in that the transfer function is formed by means of FIR (Finite Impulse Response) filters, whose 30 filter coefficients are derived from the inverse frequency curve.

4. A flat surface loudspeaker having at least one oscillating coil (3, 4) which is mounted on a 35 surface (2) in the form of a plate and having defined material characteristics and which, stimulated electrically by means of a sound source (7), causes this surface to oscillate in order to GR 99 P 1665 - 11a emit sound, characterized in that a filter device (8) is arranged between the sound source (7) and the at least one oscillating coil (3 or 4), whose GR 99 P 1665 - 12 transfer function is the inverse of the frequency response of the flat surface loudspeaker (1).

5. The flat surface loudspeaker as claimed in claim 5 4, characterized in that the filter device (8) is in the form of a digital filter.

6. The flat surface loudspeaker as claimed in claim 5, characterized in that the filter device (8) is 10 formed by FIR (Finite Impulse Response) filters.

7. The flat surface loudspeaker as claimed in one of claims 5 or 6, characterized in that the filter device (8) has a sample and hold element (9) as 15 the input element, which is connected via an analogue/digital converter (10) to the digital filter (for example 11), whose output is connected to a digital/analogue converter (12). 20

8. The flat surface loudspeaker as claimed in one of claims 5 to 7, characterized in that the filter device is equipped with a digital signal processor (11).