ELECTROACOUSTIC TRANSDUCER

Abstract: The present invention relates to an electroacoustic transducer where the voltage of a capacitive acoustic element is controlled by charging and discharging the charge of the element via an inductance (L) by fast switches (K1, K2). In that case energy unconverted into the acoustic energy of the transducer can be transferred to an energy storage of an electrical circuit formed by the inductance (L) and the capacitance (C1, C2, C0) during each control sequence and in this way the transducer can be made to operate with a good coefficient of efficiency.

Description: ELECTROACOUSTIC TRANSDUCER The invention relates to an electroacoustic transducer comprising a capacitive acoustic element and at least two switches for controlling the voltage acting on the element, in which case the switches are arranged to control the voltage acting on the element by controlling the on and off times of the switches. The coefficient of efficiency of sound reproducers based on magnetic loudspeakers is typically very low, about 0.5%, for example. It is known to control magnetic loudspeakers by so-called chopper amplifiers in which case the efficiency of the amplifier is reasonably good, but as the resistance of the coil of the loudspeaker is fairly great, it causes a great power loss and the total efficiency of the sound reproducer will thus be very low. DE-2324211 discloses a capacitive acoustic element but the reference cited does not disclose the control arrangements of the element. U.S. Patents 4,207,442, 4,286,122 and 5,161 ,128 also disclose a capacitive acoustic element and various control switchings and arrangements of the element. All the solutions mentioned above have it in common that the coefficient of efficiency will not be very good by means of them. The object of the present invention is to provide an electroacoustic transducer whose coefficient of efficiency will be very good. The transducer of the invention is characterized in that an inductance is connected to at least one electrode of the acoustic element, through which inductance voltage is arranged to act on the acoustic element, and that the transducer comprises a capacitance that together with the inductance forms an electrical circuit in such a manner that the capacitance and the inductance together operate as an energy storage for storing energy unconverted into acoustic power. The essential idea of the invention is that the capacitive acoustic element is controlled by means of at least two fast switches, in which case by controlling the off and on times of the switch, the voltage acting on the transducer is controlled. A further essential idea is that an inductance is connected to at least one electrode of the acoustic element, through which inductance voltage is arranged to act on the acoustic element. The inductance together with the capacitance of the transducer forms an oscillating circuit in such a manner that the inductance and capacitance in question are able to store energy unconverted into acoustic energy and supply it back to the transducer. The energy stored into the acoustic element is transferred almost without loss e.g. to another block of the element or to an independent storage capacitor and back to the element. The idea of one preferred embodiment is that the switches are controlled by pulses whose width is determined by means of the difference of an audio signal and the voltage of the transducer, that is, pulse width modulation is used. Furthermore, the idea of a second preferred embodiment is that the acoustic element is formed of a serial connection of two capacitors, at least one of which is acoustically active. The advantage of the invention is that the coefficient of efficiency of the equipment is very good as only that amount of energy will be consumed that the transducer emits out as acoustic power and the portion used for the switch losses of control electronics. A separate auxiliary capacitor will not be needed for the electrical circuit when the acoustic element comprises two capacitors The invention will be explained in more detail in the appended drawings, wherein Figures 1a to 1c illustrate diagrams of three different embodiments of the electroacoustic transducer of the invention, Figure 2 shows a diagram of a fourth embodiment of the electroacoustic transducer of the invention, Figure 3 shows a schematic diagram for forming control pulses of switches, Figures 4a and 4b show alternatives for coupling the transducers of the invention as sensors, Figures 5a and 5b show diagrams of a fifth and a sixth embodiment of the electroacoustic transducer of the invention, Figure 6 shows a diagram of a seventh embodiment of the electroacoustic transducer of the invention, Figures 7a to 7c show further diagrams of some embodiments of the invention, Figure 8 shows a diagram of a parallel connection of the transducers of the invention, Figures 9a and 9b are schematic views of matrix-constructed transducer systems, Figure 10 is a schematic, cross-sectional side view of a part of one capacitive acoustic element, Figure 11 shows a construction stage of the element of Figure 10, and Figures 12a and 12b shows the elements of Figure 10 placed on top of one another. Figure 1a shows the principle of the system. The system comprises capacitive acoustic elements C„ C 2 , switches K,, K 2 , diodes D,, D 2 , an inductance L and a power supply V 0 . By switching on and off the switches K 1 ( K 2 on a frequency of 1 MHz, for example, by regulating the switching times of pulses P, and P 2 , the voltage integrated into point C can be controlled, the voltage being a sound-producing voltage in the transducer. Points A and B illustrate electrodes A and B to be connected to essentially stationary surfaces of the element of Figure 10, for example, and point C illustrates an electrode C to be connected to a moving diaphragm 2. Mains voltage U is rectified, in which case the operating voltage of the transducer is 320 V, for example. This voltage is stored into capacitors Cj and C 2 , at least one of which emits sound, that is, it is an acoustically active capacitive element. The energy E c stored into the capacitor is 0.5 * CU 2 , that is, for example, if C, = C 2 = 1 μF, it is derived that E C1 is about 0.05 joules. The voltage acting on point C is controlled by the switches K, and K 2 . By switching on the switch K, at moment t,, the energy of the capacitor C 1 will start flowing to the inductance L, which flow is described by current l The energy of the inductance L depends on the attained current which is dependent on the on time t 2 of the switch K The energy E L stored into the inductance is 0.5 * LI 2 , that is, for example, if L is 100 μH and I is one ampere, E L = 50 μjoules. Thus the energy of the capacitor is reduced by 50 μjoules. The reduction of the capacitor voltage is derived from formula U = V2 * ΔE:C , that is, the capacitor voltage is reduced by 10 V. The energy stored into the inductance can now be transferred to the capacitor C 2 by switching on the switch K 2 . If the switching time is the same as above, in principle 50 μjoules is transferred to the capacitor C 2 , that is, its voltage rises by 10 V. In this way the voltage of point C in the transducer can be controlled without any great energy losses. Losses are produced in the resistances of the circuit. For example, the resistance of switching transistors can typically be about 0.2Ω. Then the power loss PL is about 0.2 W. The acoustic coefficient of efficiency α of the transducer is typically about 1%, in which case * ΔE = 0.5μJ will be transferred into acoustic energy. When the length of the control pulse has been 1 μs, 0.5 W of power has been transferred via the acoustic transducer. When the losses were 0.2 W, the efficiency of the system is 60%. The system needs to supply only the required additional energy from the power supply because the oscillating circuit formed by the inductance and capacitance acts as an energy storage. Figures 1b and 1c show alternative switching arrangements of the transducer of the invention. In these cases the acoustic element comprises a permanently charged electret diaphragm 2a, whereby the element does not have a separate electrode C. Auxiliary capacitors C 0 act as an energy storage. Figure 2 shows a solution where an audio signal S is compared in a comparator with a triangular wave produced by the oscillator, whereby pulses required for controlling the switches will be provided. The required pulses can also be formed digitally, in which case the system converts digital sound information directly into sound without digital-to-analog converters. For the sake of clarity, in the present application all the components in the figures have not been named and explained as their meaning and operation is evident to those skilled in the art. Figure 3 shows schematically the principle of pulse width modulation, that is, by comparing the signal S with a triangular wave, the widths of the control pulse P are determined in a manner known per se. For example, in the case of Figure 2, when the value of the control pulse P is high H, the switch H^ is controlled to be on and when the value is Low, the switch K 2 is controlled to be on. Because the transducer can be separated by switches K., and K 2 from a controlling signal, the transducer acts then as a sensor. In Figure 4a, by switching on the switch K 3 , it is possible to measure as a sample the moving speed V of the diaphragm of the transducer. Figure 4b shows a bridge- connected transducer where when the switches K, and K 2 are off, the moving deviation V x of the diaphragm of the transducer can be measured by switching on the switch 3. The measured signals can be used as feedback signals in the control of the transducer and sensors for other purposes. Figure 5a shows an application where the effect of switching pulses is filtered with an additional filter which is formed by the capacitor C 0 and inductance L Inductance L 2 is connected to point C. Figure 5b shows an application where the acoustic element is formed only of one capacitor C t to which a DC component is not directed. Figure 6 shows an application where a very high feedback amplification can be used, in which case distortion can be rendered very small. An input signal S is compared with the voltage of the transducer in a comparator which provides the control pulses for the switches K, and K 2 . Figures 7a to 7c show solutions where a low voltage accumulator of 12 V, for example, is used as a power supply V By switching on the switch K 1 ( energy is transferred from the accumulator to the inductance L and the amount of energy is dependent on the time the K, is switched on. By switching on the switch K 2 , the energy of the inductance L can be transferred to the element C By repeating the sequences mentioned above several times by a fast frequency of 1 MHz, for example, the desired voltage can be transferred to the element. The voltage of the element can be correspondingly discharged to the power supply by switching on the switch K 2 first, in which case the energy of the transducer is transferred to the inductance L and can be transferred therefrom to the power supply by switching on the switch K Figure 8 shows a principle of how the transducers of the invention can be connected in parallel. Figures 9a and 9b show transducers connected as matrixes, in which case the number of switches can be reduced and the characteristics of the acoustic field produced by controlling the switches in different ways can be adjusted. Figure 10 shows an acoustic element whose frame sections 1 are produced of a porous material and whose inner surface is electrically conductive. The inner surfaces form electrodes A and B. A moving diaphragm 2 is arranged between the frame sections. Figure 10 shows that the moving diaphragm 2 is an electret diaphragm which has an electrically conductive layer in the middle. The moving diaphragm can also be made of non- electrically conductive diaphragms, to the middle of which an electrically conductive diaphragm is arranged, or the diaphragm 2 can also be formed of a permanently charged electret diaphragm 2. Recesses 3 shown with broken lines can also be made to the frame section 1 of the element to lighten the plate. The electrode C of the diaphragm 2 can be divided into blocks and the electrodes A and B can also be divided as desired and the element can be controlled as a matrix, as described above. Figure 11 is a schematic view of a construction method of the element. The frame sections 1 are sintered in a mould from plastic powder and at least their inner surfaces are coated with metal. The diaphragm 2 is stretched at its edges as shown in Figure 11. After this, the frame sections 1 are pressed against one another, whereby the diaphragm 2 will be stretched tight and oriented to be thinner. In this way the distances between different electrodes can be minimized and the coefficient of efficiency can be maximized. Figures 12a and 12b show solutions where different elements are connected on top of one another so that both dipole and monopole sound sources and sensors can be produced of them. The drawings and the specification relating thereto are only intended to illustrate the idea of the invention. In its details, the invention may vary in the scope of the claims. Therefore any capacitive acoustic element may be used in connection with the invention, that is, it may be an electrostatic, a piezoelectric or an electret transducer, for example.

Claims: CLAIMS 1. An electroacoustic transducer comprising a capacitive acoustic element and at least two switches (K 1( K 2 ) for controlling the voltage acting on the element, in which case the switches (K 1t K 2 ) are arranged to control the voltage acting on the element by controlling the on and off times of the switches (K,, K 2 ), characterized in that an inductance (L) is connected to at least one electrode (A, B, C) of the acoustic element, through which inductance voltage is arranged to act on the acoustic element, and that the transducer comprises a capacitance (C^ C 2 , C 0 ) that together with the inductance (L) forms an electrical circuit in such a manner that the capacitance (C v C 2 , C 0 ) and the inductance (L) together operate as an energy storage for storing energy unconverted into acoustic power. 2. A transducer of claim 1, characterized in that the acoustic element is connected by means of the inductance (L) to a connecting point of the switches (K,, K 2 ) connected in series. 3. A transducer of claim 1 or 2, characterized in that the switches (K^ K 2 ) are controlled by the pulses formed by the difference of an audio signal (S) and the voltage supplied to the transducer. 4. A transducer of any one of the preceding claims, charac- t e r i z e d in that the acoustic element is formed of a serial connection of two capacitors, at least one of which is acoustically active. 5. A transducer of any one of the preceding claims, charac¬ terized in that the acoustic element comprises at least two porous frame sections (1) pleated at their inner surfaces and coated with metal at least at their inner surfaces, between which frame sections a moving diaphragm (2) is stretched. 6. A transducer of any one of the preceding claims, charac¬ terized in that the acoustic element is divided into several blocks which are controlled as matrixes. 7. A transducer of any one of the preceding claims, charac¬ terized in that it comprises several acoustic elements which are interconnected and arranged to be controlled as matrixes.

Inventors: KIRJAVAINEN KARI, KIRJAVAINEN, KARI
Applicants: KIRJAVAINEN KARI, KIRJAVAINEN, KARI
Application year: 1997
Main classification: H04R19
IPC codes: H04R3, H04R19