ELECTROACOUSTIC SOUND TRANSMITTER

Abstract: A sound transmitting device for the efficient production of intense sounds. It comprises a resonator device (1), and electroacoustic transducer (2) with a drive element (5) and one as an oscillator coupled amplifier. The amplifier coupled as an oscillator is provided with a circuit solution which will give an adjustment of the frequency of the drive current to the present position of the resonator frequency in the resonator device (1). This takes place by means of a negative electrical current feedback and by means of a positive feedback of the voltage at the output of the amplifier, which is instantaneously dependent of the mechanical influence of the drive element (5) of vibrations of the air column in the resonator device. By means of this the oscillator frequency is adjusted to the present position of the resonator frequency in the resonator device.

Description: Title : Electroacoustic sound transmitter Technical field : The present invention relates to an electroacoustic sound trans¬ mitter for the generation of high-intensity sound . The transmitter according to the invention is of the type comprising an open reso¬ nance unit (resonator) , an electroacoustic transducer and an amp¬ lifier. Background: Electroacoustic sound transmitters using for example horns are well known . If one in order to obtain a high efficiency wishes to utilize the natural resonance at the acoustic system the electric portion must be driven with the resonator frequency of the acoustic system . In practice, however, this causes great difficulties due to the resonance frequency will change under such conditions like temperature and pressure. This -means that a sound transmitter of this type, driven by means of a fixed frequency, will receive varied » sound level when said conditions are changed. Technical problem : The problem to be solved by this invention is to achieve a sound transmitter with mainly unchanged sound level and with high efficiency without regards of changes in temperature and other con¬ ditions . The solution : This problem is solved by this invention by a sound transmitter in which the transmitted drive frequency is governed by a resonance in the coupled transducer and resonance units so that the drive frequency always is adapted to changes of the chosen resonance frequency of the electroacoustic system. Advantages : By means of the invention it is achieved for example in con¬ nection with alarm signals one will receive the same sound level in a sound transmitter with high, unchanged efficiency regardless of the present climatic conditions . Brief description of drawings : The invention is described below and is shown in the figures 1 - 6. Fig. 1 is a schematic illustration of the mechanical input impedance of the resonance unit. Fig. 2 is a schematic illustration of the electrical input impedance of the transducer- resonance devices according to figure 5. Fig. 3 shows the principle for the connection between the amp¬ lifier oscillator and transducer-resonance devices . Fig. 4 shows an example of an intended practical embodiment. Fig. 5 shows an electroacoustic transducer of electrodynamic type with a resonance unit partly shown. Fig. 6 shows an analogue circuit for the transducer as shown in figure 5. Best mode of carrying out the invention: A typical form of embodiment of the invention is shown in fi¬ gure 4. In the figure a horn 1 is shown with an open end 6 and an electromechanical transducer 2, which is coupled to the other open end of the horn 1 opposite to the open end 6. The electromechanical, or as it may also be called electroacoustic transducer 2 is driven from an amplifier not shown, coupled like an oscillator, producing the drive frequency. The electroacoustic transducer is shown more closely in figure 5. Numeral 8 signifies the throat opening of the horn 1 at the said second to the transducer 2 adjacent portion of the horn. The throat opening 8 is connected in a direct attachment to a diaphragm 3, which is connected to a voice coil 5, which is located in an air gap 4 of a circular permanent magnet system 7. The electromechanical trans¬ ducer is consequently designed according to the principles of an electrodynamic loudspeaker, in which the coil is usually called the voice coil. Also other principles can be utilized for example the use of a piezo- electric crystal. The drive voltage from the amplifier mentioned is applied to the coil 4, which concurrently with the voltage changes by means of its alternating magnetic field, will cause vibrations the diaphragm 3 the frequency of which mainly corresponds to the frequency of the amplifier output voltage. The movements of the diaphragm 3 will in & < turn cause vibrations in the air column of the horn 1 , so that sound is radiated . An open acoustic resonator like for example a tube or a horn has an impedance characteristic which can be schematically illu¬ strated as in figure 1. The impedance characteristic is determined by the area and geometry of the transmitter mouth S M 1 (in Fig. 4 numeral 6) by the area and geometry of the throat opening S τ 2 (in Fig. 5 numeral 8) the horn function S(x) = S τ (x) where x is the. coordinate along the resonator unit (see Fig. 4) . The smaller the transmitter mouth is compared to the wave length of the sound the more resonant the impedance function becomes . For an electroacoustic transducer of for instance the electro¬ dynamic type as shown in figure 5 the input impedance on the elec¬ tric side is given by 2 z . " = z + < B1 > in E „ Z M where Zp = electrical impedance of the transducer with the voice coil 5 im¬ mobile and where BL is a product of magnetic flux density and where length L of the voice coil 5 in the air gap 4 of the transducer M MD = isphragm mass (9) = rear cavity n MS = suspension compliance (2) = front cavity PL- o = suspension mechanical resistance C MR = compliance of the rear cavity C M1 = compliance of the front cavity Z MT = l/z MT , = mechanical impedance of the horn throat Rp = electrical resistance of the voice coil e = applied voltage S- Q = diaphragm area The mechanical circuit of the suggested transducer can be drawn schematically for low frequencies as in figure 6. Z M is the mechanical impedance which the force from the transducer acts on . Z M is influenced by the impedance of the horn or the tube as shown in figure 1. At resonance the resulting velocity v for the applied force F is determined by __O PI where ^H = C A S D / S T where A is the real part of the normalized throat impedance, as shown in figure 1, where B is the imaginary part. The specific impedance of the air in the resonance device is $ c. The efficiency of the mechanoacoustic part of the transmitter is given by MH MA= R^+R^ In order to obtain a high efficiency R M ττ » O - At resonance the efficiency of the transducer is thus .2 _(BL) /R M ? EM R E +(BL)VR M The total efficiency is L = EM 7 MA In order to obtain a high efficiency BL has to be large, and (BL 2 R MS +R MH )>>R E These facts result in a large dependence of the electrical input impedance of the transducer on Rτ y r τ _τ > see figure 2. These resonance peaks, which are obtained in a correctly dimensioned transducer- resonator may be used as frequency determining elements in an oscil¬ lator as shown in figure 3. The operating principle of the amplifier/oscillator is as follows . Negative current feedback from point A of the amplifier/oscillator tries to keep the output voltage constant at point A regardless of the voltage at point B , the output terminal of the amplifier. If the load Z- , consists of a parallell-resonance circuit and a circuit with an analogous behaviour, for example a loudspeaker, the voltage at point B will be high at resonance if the paraUell resonance circuit has a high value of . The positive voltage-feedback from point B now strives to maximize the voltage at point B . With a suitable amount of feedback the circuit will oscillate at a frequency corre¬ sponding to the resonance frequency of the load impedance Z. . If the load Z. has several main resonances filtering F in the feedback from B may be used to select a chosen main frequency of oscillation . Without some kind of amplification reducing element an amplifier/ oscillator as described above would give a distorted waveform if the value of of the load impedance was low . For higher values of Q, the amplification reduction which achieved by constant circuit ope¬ ration of feedback from point A is enough . For low values of Q the feedback from point A must be carefully adjusted in order to achieve a good sinusoidal waveform of the output voltage at point B . This may be arranged by using an impedance Z„ with a high positive temperature coefficient between point A and ground. If the voltage at point B becomes too high the impedance between point A and ground will increase thus resulting in lower amplification and lower voltage at point B . In order to obtain the highest possible efficiency in the amplifier a puls-width modulation technique may be used. OMPI ^KNATlO

Claims: I claim : 1. A sound transmitting device comprising an open acoustic reso¬ nator (1) , an electroacoustic transducer (2) and one as an oscillator coupled electric amplifier CHARACTERIZED BY a circuit for positive feedback and oscillation at the resonance peaks of the electrical input impedance of the transducer, caused by the electrical, mecha¬ nical and acoustical impedance characteristics of the transducer/ resonator device, at only one predetermined main frequency of the resonator frequency of the amplifier/resonator device. 2. A sound transmitting device according to claim 1, CHARAC¬ TERIZED BY an electrical filter, which selects the choosen reso¬ nance frequency, in the electric amplifier. 3. A sound transmitting device according to claim 1 or 2, CHARAC¬ TERIZED BY negative electrical current feedback in the electrical amplifier. 4. A sound transmitting device according to claims 1, 2 or 3, CHA¬ RACTERIZED BY the coupling between the resonator device (1) and the electromechanical transducer (2) is arranged to give impedance peaks in the electric input impedance of the amplifier at resonances in the electro-acoustic mechanical system. 5. A sound transmitting device according to claims 1, 2, 3 or 4, CHARACTERIZED BY automatic limitation of the feedback by means of a current depending device in the electric amplifier. 6. A sound transmitting device according to any of the previous claims , CHARACTERIZED BY the amplifier being of the pulse width modulated type. 7. A sound transmitting device according to patent claim 4, CHA¬ RACTERIZED BY the acoustic resonator being a tube or horn (1) one end (6) of which is open and the other end (8) of which by a throat connects to the electroacoustic transducer (2) , which comp¬ rises at least one electrical driver (5) , being arranged so as to by the influence of an alternating eletrical current produce vibrations in a mechanical element such as a diaphragm (3) , which is located in connection with the said other end of the resonator device (1) , whereby the as an oscillator coupled amplifier exhibits the said cir¬ cuit solution, which gives an adaption of the frequency of the drive current to the present resonance frequency of the resonator de- vice (1) by means of said negative current feedback by a feedback of the voltage at the amplifier output, which is instantaneously de¬ pending on the mechanical influence of the drive element (5) on the vibrations of the air column in the resonator device in such a way, via the mechanical element (3) that the said voltage by the negative electrical current feedback in the amplifier will adjust the oscillator frequency to the present resonance frequency of the re¬ sonator device. "BUREA OMPI , < . WIPO

Inventors: KLEINER MENDEL, KLEINER, MENDEL
Applicants: KLEINER MENDEL, KLEINER, MENDEL
Application year: 1983
Main classification: H04R3
IPC codes: H04R1, H04R3