Patent GB-958869-A (GB)
Title: Improvements in or relating to sound level meters
Inventor: CRAGG WILLIAM DONALD
IPC: H04R19/04
Filed: 1962-11-08
Abstract: 958,869. Sound level meters; electrostatic transducers. STANDARD TELEPHONES & CABLES Ltd. Nov. 8, 1962, No. 42257/62. Headings H4D and H4J. A sound level meter comprises a normally balanced bridge circuit at least one arm of which includes an electrostatic microphone. Fig. 2 shows one embodiment in which the microphone C M  and one half of a winding on a transformer M form two arms of the bridge the other arms of which comprise the other half of the winding and the capacitor C s . In operation the bridge, which is energized by a transistor oscillator-T operating at a frequency above that to which the microphone is to be responsive, is initially balanced so that when sound waves impinge on the microphone the unbalance voltage V o , consisting of the detected amplitude modulation of the transistor oscillation, is proportional to the amplitude of the sound waves. The voltage V o  is indicated on a transistor voltmeter (see Fig. 1, in which the circuit of Fig. 2 is included in the unit " Microphone head unit ") the battery supply for which is utilized to energize an audio-frequency oscillator which is connected across terminals V A  (Fig. 2), when the arrangement is to be calibrated and may also be employed in a low D.C. to high D.C. converter to produce a polarization (bias) voltage (50 V to 200 V) for the microphone which augments the highfrequency polarization voltage produced by the transistor oscillator. Additionally, a voltage V K  may be derived from an amplifier associated with the transistor voltmeter and utilized as a negative feed-back drive for the microphone to improve the linearity of the input-output characteristic when operating at high sound levels. Fig. 3 shows a second embodiment in which adjacent arms of the bridge include microphones C 6 , C 7  the diaphragms of which are closely coupled acoustically in such manner that they move in antiphase so that push-pull operation results. One constructional form of such microphones is shown in Fig. 8 and comprises a pair of diaphragms 1 clamped by rings 3 across the ends of a tube 2 together with respective counter electrodes 4 formed by acoustically transmissive layers of conductive material deposited on the end surfaces of an electrically insulating porous plastic block 5 contained within tube 2. Fig. 4 shows a third embodiment employing a double microphone C 10  having a common diaphragm and separate counter electrodes. Such a microphone may have the form shown in Fig. 5 in which the common diaphragm 1 co-operates with counter electrodes 4 deposited on porous plastic blocks 5, 6 the former being arranged to provide the requisite acoustical network for pressure gradient operation and to have a higher acoustic impedance than block 6. Such an arrangement has a cardioid-shaped polar response. Alternatively, by making the block 5 of Fig. 8 conductive such microphone may be employed in the embodiment of Fig. 4 as a double microphone having a common counter electrode and separate diaphragms. Fig. 6 shows a composite assembly of two pairs of common diaphragm microphones C 11 , C 12  and C 13 , C 14 , each pair being similar to the single pair shown in Fig. 5. By connecting such microphones, as shown in Fig. 7a, a bi-directional response having a " figure of eight " pattern is produced whilst with the arrangement of Fig. 7b an omni-directional response is produced. Instead of employing the apparatus according to the invention as a sound level meter the transistor voltmeter (Fig. 1) may be replaced by a power amplifier and the arrangement utilized as a conventional microphone an advantage being that such amplifier may be a transistor amplifier since the apparatus is designed to work into much lower input impedances than conventional electrostatic microphone arrangements.