GB842473A - Improvements in or relating to sound receiving apparatus - Google Patents

Abstract

842,473. Measuring sound intensity. MILLTRONICS Ltd. Sept. 23, 1957 [Sept. 26, 1956], No. 29843/57. Class 40(7). [Also in Groups XXXV and XXXVIII] In apparatus for producing an electrical signal proportional to the sound emitted by a selected source the sound being mixed with extraneous sound from other sources, a first microphone generates a signal primarily proportional to sound from the selected source, a second microphone generates another signal proportional to the extraneous sound and the two signals are compared to give a difference signal proportional only to sound from the selected source. In Fig. 2, the grinding mill 1 is to be controlled according to the sound generated by grinding, this parameter being a suitable measure for optimum mill operation. Microphone MS1 is located near mill 1 to detect grinding sound, but it also picks up noise from the associated machinery and from adjacent mills. Microphone MC2 is positioned near mill 2 to detect primarily the noise extraneous to mill 1. Fig. 3 shows the circuit by which the signals from the two microphones are mixed: MS1 signal is passed through a frequency selective amplifier V 1A , V 1B and applied to integrating circuit V 3A , V 3 B, C 7 . MC2 signal passes through a similiar frequency sensitive amplifier to integrator V 4A , V 4a , C 8 . The capacitors C 7 , C 8 , which are charged with opposite polarity, form a bridge circuit together with resistors R 7 , R 8 , the output of which is applied to V 5 B through a long time constant, low pass filter P 3 , C 9 , C 10 . The output signal is used to control the ore feed and the water feed to the mill. In alternative embodiments, sound screens may be erected between adjacent mills to assist the microphone selectivity and different arrangements of microphones may be used, e.g. the compensation microphone for one mill may act as the signal microphone for another and vice-versa.