A High Speed Level Recorder and Some of Its Uses in Acoustical Measurements

Abstract

A device has been developed which will record the intensity of audiofrequency currents directly on a moving strip of paper by means of a stylus. A wide range of intensities may be covered and the deflection of the stylus is proportional to the logarithm of the input current; the recorder thus has a uniform scale in decibel units. It is capable of following fluctuations in the input current up to 150 per second, and will follow changes in intensity at a rate up to 240 db per second. When used in conjunction with a microphone the recorder lends itself to many types of acoustical measurements. It has been extensively used for reverberation time measurements for which it is particularly well adapted because it will plot the intensity of the reverberant sound on a logarithmic scale during the decay period, a complete decay curve being obtained each time the test tone is interrupted. When used with a beat frequency oscillator curves of intensity vs. frequency may be obtained. This type of curve is useful for loudspeaker and microphone calibrations for determining the distribution of sound in rooms and for intensity measurements for obtaining the sound transmission through partitions. Examples of the above types of curves and others obtained with speech and music are given.