Amplitude Cross-Section Representation with the Sound Spectrograph

Abstract

Spectrograms as normally produced by the “sound spectrograph” are three dimensional. Time is portrayed along the length of the spectrogram paper, frequency across it, and amplitude by the blackness of the marking. The time and frequency scales thus provided are adequate for most analysis purposes. The amplitude scale, however, is poorly defined, first because inherent characteristics of the spectrogram paper limit the amplitude portrayal to a highly non-linear and limited range, secondly, because reading degrees of blackness by eye is inaccurate. This inadequacy in determining amplitude led to the development of a method for portraying amplitude which was independent of the characteristics of the paper or of the ability of the eyes to delineate grades of blackness of marking. The device developed and herein described provides an auxiliary presentation which portrays, at any preselected point in time, a two-dimensional spectrogram with axes of amplitude and frequency. This device delineates a 35-db amplitude range with the scale in uniform logarithmic steps.