Considerations Relating to Warbled Frequency Films

Some warbled frequency films. intended as signal sources for acoustical response measurements, appear to have been made and used without full realization of the true nature of the warbled signal and the manner in which such a signal is affected by a non-linear transmission system. It is pointed out that the warbled signal is a frequency-modulated signal; hence the signal may be represented by a carrier frequency and a series of side-frequencies, all of which are steady and discrete. It is pointed out, and substantiated experimentally, that the signal must be regarded in this light when considering the effect on it of a non-linear transmission system. The frequency structure of one “warble film” in use is calculated and shown graphically. Fundamental requirements for a suitable warbled frequency film having sinusoidal modulation ate discussed and values for modulation rate and for modulation depth are recommended. The side-frequency array provided by the recommended modulation constants is shown in graph form. Expressions are derived giving the frequency relationship and relative amplitudes of the side frequencies resulting from the non-sinusoidal frequency modulation which contains two components of modulation rate, one component having an associated phase constant. The side-frequency structures corresponding to some assumed combinations of two rates are calculated and illustrated. Certain assumptions are made for distortion or departure from sinusoid of a modulating frequency and the effects on the side-frequency structure are shown. Prom the latter calculation recommendations are derived for tolerances of departure from sinusoidal modulation for a warbled frequency film.

Print ISSN

0097-5834

Published

1940-02

Content type

Original Research

DOI

10.5594/J11907