The selection of modulation rates for frequency modulated sound field stimuli.

Frequency modulated (FM) tones are used in sound field audiometry in order to avoid problems caused by standing waves. The FM stimuli to be used must be specified in terms of bandwidth (twice the frequency deviation), modulation rate and modulation waveform. This paper deals with selection of the modulation rate. Improved field uniformity obtained with FM tones relies upon the ear averaging the sound intensity as the frequency sweeps. The modulation rate must be less than about 1/3 of the frequency deviation in order to provide sufficient spectral components to ensure reasonable averaging across the frequency band. If the rate is too low threshold will be determined by the peak intensity during the sweep. For normal listeners a modulation rate of 5 Hz is just satisfactory but in the hearing impaired a higher rate is required because of their altered temporal integration characteristics. A suitable value for the modulation rate can be inferred from the literature on temporal integration and the acoustic characteristics of typical test rooms. The optimum rate increases with the stimulus centre frequency, but a constant rate of 20 Hz appears to be acceptable.