11.4 Signal duration

Since hearing is largely a matter of stimulus reception over time, we would expect time to influence the perception of sound. It has been known for many years that both absolute thresholds and the loudness of sounds depend upon signal duration. The studies of absolute threshold described earlier were all carried out with tone bursts of relatively long duration. For durations exceeding 500 ms, the sound intensity at threshold is roughly independent of duration. However for durations of less than 200 ms the sound intensity needed for detection increases as signal duration decreases. This also means that for sounds of less than 200 ms duration the intensity must be increased to maintain a constant level of loudness. Intensity also affects the perception of frequency. For example, if a tone of an audible frequency and intensity is presented for only a few milliseconds, it will lose its tonal character and will either be inaudible or be heard as a click. The length of time a given frequency must last in order to produce the perception of a stable and recognisable pitch is about 250 ms. We are also able to discriminate between tones of different frequencies when their duration is lengthened.