Subharmonic pitches of a pure tone at low S/N ratio

Abstract

2AFC [two-alternative forced choice] scores were obtained on 50 untrained [human] subjects who had to respond to a 3% increment or decrement of the fundamental frequency of 2 successive signals. The 1st signal in each trial was a reference, containing 6 harmonics of 200 Hz. Main variables were the number of harmonics of the 2nd signal in a trial (either 3, 2 or 1) and the presence or absence of continuous background noise. Low scores were observed in the absence of the noise (typically 65% for the 3 and 2 component signals, and virtually 50% for the 1 component signals), whereas in the presence of the noise much higher scores were observed (typically 90% and 80%, respectively). Background noise has great impact on the perceptibility of a low pitch related to the fundamental frequency, even more so than the number of components presented (within the range of 3, 2, or 1 components): at a low S/N [signal to noise] ratio, even a single harmonic may give rise to the perception of a subharmonic low pitch. Subsequent experiments with 3 trained subjects indicated that this potential of a single pure tone to evoke a subharmonic pitch deteriorates for harmonic numbers of about n = 7-11. The theoretical implications of these subharmonic pitches of a pure tone to pitch perception in general were discussed.