A sum of simultaneous pure tones with harmonic relationships (i.e., simple frequency ratios) is normally heard as a single sound, with a single pitch, even when its components are fully resolved in the auditory periphery. This perceptual phenomenon called “harmonic fusion” is thought to play an important role in auditory scene analysis as listeners often have to segregate simultaneous harmonic sounds with different fundamental frequencies. The present study explored the consequences of mild or moderate cochlear hearing loss for the sensitivity to harmonicity and the detection of inharmonicity.
The subjects were 12 normal-hearing (NH) listeners and 8 listeners with cochlear hearing loss amounting to 30 to 50 dB (mean: 42 dB) from 0.25 to 3 kHz. In each subject, thresholds for the detection of a change in the frequency ratio of simultaneous pure tones were measured with an adaptive forced-choice procedure. The standard frequency ratio was either harmonic (2:1, i.e., one octave) or inharmonic (0.8 or 1.2 octaves). The tones were presented at a low sensation level (at most 15 dB) within broadband noise, to minimize their cochlear interactions. In the main experimental conditions, the frequency register of the stimuli varied randomly within each trial, so that subjects were forced to process frequency ratios to achieve good performance; frequency discrimination was not sufficient. In other conditions, by contrast, frequency discrimination was sufficient to perform the task optimally.
For both groups of subjects, thresholds in the main experimental conditions were lower (i.e., better) when the standard frequency ratio was harmonic than when it was inharmonic. This effect, revealing sensitivity to harmonicity, was weak for some members of the hearing-impaired group, but could be observed even in subjects showing a very poor frequency discrimination ability. The two groups, however, differed from each other with respect to the detection of inharmonicity: for the NH group, in agreement with previous results, negative deviations from one octave (i.e., compressions of this frequency ratio) were better detected than positive deviations (stretchings); for the hearing-impaired group, on the other hand, the sign of the deviations had no effect on performance.
Sensitivity to harmonicity appears to be remarkably robust. However, it can be reduced in some listeners with mild or moderate cochlear damage. Moreover, as inharmonicity detection is asymmetric for NH listeners but apparently becomes symmetric in case of cochlear damage, it may be that listeners with cochlear damage do not detect inharmonicity in the same manner as NH listeners do. In some circumstances, inharmonicity can be detected on the basis of “beat” cues available in single frequency channels; however, the subjects tested here were unlikely to use cues of this type.