Enhancing and unmasking the harmonics of a complex tone.

Alternately eliminating and reintroducing a particular harmonic of a complex tone can cause that harmonic to stand out as a pure tone-separately audible from the rest of the complex-tone background. In the psychoacoustical literature the effect is known as "enhancement." Pitch matching experiments presented in this article show that although harmonics above the 10th are not spectrally resolved, harmonics up to at least the 20th can be enhanced. Therefore, resolution is not required for enhancement. Further, during those experimental intervals in which a harmonic is eliminated, excitation pattern models suggest that listeners should be able to hear out a neighboring harmonic-separately audible from the background. The latter effect has been called "unmasking." In the present article we provide the first experimental evidence for unmasking. Harmonics of 200 Hz, with harmonic numbers between about 5 and 16, are readily unmasked. Their pitches are usually matched by sine tones with frequencies that are not exactly those of the unmasked harmonics but are shifted in a direction away from the frequency of the pulsed harmonic. Phase relationships among the harmonics that produce temporally compact cochlear excitation lead to reduced enhancement but greater unmasking.