Combination tones and unmasking

Some important relationships between two auditory nonlinearities, unmasking and combination tone (CT) production, are described. An example of unmasking is provided by using a forward-masking paradigm where a baseline is first obtained by determining the threshold of a 2000 Hz signal preceded by a 2000 Hz masker. When a second tone (the suppressor) is then added concurrently with the masker, the signal can be easier to hear. At a suppressor level of 80 dB SPL, this unmasking occurs for suppressor frequencies around 2300 Hz. At a suppressor level of 55 dB SPL, the unmasking effect occurs at suppressor frequencies closer to the masker frequency. An example of CT production is provided by presenting two sinusoids (f1 and f2) as maskers in a forward-masking experiment. The threshold of the 2000 Hz signal is shown to decrease as f2/f1 is increased keeping 2f1 - f2 = 2000 Hz. This is consistent with the notion that the magnitude of the CR decreases as f2/f1 increases. We then provide evidence that CTs can produce ummasking effects similar to acoustic tones. Again a baseline was determined with a 2000 Hz signal amd masker (f1). Two higher-frequency sinusoids (f2 and f3) were added simultaneously to the masker, neither of which produced unmasking when presented individually. When 2f2 - f3 or f3 - f2 was approximately 2300 Hz, unmasking was observed. Next was explored 2f2 - f3 unmasking as a function of f3/f2, keeping 2f2 - f3 frequency fixed at 2300 Hz. As f3/f2 increases, the magnitude of the unmasking decreases. These CT unmasking effects suggest that the generation of CTs must be at the same site or peripheral to the site of suppression.