response to a signal due to the presence of another. this happens because the original neural activity caused by the first signal is reduced by the neural activity of the other sound. combination tones are products of a signal and a masker. this happens when the two sounds interact causing new sound, which can be more audible than the original signal. this is caused by the non linear distortion that happens in the ear. for example, the combination tone of two maskers can be a better masker than the two original maskers alone. the sounds interact in many ways depending on the difference in frequency between the two sounds. the most important two are cubic difference tones and quadratic difference tones . cubic difference tones are calculated by the sum. 2f1 – f2 (f1 being the first frequency, f2 the second) these are audible most of the time and especially when the level of the original tone is low. hence they have a greater effect on psychoacoustic tuning curves than quadratic difference tones. quadratic difference tones are the result of f2 – f1 this happens at relatively high levels hence have a lesser effect on psychoacoustic tuning curves. combination tones can interact with primary tones resulting in secondary combination tones due to being like their original primary tones in nature, stimulus like. an example of this is 3f1 – 2f2 secondary combination tones are again similar to the combination tones of the primary tone. off frequency listening off frequency listening is when a listener chooses a filter just lower than the signal frequency to improve their auditory performance. this “off frequency” filter reduces the level of the masker more than the signal at the output level of the filter, which means they can hear the signal more clearly hence causing an improvement of auditory performance. applications auditory masking is used in tinnitus maskers to suppress annoying ringing, hissing, or buzzing or tinnitus often associated with hearing loss. it is also used in various kinds of audiometry, including pure tone audiometry, and the standard hearing test to test each ear unilaterally and to test speech recognition in the presence of partially masking noise. auditory masking is exploited to perform data compression for sound signals (mp3).