When the ear is exposed to two or more different tones, it is a common experience that one tone may mask the others. Masking is probably best explained as an upward shift in the hearing threshold of the weaker tone by the louder tone and depends on the frequencies of the two tones. Pure tones, complex sounds, narrow and broad bands of noise all show differences in their ability to mask other sounds. Masking of one sound can even be caused by another sound that occurs a split second before or after the masked sound.
Some interesting conclusions can be drawn from the many masking experiments that have been performed:
1. Pure tones close together in frequency mask each other more than tones widely separated in frequency.
2. A pure tone masks tones of higher frequency more effectively than tones of lower frequency.
3. The greater the intensity of the masking tone, the broader the range of frequencies it can mask.
4. Masking by a narrow band of noise shows many of the same features as masking by a pure tone; again, tones of higher frequency are masked more effectively than tones having a frequency below the masking noise. .
5. Masking of tones by broadband ("white") noise shows an approximately linear relationship between masking and noise level (that is, increasing the noise level 10 dB raises the hearing threshold by the same amount). Broadband noise masks tones of all frequencies.
6. Forward masking refers to the masking of a tone by a sound that ends a short time (up to about 20 or 30 milliseconds) before the tone begins. Forward masking suggests that recently stimulated cells are not as sensitive as fully rested cells.
7. Backward masking refers to the masking of a tone by a sound that begins a few millie:econds later. A tone can be masked by a noise that begins up to 10 milliseconds later, although the amount of masking decreases as the
time interval increases (Elliot, 1962). Backward masking apparently occurs
at higher centers of processing where the later-occurring stimulus of greater
intensity overtakes and interferes with the weaker stimulus.
Some of the conclusions just stated can be understood by considering the way in which pure tones excite the basilar membrane. High-frequency tones excite the basilar membrane near the oval window, whereas low-frequency tones create their greatest amplitude at the far end. The excitation due to a pure tone is asymmetrical, however, having a tail that extends toward the high-frequency end. Thus it is easier to mask a tone of higher frequency than one of lower frequency. As the intensity of the masking tone increases, a greater part of its tail has amplitude sufficient to mask tones of higher frequency. This "upward spread" of masking tends to reduce the perception of the high-frequency signals that are so important in the intelligibility of speech.