11.3.1 A masking experiment
The procedure for a masking experiment is shown in Figure 38. First, the threshold for hearing is determined across a range of frequencies (Figure 38a). Then, a masking stimulus is presented at a particular place along the frequency scale and while the masking stimulus is sounding, the thresholds for all frequencies are re-determined (Figure 38b). The masking stimulus used can be a pure tone or, more commonly white noise. A white noise stimulus is simply one that contains a band of frequencies with equal sound pressure at each frequency. It sounds something like the ‘shhhhhhh’ sound you can make by blowing air across your teeth when they are slightly separated. The band of frequencies used can vary in width. For example, a band of frequencies 90 Hz wide centred on 410 Hz would contain frequencies ranging from 365 to 455 Hz.
When thresholds are measured in the presence of a masking noise the original thresholds are raised. Figure 39 shows the result of an experiment using a 90 Hz band of noise centred at 410 Hz.
There are two things to note about Figure 39. First, the threshold increases most for frequencies near the frequencies in the masking tone. Second, the curve is not symmetrical; the masking effect spreads more to high frequencies than to low frequencies. So, lower frequencies mask higher-frequency sounds much more effectively than the reverse.
How is this masking effect explained?
Many masking effects can be explained in a very simplified way by analysing the interaction of displacement patterns in response to sound on the basilar membrane. Figure 40 shows the vibration patterns on the basilar membrane caused by a 400 Hz, 800 Hz and 1000 Hz tone. You can see that the vibration pattern of the 800 Hz tone overlaps those of the 400 and 1000 Hz tones. Note also that the pattern for the 800 Hz tone (which is shaded) almost totally overlaps the pattern for the higher frequency, 1000 Hz tone, but does not overlap the place of peak vibration of the lower frequency, 400 Hz tone. We would therefore expect the 800 Hz tone to mask the 1000 Hz tone more effectively than the 400 Hz tone. This is what happens, providing support for the place mechanism of frequency tuning on the basilar membrane.
Masking has also been used to determine psychophysical tuning curves. A low intensity tone, called the test tone, is presented throughout the experiment. A series of masking tones (also pure tones) are then presented, one at a time, with the test tone. One of the masking tones is the same frequency as the test tone and the others are higher or lower in frequency. The level of each masking tone is reduced until the test tone is just audible. To understand the rationale behind this experiment, imagine that the horizontal line in Figure 41 is the basilar membrane. Each masking tone, because it is a pure tone (single frequency), will cause vibration mainly at one point on the basilar membrane. If the masking tone is the same, or of similar frequency to the test tone, it will mask the test tone more effectively than a masking tone with a frequency far away from that of the test tone. The results of this kind of masking experiment are shown in Figure 42a. They show what we would expect according to the place theory of frequency coding on the basilar membrane: when the masking tone is the same frequency as the test tone it doesn't need to be very intense in order to mask the test tone. However, when the masking tone is higher or lower in frequency than the test tone, higher intensities are required to mask the test tone. You can compare the psychophysical tuning curves in Figure 42a with those obtained from auditory nerve fibres of a cat seen in Figure 42b.
The close match between them suggests that both reflect the same process – a place code for frequency on the basilar membrane.