The Human Ear
2.4 SUBJ ECTIVE RESPONSE TO SOUND PRESSURE LEVEL
2.4.1 Mask ing
Masking is the phenomenon of one sound interfering with the perception of another sound. For example, the interference of traffic noise with the use of a public telephone on a busy street corner is probably well known to everyone. Examples of masking are shown in Figure 2.7, in which is shown the effect of either a tone or a narrow band of noise upon the threshold of hearing across the entire audible spectrum. The tone or narrow band of noise will be referred to as the masker.
Referring to Figure 2.7, the following may be observed:
1. The masker is an 800 Hz tone at three sound pressure levels. The masker at 80 dB has increased the level for detection of a 600 Hz tone by 25 dB and the level for detection of a 1,100 Hz tone by 52 dB. The masker is much more effective in masking frequencies higher than itself than in masking frequencies lower than itself.
2. The masker is a narrow band of noise 90 Hz wide centred at 410 Hz. The narrow band of noise masker is seen to be very much more effective in masking at high frequencies than at low frequencies, consistent with the observation in (a).
As shown in Figure 2.7, high frequencies are more effectively masked than are low frequencies. This effect is well known and is always present, whatever the masker.
The analysis presented here suggests the following explanation. The frequency component energies of any stimulus will each be transported essentially without loss at a relatively constant group speed, to a place of resonance. As a component
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Figure 2.7 Example of masked audible spectra where the masker is either a tone or a narrow band of noise. The masker at three levels is: (a) 800 Hz tone; (b) a narrow band of noise 90 Hz wide centred on 410 Hz.
approaches a place of resonance, the group speed of the component slows down and reaches a minimum at the place of resonance, where the component’s energy is dissipated doing work to provide a stimulus, which is transmitted to the brain. Only in the region of resonance of the masker, will the masker and test tone components be summed, giving rise to high threshold levels for detection of the test tone.
Evidently, the higher levels of threshold shift at high frequencies are due to the passage of the masker components through the places of resonance for high
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Figure 2.8 Comparison of a tone and a narrow band of noise as maskers.
frequencies. It is suggested here that the most likely explanation is that the outer hair cells, which act to amplify a test stimulus at low levels, are inhibited by the high levels of excitation resulting from transmission of the masker. Consequently, the threshold level is elevated. By contrast, any residual components of the masker must decay very rapidly so that little or no masker is present on the apical side of the place of masker stimulation. Masking, which is observed at low frequencies, is due to the low frequency response of the filter. In a linear filter, unique relations exist which can provide guidance, but for the ear, the system is active and such relations are unknown.
In all of the curves of Figure 2.7(a), where the masker is a tone, a small dip is to be noted when the variable tone approaches the masking tone. This phenomenon may be interpreted as meaning that the tones are close enough for their critical bands to overlap. In the frequency range of critical bandwidth overlap, one tone of modulated amplitude will be heard. For example, consider two closely spaced but fixed frequencies. As the phases of the two sound disturbances draw together, their amplitudes reinforce each other, and as they subsequently draw apart, until they are of opposite phase, their amplitudes cancel. The combined amplitude thus rises and falls, producing beats (see Section 1.11.3).
Beats are readily identified, provided that the two tones are not greatly different in level; thus the dip is explained in terms of the enhanced detectability due to the phenomenon of beating. In fact, the beat phenomenon provides a very effective way of matching two frequencies. As the frequencies draw together the beating becomes slower and slower until it stops with perfect matching of tones. Pilots of propeller driven twin-engine aircraft use this phenomenon to adjust the two propellers to the same speed.
Reference is made now to Figure 2.8 where the effectiveness, as masker, of a tone and a narrow band of noise is compared. The tone is at 400 Hz and the band of noise is 90 Hz wide centred at 410 Hz. Both maskers are at 80 dB sound pressure level. It is evident that the narrow band of noise is more effective as a masker over most of the audio frequency range, except at frequencies above 1000 Hz where the tone is slightly more effective than a narrow band of noise.
It is of interest to note that the crossover, where the narrow band of noise becomes less effective as a masker, occurs where the ratio of critical bandwidth to centre band frequency becomes constant and relatively small (see Section 2.2.6 and Figure 2.6).
In this range, the band filters are very sharply tuned. That is, the tone is more effective as a masker in the frequency range where the cochlear response is most sharply tuned, suggesting that the band pass filter is narrow enough to reject part of the narrow band of noise masker.
In the foregoing discussion of Figures 2.7 and 2.8, a brief summary has been presented of the effect of the masking of one sound by another. This information is augmented by reference to the work of Kryter (1970). Kryter has reviewed the comprehensive literature which was available to him and based upon his review he has prepared the following summary of his conclusions.
1. Narrowband noise causes greater masking around its frequency than does a pure tone of that frequency. This should be evident, since a larger portion of the basilar membrane is excited by the noise.
2. Narrowband noise is more effective than pure tones in masking frequencies above the band frequency.
3. A noise bandwidth is ultimately reached above which any further increase of bandwidth has no further influence on the masking of a pure tone at its frequency.
This implies that the ear recognises certain critical bandwidths associated with the regions of activity on the basilar membrane.
4. The threshold of the masked tone is normally raised to the level of the masking noise only in the critical bandwidth centred on that frequency.
5. A tone, which is a few decibels above the masking noise, seems about as loud as it would sound if the masking noise were not present.