Audio Engineering

Audio Principles 11

Nerve fi rings are not a perfect analog of the basilar membrane motion. On continuous tones,
a nerve fi ring appears to occur at a constant phase relationship to the basilar vibration, a
phenomenon called phase locking, but fi rings do not necessarily occur on every cycle. At
higher frequencies fi rings are intermittent, yet each is in the same phase relationship.

The resonant behavior of the basilar membrane is not observed at the lowest audible
frequencies below 50 Hz. The pattern of vibration does not appear to change with
frequency and it is possible that the frequency is low enough to be measured directly
from the rate of nerve fi rings.

1.6 Mental Processes .....................................................................................................

The nerve impulses are processed in specifi c areas of the brain that appear to have
evolved at different times to provide different types of information. The time domain
response works quickly, primarily aiding the direction-sensing mechanism and is older
in evolutionary terms. The frequency domain response works more slowly, aiding the
determination of pitch and timbre and evolved later, presumably as speech evolved.

The earliest use of hearing was as a survival mechanism to augment vision. The most
important aspect of the hearing mechanism was the ability to determine the location
of the sound source. Figure 1.7 shows that the brain can examine several possible
differences between the signals reaching the two ears. In Figure 1.7(a) , a phase shift is
apparent. In Figure 1.7(b) , the distant ear is shaded by the head, resulting in a different
frequency response compared to the nearer ear. In Figure 1.7(c) , a transient sound arrives
later at the more distant ear. The interaural phase, delay, and level mechanisms vary in
their effectiveness depending on the nature of the sound to be located. At some point
a fuzzy logic decision has to be made as to how the information from these different
mechanisms will be weighted.

There will be considerable variation with frequency in the phase shift between the ears.
At a low frequency such as 30 Hz, the wavelength is around 11.5 m so this mechanism
must be quite weak at low frequencies. At high frequencies the ear spacing is many
wavelengths, producing a confusing and complex phase relationship. This suggests a
frequency limit of around 1500 Hz, which has been confi rmed experimently.

At low and middle frequencies, sound will diffract round the head suffi ciently well that
there will be no signifi cant difference between the levels at the two ears. Only at high