Audio Principles 7of the original disturbance are inaudible to us and are said to be masked. If our goal is
the highest quality, we can design our imperfect equipment so that the shortcomings
are masked. Conversely, if our goal is economy we can use compression and hope that
masking will disguise the inaccuracies it causes.
A study of the fi nite resolution of the ear shows how some combinations of tones sound
pleasurable whereas others are irritating. Music has evolved empirically to emphasize
primarily the former. Nevertheless, we are still struggling to explain why we enjoy music
and why certain sounds can make us happy whereas others can reduce us to tears. These
characteristics must still be present in digitally reproduced sound.
The frequency range of human hearing is extremely wide, covering some 10 octaves (an
octave is a doubling of pitch or frequency) without interruption.
By defi nition, the sound quality of an audio system can only be assessed by human
hearing. Many items of audio equipment can only be designed well with a good knowledge
of the human hearing mechanism. The acuity of the human ear is fi nite but astonishing. It
can detect tiny amounts of distortion and will accept an enormous dynamic range over a
wide number of octaves. If the ear detects a different degree of impairment between two
audio systems in properly conducted tests, we can say that one of them is superior.
However, any characteristic of a signal that can be heard can, in principle, also
be measured by a suitable instrument, although in general the availability of such
instruments lags the requirement. The subjective tests will tell us how sensitive the
instrument should be. Then the objective readings from the instrument give an indication
of how acceptable a signal is in respect of that characteristic.
The sense we call hearing results from acoustic, mechanical, hydraulic, nervous, and
mental processes in the ear/brain combination, leading to the term psychoacoustics. It
is only possible to briefl y introduce the subject here. The interested reader is referred to
Moore^1 for an excellent treatment.
Figure 1.4 shows that the structure of the ear is divided into outer, middle, and inner ears.
The outer ear works at low impedance, the inner ear works at high impedance, and the
middle ear is an impedance matching device. The visible part of the outer ear is called the
pinna, which plays a subtle role in determining the direction of arrival of sound at high
frequencies. It is too small to have any effect at low frequencies. Incident sound enters
the auditory canal or meatus. The pipe-like meatus causes a small resonance at around