Page 86 - The Master Handbook Of Acoustics

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THE EAR AND THE PERCEPTION OF SOUND

occur, these consonants are genuine transients having durations on the
order of 5 to 15 msec. A glance at Fig. 3-12 tells you that transients this
short must be louder to be comparable to longer sounds. In the above
words, each consonant is not only much shorter than the rest of the
word, it is also at a lower level. Thus you need good listening conditions
to distinguish between such sets of words. Too much background noise
or too much reverberation can seriously impair the understandability of
speech because of the consonant problem. 12

Audibility of Loudness Changes

Modern faders are of the composition type giving gradations in level
so small as to be inaudible. Wire-wound faders of early mixing con-
soles produced discrete steps in level that could be audible. Steps of
5 dB were definitely audible, steps of 0.5 dB were inaudible, but these
steps cost too much to produce and 0.5 dB steps were not necessary.
Steps of 2 dB, an economic compromise, produced changes in signal
level that were barely detectable by an expert ear. Detecting differ-
ences in intensity varies somewhat with frequency and also with
sound level.
At 1 kHz, for very low levels, a 3-dB change is the least detectable
by the ear, but at high levels the ear can detect a 0.25-dB change. A
very low level 35-Hz tone requires a 9-dB level change to be
detectable. For the important midfrequency range and for commonly
used levels, the minimum detectable change in level that the ear can
detect is about 2 or 3 dB. Making level changes in increments less than
these is usually unnecessary.

Pitch vs. Frequency

Pitch, a subjective term, is chiefly a function of frequency, but it is not
linearly related to it. Because pitch is somewhat different from fre-
quency, it requires another subjective unit—the mel. Frequency is a
physical term measured in cycles per second, now called Hertz.
Although a weak 1,000-Hz signal is still 1,000 Hz if you increase its
level, the pitch of a sound may depend on sound-pressure level. A ref-
erence pitch of 1,000 mels has been defined as the pitch of a 1,000-Hz
tone with a sound-pressure level of 60 dB. The relationship between