12.
Understanding Decibels
(A Power Point slide show)
There are a number of differences in the fifth and sixth editions of Peter Ladefoged's A Course in Phonetics, compared with the 4th and earlier editions. One is the mention of various issues relating to amplitude and intensity. The following two paragraphs from the fourth edition (p. 165-166) do not appear in the fifth or sixth:
LOUDNESS AND INTENSITY
In
general, the loudness of a sound depends on the size of the
variations in air pressure that occur. Just as frequency is the acoustic measurement
most directly corresponding to the pitch of a sound, so acoustic intensity
is the appropriate measure corresponding to loudness. The intensity
is proportional to the average size, or amplitude, of the variations in air
pressure. It is usually measured in decibels (abbreviated as dB) relative
to the amplitude of some other sounds. Technically, to get the dB difference
one has to compare the power ratio, where the power is defined as the square
of the mean amplitude (the mean variation in air pressure). The difference
in dB is 10 times the common logarithm of the power ratio of the two sounds
or 20 times the log of the amplitude ratio. The human ear can hear (perhaps
tolerate would be a better word) a range of about 120 dB, although if you
persist in listening to sounds 110 to 120 dB above the quietest sound you
can hear you will soon go deaf, as many rock musicians have found out. When
one sound has an intensity of 5 dB greater than another, then it is approximately
twice as loud. A change in intensity of 1 dB is a little more than the just
noticeable difference in loudness.
Figure 8.3 shows the waveform of the phrase
"We saw three dogs" and underneath it a record of the intensity
in dB. Intensity is always a relative measure – one sound has an intensity
of so many dB more or less than another. The zero point in Figure 8.3 is arbitrarily
taken to be the level recorded for the silence (actually the quiet room noise)
at the beginning and end of the utterance. The vowels in "We" and
"see" have a level of about 34 dB above this. The vowels in "saw"
and "dogs" have intensities about 10 dB greater still, making them
well over twice as loud. The two fricatives, [s]
and [θ],
are much lower, [s] having
a mean of about 17 dB and [θ] being about 13 dB above the
base line. Generally, vowels such as [ɑ]
and [ɔ], in which the mouth is
more open, are about 5 dB greater than the vowels [i]
and [u], assuming all these vowels
have been said with an equal degree of stress. In this particular phrase,
the words "saw" and "dogs" were stressed, so their vowels
have an even higher intensity. In any utterance, the actual intensity of a
segment will depend on many factors, such as its position in the sentence,
the degree of stress on each word, and the personal characteristics of the
speaker.
Perhaps Professor Ladefoged omitted it because
it introduced a number of new issues that he felt he didn't have space to
go into deeply in the book (he does explain them in another book, Elements
of Acoustic Phonetics; Professor Keith Johnson, co-author of edition
6, also covers them in his Acoustic
and Auditory Phonetics). And it was because I felt that considerable
additional background knowledge was required in order to understand this paragraph
that I prepared the following slide show on decibels. Now, without these paragraphs
as a reminder, it might be easy to forget about decibels entirely. But they
are very important in understanding acoustics, including acoustic phonetics.
So we will proceed.
If you feel you have a pretty good handle on
how logarithms work (it wasn't that hard, was it?), it's time to begin
the decibels slide show. Be prepared to work through the material slowly and
carefully. You may need to go over it a number of times to absorb it all,
or at least the main points! Here is the link:
Understanding
decibels
The slide show contains names of units of measurement
that may be unfamiliar to you. Here is a site that helps you convert one type
of unit to another, for example, micropascals to microbars:
http://www.convertauto.com/
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