PHYSICS

-------··CU
~ ..19· ~~
.0 '\
! Spores Growth \
Fig. 7.4. Formation of colony by the
amoebae Dictyostelium discoidem
.. '\l+;/ spores which are separating
Yr·"
~ Plasmodium ----.. 17. +'- from it into the medium. When
~.. Cell they encounter a nutrient rich
~ . agregation environment, they form a new
colony by way of chemotactic
M::;iO:/ migration (fig. 7.4).
7.3. ACOUSTIC WAVES
7.3.1. Classification of Acoustic Waves
A longitudinal wave that consists of a sequence of pressure
pulses or an elastic displacement of the material, whether gas,
liquid, or solid, is called an acoustic wave. There are three
categories of acoustic waves that cover different ranges of
frequency - sound, ultrasound, and infrasound waves.
7.3.2. Physical Characteristics of Sound Waves
Sound has three physical quannties: frequency (Q), intensity
(I), and waveform which correspond to the characteristics of
sound that we hear - pitch, loudness, and tone quality. Sound
waves that lie within the range of sensitivity of the human ear are
called audible sound waves; these waves have a frequency ranging
between approximately 20 Hz to 20000 Hz.
The pitch of a note is determined by the frequency; the more
vibrations per second of the sound, the higher will be the pitch
of the note.
The intensity or loudness of a sound is a measure of the energy
impinging on a unit area of receiver surface per unit of time. The
units of intensity are therefore W/m 2 • The intensity of the faintest
sound which can just be heard is about 10- 12 W/m2; the loudest
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tolerable sound has an intensity of approximately 1 W1m 2 •
Because of this wide range in intensities over which the ear operates,
and because the ear can only discriminate between sounds
of a certain intensity ratio whether they are loud or soft, a logarithmic
rather than a linear intensity scale is used. The intensity
level (B) of a sound wave is defined by the equation:
I
B = 19­
1 0
(7.15)
where 1 0
is an arbitrary reference intensity and is conventionally
taken as the threshold for hearing (i.e., 10- 12 W1m 2 ) . The intensity
7.1. Typical values ofthe sound level is a dimensionless
. levels from various sources quantity and the unit is
or causing specific symptoms the bel (B) or more
commonly, the decibel
Source of Sound ] Sound Level,dBJ (dB).
Damage of eardrum 160
Nearby jet airplane
Threshold of pain
Rock concert
Subway
150
130
120
100
I
dB=10Ig­
1 0
Table 7.1 gives
(7.16)
some
Busy traffic 80 typical values for the
Vacuum cleaner 70 sound levels from various
Normal conversation 50 sources or causing spe­
Mosquito buzzling 40 cific symptoms in hu­
Rustling leaves 10 mans.
Example. The intensity of an ultrasound pulse produced by a bat
during echolocation is about 10- 2 W1m 2 • Express this in decibels.
Solution. Using Equation (7.16), we shall find:
I 10- 2
dB = 10 Ig/ = 10 Ig 10- 12
= 10,10 = 100 dB
o
Tone quality corresponds to the complexity of the frequency
composition of the sound produced by the source. A precision
tuning fork will vibrate with only one frequency, producing a pure
tone. However, most vibrating bodies, in addition to the fundamental
or lowest frequency, have harmonics which are frequencies
that are single multi plies of the fundamentals. If f is the
fundamental frequency, then 2f is calIed the second harmonic. The
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