intensity or pressure. By indicating the accepted standard reference values
for intensity or pressure, it will be immediately obvious which measure is
being reported for either air or water.
The relationship between intensity and pressure is easily confused. Remember that: (1) as sound intensity increases by some factor, pressure
increases only by the square root of the factor; and (2) as pressure increases
by some factor, sound intensity increases by the square of that factor (after
Speaks 1992, p. 130).
2.2. Sound Velocity
Sound velocity varies with the transmission medium and, in general, is
higher in liquids than gases but highest in solids. It can be affected by temperature, salinity, and pressure or depth (see Beranek 1988); typically, these
variables are designated when reporting sound velocity. Thus, in air, at sea
level and 20°C, the speed of sound is 343 m/sec. In this case, velocity, c, is
determined as (Kinsler and Frey 1962, pp. 116 and 503)
(4)
where g is the ratio of specific heats (1.402), P 0 is the standard barometric
pressure (1.013 ¥ 10
5 N/m
2 ), and r 0 is the density (1.21 kg/m
3 ).
At sea level and 32% salinity, the speed of sound in water is 1,518.06 m/
sec at 20°C, almost fivefold greater than in air. At first glance, we would
expect that sound speed would be less in water than air, given water’s higher
c
P
= g
r
0
0
2. Physical Acoustics of Underwater Sound Communication
19
Table 2.1. Acoustic Measurements. The standard references are in bold. In air, the
standard reference is 20 mPa, whereas in water it is 1 mPa. Thus, the difference
between the in-air and in-water standard reference translates into a difference of
26 dB. Modified from Table 2.1 in Richardson et al. (1995).
dB re 20 mPa or 0.0002
Pascal (Pa)
dynes/cm
2
dB re 1 mPa
dynes/cm
2
1,000,000
10
7
240
214
100,000
1,000,000
220
194
10,000
100,000
200
174
1,000
10,000
180
154
100
1,000
160
134
10
100
140
114
1
10
120
94
0.1
1.0
100
74
0.01
0.1
80
54
0.001
0.01
60
34
0.0001
0.001
40
14
20 m
0.0002
26
0
10 m
0.0001
20
-6
1 m
0.00001
0
-26
for intensity or pressure, it will be immediately obvious which measure is
being reported for either air or water.
The relationship between intensity and pressure is easily confused. Remember that: (1) as sound intensity increases by some factor, pressure
increases only by the square root of the factor; and (2) as pressure increases
by some factor, sound intensity increases by the square of that factor (after
Speaks 1992, p. 130).
2.2. Sound Velocity
Sound velocity varies with the transmission medium and, in general, is
higher in liquids than gases but highest in solids. It can be affected by temperature, salinity, and pressure or depth (see Beranek 1988); typically, these
variables are designated when reporting sound velocity. Thus, in air, at sea
level and 20°C, the speed of sound is 343 m/sec. In this case, velocity, c, is
determined as (Kinsler and Frey 1962, pp. 116 and 503)
(4)
where g is the ratio of specific heats (1.402), P 0 is the standard barometric
pressure (1.013 ¥ 10
5 N/m
2 ), and r 0 is the density (1.21 kg/m
3 ).
At sea level and 32% salinity, the speed of sound in water is 1,518.06 m/
sec at 20°C, almost fivefold greater than in air. At first glance, we would
expect that sound speed would be less in water than air, given water’s higher
c
P
= g
r
0
0
2. Physical Acoustics of Underwater Sound Communication
19
Table 2.1. Acoustic Measurements. The standard references are in bold. In air, the
standard reference is 20 mPa, whereas in water it is 1 mPa. Thus, the difference
between the in-air and in-water standard reference translates into a difference of
26 dB. Modified from Table 2.1 in Richardson et al. (1995).
dB re 20 mPa or 0.0002
Pascal (Pa)
dynes/cm
2
dB re 1 mPa
dynes/cm
2
1,000,000
10
7
240
214
100,000
1,000,000
220
194
10,000
100,000
200
174
1,000
10,000
180
154
100
1,000
160
134
10
100
140
114
1
10
120
94
0.1
1.0
100
74
0.01
0.1
80
54
0.001
0.01
60
34
0.0001
0.001
40
14
20 m
0.0002
26
0
10 m
0.0001
20
-6
1 m
0.00001
0
-26