6. SOUND PRODUCTION AND DETECTION
139
Sound velocity ft/sec
0
50
100
200
Fig. 1. Velocity of sound in the sea. The two series of determinations show that
velocity varies from about 5070 ft/sec (1546 meters/sec) at the surface, to about
4800 ft/sec (1464 metedsec) at depths below 500 fathoms (915 meters). After
Tschiegg and Hays (1959).
the sea at dead calm and with no vessels or sound-producing animals
about, there is still an ambient noise whose pressure is about 0.18 to 0.20
dyne/cm2. This is usually expressed in terms of decibels (dB) with respect to a reference level of 1 dyne/cm2 [ = 1 pb (microbar)], and, in this
case, the noise level would be about 15 dB below this reference level.
In air, the reference level is usually taken to be 0.0002 pb, since this is
the standard threshold of human hearing for a frequency of 1000 cps
(Hz). This reference level has little objective meaning in underwater
acoustics. Thus the 1-pb level is now almost universally used as the reference level in water and all the sound pressure values given in this report
will be in decibels re 1 pb (abbreviated to dB pb) (Table I ) .
139
Sound velocity ft/sec
0
50
100
200
Fig. 1. Velocity of sound in the sea. The two series of determinations show that
velocity varies from about 5070 ft/sec (1546 meters/sec) at the surface, to about
4800 ft/sec (1464 metedsec) at depths below 500 fathoms (915 meters). After
Tschiegg and Hays (1959).
the sea at dead calm and with no vessels or sound-producing animals
about, there is still an ambient noise whose pressure is about 0.18 to 0.20
dyne/cm2. This is usually expressed in terms of decibels (dB) with respect to a reference level of 1 dyne/cm2 [ = 1 pb (microbar)], and, in this
case, the noise level would be about 15 dB below this reference level.
In air, the reference level is usually taken to be 0.0002 pb, since this is
the standard threshold of human hearing for a frequency of 1000 cps
(Hz). This reference level has little objective meaning in underwater
acoustics. Thus the 1-pb level is now almost universally used as the reference level in water and all the sound pressure values given in this report
will be in decibels re 1 pb (abbreviated to dB pb) (Table I ) .