|
One
of the more popular models used to describe the propagation
of sound through water or air is the "source, path, receiver"
model (Richardson 1995). The basic parameters (there
are many we will not discuss) in this model related to the
receiver's perception of loudness are
-
source:
source level (SL)
-
path
or medium: transmission loss (TL), ambient noise level
(NL)
-
receiver:
signal to noise ratio (SNR), received level (AIL), detection
threshold (DT)
A
simple definition of sound propagation is:
RL
= SL - TL
where
TL = 10 log (Intensity at 1 meter/Intensity at r meters away
from the source, assuming spherical spreading)
Transmission
loss can also be estimated by adding the effects of geometrical
spreading, absorption and scattering. For our purposes we'll
deal only with spreading (TLg) and absorption loss
(TLa):
TL
= TLg + TLa
where
TLg
= 20 log (r/rref)
(for
geometrical spherical spreading; r is in meters), and
TLa
= a r x 10-3
where
a is the absorption coefficient (units are dB/km) and is a
function of frequency, r is in meters, and 10-3
is a conversion factor for m to km
The
rate at which sound is absorbed by water is related to the
square of frequency (a
µ f 2); lower
frequency sounds have low absorption coefficients and therefore
propagate long distances. If you know the frequency of the
sound you're dealing with, the attenuation coefficient (a)
can be looked up in the appropriate table or graph in any
acoustics textbook.
An
example.....
If
I am diving at a depth of 1 km (I'm a good diver!) and a humpback
whale is vocalizing (120 Hz) at a source level of 150 dB re
1µPa @ 1 m, what is my received level (assuming spherical
spreading)?
source
level = 150 dB re 1µPa @ 1 m, frequency = 120 Hz therefore
a ~
.003 dB/km
transmission
loss = TLg + TLa = 20 log (1 km/1 m)
+ .003(50????) = 60 +.15 = 60.15
dB re 1µPa @ 1 meter.
RL = SL - TL
= 150 - 60.15
RL
is about 90 dB
|
