Substituting this into equation 1 we have:
A devise which generates
acoustic energy is called a projector. An acoustic projector can take several
forms. Sound transducers convert electrical energy into mechanical
(kinetic) energy which then propagates through the water as a longitudinal
pressure wave. This process is known as acoustic transduction. The
same effect can be created by using underwater explosions, electrical sparks or
air guns, although these approach is less well controlled. Here we will
concentrate primarily on the acoustic transduction.
The process of acoustic transduction is a two-way process as illustrated below:
Electrical Energy <=> Mechanical Energy <=> Longitudinal Pressure Wave
Transducer
=========================================>
<======================================== Hydrophone
A hydrophone is an acoustic listening devise used for sensing sound underwater. The hydrophone relies on the reverse process where longitudinal sound waves stress the surface of the hydrophone sensor (mechanical energy) that in tern converts the physical displacement into an electrical signal. This electrical signal can be easily logged and interpreted by a specialized electrical unit or computer system. Often the same instrument is used to generate and receive acoustic energy. In this case the system is termed, 'monostatic'. If a separate projector and hydrophone are used the system is referred to as 'bistatic'.
Both the process of transduction (hydrophones) and reverse transduction (transducers) can be achieved by using either the piezo-electric effect or the principle of magnetostriction.
The piezo-electric effect: Definition. 'The generation of a potential difference (voltage) across the opposite faces of certain non-conducting crystals as a result of the application of mechanical stress.' The reverse piezo-electric effect is the opposite to this and is the principle of acoustic transduction.
About 2/3 of the 32 crystalline classes exhibit the piezo-electric effect. Quartz is a well known example which has been important in the field of acoustic transduction in the past. More recently piezo-electric ceramics like lead zirconate titanate (PZT) are used.
Magnetostriction. This effect utilizes the physical changes of length of a ferromagnetic material due to the application of a magnetic field. A simple example is a nickel iron hoop bound with a magnetizing coil. When an alternating current energizes the coil a magnetic field is set up which in tern causes a change in shape of the ferromagnetic material. As the metal hoop continually changes shape when the alternating current is applied, a longitudinal pressure wave is generated in the water. This sort of transduction is particularly useful for low frequency applications.
Upper Limits To Source
Level
You
might imagine that the higher the acoustic source level the more powerful the
system, and the longer the detection range will be. However, there is a distinct
upper limit to this relationship. Generally, sonar range will increase with the
source power until one of two things happen. Firstly, if the system is used
close to the sea surface or bed, or if there are large amounts of particulate in
the water column, back scattered acoustic reflections (reverberations) may mask
echoes from the intended target. Thus, the sonar range may actually decrease
with further increase in power. Secondly, if the source level is too high
cavitation may occur. This results in the formation of bubbles on the surface of
the transducer which reduces the efficiency of the transducer and disrupts the
beam pattern of the acoustic pulse.
Calculation of Source
Level
Remember that intensity
(I) is defined as power (P) per unit area (A). Also remember that the a
quantity expressed on the decibel
scale is given by; N = 10 log(I/Io), where Io is the standard intensity (=
6.504x10-19W/m2). When
calculating the source level (SL) we consider that sound from an acoustic source
(S) spreads omnidirectionally (equal in all directions) outwards from the
source. We also take a standard reference distance away from the source of 1m.
Thus, our definition of SL on the dB scale becomes:
SL = 10 log(I1 / Io)
SL = 10 log([P1/A1] / Io) Equation 1
Here I1, P1and A1 are the intensity, power and area respectively measured one metre away form the source. The appropriate area here is the surface-area of a sphere with a radius (R) of 1 metre (A = 4pR2 = 4p for R=1).
Substituting this into equation 1 we
have:
SL = 10 log([P/4p] / 6.504x10-19) Equation 2
Where P is the power of the acoustic source. Rearranging this equation we have;
SL = 10 log(P/ [4p6.504x10-19])
or equivalently;
SL = 10 log(P) - 10log(4p6.504x10-19)
Evaluating this we have:
SL = 10 log(P) + 171 (dB re. 1mPa) Equation 2