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The principle of sonar detection is rather simple (the implementation is the difficult part). A high-frequency sound is emitted from a transmitter toward the direction of a suspected unseen target. The transmitted sound reflects off the surface of the target, forming an echo that is detected by sensitive audio receivers. The target's range can then be calculated by measuring the time interval between the sound reflection and detection of the echo.

Figure 3-26 How sonar works onboard a military vessel.

Figure 3-26 shows a diagram of a typical sonar installation aboard a military ship sending out sound waves that strike an object, creating return echoes. A similar application is used in the operation of depth sounders used by fishermen for the purpose of locating schools of fish and keeping track of the bottom of the waterway.

Let's take a look now at the transmitter and receiver used in these applications.

The transmitter consists of a "speaker" device capable of emitting several tens or hundreds of watts of sound power. This system can also act as a receiving device, depending on the type of transmitting element used, or the transmitter and receiver can be two separate devices. In one application the transmitter consists of a piezoelectric material such as a ceramic element or crystal element. When subjected to a voltage the piezoelectric material is caused to change its physical size at a rate equal to the voltage's frequency and duration. In other words, the piezoelectric sensor oscillates at this frequency and the energy produced is "acoustically coupled" to the water, much as an ordinary loudspeaker is "coupled" to the air for sound transmission. The acoustic coupling is done by submerging the oscillating piezoelectric transducer into the water and then using specially designed sound baffling to concentrate the sound source in one direction. This type of transmitter can also be used as a sonar receiver system, in that the transmitting diaphragm can act as a microphone or hydrophone for receiving returning sound echoes. Figure 3-27 illustrates a typical transmitting and receiving pattern for a sonar system. The sound baffling referred to above is responsible for the highly directional characteristics seen in this pattern.

Figure 3-27 Transmitting and receiving pattern for a Sonar on board a ship.

Another popular form of sonar transmitter is the dynamic coil and diaphragm. Again, this is very similar to a dynamic loudspeaker in that a coil suspended inside a magnetic field is energized with a signal voltage. In this case, however, the voltage's frequency and pulse duration is such that a water-submerged diaphragm is made to oscillate rather than an air-suspended voice cone. Like the piezoelectric system described above, the moving-coil or dynamic coil system can also serve as a hydrophone for receiving return signals.

Sonar usually deals with the reception of very weak signals superimposed on a background of spurious noise signals. These noise signals come from at least three major sources:

(1) self-noise (noise from platform or vehicle where the sonar is installed, i.e., noise from the engine, pumps, propellers, etc.),

(2) ambient noise (noise from waves, ice motion, fish, human-made noise, etc.), and

(3) reverberation noise (noise from spurious reflections of the transmitted signal, i.e., reflections from the bottom, surface, or scattered layers of varying water densities due to temperature variations, etc.)

It is extremely difficult to design a transducer that can distinguish between background noise and the desired echo or sound source. Figure 3-28 demonstrates this problem.

In Figure 3-28(a) we see a graphical representation of three desirable signals we would like to detect. However, when noise is added to our signals we could get the pattern shown in Figure 3-28(b). This illustration could be from an oscilloscope pattern actually received from the output of a hydrophone. It is obvious from what we see here that it would be very difficult to distinguish the desired signals from the noise.

Figure 3-28 (a) Desirable signals;

(b) noise added to the desired signals;

(c) setting the detection threshold to detect only the desired signals.

One method frequently used with sonar transducers for separating signals from noise is to incorporate a threshold setting circuit. This circuit is set so that any signals

"peaking" above the background noise at a certain predetermined voltage level will be singled out for identification. This is illustrated in Figure 3-28(c)-an over simplification of the actual method used but depicting essentially what is done. In reality, a rather complicated statistical analysis is made of the noise patterns, and based on these data, the probability of what is noise is subtracted from the signal-plus-noise ' signals, leaving only the signal amplitudes desired. The threshold level is then set to indicate any signals occurring at or above this calculated level. To determine the range of an object that has reflected a signal back to the sonar's receiver, it is first necessary to know the signal's velocity as it travels through water. Having obtained that piece of information, we can then use the equation

= 0.5 vt

d ^(3-7)

where

d = distance (m, ft, etc.)

v = propagation velocity of sound (m/s, ft/s, etc.) t = time (s)

The factor 0.5 in eq. (3-7) is necessary to halve the results, since the distance over which the sound travels represents a double path. That is, the incident or outgoing path of the transmitted signal, plus the reflected path of travel of the incoming reflected signal, make up a path that is double the distance that separates the sound source and the reflecting surface.