Sound Navigation and Ranging(SONAR)
It is based on the principle of Echo – Sounding. When the Ultrasonic waves are transmitted through water, it is reflected by the objects in the water and will produce an echo signal. The change in frequency of the echo signal, due to Doppler Effect helps us in determining the velocity and direction of the object.
It consists of timimg section which triggers the electric pulse from the pulse generator. This pulse generator is connected to the transducer so that ultra sonic can be produced. The transducer is further connected with the CRO for display. The timing section is also connected to the CRO display or reference of the timing at which the pulse is transmitted as shown in the block diagram(Figure 1.6.1).
The transducer is mounted on the ship’s hull without any air gap between them as shown. The timing at which the pulse generated is recorded at the CRO or reference and this electrical pulse triggers the transducer which is kept in hull of the ship to produce ultrasonic waves due to the principle of inverse piezo electric effect.
These ultrasonic waves are transmitted through the water in sea. On stricking the object the ultrasonic waves (echo pulses) are reflected in all directions as shown in the figure 1.6.2
Definition: The Ultrasonic sound waves that propagate into the liquid media result in alternating high pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency.
During the low pressure cycle, high intensity ultrasonic waves create small vcuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high pressure cycle. This phenomenon is termed cavitation.
During the low pressure cycle, high intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high pressure cycle. This phenomenon is termed cavitation.
During the implosion very high temperature(approx 5,000K) and pressures (approx 2,000atm) are reached locally, The implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity.
The ultrasonic waves generated with the help o a quartz crystal inside the liquid in a container sets up standing wave pattern consisting of nodes and anti-nodes. The nodes are transparent and anti-nodes are opaque to the incident light.
In effect the nodes and anti-nodes acts like grating(a setup of large number of slits of equal distance) similar to that of ruling in diffraction grating. It is called as acoustic grating or aqua grating. At nodes the density of the liquid is maximum and at antinodes density is minimum. This arrangement is very much similar to the diffraction grating and is called acoustic grating.
Hence, by using the condition for direction, we can ind the wavelength of ultrasound and thereby the velocity of sound in the liquid medium.
When ultrasonic waves are generated in a liquid kept in rectangular vessel, the wave can be reflected from the walls of the vessel. The direct and reflected waves get superimposed, which causes a standing wave to be formed. The density o the liquid at the node will be more than the density at an antinode. Under these conditions, if a beam of light is passed through the liquid at right angles to the wave the liquid acts as a diffraction grating. Such a grating is known as an acoustical grating.
Here, the antinode acts as the transmitting slit and the node acts as the opaque part… thus resembling a normal ruled diffraction grating. This is obvious because the nodes have points o minimum density and hence allow more amount o light to pass through them than those at antinodes. Thus, the nodes act like slits.
A Diffraction grating is an extremely useful device. It consists of large number of narrow slits side by side. The slits are separated by opaque surfaces. When a wavefront is incident on the grating surface, light is transmitted through the slits and obstructed by the opaque spaces. Such a grating is called transmission grating.
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