Hello Everyone, today we discuss the detection of ultrasound and application ultrasonic waves.

Detection of Ultrasonics:

We cannot directly detect the ultrasonic although some animals especially the bat can do so. However, the ultrasonic can be detected by the following methods:

1.Kundt’s tube method:

– restricted to low-frequency waves

– when ultrasonic wave go through the glass tube filled with hycopodium powder sprinkled in the tube, the powder gets blown off and forms heaps at nodal points

– the distance between any two nodes is equal to half of the wavelength.

D = €/2

€= wavelength

– the wavelength of an ultrasonic wave is estimated from above equation.

2.Thermal detector method:

Often used as a fine platinum wire is moved through the medium. Due to compressions and rarefactions, the temperature changes at the nodal points and resistance of the wire will be changed at that point whereas it will remain constant at the antinodes. The change in the resistance of the platinum wire with respect to time can be detected by using a sensitive bridge arrangement. The bridge will be in the balanced position when the platinum wire is at anti-nodes.

3.Sensitive flame method:

Ultrasonic waves are detected by moving a sensitive flame in the medium. At nodes points, the flame will flicker and at the antinodes, it will remain stationary.

By knowing the mean distance between two consecutive nodes, the ultrasonic wavelength can be determined as usual.

4.Quartz Crystal method:

This method depends upon the piezoelectric effect. when one pair of faces of a quartz crystal is exposed to ultrasonic waves, electric charges are developed on the other pair of opposite faces which are perpendicular to the previous one. these charges are amplified and detected by using electric circuits.

Determination of wavelength and velocity of ultrasonic waves in liquids:

The phenomenon of diffraction of light by ultrasonic waves passing through a liquid was first observed by Debye and Sears in America in 1932.

Ultrasonic waves are generated and propagate through the liquid medium by compression and rarefaction, the density varies from layer to layer due to a periodic variation in pressure. The change in density of the liquid, in turn, leads to a variation in the refractive index of the liquid. The density of the liquid will be maximum at nodal planes, while at the antinodal planes the density of the liquid will be minimum. Such a liquid column subjected ultrasonic waves behave like a grating. Under this condition, if a parallel beam of light is passed through this medium, the liquid behaves like a diffraction grating. This is known as Acoustical grating’.

Experiment:

In this liquid tank, ultrasonic quartz is kept which produces ultrasonic waves. Due to the reflection of ultrasonic waves from the surface of the tank, a stationary wave is formed in the liquid. Then light from a monochromatic source is collimated with the help of two lenses and can be passed through the liquid column which is in the direction perpendicular to the liquid column. We can observe maxima and minima pattern due to diffraction of light and distance between two consecutive maxima.

Using this acoustic diffraction method, the wavelength and hence the velocity of ultrasonic waves through liquids and gases at various temperatures can be determined.

Applications of ultrasound:

Science and Engineering:

• Used to detect flaws or cracks in metals
• used to detect ships, submarines, iceberg etc. in ocean
• used for soldering aluminum coil capacitors, aluminum wires and plates without
• using any fluxes.
• used for cutting, drilling holes and welding of metals
• used as a catalytic agent and accelerate chemical reactions
• also used to inspect any object without damage which is called Non-Destructive Testing.

Medicine:

• Used to remove kidney stones and brain tumors without shedding any blood
• used to remove broken teeth
• used for sterilizing milk and to kill bacteria
• used to study the blood flow velocities in blood vessels of our body
• used as a diagnostic tool to detect tumors and for cancer treatment.

It is also used to find the depth of the sea through SONAR technique.