# Concept and nature of sound.Wave length and frequency, infrasonic and Ultrasonic sounds, Sources of Infrasonic sound In nature, Ultrasonic sound characteristics and some applications.

Sound is a longitudinal wave, in which the particles oscillate to and fro in the same direction of wave propagation. Sound waves cannot be transmitted through vacuum. The transmission of sound requires at least a medium, which can be solid, liquid, or gas.

Newton to derive a relation for the velocity of sound in a gaseous medium, assumed that compression and rarefaction caused by the sound waves during their passage through the gas take place under isothermal condition. This means that the changes in volume and pressure take place at constant temperature. Under such conditions, Newton agreed that the velocity of sound wave in a gas is given by v = ?(P/ ?)

Frequency is the number of oscillations completed by an oscillator in one second. It is denoted by v. The SI unit of frequency is hertz (symbol Hz). Since v is the number of oscillations per second, the time taken to complete one oscillation is 1/v. Hence T =1/v or v = (1/T) s–1.

Sound waves are divided into three categories that cover different frequency ranges.

(1) Audible waves lie within the range of sensitivity of the human ear. They can be generated in a variety of ways, such as by musical instruments, human voices, or loudspeakers.

(2) Infrasonic waves have frequencies below the audible range. Elephants can use infrasonic waves to communicate with each other, even when separated by many kilometers. Various animals like Elephants, Whale and Rhino produces Infrasonic waves. Sources of infrasound in nature include volcanoes, avalanches, earthquakes and meteorites.

(3) Ultrasonic waves have frequencies above the audible range. You may have used a “silent” whistle to retrieve your dog. The ultrasonic sound it emits is easily heard by dogs, although humans cannot detect it at all. Ultrasonic waves are also used in medical imaging

Ultrasonic sound characteristics

• They have a high energy content.
• Just like ordinary sound waves, ultrasonic waves   get reflected, refracted and    absorbed.
• They can be transmitted over large distanceswith no appreciable loss of energy.
• If an arrangement is made to form stationary waves of ultrasonics in a liquid, it serves as a diffraction grating.  It is called an acoustic grating.
• They produce intense heating effect when passed through a  substance.

Ultrasonic waves are used for the following purposes.

1. Ultrasonic flaw detection.
2. Cutting and matching of hard materials.
3. Ultrasonic soldering and welding.
4. Measurement of flow devices.
5. Applications in medicine.
6. Thermal effect.
7. Ultrasonic as means of communication.

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