Revision: Sound Science English Medium Class 9 CBSE

The sound of frequency above 20,000 Hz is called ultrasonic.

When a sound wave travels in a medium, the maximum displacement of the particle of the medium on either side of its mean position, is called the amplitude (a) of the wave.

The time taken by the particle of the medium to complete its one vibration, is called the time period (T) of the wave.

The number of vibrations made by the particle of the medium in one second, is called the frequency (j) of the wave.

The distance travelled by a wave in one time period of vibration of the particle of the medium, is called the wavelength (λ).

The distance travelled by the wave in one second is called the wave velocity (V).

If the vibrations of medium particles are along the direction of propagation of the wave, thus forming compressions and rarefactions in the medium, the wave is called a longitudinal wave.

The wavelength of a sound wave is the spatial distance over which the wave's shape repeats. Because sound is a longitudinal wave, it is specifically defined as the distance between two consecutive points in the same phase of vibration.

The process by which bats and dolphins detect the presence of an obstacle by hearing the echo of the sound produced by them is called sound ranging. 

The time period of a particle executing S.H.M. is the time taken to complete one cycle and is denoted by T.

The linear distance between the two consecutive particles of a vibrating medium in the same phase is called its wavelength. It is denoted by the Greek letter lambda (λ).

Our ears are sensitive only to a limited range of frequencies from 20 Hz to 20,000 Hz, which is called the audible range of frequency.

The sound of frequency below 20 Hz is called infrasonic.

If the medium particles vibrate normal to the direction of propagation of the wave, forming crests and troughs, the wave is called a transverse wave.

The linear distance between the two consecutive particles of a vibrating medium in the same phase is called its wavelength. It is denoted by Greek letter lambda (λ).
OR
The distance travelled by the wave in one time period of vibration of the particles of medium is called its wavelength.
OR
In a longitudinal wave, distance between the two consecutive compressions or rare fractions is called wavelength.

The frequency of a particle executing S.H.M. is equal to the number of oscillations completed in one second. 

The return of a sound wave on striking a surface such as wall, metal sheet, plywood etc. back in the same medium is called the reflection of sound wave.

The distance travelled by the wave in one time period of vibration of a particle of medium is called its wavelength. It's S.I. unit is metre (m).

The sound heard after reflection from a rigid obstacle (such as cliff, a hillside, a wall of a building, edge of a forest etc.), is called an echo.

The number of vibrations per second taken by a wave is called its frequency.

The number of vibrations made by a particle of the medium in one second is called the frequency of the waves. It is also defined as the number of waves passing through a point in one second. It's S.I. unit is hertz (Hz).

The maximum displacement of the particle of medium on either side of its mean position is called the amplitude of the wave. Its S.I. unit is a meter (m).

The distance travelled by a wave in one second is called its wave velocity. It's S.I. unit is metre per second (ms^-1).

The sound heard after reflection from a distant obstacle (such as a cliff, a hillside, wall of a building, edge of a forest, etc.) after the original sound has ceased, is called an echo.