Revision: Sound: Production of Sound Science SSC (English Medium) 7th Standard Maharashtra State Board

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).

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.

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.

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 above 20,000 Hz is called ultrasonic.

The sound of frequency below 20 Hz is called infrasonic.

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

Pitch is that characteristic of sound by which an acute (or shrill) note can be distinguished from a grave (or flat) note of the same loudness and quality.

The intensity of a sound wave at any point of the medium is measured as the amount of sound energy passing per second, normally through a unit area at that point. 

Timbre is the characteristic that distinguishes two sounds of the same loudness and pitch of two different instruments.

Intensity is defined as the amount of energy per unit of time perpendicular to the direction of propagation of the wave.

Loudness is the characteristic by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch and quality.

• Loudness of sound depends on the amplitude of the wave, and a loud sound corresponds to a wave of large amplitude.
• Intensity of a sound wave is the amount of sound energy passing per second, usually through a unit area, and its unit is watts per square metre² (W m⁻²).
• Loudness is subjective because it depends on the listener's ear sensitivity, whereas intensity is objective.
• Loudness depends on the square of the amplitude, the distance from the source, the surface area of the vibrating body, the density of the medium, and the presence of resonant bodies.
• Sound level is expressed in decibels (dB), and noise pollution is caused by undesirable loud and harsh sounds at levels above 120 dB.