Class 9 Science

# Sound

## Characteristics of Sound Wave

Following are the main characteristics of sound wave

• Wavelength
• Amplitude
• Time period
• Frequency
• Speed

To discuss the characteristics of wave, a graphical representation of sound wave is considered. The peak of a wave is called compression or crest. The valley of a wave is called rarefaction or trough.

### WAVELENGTH:

Wave length is the length between two consecutive peaks, i.e. crest or two consecutive valleys, i.e. trough of a wave. Wave length is represented by Greek letter λ (lambda). Louder sound has shorter wavelength and softer sound has longer wavelength. The SI unit of wavelength is metre (m).

### AMPLITUDE:

Magnitude of maximum disturbance on either side of the normal position or mean value in a medium is called amplitude. In other words, amplitude is the distance from normal to the crest or trough. Amplitude is the energy of sound. Louder sound has greater amplitude and softer sound has shorter amplitude. Thus, louder or softer sound is determined by its amplitude. Since louder sound has greater energy consequently greater amplitude, thus it travels to a longer distance. Softer sound has smaller energy consequently shorter amplitude, thus it travels to a shorter distance.

Amplitude is denoted by letter 'A'. The SI unit of amplitude is metre (m).

### TIME PERIOD:

Time required to produce one complete wave is called time period or time taken to complete on oscillation is called the time period of the sound wave. In other words, time in which a wave moves a distance equal to its wavelength is called time period.

The time period of sound wave is represented by letter 'T'. The SI unit of time period is second (s).

### FREQUENCY:

The number of sound waves produced in unit time is called the frequency of sound waves. For example, if a source of sound produces 20 sound waves in one second then the frequency is 20 Hz.

Time taken to calculate frequency is in second. Frequency is denoted by Greek letter ‘ν’ (nu). The SI unit of frequency is 'hertz'.

This name had been given after the German Scientist Heinrich Rudolph Hertz.

#### Relation between time-period and frequency

If 1 sound wave is produced by a source, in T second.

Therefore, in 1 second number of wave produced =1/T

Since, frequency is the rate of production of wave

∴ text{Frequency}=1/T

=>text{Frequency}=1/text{Time period}

=>v=1/T

Where, ν = frequency and T = time period

Thus frequency is the reciprocal of the time period of wave. This means the frequency is increased with decrease in time and vice versa.

### VELOCITY:

Distance covered by sound wave in unit time is called the velocity of sound wave.

∴ text{Velocity}=(text{Distance})/(text{Time taken})

If distance =lamda and Time =T

∴ text{Velocity}=lamda/T

Or, v=lamda/T ---(i)

SI unit of lamda is meter (m) and SI unit of time is second (s)

Therefore, SI unit of velocity =ms^(-1)

Therefore, velocity can be defined as distance travelled per second by sound wave.

Since, text{Frequency}\ ( nu) = 1/T

Therefore, equation (i) can be written as

v = lamda/T = 1/T = nu * lamda

=> v = nu*lamda

Where, v = velocity

lamda = Wavelenght

And nu text{(nu)} = Frequency

Thus, velocity of sound wave = frequency X wavelength

This is called WAVE EQUATION. Wave equation is applied to all types of waves.

Thus, velocity of sound wave is the product of frequency and sound wave.