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Characteristics of a Sound Wave

A wave can be completely described by certain terms like wavelength, wave velocity, wave frequency, time period, amplitude, etc. A sound wave is represented graphically in the following figure.

Density and Pressure varies when the sound wave moves in the medium. The density and pressure of the medium at a given point of time varies with distance above and below the average values for both. Compressions are the regions where the particles are close together and are represented by the upper portion of the curve in the graph. The peak represents the maximum compression. Hence compressions are the regions where pressure and density are very high. Rarefactions, on the other hand, are low pressure regions where the particles of the medium are spread apart. In the below figure they are represented by the valleys in the lower portion of the curve. The peak forms the crest and the valley is the trough of a wave.

The distance between two consecutive compressions or two consecutive rarefactions is called a wavelength. This is shown in the figure. It is represented by the Greek letter lambda (λ). The SI unit for wavelength is m.

Frequency tells us rate at which the waves are produced by their source. 'The number of complete waves (or cycles) produced in one second or the number of vibrations per second' is called frequency of the wave. The unit of frequency is hertz (Hz). One hertz is one vibration per second. Frequency is denoted by the Greek letter 'nu' (n).

'Time period of a wave is the time taken by two consecutive compressions or two consecutive rarefactions to cross a fixed point'. Or the time taken for one complete oscillation in the density of the medium ( move from the maximum value to the minimum value and back to the maximum value) is known as time period of the wave. It is denoted by T and the SI unit is second (s). 'The frequency of a wave and its time period are reciprocal of one another'. Suppose the time period of a wave is T seconds.

In T seconds, the number of wave produced is 1.

Then in 1 s the number of waves produced = 1/T. But the number of waves produced in 1 second is called the frequency of the wave. Therefore frequency and time period are reciprocal of one another.

The maximum displacement of the particles of the medium from their mean position, when a wave passes through the medium, is called the amplitude of the wave. It measures the height of a crest or the depth of a trough. It is represented by 'A'. The force with which an object is made to vibrate determines the amplitude of vibrations.

The distance travelled by a wave in one second is known as wave velocity. It is denoted by V. The SI unit for wave velocity is m/s. Speed = Distance / Time

Therefore V = λ/T
= λ x 1/T
= λ ν


Characteristics of Sound

Any sound has the following characteristics:

  • Its loudness – which depends on the amplitude of vibrations. Greater the amplitude the louder the sound. Loud sounds can travel larger distances as they have more energy.
  • Its pitch – which depends on the frequency of vibrations. The faster the vibrations, the higher the frequency and so the higher the pitch. A high pitch sound corresponds to more number of compressions and rarefactions passing a fixed point per unit of time. Different objects of varying sizes and conditions vibrate at different frequencies to produce sounds of different pitch.
  • Its quality or timbre – this helps us to identify one sound from another, one voice from another having the same pitch and loudness. Pleasant sounds have rich quality.
  • Its intensity – it is the amount of energy that passes through unit area each second. Loudness and intensity are not the same. Loudness is a measure of the response of the ear to the sound. Two sounds may be of the same intensity but we may be able to hear one as louder than the other because our ear detects it better.

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