• Sound Wave: A Longitudinal Wave

    When the particles of a medium oscillate (particles of the medium vibrate back and forth) in same direction in which the wave is being propagated, then the wave so formed is called a longitudinal wave.

    Example-01: A long and light spring attached to hook in a wall and held horizontally, such that there is no tension in the spring. When this spring is pushed towards the wall and then pulled away from the wall rapidly, there appear regions, where the coils of the spring come very close. These regions where the coils of the spring come closer than normal are called compressions and denoted by C. Conversely, there appear regions where the coils of the spring are farther apart. These regions where the coils of the spring are farther apart than normal are called rarefactions and denoted by R.

    Example-02: The sound waves produced in air is an example of longitudinal wave. When the sound wave propagates through air, the particles of air oscillate in the direction of wave propagation, with the result the regions of compressions and rarefactions are formed in the air which alternate with one another. Thus, sound energy moves outward.

    Compression: A part of a longitudinal wave in which particles of air are closer to one another than normal, such that there is a momentary decrease in volume and increase in pressure is called the region of compression.

    Rarefactions: A part of a longitudinal wave in which particles of air are farther away from one another than normal, such that there is a momentary increase in volume and decrease in pressure is called the the region of faction.


    We can describe a sound wave by wavelength, frequency, amplitude, time period and velocity.


    (i) Wavelength

    In sound waves the combined length of a compression and an adjacent rarefaction is called its wavelength. The distance between the centres of two consecutive compressions or two consecutive rarefactions is also called its wavelength. It is denoted by the Greek letter lamda λ. Its SI unit is metre.

    (ii) Frequency

    No. of complete waves produced in one second or number of vibrations per second is called frequency. Number of compressions or rarefactions passed in one second is also frequency.

    Frequency of wave is same as the frequency of the vibrating body which produces the wave. The SI unit of frequency is hertz (Hz). The symbol of frequency is v (nu). One Hz is equal to 1 vibration per second. Bigger unit of frequency is kilohertz kHz = 1000 Hz.

    (ii) Time Period

    Time taken to complete one vibration is called time period. Time required to pass two consecutive compressions or rarefactions through a point is called time period. SI unit of time period is second (s). Time period is denoted by T. The frequency of a wave is the reciprocal of the time period.

    (iv) Amplitude

    The maximum displacement of the particle of the medium from their original undisturbed position is called amplitude of the wave. Amplitude is denoted by A and its SI unit is metre (m). 

    (v) Speed

    The distance travelled by a wave in one second is called velocity of the wave. Its SI unit is metre per second (ms-1). 

    Speed = Distance travelled ÷ Time taken

    So,  v = λ  ÷ T

    (λ = Wavelength of the waves travelled in one time period T)

    or,  v = λ × f

    Thus,    Speed = Wavelength ×  frequency

    This is the wave equation. 


    Characteristics of a Sound Wave

    Pitch: The pitch of sound depends on the frequency of sound (vibration). It is directly proportional to its frequency. Greater the frequency, higher is the pitch and lesser the frequency, lower is the pitch.

    A woman’s voice is shrill having a high pitch while a man’s voice is flat having low pitch. High pitch sound has large number of compressions and rarefactions passing a fixed point per unit time.


    Loudness: The loudness depends on the amplitude of the sound wave. It is the measure of the sound energy reaching the ear per sec.

    Greater the amplitude of sound wave, greater is the energy, louder the sound; short is the amplitude, less is the energy, soft is the sound. Loudness is measured in decibel ‘dB’.


    Quality or Timbre: The timbre of a sound depends on the shape of sound wave produced by it. It is the characteristic of musical sound. It helps us to distinguish between two sounds of same pitch & loudness.

    Sound of single (same) frequency is called tone while a mixture of different frequencies is called note.

    Noise is unpleasant to hear while music is pleasant to hear and it is of good quality. The distinction between music and noise is mathematical form. Music is ordered sound while noise is disordered sound. Music and noise are both mixtures of sound waves of different frequencies.

    The component frequencies of music are discrete (separable) and rational (their ratios form simple fractions) with a discernible dominant frequency. The component frequencies of noise are continuous (every frequency will be present over some range) and random (described by a probability distribution) with no discernible dominant frequency.