Propagation of sound

Tanusri Gururaj, Academic content writer of Physics at Edumarz

  • Medium:

The substance through which sound is transmitted.

It can be solid, liquid, or gas.

  • How does sound reach our ears?

Sound moves from the point of generation to the listener through a medium. When an object vibrates, it sets the particles nearby vibrating. A particle in contact with the vibrating object is first displaced from its equilibrium position and then exerts a force on the adjacent particle. This causes the adjacent particle to get displaced from rest. This process goes on till the sound reaches the ear and, after the adjacent particle is displaced, the first particle comes back to its initial position.   

When particles in a medium set adjacent particles in motion, it is a disturbance that moves through the medium. The disturbance is carried forward and, the particles themselves do not move. 

  • Mechanical waves: 

These waves require a medium to propagate. 

Sound waves are mechanical waves. 

When a vibrating object moves forward, it pushes and compresses the air in front of it, resulting in a high-pressure region. This region is called compression. 

When the vibrating object moves backward, a region of low pressure is created. This region is called rarefaction. 

When the object moves forward and backward, it creates a series of compressions and rarefactions. This allows sound to propagate through the medium. 

  • Pressure is directly proportional to the density of the particles of a medium. 

  • Let us perform an experiment to prove that sound requires a medium to travel.

Take an electric bell and suspend it inside an airtight glass bell jar. Connect this bell jar to a vacuum pump. After starting the vacuum pump, the air is pumped out of the jar and the sound becomes fainter. Even though the current remains the same, the sound becomes very feeble as the air becomes less and less. 

  • Sound waves are longitudinal because the particles of the medium travel in a direction parallel to the direction of propagation of the disturbance. 

  • In transverse waves, the particles of the medium move at right angles to the direction of wave propagation. Example: light wave.

  • Figures a and b show the variation of density and pressure as sound waves propagate through a medium. 

Figure c shows the sound wave in graphic form. 

The peak is called the crest and it represents maximum compression. In regions of compression, the pressure and density are high.

The valley is called the trough. Here the particles are spread apart, and this region has low pressure. 

The distance between two consecutive rarefactions or compressions. It is represented by lambda (𝝀). 

SI unit: metre (m)

  • Frequency: 

The number of oscillations per unit time. One oscillation is completed when there is a change in density from the maximum to the minimum value, to the maximum again. It is represented by nu (𝛎). 

SI unit: hertz (Hz)

  • Time period:

The time taken to complete one oscillation. It is represented by ‘T.’ 

SI unit: second (s)

Frequency = 1/Time Period

  • Pitch:

Our brain’s interpretation of the frequency of a sound that is emitted.

Higher is the pitch, higher is the number of compressions and rarefactions passing a fixed point per unit time.

Faster vibration = higher frequency = higher pitch 

  • Amplitude:

The maximum disturbance magnitude on either side of the mean value. It is represented by ‘A.’

Unit of amplitude for sounds: Same as the unit of density or pressure

  • Loudness:

The measurement of the ear’s response to a sound.

Directly proportional to amplitude. 

Depends on the force with which we make an object vibrate. 

Loud sounds travel larger distances as they have more energy. 

As a sound moves away from its source, the loudness decreases. 

  • Quality/timber of sound:

Helps to differentiate two sounds having the same pitch and loudness. 

If a sound is more pleasant, it is said to have a rich quality. 

  • Tone:

A sound having a single frequency. 

  • Note:

A sound that has multiple frequencies. 

  • Speed of sound:

The distance travelled per unit time by compression or rarefaction.

Speed = distance/time

= 𝝀/T

= 𝝀 𝛎 (1/T = 𝛎)

Hence, speed = wavelength x frequency

It remains the same for almost all frequencies in a given medium under the same physical conditions. 

It depends on the following-

  1. Temperature: As it increases, the speed of sound also increases.

  2. Medium: As the medium changes from solid to gas, the speed of sound decreases. 

  • Intensity of sound:

The amount of sound energy that passes through a unit area every second.

Pictures courtesy:

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