PROPAGATION OF SOUND

Published on: February 12, 2024

Propagation of sound

Vibrating objects produce sound. A medium is necessary for the propagation of sound. Medium is a substance through which the sound is traveling. That medium can be solids, liquids, or gases. Air is the most common medium through which sound travels.

When an object starts vibrating, the particles of the medium near it also vibrate. This vibration makes the next adjacent particles vibrate. This process continues until the sound reaches our ears. When a disturbance is produced in any part of a medium, the disturbance moves through the medium without the translatory motion of particles. A wave is defined as a disturbance or vibration that moves through a medium. Hence, sound can be treated as a wave.

Sound waves are called mechanical waves because

  • The disturbance is periodic. i.e., a similar motion is repeating.

  • Medium is needed for propagation.

Compression and rarefaction

Compression (C): when a vibrating object is moved forward through a medium, it compresses the particles of the medium, and the pressure in that region increases. This region of high pressure is called compression.

Rarefaction (R): When the vibrating object moves backward, the pressure in that region decreases. This region is called rarefaction.

A series of compressions and rarefactions are created in the medium as the object moves back and forth. Thus, a sound wave is an example of a longitudinal wave.

Sound needs a medium to travel.

Sound cannot travel through a vacuum. A medium is necessary for its propagation. The following activity demonstrates that sound needs a medium to travel.

Bell in a bell jar: Take an electronic bell and suspend it inside an airtight glass bell jar. The jar is connected to a vacuum pump. Switch on the pump. You can observe that the sound becomes fainter gradually, even though it is passing through the bell. After the air is completely removed, you cannot hear any sound. From this, we can infer that sound cannot travel through a vacuum. It needs a medium to travel.

 

Sound waves are longitudinal waves.

There are two types of waves:

  1. Longitudinal waves

  2. Transverse waves

Longitudinal waves

Transverse waves

  • If the particles of the medium vibrate in a direction parallel to the direction of propagation of the disturbance or wave, it is a longitudinal wave.

  • It is composed of compression and rarefaction.

  • Here, the particles do not move, but they oscillate back and forth from their equilibrium position.

  • Example: Sound waves

Take a slinky. Stretch it on the ground or a long desk. Move one end of the slinky forward and backward (push and pull) in quick succession. We can see alternate regions of slinky getting compressed and alternate regions of slinky getting rarefied.

  • If the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the disturbance or wave, it is a longitudinal wave.

  • It is composed of crests and troughs.

  • Here, the particles oscillate up and down from their equilibrium position.

  • Example: Light waves

Take a trough. Fill three-fourths of it with water. Disturb any one point on the water's surface. We can see waves on the surface of the water, which are transverse waves.

 

Characteristics of a Sound Wave

Crest: A peak is called the crest of a wave.

Trough: A valley is called the trough of a wave.

Terms

Description

Compression

A region of high pressure and high density is called compression. Here, the particles are crowded together. It represents the upper portion of the curve.

Rarefaction

A region of low pressure and low density is called rarefaction. Here, particles are spread apart.

Wavelength

It is the distance between two consecutive compressions or rarefactions.

It is denoted by the Greek letter λ (lambda).

SI unit: meter (m)

Frequency

Definition: The number of oscillations per second is called the frequency of oscillation. ν

Unit: Hertz (Hz).

If an object oscillates twice per second, then its frequency is 2 Hz.

Time

Definition: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time of the wave. The time taken for one complete oscillation in the density of the medium is called the time of the sound wave.

Unit: Seconds (s)

          N = 1T        

 Relationship between frequency and time

Amplitude

Definition: The amplitude of an oscillation is the distance from the center of the oscillation to the extreme point. It is denoted by the letter A. It depends on the force that makes the object vibrate.

Unit: meter (m)

Pitch(Shrillness)

Definition: It is a property of sound that determines how high or low a sound is. Pitch or shrillness depends on the frequency of the sound. High-frequency sound is known as high-pitched sound or sharp sound. Low-frequency sound is known as low-pitched sound or flat sound. A high-pitched sound corresponds to a greater number of compressions and rarefactions passing a fixed point per unit of time.

Unit :Hertz(Hz)

Loudness

Definition: It is a property of sound that determines how loud or soft a sound is. Loudness depends on the amplitude of the oscillation. The larger the amplitude, the louder the sound, and vice versa. The loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. Loud sounds have higher energy, so they can travel a large distance.
 

Unit: Decibel

Symbol: dB

Timber or Quality of Sound

It is the characteristic that enables us to distinguish one sound from another that has the same pitch and loudness.

Tone

A sound with a single frequency is called a tone.

Note

A sound with a mixture of different frequencies is called a note. It is a pleasant sound.


 

Noise

A sound produced by irregular vibration is called noise. It is unpleasant and short-lived.

Music

Musical sound is one produced by regular vibration, which is pleasing.

 

Speed of the sound

The speed of sound is defined as the distance at which a point on a wave, such as a compression or a rarefaction, travels per unit of time.

Speed, v = distancetime

The distance traveled by the sound wave in one period (T) is its wavelength (λ)

So, 

               v = λTv = λ × 1Tv = λ × νv = λν

Speed = Wavelength × Frequency

The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.

The intensity of the sound

The amount of sound energy passing each second through a unit area is called the intensity of sound.

All sounds are not audible to us. A man with an average ear may hear a sound of frequency ranging from 20 Hz to 20000 Hz. Hence, 20 Hz is called the lower limit of audibility, and 20000 Hz is called the upper limit of audibility.

SI Unit : Wattmetre2

Difference between the loudness and intensity of the sound

Observe the two graphs. Here, we can see that the intensity of the two sounds is the same, but the amplitude is different. ie., loudness is different for the two sounds having equal intensity. Loudness is a measure of the response of our ears to sound. Intensity is the amount of sound energy transferred through a unit area.

 

Speed of Sound in Different Media

The speed of sound depends on the properties of the medium through which it travels.

The speed of sound depends

1. Temperature of the medium

2. State of the medium (the speed of sound decreases when we go from a solid to a gaseous state).

State

Substance

Speed in m/s at 250 c

Solids

Aluminium

Nickel

Steel

Iron

Brass

Flint Glass

6420

6040

5960

5950

4700

3980

Liquids

Sea Water

Distilled Water

Ethanol

Methanol

1531

1498

1207

1103

Gases

Hydrogen

Helium

Air

Oxygen

Sulfur Dioxide

1284

965

346

316

213

 

 

 

 

At 00C

At 220C

Speed of sound in air

331ms-1

344ms-1

Sonic boom

  • An object is said to be traveling at supersonic speed if the object’s speed is greater than the speed of sound.

  • Objects traveling at supersonic speed produce shock waves with a large amount of energy. This in turn causes a great pressure variation, which produces a very sharp, loud sound called the sonic boom’.

  • As shock waves carry a very large amount of energy, they can damage buildings, shatter glasses, etc.

 

 

Frequently Asked Questions

 

Q1. How does sound travel through different mediums?

Ans: Sound travels through solids, liquids, and gases. It travels fastest through solids, slower through liquids, and slowest through gases.

Q2. Define compression and rarefaction in a sound wave.

Ans: Compression is the region of a sound wave where air particles are closely packed together, while rarefaction is where particles are spread apart.

Q3. Explain the term 'frequency' in the context of sound.

Ans: Frequency is the number of vibrations or cycles of a sound wave per second, measured in hertz (Hz).

Q4. Describe the process of hearing in the human ear.

Ans: Sound waves enter the ear canal, causing the eardrum to vibrate. These vibrations are then transmitted to the three small bones in the middle ear, which amplify the sound. The vibrations are finally converted into electrical signals in the cochlea and transmitted to the brain via the auditory nerve.

Q5. What is a sonic boom?

Ans:

  • An object is said to be traveling at supersonic speed if the object’s speed is greater than the speed of sound.

  • Objects traveling at supersonic speed produce shock waves with a large amount of energy. This in turn causes a great pressure variation, which produces a very sharp, loud sound called the sonic boom.

  • As shock waves carry a very large amount of energy, they can damage buildings, shatter glasses, etc.