Sound

Sound is a form of energy. Sound produces sensation in our ears and we hear the sound.

According to Law of Conservation of Energy, energy can only be converted from one form to another; it can neither be created nor destroyed.

Thus, sound energy also is produced using other form of energy.

Examples

When we clap, a sound is produced. In clapping muscles energy is used.

Figure: Sound produced by clapping¹

Figure: Beating of drum²

When a drum is beaten with stick, a sound is produced.

When a wooden log is beaten using a hammer or a stone, a sound is produced. While beating some energy is used, and some part of that energy is converted into sound energy.

In car or bike horns, electrical energy is used to produce sound energy. In other word, in horns, electrical energy is converted into sound energy.
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Similarly in loudspeakers electrical energy is used to produce sound energy.

Thus, sound is produced using a energy. That is to produce sound energy a energy is required.

Production of Sound

Sound is produced by a vibrating object. In other words; when an object is set to be vibrating, sound is produced.

Example:

(a) When air passes through the leaves of trees; leaves are started vibrating, and it creates a hiss sound.

Figure: Sound produced when air passes through leaves³

(b) When wire of a music instrument is plucked, it starts vibrating and produces sound.

Figure: Guitar⁴

(c) Beating of a drum vibrates its diaphragm, which produces sound.

(d) We can produce sound because of vibration in our vocal cords, which are found in our throat.

(e) Bee produces buzzing sound because of vibrating of its wings.

Production of sound after feather flapping by a bird⁵

Reference: By Dan Pancamo - originally posted to Flickr as Hummingbird aerodynamics, CC BY-SA 2.0, Link

(f) When a bird flaps its wings, i.e. vibrates its wings, sound is produced.

(g) When a stretched rubber band is plucked it starts vibrating and produces sound.

Thus, we can produce sound by plucking, scratching, rubbing, blowing or shaking different objects.

Propagation of Sound

Reaching of sound from source to us is called Propagation of Sound. This means transmission of sound is called Propagation of Sound and not the travelling of sound.

Sound does not transmit in vacuum. For transmission of sound, a medium is must. Without medium sound does not transmit from source to anywhere.

Te matter or substance through which sound is transmitted is called Medium. Medium can be solid, liquid or gas.

How does Sound Propagate?

Sound moves through a medium from the point of generation to the listener.

We know that a medium constitutes of particles; the sound reaches from source to destination through these particles of medium. In propagation, particles of medium do not travel. Rather vibration of sound producing object disturbs particles or medium. This disturbance continues and sound reaches from one place to another place.

Steps of Propagation of Sound from Source to Listener

(a) When an object vibrates, it sets the particles of the medium around it vibrating.

(b) A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position.

Figure: Propagation of sound⁶

(c) It then exerts a force on the adjacent particles.

(d) As a result of which the adjacent particle gets displaced from its position of rest.

(e) After displacing the adjacent particles the first particle comes back to its original position.

(f) This process continues in the medium till the sound reaches to the listener.

(g) The disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium.

(h) Thus the particles do not travel all the way from the vibrating object to the ear of the listener. Rather only disturbance due to vibrating object reaches to the listener.

What is a Wave?

A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion. The neighbouring particles in turn produce similar motion in others. The particles of the medium do not move forward themselves, but the disturbance is carried forward.

Example

Figure: Ripples in water⁷

Reference: By Dan Copsey (DanCopsey1 at English Wikipedia) [Public domain], from Wikimedia Commons

When a stone is thrown in a pond, this creates a disturbance in the water. This disturbance creates wave in water which can be seen in the form of ripples in water. These ripples in circles create more circles and finally vanish out after reaching to a some extent.

Type of Waves

Waves can be divided into Two types, Longitudinal Wave and Transverse Wave.

Longitudinal Wave

Wave in which individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance is called Longitudinal Wave.

Figure: Longitudinal Wave⁸

Reference: By Christophe Dang Ngoc Chan (cdang) - Own work, CC BY-SA 3.0, Link

Transverse Wave

Wave in which individual particles of the medium oscillate up and down about their mean position as the wave travels, is called Transverse Wave.

Figure: Transverse wave⁹

Reference: By Christophe Dang Ngoc Chan (cdang) - Own work, CC BY-SA 3.0, Link

Sound is a Wave

Same formation of ripple like situation happens during propagation of sound in a medium and sound reaches to destination. And hence sound can be visualized as a Wave.

Sound waves are characterized by the motion of particles in the medium and are called mechanical waves.

Sound is Which type of Wave

Sound wave is Longitudinal Wave.

We hear most of the Sound coming through air. For example: calling by someone using our name, sound coming from the TV set, sound coming from car horn, sound coming from music system, etc.

Here air is medium which propagates the sound wave. Without any medium sound wave does not propagate.

How Sound Wave Travel

Sound is produced by a vibrating object. When vibrating object moves forward, it pushes and compresses the particles of medium in from of it. This pushing and compression creates a region of high pressure.

This region of high pressure is called compression (C). Compression is generally denoted by letter "C".

This compression starts to move away from the vibrating object.

When vibrating object moves backward, it creates a low pressure zone. This low pressure zone of particles of medium is called Rarefaction (R). The Rarefaction is denoted by letter "R".

With moving of vibrating object back and forth rapidly, a series of compression and rarefactions is created in the air. These series of compressions and rarefactions make the sound wave that propagates through the medium.

Compression: The region of High Pressure is called Compression.

Rarefaction: The region of Low Pressure is called Rarefaction.

Density of Medium and Speed of Sound Wave

Pressure is related to the number of particles of a medium in a given volume. More density of particles in the medium gives more pressure and less density of particles in the medium gives less pressure.

Thus, sound waves move fastest in solid and slowest in air. The speed of sound wave in liquid is moderate that is in between air and solid.

This means, order of speed of sound wave is

Solid > Liquid > Air

Sound waves travel fastest in solid. In liquid; sound wave travels slower than solid but faster than air. Sound waves travel slowest in air. Sound wave does not propagate in vacuum.

Sound Needs a Medium To Travel

Sound is mechanical waves and hence needs a material medium like air, water, liquid, etc. for its propagation. Sound waves cannot travel through vacuum.

On the other hand electronic waves can travel without a medium.

Demonstration of propagation of sound wave in air and vacuum

Bell Jar Experiment of propagation of sound waves

Materials required

An electric bell, wire, switch, electric cell, an air tight glass bell jar, a vacuum pump

Figure: Bell Jar Experiment¹⁰

Method

(a) First electric bell is fitted in the air tight.

(b) After that, bell is connected with electric cell.

(c) Vacuum pump is also connected with glass bell jar.

(d) Now; switch of bell is put on.

(e) Observe the sound of bell

(f) Now; air inside the glass bell jar is gradually vacuumed out.

(g) While vacuum of air, the sound of bell is kept observing.

(h) After vacuumed out of air completely from bell jar, sound of bell has been observed.

Observation

(a) When air was full in the bell jar, sound of bell heard clearly.

(b) Sound of bell gradually becoming slower with vacuuming out of air.

(c) After completely vacuum out of air, no sound of bell was coming out even bell was connect and in working.

Inference

This shows that, sound does not travel in vacuum. And travel in more speed with more density of air.

Graphic Representation of Sound Wave

Sound waves can be represented graphically using pressure and density variations.

Figure: compression and rarefaction of sound waves¹¹

In figure (a) dotted lines represent the particles of medium. And sound wave is shown in the form of dotted form. Areas with high density of dots show the high density and high pressure of sound wave. And areas with low density of dots show the low density and pressure of sound wave.

In figure (b) sound wave is shown in the form of vertical lines. Vertical lines represent the particles of medium. Area of high density of vertical lines represents the high density and high pressure of sound wave. And area of low density of vertical lines represents the low density and low pressure of sound wave.

Figure: compression and rarefaction of sound waves¹²

High density and high pressure of sound wave is called Compression and is represented by letter "C". Low density and low pressure of sound wave is called Rarefaction and is represented by letter "R".

Figure: Crest and Trough formed by sound wave¹³

In the figure (c) the sound wave is shown using wavy lines. The peak of line shows the area of high pressure i.e. Compression. And valley, i.e. extreme lower end of line shows the area of low pressure i.e. Rarefaction.

A peak of the wave is called Crest and, A valley of the wave is called Trough.