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Sound Absorption

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Introduction

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In our daily life every day we hear sound from various sources like humans, birds, bells, machines, vehicles, televisions, radios etc. A sound can be considered as the form of energy that produces the sensation of hearing. Like all wave, the sound wave also travels at a certain speed and has the properties of frequency and wavelength. Sound waves are caused by vibrating an object. In this chapter, we are going to learn how sound is produced, the wavelength of sound waves, how it is transmitted through a medium.


Definition of Sound Absorption

Sound absorption is defined as the loss of sound energy when sound waves come into contact with an absorbent material such as ceilings, walls, floors and other objects.  As a result of that, the sound is not reflected back into space. The wavelength of sound waves is not directly sensed, but indirectly evidence is found in the correlation of the size of musical instruments with their pitch. 

Sound absorbent materials are used to create a suitable acoustic environment within a space by reducing the ‘reverberation time’. Reverberation affects the way space 'sounds'. A long reverberation time produces a loud and noisy sound in the room. Rooms that are designed for speech typically have a short reverberation time of less than 1 second. Just in the reverse process, a longer reverberation time can enhance a music hall by adding richness, depth and warmth to the music. Sound waves are caused by the simple but rapid mechanical vibrations of various elastic bodies.

Sound absorption can be an important factor for spaces such as:

  • Sports halls.

  • Schools.

  • Recording studios.

  • Lecture theatres.

  • Concert venues, cinemas and theatres.

How Does Sound Absorption Work?

Sound waves when encounters with an object, one of two things will happen- it can be absorbed, or it can be reflected. When sound is reflected, it is sent back into the medium and when it’s absorbed by a sound-absorbing material, it turns into a small amount of heat energy. Acoustics science involved in finding the right balance between absorption and reflection.

Let’s understand this with an example. Imagine you are watching a band play in the auditorium. If the entire space were covered with sound-absorbing materials, then the walls would have absorbed too much sound and make the music sound flat. The musicians would also have to work hard in order to not make any mistakes. However, some reverberation would help the music ring sounds beautiful, as long as there’s not too much echo produced. There is also a chance of sound transmission from one room to another. Just like sound absorption, certain materials are used for blocking sound. sound insulation is used to control sound between rooms. 


Difference Between Reflection and Absorption of Sound

Reflection of Sound:

When sound travels in a provided medium it hits the surface of another medium and returns back in some various way. This process is called a reflection of sound waves.

There are things like a wall, a window that prevents sound from moving across them and these things reflect the sound. This is known as sound reflection.

The reflection of sound is used in horns, megaphone and in shehnais.

Applications of reflection of sound are an echo, hearing aid, soundboard etc.


Absorption of Sound:

It is defined as the tendency of material absorbs light.

When the sound strikes the surface most of them are absorbed and some amount of sound energy is reflected back.

There are things like sofa cover, the curtain which can absorb sound and this is known as sound absorption.

These are required in concert halls, recording studios.


How is Sound Propagated in a Medium?

Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is known as a medium. It can be solid, liquid or gas.

Sound moves through a medium from the point where it is generated to the listener point. When an object vibrates, the particles of the medium around it also start vibrating. The particles do not travel all along from the vibrating object to the ear. When a particle of the medium comes in contact with the vibrating object first it is displaced  by the vibrating object from its equilibrium position. After that, it exerts a force on the adjacent particle. So, the adjacent particle also gets displaced from its position of rest. After displacing the adjacent particle in the medium the first particle comes back into its original position. This process continues in the medium till the sound reaches our ear. The disturbance produced here is by a source of sound in the medium but not the particles of the medium.

A wave is a disturbance that moves through a medium when the particles of the medium set will bring neighbouring particles into motion. They in turn produce similar motion in others. The particles of the medium do not move forward by themselves; they are moved by the disturbance produced and it is carried forward. This happens during the propagation of sound in a medium and hence sound can be visualised as a wave.

FAQ (Frequently Asked Questions)

1.What Does Sound Absorption Mean?

Ans: Sound absorption means that sound energy is converted into mechanical vibration energy and/or heat energy.

2.What are the Physical Factors that Affect Sound Propagation?

Ans: Following are a few factors that affect the propagation of sound:

  •  Atmospheric Turbulence: If the atmosphere in which the sound wave is travelling is turbulent, then the sound waves would scatter due to velocity fluctuations of the medium.

  • Wind Gradient: Sound propagating along the wind would bend downwards whereas the sound propagating against the wind would bend upwards.

  • Temperature Gradient: Sound waves travel faster in a warm atmosphere such as near the surface of the earth. Here, there is an increase in temperature so it is upward refraction of sound waves. At higher altitudes, there is a decrease in temperature in that case the refraction would be downwards.

3. Name the Property of Sound that is Affected When We Change in Temperature.

Ans: The wavelength of the sound is affected when we change in temperature.