# Sound: Production, Propagation, Properties, Echoes, and Reverberation

A sound is a form of energy and it is made up of vibrations. When particles of air vibrate in a particular pattern, sound waves are produced. It travels through a medium such as gas, liquid, and solids. Sound waves are longitudinal waves, it needs medium to travel. Sound waves have an amplitude or volume, frequency or pitch, wavelength or speed!

You all know that energy exists in many forms such as:

1. Light
2. Electrical
3. Nuclear
4. Chemical
5. Mechanical

And one of the important forms of energy is sound. Yes, sound is also a form of energy that is essential for communication.

If there was no sound, then there would have been a lot of misunderstandings right from individuals to the nations as no one could have been able to speak to anyone else. Two individuals engaged in an effort to talk to each other might have been misunderstood as jeering at each other like monkeys, resulting possibly in a quarrel.

Now the question arises of how sound is produced?

To understand this let us do an activity:

1. Let us take a metallic wire AB
2. Stretch it tightly between two nails fixed on a tabletop
3. Pluck the wire
4. What do you observe? A sound is heard.
5. Now place a ‘V’-shaped paper rider (R) on the wire near the center.
6. What do you observe? The rider starts vibrating.

Propagation of Sound

We know what is sound now, let’s know how the sound is produced by vibrating objects.
• In order to reach the listener, sound passes through a medium which may be a liquid, solid, or gas.
• Let us understand how sound travels from the source to the listener.

When an object vibrates, it sets the particles of the medium around it into vibration. These particles do not travel all the way from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result of which the adjacent particle gets displaced from its position of rest.

After displacing the adjacent particle, the first particle comes back to its original position.
This process continues in which each particle strikes its, neighbor while vibrating without leaving its mean position in the medium till the sound reaches your ear. It is important to note that the particles of the medium only vibrate and do not actually move from the vibrating body to the listener.

Once the vibrations of the source stop, the particles no longer vibrate and no sound reaches the listener. In fact, Disturbance carrying sound energy is called a sound wave and the motion associated with this wave is called wave motion.

A form of disturbance is due to repeated vibration of the particles of the medium about their position and the motion is handed over from one particle to the other without any net transport of the medium. Since sound waves are produced by the vibrations of the particles of the medium these are called mechanical waves.

Properties of Sound

Let us understand the characteristics of Sound:

1. Speed of Sound: The distance traveled by the sound wave in a given amount of time is known as the speed of the sound. Sound travels slower than light which is why you hear the lightning before you see it. On the basis of the medium through which the sound travels, the speed of the sound also changes. Sound travels faster in solids.
2. Wavelength: Wavelength is the distance between two successive compressions or rarefaction of a wave. It is measured in meters (m). Wavelength is denoted by a greek letter lambda (λ).
3. Amplitude or Intensity of Sound: The amount of energy present in sound waves is known as intensity. It is measured in decibel or dB. Amplitude refers to the size of the wave. It is measured in meters (m).
4. Frequency (Pitch): The number of waves produced in a second is called frequency. The frequency of sound is perceived as pitch. Pitch refers to a sound being high or low. When the molecules in the medium oscillate rapidly, the sound which is produced is a high-pitched sound. The S.I unit of frequency is hertz.
5. Timbre: Timbre is the difference in the sound produced even if two different sounds are said to have the same frequency and amplitude.

Reverberation

1. When a sound is made in a hall, the listener can not understand it as such, due to repeated reflection from the walls & ceiling. It is audible for a longer period of time.
2. The intensity of sound keeps falling till it becomes inaudible.
3. This is because the sound energy goes on decreasing with successive reflection till it is no longer audible.
4. This phenomenon of persistence or prolongation of audible sound even after the source has stopped emitting sound is called reverberation.
5. The time, for which reverberation persists until it becomes inaudible, is called reverberation time.
6. Reverberation can be reduced by increasing the absorption of sound energy.

To achieve this –

1. The walls are covered with some sound-absorbing material like fiberboard, glass wool, etc.
2. Heavy curtains with folds are used
3. The floor is carpeted
4. The furniture is upholstered
5. False ceilings of suitable sound-absorbing material are used.

Echo

You must Have experienced that when we utter a few words in a high domed hall, the words are repeatedly heard because of reflection from the walls of the dome. This phenomenon is called echo.

An echo is a phenomenon of repetition of sound of a source by Reflection from an obstacle.

To distinguish an echo from the original Sound, the obstacles must be situated at a suitable distance from the source of the sound. The persistence of hearing i.e. The sensation of sound lasts in our brain for (1/10 of a second). So, to hear a distinct echo of a sound the time taken by the sound to reach the listener after reflection should be at least 1/10 of a second.

Let d be the minimum distance of the obstacle from the source (s).

v is the speed of sound in air

t is the total time taken by the sound to reach the listener (L) after reflection.

The total distance travelled by the sound from the point of generation to the obstacle & back is the product of speed of the sound and total time t.

Substituting the values.

v=344m/s

t=1/10m/s

2d = 344m/s * 1/10s

= 34.4 m

So d = 17.2m

Hence for hearing a distinct echo, the minimum distance of the obstacles from the source of sound should be 17.2m

This distance changes with the temperature.

Multiple Echoes

When sound is repeatedly reflected from a number of obstacles at suitable distances, many Echoes are heard one after the other, this constitutes multiple Echoes.

Such as-

• Rolling sound of thunder due to successive reflection from a number of reflecting surfaces such as Mountain, clouds, land and rocks.

The Phenomenon of multiple reflections of sound is put in use at many places such as –

1. Speaking tube or megaphone where sound waves are confined in a tube and by multiple reflections from the walls of the tube, it travels a larger distance. Musical instruments like trumpets, shehnai, etc. are based on the same principle.
2. The ceilings of concert halls are made curved so that sound can reach all corners of the hall after reflection.
3. The soundboard, which is a curved sound-reflecting surface, is placed behind the stage. The source of sound is located at the focus of this reflecting surface.
4. Sound waves coming from the source become parallel after reflection from the soundboard and spread evenly throughout the width of the hall.
5. In the stethoscope used by the doctor; Vibration of the Diaphragm reaches earphones after multiple reflections.