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Chapter: Sound

Sound is termed as a mechanical disturbance from a state of equilibrium that propagates through an elastic material as medium. In simple terms, it can be defined as the vibrations that travel through the air or another medium and can be heard when they reach a person's or an animal's ear.



Sound plays a predominant role in our life. It is the reason behind our communication. We hear so many sounds in our daily life. Some of the applications like alarm is being used to wake us up on time, school bell notifies you about the end of a particular session, horn of a vehicle makes you notice the vehicles in traffic. It is because of sound that we are able to hear what others are speaking and to the sounds of musical instruments which entertains us etc.



Sound is basically produced by vibration of any material. We can say any vibrating object produces sound. In some cases, the vibrations are easily visible to us. But in most cases, their amplitude is very small that we cannot even see them. However, we can feel them. Everything has some vibrating part which produces sound. Following is a list of some things and instruments with their vibrating parts which produce sound.




Vibrating Part which produces sound



Air column



Stretched Strings



Air column


School bell



Human Body



Sound Produced by Humans

Sound is produced, in humans, by an organ called the voice box or the larynx. You can find a hard bump called voice box, by placing a finger on the throat and it seems to move when you swallow. This is called voice box and is located at the upper end of the wind pipe. Two vocal cords are stretched across the voice box or larynx, in such a way that it leaves a narrow slit between them for the air to pass through.



Sound will be produced when the lungs force air through the slit which makes the vocal cords vibrate. Muscles attached to the vocal cords will help to make the cords tight or loose. Variation of the voice in type, volume or quality will depend on the muscle action of these vocal cords.

Sound Propagation

Sound is like a mechanical wave and it needs a medium to travel. The medium can be anything like air, water, steel etc. Have you ever heard the word Vacuum? When air is removed completely from a vessel, we say that there is vacuum in the vessel. The sound cannot travel through vacuum. It can be demonstrated by the following experiment.

As shown in figure, take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected to a vacuum pump, as shown in Fig below.



Now you will be able to hear the bell placed inside a jar if you press the switch. Now start to pump out all the air inside the jar. When the air in the jar is pumped out slowly, the sound becomes less audible, although the same current is passing through the bell. Let some time pass. And you can notice that you hear a very weak sound. This is due to the less amount of air present in the jar. Now let’s remove the air completely and see what happens. Note down if you are still being able to hear the sound of the bell?

No sound is heard when the air is removed completely. Hence, it is proved that some medium is required for the propagation of sound. Sound travels through solids and liquids also. Sound can be heard when we ring bell in a bath tub and ear placed on surface of the water. Similarly, a piece of wood or metal can be scratched and sound can be heard.

Human Ear

Human beings hear sounds using ears. Ears are extremely sensitive. The shape of ear is as shown in figure below:


The shape of the outer part of the ear is like a funnel, which is called Pinna. Firstly, it collects the sound from the surroundings and it passes through the auditory canal at the end of which, a thin membrane is stretched tightly. It is called the eardrum and it performs a vital function. The eardrum looks like a stretched rubber sheet and it is vibrated by the Sound vibrations. These vibrations are sent to the middle part of the ear by ear drum and the middle ear further transmits the sound wave to the inner ear. The sound wave produces amplified pressure variations. These pressure variations are converted into electrical signals by an organ called cochlea which is present in the inside ear. Further, brain receives these electrical signals and interprets as sound. This is how we are able to hear.

Characteristics of a sound wave

We can describe a sound wave by three important terms:

1. Frequency:

Frequency is defined as the number of occurrences of a repeating event per unit time. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency. Frequency is a significant parameter used in science and engineering to denote the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio (sound) signals, radio waves, and light. The SI unit of frequency is hertz.


Frequency = 1/Time period

ν = 1/T

2. Amplitude:

The amplitude of a vibrating object is the maximum displacement on either side of its mean position. The SI unit of amplitude is meter (m).

3. Speed:

Generally the speed of an object is the magnitude of its velocity (the rate of change of its position); it is thus a scalar quantity. The SI unit of speed is m/s.

A sound wave is shown as below:


The regions in which the particles are crowded are called as compression. The region of maximum compression is represented by the peak. Thus, compression is region where density as well as pressure is high. The regions of low pressure are called rare factions and in this area, the particles are spread apart.
The distance between two consecutive compressions (C) or two consecutive rare factions (R) is called the wavelength. The wavelength is usually represented by λ (Greek letter lambda). Its SI unit is metre (m).

Sounds of different pitches are produced by different objects which are of different sizes and vibrate at different conditions and at different frequencies. Amplitude is a factor which determines the loudness or softness of a sound. This amplitude depends on the force which makes an object to vibrate. We hear weak or soft sound when we strike any object lightly as it produces a sound wave of less amplitude and less energy. And we hear loud sound if the object is hit harder. A sound wave always spreads out from its source. We notice that the amplitude and loudness reduces slowly but surely decreases as the sound wave moves away from the source.

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

speed, v = distance / time

v = λ/T

Here λ is the wavelength of the sound wave. It is the distance travelled by the sound wave in one time period (T) of the wave. Thus,

v = λ ν

That is, speed = wavelength × frequency


Have you ever shouted or clapped near a mountain and noticed what happened? We hear the same sound with some time interval. This is called echo. This sensation of sound persists in our brain for 0.1 second. So the time interval between the original sound and the reflected sound from mountain should be 0.1 S for hearing a distinct echo. For example, we consider the speed of sound at ambient temperature as 344m/s. and the time interval for the sound to go to the mountain and get reflected back is 0.1s. So the total distance is at least 344 m/s × 0.1 s = 34.4 m. Therefore, we can say that the minimum distance of the mountain from us should be half of this distance i.e. 17.2m.

Range of Hearing

The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle/s). Children under the age of five and some animals, such as dogs can hear up to 25 kHz (1 kHz = 1000 Hz). As people grow up, their ears become less sensitive to higher frequencies. Sounds of frequencies below 20 Hz are called infrasonic sounds or infra sound.

Examples of infra sound are vibrations of wings of an insect, vibrations of pendulum etc. Animals such as rhinoceros, whales, and elephants produce sound in this range. We can notice that some animals get disturbed before earthquakes as if they get some indications. This is because of the low frequency infra sound that starts before the main huge waves and these infra sound waves alert the animals. Ultrasonic sounds are the sound waves whose frequencies are higher than 20 kHz. They are also called as ultrasound.

Applications of Ultrasonic

Ultrasound is used in many fields. Ultrasonic devices are used to identify objects and measure distances. Ultrasonic imaging (sonography) is used in both veterinary medicine and human medicine. Ultra sound is used to identify invisible defects in NDT of materials. It is also used for cleaning, mixing and to initiate some chemical processes.

Ultrasonic is termed as the application of ultrasound. Ultrasound can be used for medical imaging, detection, measurement and cleaning. Ultrasonic is also used for changing the chemical properties of substances at higher power levels.

Ultrasonic in detection and ranging

Non-contact sensor:

An ultrasonic level or sensing system does not require any contact with the target. This is a huge advantage for many processes in medical, military and some general industries as the inline sensors may pollute the fluids inside the vessel or may be clogged.

Motion sensors and flow measurement:

Automatic door opener is a device in which the sensor detects people approaching and automatically opens the door for them. This is some common application of ultrasound. Ultrasonic flow meters are used to measure flow in pipes, channels and can measure the average velocity of the fluid which is flowing inside the pipe or channel. An acoustic rheometer is also a device which depends on the principle of ultrasound. This is used in the field of rheology.

Ultrasonic range finding:

Finding range in underwater is a difficult task and an ultra sound simplifies it. Sonar is a device which uses ultra sound to detect the pulse which is generated in a particular direction and gets reflected back to the transmitter if an object is hindering its path. This is reflected back as an echo.


We can apply ultrasounds in non destructive testing, quality control and some medical uses like study of small structures and tissues to a very detailed internal analysis and it avoids heating and cavitations effects as the power density is very less.

Bio medical applications

Ultra sound is also used in various bio medical applications to measure elastic properties of tissue, breaking of stony deposits, hasten the effect of medicines in a targeted area and to arrange cells for performing research.

Ultrasonic cleaning

Ultrasonic cleaners are also called as supersonic cleaners. They are under frequencies from 20 to 40 kHz and they are used for cleaning jewelry, lenses and other optical parts, watches, dental instruments, surgical instruments and industrial parts. An ultrasonic cleaner works by the energy released when some millions of microscopic cavitations collapse near the surface which is dirty. The bubbles made by these cavitations disintegrate forming tiny jets which are directed at the surface.

Ultrasonic welding

Low amplitude vibrations which are of high frequency of around 15 kHz to 40 kHz are used for welding of plastics. This is called ultrasonic welding. These vibrations produce heat by the friction produced between the materials which are to be joined.