Sound: Most of us have been gifted with five wonderful senses. The sensation of sight, taste, touch, smell, and hearing- all of these senses put together, help us experience the world around us comprehensively. Out of these, the sensation of hearing is probably the easiest to experience because to see we have to open our eyes, to taste we have to have food, to feel the touch our skin needs to make contact with a surface, to smell there must be some aroma around, but sounds travel to our ears continuously even from the smallest objects and provide us with the sensation of hearing. From the buzzing of a mosquito to the chirping of birds to the loud rap music – sounds are associated with them all.

A sound is a form of energy that we associate with the sensation of hearing. From the law of energy conservation, we know that energy can neither be created nor be destroyed; thus, when we clap our hands, the mechanical energy is converted into sound energy, producing a sound. Let us learn in detail how sound is produced, propagated and heard.

Learn about Propagation of Sound in detail here

Production of Sound

Sound is produced due to the vibration of objects. Vibration is rapid to and fro motion of an object. Vibrations can be produced by striking, plucking, scratching, rubbing, shaking, or blowing different objects. The vibration of our vocal cords produces sound as we talk, plucking of guitar strings produces the sound of music, flapping of a mosquito’s wings produces the buzzing sound. Vibrations are produced when a stretched rubber band is plucked, producing sound, which can be seen in musical instruments like a table.

Propagation of Sound

Sounds are associated with vibrating objects. The matter or the substance through which sound travels and get transmitted is called a medium. This medium through which sound propagates from the source to the listener can be solid, liquid or gas. As an object vibrates, the particles of the medium around it vibrate along with it. The vibrating particles do not leave their places to travel into our ears. What happens is, as the vibrations start in an object, the particles of the medium which are in direct contact with it also start vibrating, due to which they get displaced from their equilibrium positions. These displaced particles then exert a force on the particles adjacent to them, displacing those particles from their position. While the adjacent particles get displaced, the original particles return to their equilibrium positions, and this process continues through the medium till we hear the sound.

Thus, the disturbance generated by the source propagates through the medium and not by the particles of the medium.  This is why we often refer to sound as a sound wave because a wave is a disturbance that travels through a medium when particles of the medium set the particles adjacent to them into motion and so on. Here, the medium particles do not leave their place, just the disturbance moves forward, and it’s exactly what happens when a sound wave moves through a medium. Sound waves are called mechanical waves since they are associated with the motion of the particles of the medium.

In our daily life, we can see that sound waves travel through the air. As an object starts vibrating, it sets the particles of air adjacent to it into motion. The vibrating air molecules move back and forth as the disturbance travels through them. As these particles are pushed forward, they get compressed, generating a high-pressure region called compression. As the compression moves through the medium, away from the particles, the vibrating particles move backwards; they move apart, creating a low-pressure region called rarefaction.

These compressions and rarefactions are rapidly created in the air as the object vibrates back and forth, causing the sound waves to propagate through the medium. Since the pressure is associated with the total number of particles in a specified volume of the medium, greater is the density of particles in a region; higher would be the pressure in that region and vice versa. The propagation of sound waves through a medium can be understood as the propagation of pressure or density variations in the medium.

Is Sound Wave a Longitudinal Wave?

Waves are of two types: Transverse and Longitudinal. In the case of a transverse wave, the particles of the medium move perpendicular to the direction of disturbance propagation. In the case of a longitudinal wave, the particles of the medium move parallel to the direction of disturbance propagation. During the propagation of a longitudinal wave, individual particles of the medium oscillate back and forth about their equilibrium position, but they do not move to leave their positions. That is why a sound wave is a longitudinal wave.

Characteristics of a Sound Wave

The density and pressure of a medium vary as the sound wave moves through the medium. The values of density and pressure go above and below their average values as the sound waves propagate through the medium. The motion of a sound wave through a medium can be graphically represented as follows:

This curve presents the motion of the sound wave. The upper part of the curve represents the region where particles are much crowded, representing a compression. The highest point or peak of the curve, which indicates the region of maximum compression, is called a crest of the wave.
The lower part of the curve represents the region where particles are much farther apart, representing a rarefaction. The lowest point or the valley of the curve, which indicates the region of the lowest pressure, is called a trough of the wave.

Following are the characteristics of a sound wave:

1. Wavelength: It is defined as the distance between two consecutive compressions or two consecutive rarefactions. Wavelength is represented by the letter \(\lambda\) (lambda). The SI unit of wavelength is metre \((\rm{m})\).
2. Frequency: It is defined as the number of oscillations of a particle per unit of time. As a sound wave moves through the medium, its density changes from the maximum value to the minimum value, again to the maximum value, making one complete oscillation. Thus, the frequency of a sound wave can be calculated by counting the number of compressions or rarefactions that pass a point per unit. The SI unit of frequency is hertz \((\rm{Hz})\), represented by the letter \(\nu\) (nu).
3. Time period:  It is defined as the time taken to complete one oscillation in a medium. In simple terms, the time period of a sound wave can be understood as the time taken by two consecutive rarefactions or compressions to cross a fixed point. It is represented by \(T\) and its SI unit is second \((\rm{s})\). The frequency and time period of a sound wave are related as:
   \( v = \frac {1}{T}\)
4. Amplitude is defined as the magnitude of maximum displacement of the disturbance on either side of the mean position. It is represented by \(A\). For a given sound wave, it’s unit will same as the unit of either density or pressure. The amplitude of a wave depends upon the force applied to an object to start its vibration.
   
5. Pitch is defined as the characteristic of a sound that describes how our brain interprets a sound. Faster is the speed at which the source vibrates; higher will be the frequency and the pitch of sound waves produced from it.
6. The loudness or softness of a sound wave is associated with the amplitude of the wave. Soft sound Is associated with the sound wave of less amplitude or energy, and it can only travel a small distance while the reverse is applicable for a loud sound. Sound waves spread out from the source, and its amplitude and loudness reduce as it moves away from the source.
7. The quality or timber is an important feature of a sound wave. It helps to differentiate between two sounds having the same pitch and loudness.
8. A sound that is more pleasant to hear is considered to be of rich quality. Music is of rich quality, and it is pleasant to hear.
9. Tone: Sound consisting of a single frequency is called a tone.
10. Note: Sound consisting of a mixture of several frequencies is called a note.
11. Noise is defined as a sound that is quite unpleasant to our ears.
12. Intensity of sound is defined as the amount of sound energy passing each second through a unit area. Loudness and intensity may seem similar, but these describe different qualities of a sound wave. Loudness is associated with the response of the human ear to a given sound. Out of two equally intense sound waves, our ear would detect the louder sound better.

Speed of Sound in a Medium

A sound wave travels at a finite speed through a medium. The speed of sound in air is less than the speed of light. That’s why lightning is seen before thunder is heard. Speed of sound depends on:

1. Nature of medium: Speed of sound is fastest in solids and slowest in gases.
2. Temperature of medium: As the temperature of the medium increases, the speed of sound waves through it increases.

Reflection of Sound

A sound wave gets reflected at the surface of a solid or liquid medium. It follows the same laws of reflection as the light, i.e. the angle of incidence is equal to the angle of reflection, and the reflected ray, incident ray and the normal all lie in the same plane. An obstacle of large size, which may be polished or rough, is needed to reflect sound waves.

Echo

An echo is defined as the reflection of a sound from a hard surface that is heard after a small delay from the direct sound. For a given sound, its sensation in our brain persists for about \(0.1\,{\rm{s}}\). Thus, a distinct echo is heard if the time interval between the original and reflected sound is at least \(0.1\,{\rm{s}}\).
For the speed of sound to be \(344\,{\rm{m/s}}\), at a temperature of \(22^\circ {\rm{C}}\) in the air, the minimum distance between the obstacle and the source of sound must be \(17.2\,{\rm{m}}\) to hear the echoes distinctly. This distance can vary with the temperature and humidity of the air. We can hear echoes more than once due to successive or multiple reflections.

Reverberation

The phenomenon associated with the repeated reflection that results in the persistence of sound is called reverberation; it is experienced in movie halls and auditoriums and is often undesirable. The roof and walls and the seat materials of the auditorium are covered with specifically chosen sound-absorbent materials like compressed fibreboard, rough plaster or draperies to reduce the effect of reverberation.

Uses of Multiple Reflections

1. Musical instruments like trumpets and shehnais or megaphones and loud hailers are designed to send sound along a single direction. Sound undergoes multiple reflections in the tubes within these instruments and moves forward.
2. In stethoscopes, doctors can hear our heartbeats due to multiple reflections of the sound.
3. Ceilings of concert, conference and cinema halls are curved to allow multiple reflections of the sound waves so the sound can spread evenly throughout the space.

Range of Hearing

The audible range of normal human beings lies within the range of \(20\;{\rm{Hz}}\) to \(20000\;{\rm{Hz}}\). The audible range of human hearing varies with age. Older people find it harder to hear sounds of higher frequencies, while children or babies can hear sounds up to \(25\;{\rm{kHz}}\).
Infrasound: The sounds of frequencies less than \(20\;{\rm{Hz}}\) are called infrasonic sound or infrasound. Rhinoceroses, whales, and elephants use infrasounds for communication; bees’ wings and pendulum’s swinging occur at infrasonic frequencies. Some animals like dogs can detect infrasounds produced before earthquakes.
Ultrasound: The sounds of frequencies greater than \(20\;{\rm{kHz}}\) are called ultrasonic sound or ultrasound. Dolphins, bats, porpoises and certain moths are sensitive to ultrasounds. In fact, animals like rats play games at ultrasonic frequencies.

Applications of Ultrasounds

Ultrasounds are waves of high frequency that can travel along well-defined paths even if there are obstacles in them. Ultrasounds have the following applications:

1. These are used to clean the places that are not easily accessible in certain electrical types of equipment, spiral tubes and oddly shaped objects.
2. These are used to detect cracks in huge metal equipment.
3. Ultrasonic waves are used in echocardiography to get an image of various parts of the heart.
4. Ultrasonic scanners are used to get the image of various parts of the human body. These images are then produced on a screen, and this technique is called ultrasonography.
5. Ultrasounds are used to break stones in kidneys or gallbladders.
6. Bats and porpoises use ultrasounds to detect food and obstacles.

SONAR

Sound Navigation And Ranging is a device that employs ultrasonic waves to measure the distance, direction and speed of the objects moving underwater. It is based on the technique of echo-ranging, where the detector detects the sound produced at ultrasonic frequencies by a transmitter after it’s reflected from an obstacle present underwater. This technique is used to determine the sunken ships,  depth of the sea and to locate underwater hills, valleys, submarines, icebergs etc.

Structure of Human Ear

We hear sounds coming from all around us using an extremely sensitive device called the ear. Our ears detect the pressure variations, in the air around them, of appropriate audible frequencies and convert them into electrical signals that travel to the brain via the auditory nerve; the brain decodes the electrical signals and helps us understand those sounds.

Parts of the human ear

Pinna: Outer ear that collects the sound from the surroundings is called the pinna.
Auditory canal: The sound collected in the outer ear travels through the auditory canal to reach the eardrum.
Ear-drum: The eardrum is also called the tympanic membrane. It is a thin membrane that moves inwards in the presence of compression, and it moves outwards in the presence of a rarefaction.
Middle ear: This inward and outward motion of the eardrum causes it to vibrate, and these vibrations are further amplified by three bones hammer, anvil and stirrup present in the middle ear. From the middle ear, this amplified sound travels into the inner ear.
Inner ear: Cochlea present in the inner ear converts these pressure variations into suitable electrical signals, which are then sent to the brain for interpretation using the auditory nerve.

Summary

A sound is a form of energy that we associate with the sensation of hearing. Sound is produced due to the vibration of objects. The matter or the substance through which sound travels and get transmitted is called a medium. The speed of sound depends on the nature and temperature of the medium. Sound propagates through a medium in the form of compressions and rarefactions.

In this article, we learned about Sound, the characteristics that define a sound wave, like its wavelength, frequency, time period, pitch and intensity.
An echo is defined as the reflection of a sound from a hard surface that is heard after a small delay from the direct sound. The phenomenon associated with the repeated reflection that results in the persistence of sound is called reverberation.

The audible range of normal human beings lies within the range of \(20\;{\rm{Hz}}\) to \(20000\;{\rm{Hz}}\). The audible range of human hearing varies with age. Older people find it harder to hear sounds of higher frequencies, while children or babies can hear sounds up to \(25\;{\rm{kHz}}\).
Our ears detect the sounds from all around us. Our ears consist of three parts called the outer, middle, and inner ear, which collect the sound waves, convert them into electrical signals, and send them further to the brain for processing.

Frequently Asked Questions on Sound

Q.1. How does sound propagate?
Ans: Sound waves propagate through a medium in the form of pressure variations called compressions (high-pressure region) and rarefactions (low-pressure region).

Q.2. What is the amplitude of a sound wave?
Ans: The maximum displacement of a disturbance in a given medium is called the amplitude of the sound wave.

Q.3. List some uses of ultrasounds.
Ans: Ultrasounds are used for:
1. cleaning inaccessible parts in electrical equipment
2. detecting cracks in metal blocks
3. detecting food and obstacle by animals like bats
4. breaking stones in kidneys.

Q.4. Name the part of the ear that collects the sound from the surroundings.
Ans: Pinna, part of the outer ear, collects the sound from the surroundings

Q.5. What is the audible range of the human ear?
Ans: The audible range of the human ear is \(20\;{\rm{Hz}}\) to \(20000\;{\rm{Hz}}\).

NCERT Solutions For Science Chapter: Sound

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