Sound

Good morning, dear students! Welcome to today's science class. I'm so happy to see you all here, ready to learn about something we experience every single day but rarely stop to think about. Today, we're going to study Chapter 11: Sound. This is a fascinating chapter that will help you understand how you hear music, how your favorite songs reach your ears, and many other interesting phenomena. So let's begin our journey into the world of sound!

Every single day, we hear sounds from so many different sources. Think about it - when you talk to your friends, when birds chirp in the morning, when the school bell rings, when vehicles pass by on the road, when the television plays your favorite show, when someone plays the harmonium or guitar - all of these produce sound. But have you ever wondered what exactly sound is? Let me tell you, sound is a form of energy that produces a sensation of hearing in our ears. Isn't that interesting? Just like we have mechanical energy and light energy, sound is another form of energy.

Now, you have already learned about mechanical energy in your previous chapters, and you know the important principle of conservation of energy. This principle tells us that we can neither create nor destroy energy - we can only change it from one form to another. So when you clap your hands, you use your muscular energy to produce sound. Can you produce sound without using any energy? Of course not! You need to put in some energy to create sound. In this chapter, we are going to learn how sound is produced, how it travels through a medium, and how our ears receive it. Let's start with the first and most fundamental concept - how is sound produced?

## 11.1 Production of Sound

Now, let's do some activities to understand how sound is produced. I want you to imagine you're doing these activities with me in the classroom.

### Activity 11.1

Take a tuning fork - you must have seen one in the physics laboratory. Now, strike its prong on a rubber pad and set it vibrating. Bring it near your ear. Do you hear any sound? Of course you do! Now, touch one of the prongs of the vibrating tuning fork with your finger. What do you feel? You feel a vibration, right? The prongs are vibrating rapidly.

Now, let's do another interesting activity. Suspend a table tennis ball or a small plastic ball by a thread from a support. You can do this by taking a big needle and a thread, putting a knot at one end of the thread, and then passing the thread through the ball using the needle. Now, gently touch the ball with the prong of a vibrating tuning fork. What do you observe? You will see that the ball moves away from the tuning fork! This happens because the vibrating prong pushes the ball due to the vibrations. This shows that the tuning fork is indeed vibrating and these vibrations are being transferred to the ball.

### Activity 11.2

Now, let's do another activity. Fill water in a beaker or a glass up to the brim. Gently touch the water surface with one of the prongs of the vibrating tuning fork, as shown in the figure in your book. What do you observe? You will see that the water gets disturbed - there are ripples formed on the water surface. Now, dip the prongs of the vibrating tuning fork in water, as shown in another figure. What happens now? You will see more vigorous splashing of water! This happens because when the vibrating prongs touch the water, they transfer their vibrations to the water, causing disturbances.

So, what do we conclude from these activities? Can we produce sound without a vibrating object? The answer is NO! Sound is always produced by a vibrating object. When you strike the tuning fork, it vibrates and produces sound. When you pluck a guitar string, it vibrates and produces sound. When you clap, your hands vibrate and produce sound.

Now, let's think about other ways we can produce sound. We can produce sound by plucking, scratching, rubbing, blowing, or shaking different objects. What do we do to all these objects? We set them vibrating, and that produces sound. So, what exactly is vibration? Vibration means a kind of rapid to-and-fro motion of an object. When an object moves back and forth very quickly, it is said to be vibrating.

Let me give you some examples from everyday life. When you speak, the sound of your voice is produced due to vibrations in your vocal cords inside your throat. Have you ever thought about how a bee produces that buzzing sound? It's because of the rapid vibration of its wings. When you pluck a stretched rubber band, it vibrates and produces sound. If you have never tried this, I strongly recommend you try it at home - take a rubber band, stretch it, and pluck it. You will see it vibrating and hear the sound it produces.

### Activity 11.3

Now, let's think about musical instruments. Make a list of different types of musical instruments and discuss with your friends which part of the instrument vibrates to produce sound. There are many musical instruments - the sitar has strings that vibrate, the tabla has a membrane that vibrates, the flute has air inside that vibrates, the harmonium has reeds that vibrate, and so on. In all musical instruments, some part vibrates to produce sound. This is a very important concept to remember - sound is produced by vibrating objects.

Now, let's move on to the next important concept - how does sound travel from one place to another?

## 11.2 Propagation of Sound

We have learned that sound is produced by vibrating objects. But how does the sound reach our ears? This is what we are going to learn now.

The matter or substance through which sound is transmitted is called a medium. This medium can be solid, liquid, or gas. Sound moves through a medium from the point of generation to the listener. Let me explain how this happens.

When an object vibrates, it sets the particles of the medium around it vibrating. Now, here's a very important point - the particles do not travel all the way from the vibrating object to the ear. Think about it - when someone speaks in front of you, do you think the air particles travel from their mouth to your ear? No, that doesn't happen! Let me explain the actual process.

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, 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 the medium till the sound reaches your ear. So, the disturbance created by a source of sound in the medium travels through the medium, but not the particles of the medium. The particles just vibrate in place, but the disturbance moves forward. This is very similar to what happens when you create a wave in a rope - the rope moves up and down, but the wave travels forward along the rope.

A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. They, in turn, produce similar motion in others. The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is exactly what happens during propagation of sound in a medium. So, we can visualize sound as a wave. Sound waves are characterized by the motion of particles in the medium, and because they require a medium to travel, they are called mechanical waves.

Now, let's understand how sound travels through air, which is the most common medium. When a vibrating object moves forward, it pushes and compresses the air in front of it, creating a region of high pressure. This region is called a compression. When the vibrating object moves backward, it creates a region of low pressure called rarefaction. As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium.

Let me explain this with the help of a slinky. You might have played with a slinky at some point. When you compress one end of the slinky and release it, you can see the coils getting closer together in some places and further apart in others. The regions where the coils become closer are called compressions, and the regions where the coils are further apart are called rarefactions. This is exactly how sound propagates in a medium - as a series of compressions and rarefactions.

Now, there are two types of waves - longitudinal waves and transverse waves. In longitudinal waves, the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another, but they simply oscillate back and forth about their position of rest. This is exactly how a sound wave propagates. So, sound waves are longitudinal waves.

In transverse waves, particles do not oscillate along the direction of wave propagation but oscillate up and down about their mean position as the wave travels. A good example of a transverse wave is the waves you see on the water surface when you drop a pebble in a pond. Light is also a transverse wave, but for light, the oscillations are not of the medium particles or their pressure or density - it is not a mechanical wave. You will learn more about transverse waves in higher classes.

Now, let's answer some questions to check our understanding.

### Questions from the Section

**Question 1:** How does the sound produced by a vibrating object in a medium reach your ear?

**Answer:** When a vibrating object moves forward, it compresses the air in front of it, creating a region of high pressure called compression. When it moves backward, it creates a region of low pressure called rarefaction. These compressions and rarefactions travel through the medium as a sound wave. When these waves reach our ear, they cause our eardrum to vibrate, and this vibration is interpreted by our brain as sound.

**Question 2:** Explain how sound is produced by your school bell.

**Answer:** When the hammer strikes the bell, it sets the bell vibrating. These vibrations are transferred to the surrounding air particles, creating compressions and rarefactions. These travel through the air as sound waves and reach our ears, allowing us to hear the bell.

**Question 3:** Why are sound waves called mechanical waves?

**Answer:** Sound waves are called mechanical waves because they require a material medium (solid, liquid, or gas) to travel. They are produced by the vibration of objects and propagate through the medium by the vibration of particles. Without a medium, sound cannot travel.

**Question 4:** Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

**Answer:** No, we will not be able to hear any sound on the moon. This is because the moon does not have an atmosphere - there is no air or any other medium to carry the sound waves. Sound requires a medium to travel, and in the vacuum of space, there is no medium. So, even if your friend shouts or makes any sound, you won't be able to hear it. This is why astronauts communicate using radio signals when they are in space.

### 11.2.1 Sound Waves are Longitudinal Waves

Let me demonstrate this with another activity.

### Activity 11.4

Take a slinky. Ask your friend to hold one end, and you hold the other end. Now, stretch the slinky. Then give it a sharp push towards your friend. What do you notice? You will see a compression traveling along the slinky. If you move your hand pushing and pulling the slinky alternatively, you will observe a series of compressions and rarefactions traveling along the slinky. If you mark a dot on the slinky, you will observe that the dot moves back and forth parallel to the direction of the propagation of the disturbance. This confirms that sound waves are longitudinal waves - the particles of the medium move parallel to the direction of wave propagation.

### 11.2.2 Characteristics of a Sound Wave

Now, let's learn about the characteristics of a sound wave. We can describe a sound wave by its frequency, amplitude, and speed. Let's understand each of these in detail.

When we look at a sound wave in graphic form, we can see how density and pressure change when the sound wave moves in the medium. The density as well as the pressure of the medium at a given time varies with distance, above and below the average value of density and pressure.

Compressions are the regions where particles are crowded together. In a graph, they are represented by the upper portion of the curve. The peak represents the region of maximum compression. Compressions are regions where density as well as pressure is high. Rarefactions are the regions of low pressure where particles are spread apart. They are represented by the valley, that is, the lower portion of the curve. A peak is called the crest, and a valley is called the trough of a wave.

The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength. It is usually represented by the Greek letter lambda (λ). Its SI unit is metre (m).

Now, let's talk about frequency. Frequency tells us how frequently an event occurs. Suppose you are beating a drum. How many times you are beating the drum in unit time is called the frequency of your beating the drum. When sound is propagated through a medium, the density of the medium oscillates between a maximum value and a minimum value. The change in density from the maximum value to the minimum value, then again to the maximum value, makes one complete oscillation. The number of such oscillations per unit time is the frequency of the sound wave. If we can count the number of compressions or rarefactions that cross us per unit time, we will get the frequency of the sound wave. Frequency is usually represented by the Greek letter nu (ν). Its SI unit is hertz (symbol Hz).

The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave. In other words, the time taken for one complete oscillation is called the time period of the sound wave. It is represented by the symbol T. Its SI unit is second (s).

Frequency and time period are related by the following formula:

ν = 1/T

This means frequency is the reciprocal of time period. If the time period is less, frequency is more, and vice versa.

Now, let's talk about amplitude. The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. If we strike a table lightly, we hear a soft sound because we produce a sound wave of less energy, which means less amplitude. If we hit the table hard, we hear a louder sound because we produce a sound wave with more energy and more amplitude. A sound wave spreads out from its source. As it moves away from the source, its amplitude as well as its loudness decreases. Louder sound can travel a larger distance because it is associated with higher energy.

Now, let's talk about the quality or timber of sound. This is the characteristic that enables us to distinguish one sound from another having the same pitch and loudness. The sound which is more pleasant is said to be of a rich quality. A sound of single frequency is called a tone. The sound which is produced due to a mixture of several frequencies is called a note and is pleasant to listen to. Noise is unpleasant to the ear! Music is pleasant to hear and is of rich quality.

Let me give you an example. When a musical instrument like a guitar plays a note, it produces a sound that is pleasant to hear. This is because it produces a mixture of different frequencies that are in harmony. But when someone scratches a chalk on a blackboard, it produces a noise that is unpleasant. This is because it produces random frequencies that don't harmonize well.

Now, let's answer some questions.

### Questions from the Section

**Question 1:** Which wave property determines (a) loudness, (b) pitch?

**Answer:** (a) Loudness is determined by the amplitude of the sound wave. Greater amplitude means louder sound. (b) Pitch is determined by the frequency of the sound wave. Higher frequency means higher pitch, and lower frequency means lower pitch.

**Question 2:** Guess which sound has a higher pitch: guitar or car horn?

**Answer:** Generally, a guitar produces sound with higher frequency than a car horn, so the guitar has a higher pitch. However, this can vary depending on how the guitar is played or what type of car horn it is.

Now, let's talk about the speed of sound. The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time.

We know that speed = distance/time. For a wave, the distance traveled in one time period is the wavelength (λ). So,

speed, v = distance/time = λ/T

Since frequency ν = 1/T, we can write:

v = λ × ν

or v = λν

That is, speed equals wavelength multiplied by frequency. This is a very important formula that you must remember!

The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.

Now, let's look at an example problem.

### Example 11.1

A sound wave has a frequency of 2 kHz and wavelength 35 cm. How long will it take to travel 1.5 km?

**Solution:**

Given, Frequency, ν = 2 kHz = 2000 Hz Wavelength, λ = 35 cm = 0.35 m

We know that speed, v of the wave = wavelength × frequency v = λν = 0.35 m × 2000 Hz = 700 m/s

The time taken by the wave to travel a distance, d of 1.5 km is

t = d/v = (1.5 × 1000 m) / (700 m/s) = 1500/700 s = 15/7 s = 2.1 s

Thus, sound will take 2.1 seconds to travel a distance of 1.5 km.

Great! Now let's practice some more questions.

### Questions from the Section

**Question 1:** What are wavelength, frequency, time period, and amplitude of a sound wave?

**Answer:** Wavelength (λ) is the distance between two consecutive compressions or two consecutive rarefactions. Its SI unit is metre.

Frequency (ν) is the number of complete oscillations per unit time. Its SI unit is hertz (Hz).

Time period (T) is the time taken for one complete oscillation. Its SI unit is second (s).

Amplitude is the maximum displacement of the particles of the medium from their mean position. It determines the loudness of sound.

**Question 2:** How are the wavelength and frequency of a sound wave related to its speed?

**Answer:** The speed of a sound wave is equal to the product of its wavelength and frequency. That is, v = λν.

**Question 3:** Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

**Solution:**

Given, Frequency, ν = 220 Hz Speed, v = 440 m/s

We know that v = λν So, λ = v/ν = 440/220 = 2 m

Therefore, the wavelength is 2 metres.

**Question 4:** A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

**Solution:**

The time interval between successive compressions is the time period of the sound wave.

Given, frequency ν = 500 Hz

We know that time period T = 1/ν = 1/500 = 0.002 s = 2 × 10⁻³ s

So, the time interval between successive compressions is 0.002 seconds or 2 milliseconds.

Now, let's talk about the intensity of sound. The amount of sound energy passing each second through unit area is called the intensity of sound. We sometimes use the terms "loudness" and "intensity" interchangeably, but they are not the same. Loudness is a measure of the response of the ear to the sound. Even when two sounds are of equal intensity, we may hear one as louder than the other simply because our ear detects it better. This is because our ears are more sensitive to some frequencies than others.

### Question from the Section

**Question:** Distinguish between loudness and intensity of sound.

**Answer:** Intensity of sound is the amount of sound energy passing per second through unit area. It is a physical quantity that can be measured objectively. Loudness, on the other hand, is a physiological response of the ear to the sound. It is a subjective quantity - what sounds loud to one person may sound less loud to another. Two sounds with the same intensity may be perceived as having different loudness because our ears are more sensitive to some frequencies than others.

### 11.2.3 Speed of Sound in Different Media

Sound propagates through a medium at a finite speed. You might have noticed that when there's lightning, we see the flash first and hear the thunder later. This is because light travels much faster than sound. So, sound travels with a speed that is much less than the speed of light.

The speed of sound depends on the properties of the medium through which it travels. You will learn about this dependence in more detail in higher classes.

The speed of sound in a medium depends on the temperature of the medium. The speed of sound decreases when we go from solid to gaseous state. In any medium, as we increase the temperature, the speed of sound increases. For example, the speed of sound in air is 331 m/s at 0°C and 344 m/s at 22°C.

The speeds of sound at a particular temperature in various media are listed in Table 11.1 in your book. Let me go through some of these values with you.

In solids: - Aluminium: 6420 m/s - Nickel: 6040 m/s - Steel: 5960 m/s - Iron: 5950 m/s - Brass: 4700 m/s - Glass (Flint): 3980 m/s

In liquids: - Water (Sea): 1531 m/s - Water (Distilled): 1498 m/s - Ethanol: 1207 m/s - Methanol: 1103 m/s

In gases: - Hydrogen: 1284 m/s - Helium: 965 m/s - Air: 346 m/s - Oxygen: 316 m/s - Sulphur dioxide: 213 m/s

Notice something interesting? Sound travels fastest in solids, then in liquids, and slowest in gases. This is because the particles in solids are closely packed, so the vibrations are transferred quickly from one particle to the next. In gases, the particles are far apart, so it takes longer for the vibrations to transfer.

You don't need to memorize these values, but it's good to remember the general trend - sound travels fastest in solids and slowest in gases.

### Question from the Section

**Question:** In which of the three media, air, water, or iron, does sound travel the fastest at a particular temperature?

**Answer:** Sound travels the fastest in iron (a solid) among air, water, and iron at a particular temperature. This is because the particles in iron are closely packed, allowing the vibrations to transfer quickly.

Now, let's move on to the next topic - Reflection of Sound.

## 11.3 Reflection of Sound

Just like light bounces off a surface, sound also bounces off a solid or a liquid. When a rubber ball is thrown against a wall, it bounces back. Similarly, when sound waves hit a solid or liquid surface, they reflect back. Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection as you have studied in earlier classes.

The laws of reflection state that the directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane. An obstacle of large size which may be polished or rough is needed for the reflection of sound waves.

### Activity 11.5

Let's do an activity to understand the reflection of sound. Take two identical pipes. You can make the pipes using chart paper. The length of the pipes should be sufficiently long. Arrange them on a table near a wall. Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe. Adjust the position of the pipes so that you can best hear the sound of the clock. Now, measure the angles of incidence and reflection and see the relationship between the angles. You will find that the angle of incidence equals the angle of reflection. Now, lift the pipe on the right vertically to a small height and observe what happens. The sound becomes less clear. This is because the reflected sound doesn't reach the pipe properly anymore.

In place of a clock, a mobile phone on vibrating mode may also be used.

### 11.3.1 Echo

Now, let's talk about a very interesting phenomenon - echo. If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo.

The sensation of sound persists in our brain for about 0.1 seconds. To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 seconds. If we take the speed of sound to be 344 m/s at a given temperature, say at 22°C in air, the sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1 seconds. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344 m/s) × 0.1 s = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m. This distance will change with the temperature of air.

Echoes may be heard more than once due to successive or multiple reflections. The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.

Now, let's solve a problem related to echo.

### Problem

What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m/s?

**Solution:**

Given, Speed of sound, v = 346 m/s Time taken for hearing the echo, t = 2 s

Distance travelled by the sound = v × t = 346 m/s × 2 s = 692 m

In 2 seconds, sound has to travel twice the distance between the cliff and the person. Hence, the distance between the cliff and the person = 692 m / 2 = 346 m.

### Question from the Section

**Question:** An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m/s?

**Solution:**

Given, Speed of sound, v = 342 m/s Time taken for echo, t = 3 s

Distance travelled by sound = v × t = 342 × 3 = 1026 m

This distance is twice the distance between the source and the reflecting surface.

So, distance of the reflecting surface from the source = 1026/2 = 513 m

### 11.3.2 Reverberation

Now, let's talk about reverberation. A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation.

In an auditorium or big hall, excessive reverberation is highly undesirable because it makes the sound muddy and unclear. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster, or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

### Example 11.2

A person clapped his hands near a cliff and heard the echo after 2 s. We already solved this problem earlier - the distance was found to be 346 m.

### 11.3.3 Uses of Multiple Reflection of Sound

Now, let's learn about some practical applications of multiple reflection of sound.

1. **Megaphones or loudhailers, horns, musical instruments such as trumpets and shehnais** - These are all designed to send sound in a particular direction without spreading it in all directions. In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience. This helps to amplify the sound and direct it where we want it to go.

2. **Stethoscope** - This is a medical instrument used for listening to sounds produced within the body, mainly in the heart or lungs. In stethoscopes, the sound of the patient's heartbeat reaches the doctor's ears by multiple reflection of sound. The sound travels through the tube, reflects off the walls, and reaches the doctor's ears.

3. **Concert halls and cinema halls** - Generally, the ceilings of concert halls, conference halls, and cinema halls are curved so that sound after reflection reaches all corners of the hall. Sometimes, a curved soundboard may be placed behind the stage so that the sound, after reflecting from the soundboard, spreads evenly across the width of the hall.

### Question from the Section

**Question:** Why are the ceilings of concert halls curved?

**Answer:** The ceilings of concert halls are curved so that sound after reflection from the ceiling reaches all parts of the hall evenly. This ensures that everyone in the audience, whether sitting near the stage or far from it, can hear the sound clearly. The curved surface helps to distribute the sound uniformly throughout the hall.

## 11.4 Range of Hearing

Now, let's talk about the range of hearing. The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one Hz = one cycle per second). 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 older, their ears become less sensitive to higher frequencies.

Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound. If we could hear infrasound, we would hear the vibrations of a pendulum just as we hear the vibrations of the wings of a bee. Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin, which possibly alert the animals.

Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound is produced by animals such as dolphins, bats, and porpoises. Moths of certain families have very sensitive hearing equipment. These moths can hear the high-frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture. Rats also play games by producing ultrasound.

Now, let's talk about hearing aids. People with hearing loss may need a hearing aid. A hearing aid is an electronic, battery-operated device. The hearing aid receives sound through a microphone. The microphone converts the sound waves to electrical signals. These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. The speaker converts the amplified electrical signal to sound and sends it to the ear for clear hearing.

### Questions from the Section

**Question 1:** What is the audible range of the average human ear?

**Answer:** The audible range of the average human ear is from about 20 Hz to 20000 Hz.

**Question 2:** What is the range of frequencies associated with (a) Infrasound? (b) Ultrasound?

**Answer:** (a) Infrasound: frequencies below 20 Hz (less than 20 Hz) (b) Ultrasound: frequencies above 20000 Hz (more than 20 kHz)

## 11.5 Applications of Ultrasound

Ultrasounds are high-frequency waves. Ultrasounds are able to travel along well-defined paths even in the presence of obstacles. Ultrasounds are used extensively in industries and for medical purposes.

Let me tell you about some important applications of ultrasound:

1. **Cleaning** - Ultrasound is generally used to clean parts located in hard-to-reach places, for example, spiral tubes, odd-shaped parts, electronic components, etc. Objects to be cleaned are placed in a cleaning solution, and ultrasonic waves are sent into the solution. Due to the high frequency, the particles of dust, grease, and dirt get detached and drop out. The objects thus get thoroughly cleaned.

2. **Detecting cracks and flaws in metal blocks** - Ultrasounds can be used to detect cracks and flaws in metal blocks. Metallic components are generally used in the construction of big structures like buildings, bridges, machines, and also scientific equipment. The cracks or holes inside the metal blocks, which are invisible from outside, reduce the strength of the structure. Ultrasonic waves are allowed to pass through the metal block, and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back, indicating the presence of the flaw or defect. Ordinary sound of longer wavelengths cannot be used for such purpose as it will bend around the corners of the defective location and enter the detector.

3. **Medical imaging** - Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called 'echocardiography'. An ultrasound scanner is an instrument that uses ultrasonic waves for getting images of internal organs of the human body. A doctor may image the patient's organs, such as the liver, gall bladder, uterus, kidney, etc. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumors in different organs. In this technique, the ultrasonic waves travel through the tissues of the body and get reflected from a region where there is a change of tissue density. These waves are then converted into electrical signals that are used to generate images of the organ. These images are then displayed on a monitor or printed on a film. This technique is called 'ultrasonography'. Ultrasonography is also used for examination of the foetus during pregnancy to detect congenital defects and growth abnormalities.

4. **Breaking kidney stones** - Ultrasound may be employed to break small 'stones' formed in the kidneys into fine grains. These grains later get flushed out with urine. This is a non-invasive procedure that helps patients avoid surgery.

Now, let's solve the exercises at the end of the chapter.

## Exercises

**Exercise 1:** What is sound and how is it produced?

**Answer:** Sound is a form of energy that produces a sensation of hearing in our ears. It is produced by vibrating objects. When an object vibrates, it causes the surrounding particles of the medium to vibrate, creating compressions and rarefactions that travel through the medium as sound waves.

**Exercise 2:** Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

**Answer:** When a vibrating object moves forward, it pushes the air molecules in front of it, creating a region where the molecules are close together. This region of high pressure is called a compression. When the object moves backward, it leaves a region where the molecules are far apart. This region of low pressure is called a rarefaction. As the object continues to vibrate back and forth, it creates a series of compressions and rarefactions that travel outward from the source as sound waves.

**Exercise 3:** Why is sound wave called a longitudinal wave?

**Answer:** A sound wave is called a longitudinal wave because the individual particles of the medium move back and forth along the same direction in which the wave is propagating. The particles oscillate about their mean positions but do not move from one place to another. The disturbance (compression and rarefaction) travels forward through the medium.

**Exercise 4:** Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

**Answer:** The characteristic of sound that helps identify your friend by his voice is the quality or timber of sound. Quality or timber is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. Each person has a unique quality of voice, which is why we can recognize our friends even in a dark room.

**Exercise 5:** Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

**Answer:** Flash and thunder are produced simultaneously, but we see the flash almost immediately because light travels at a very high speed (approximately 3 × 10⁸ m/s). Sound travels much slower (about 344 m/s in air). Therefore, sound takes more time to reach us than light. The time difference between seeing the flash and hearing the thunder depends on how far away the lightning strike occurred.

**Exercise 6:** A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m/s.

**Solution:**

For frequency 20 Hz: Wavelength λ = v/ν = 344/20 = 17.2 m

For frequency 20 kHz = 20000 Hz: Wavelength λ = v/ν = 344/20000 = 0.0172 m = 1.72 cm

So, the typical wavelengths range from about 17.2 metres for 20 Hz to about 1.72 centimetres for 20000 Hz.

**Exercise 7:** Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

**Solution:**

We need to find the time taken for sound to travel through air and through aluminium for the same distance. Let the length of the rod be L.

Speed of sound in air = 346 m/s (approximately) Speed of sound in aluminium = 5960 m/s (from Table 11.1)

Time taken in air, t_air = L/346 Time taken in aluminium, t_aluminium = L/5960

Ratio of times, t_air : t_aluminium = (L/346) : (L/5960) = 1/346 : 1/5960 = 5960 : 346 = 5960/346 : 1 = approximately 17.23 : 1

So, the ratio is approximately 17.23 : 1, meaning sound travels about 17 times faster in aluminium than in air.

**Exercise 8:** The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

**Solution:**

Frequency = 100 Hz means 100 vibrations per second. In one minute (60 seconds), number of vibrations = 100 × 60 = 6000

So, the source vibrates 6000 times in a minute.

**Exercise 9:** Does sound follow the same laws of reflection as light does? Explain.

**Answer:** Yes, sound follows the same laws of reflection as light. The laws of reflection state that: 1. The angle of incidence equals the angle of reflection. 2. The incident sound wave, the reflected sound wave, and the normal to the reflecting surface at the point of incidence all lie in the same plane.

Both light and sound are waves and follow these laws when reflecting off a surface.

**Exercise 10:** When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?

**Answer:** On a hotter day, the speed of sound in air is higher (because sound speed increases with temperature). Since the distance remains the same, the time taken for sound to travel to the reflecting surface and back will be less. However, as long as the time interval is at least 0.1 seconds, we will still hear an echo. But if the distance is just at the minimum required for echo at normal temperature, on a hotter day the sound might return too quickly (less than 0.1 seconds) and we might not hear a distinct echo. So, it depends on the exact distance and temperature.

**Exercise 11:** Give two practical applications of reflection of sound waves.

**Answer:** Two practical applications of reflection of sound waves are: 1. Megaphones and loudhailers - These use multiple reflections to direct sound towards the audience without spreading it in all directions. 2. Stethoscope - It uses multiple reflections of sound to transmit the sound of heartbeat from the patient's chest to the doctor's ears. 3. Concert halls and cinema halls - Curved ceilings and walls are used to ensure even distribution of sound throughout the hall.

**Exercise 12:** A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m/s² and speed of sound = 340 m/s.

**Solution:**

First, let's find the time taken by the stone to fall to the pond. Using the equation of motion: s = ut + (1/2)gt² Here, s = 500 m, u = 0, g = 10 m/s² 500 = 0 × t + (1/2) × 10 × t² 500 = 5t² t² = 100 t = 10 s

So, the stone takes 10 seconds to reach the pond.

Now, the sound of the splash travels back to the top of the tower. Distance = 500 m Speed of sound = 340 m/s Time taken by sound = distance/speed = 500/340 = 1.47 s (approximately)

Total time = time for stone to fall + time for sound to travel = 10 + 1.47 = 11.47 s

So, the splash is heard at the top after approximately 11.47 seconds.

**Exercise 13:** A sound wave travels at a speed of 339 m/s. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

**Solution:**

Given, Speed v = 339 m/s Wavelength λ = 1.5 cm = 0.015 m

We know v = λν So, ν = v/λ = 339/0.015 = 22600 Hz = 22.6 kHz

This frequency is above 20000 Hz, which is the upper limit of human hearing. Therefore, it will not be audible to humans. This is an ultrasound wave.

**Exercise 14:** What is reverberation? How can it be reduced?

**Answer:** Reverberation is the persistence of sound in a big hall or enclosure due to repeated reflections from the walls, ceiling, and other surfaces. It makes the sound muddy and unclear.

Reverberation can be reduced by: 1. Covering the walls and ceiling with sound-absorbent materials like compressed fibreboard, rough plaster, or draperies. 2. Using seat materials that absorb sound. 3. Installing acoustic panels designed to absorb sound energy. 4. Keeping the hall properly furnished to reduce reflections.

**Exercise 15:** What is loudness of sound? What factors does it depend on?

**Answer:** Loudness of sound is the subjective perception of how loud a sound is. It is a measure of the response of our ear to the sound.

Loudness depends on: 1. Amplitude of the sound wave - Greater amplitude means louder sound. 2. Distance from the source - Sound becomes quieter as it travels farther from the source. 3. Physical condition of the medium - Some media conduct sound better than others.

Note: Loudness is different from intensity. Intensity is the objective measure of sound energy, while loudness is the subjective perception by the ear.

**Exercise 16:** How is ultrasound used for cleaning?

**Answer:** Ultrasound is used for cleaning in the following way: 1. The objects to be cleaned are placed in a cleaning solution. 2. Ultrasonic waves are sent into the solution. 3. Due to the high frequency of ultrasound, the solution vibrates rapidly. 4. This creates tiny bubbles in the solution that collapse and produce strong jets of liquid. 5. These jets of liquid help to dislodge the particles of dust, grease, and dirt from the surface of the objects. 6. The objects thus get thoroughly cleaned, even in hard-to-reach places like spiral tubes and odd-shaped parts.

**Exercise 17:** Explain how defects in a metal block can be detected using ultrasound.

**Answer:** Ultrasound is used to detect defects in metal blocks as follows: 1. Ultrasonic waves are passed through the metal block. 2. A detector is placed on the other side to detect the transmitted waves. 3. If the metal block is perfect (without any defects), the ultrasound passes through and reaches the detector. 4. If there is a crack, hole, or any other defect inside the metal block, the ultrasound gets reflected back from the defective location. 5. The reflected waves are detected by a receiver. 6. By analyzing the pattern of reflected waves, the location and size of the defect can be determined.

This method is very effective because ultrasound can travel through metal but gets reflected by even small defects. Ordinary sound waves cannot be used for this purpose because they would bend around the corners of the defective location and enter the detector, making it difficult to detect the flaw.

Now, let's summarize what we have learned in this chapter.

## What You Have Learnt

- Sound is produced due to vibration of different objects. - Sound travels as a longitudinal wave through a material medium. - Sound travels as successive compressions and rarefactions in the medium. - In sound propagation, it is the energy of the sound that travels and not the particles of the medium. - The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation. - The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, denoted by λ. - The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, denoted by T. - The number of complete oscillations per unit time is called the frequency, denoted by ν. Frequency ν = 1/T. - The speed v, frequency ν, and wavelength λ of sound are related by the equation, v = λν. - The speed of sound depends primarily on the nature and the temperature of the transmitting medium. Sound travels fastest in solids, then in liquids, and slowest in gases. - The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three lie in the same plane. - For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 seconds. This means the minimum distance for hearing an echo is 17.2 metres (at 22°C). - The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation. - Sound properties such as pitch, loudness, and quality are determined by the corresponding wave properties. - Loudness is a physiological response of the ear to the intensity of sound. - The amount of sound energy passing each second through unit area is called the intensity of sound. - The audible range of hearing for average human beings is in the frequency range of 20 Hz to 20 kHz. - Sound waves with frequencies below the audible range are termed "infrasonic" and those above the audible range are termed "ultrasonic". - Ultrasound has many medical and industrial applications, including cleaning, detecting flaws in metals, medical imaging, and breaking kidney stones.

This concludes our lesson on Chapter 11: Sound. I hope you have understood all the concepts clearly. Remember, sound is all around us, and understanding its properties helps us appreciate the world of physics even more. Thank you for your attention, and I'll see you in the next class!