Contents
The study of Physics Topics can help us understand and solve real-world problems, from climate change to medical imaging technology.
What is Reflection of Sound and How does the Reflection of Sound Produce an Echo ?
We have studied the reflection of light in earlier classes. Just like light, sound can also be made to change its direction and bounce back when it falls on a hard surface. The bouncing back of sound when it strikes a hard surface is called reflection of sound. Hard, solid surfaces are the best for reflecting sound waves.
For example, sound is reflected well from hard surfaces like a wall, a metal sheet, hard wood and a cliff (A cliff is a steep rock, especially at the edge of the sea). Soft surfaces are bad reflectors of sound. Soft surfaces absorb sound.
The reflection of sound does not require a smooth and shining surface like that of a mirror. Sound can be reflected from any surface, whether smooth or rough. Sound waves are much longer than light waves, so they require a much larger area for reflection.
Sound is reflected in the same way as light. The laws of reflection of light are obeyed during the reflection of sound. We can write down the laws of reflection of sound as follows :
- The incident sound wave, the reflected sound wave, and the normal at the point of incidence, all lie in the same plane.
- The angle of reflection of sound is always equal to the angle of incidence of sound.
We will now describe an experiment to study the reflection of sound. We fix a drawing board DD’ vertically on a table (see Figure). This drawing board acts as a reflecting surface for sound. Two cardboard tubes T1 and T2, about 50 cm long and 3 cm in diameter, are taken and kept in inclined positions with respect to the drawing board (as shown in Figure). The ends of cardboard tubes should not touch the drawing board, they should be at a distance of about 5 cm each from the drawing board.
We keep a clock near the outer end of the tube T1 (see Figure). The clock makes a ticking sound. The sound waves of ticking of clock pass through the tube T1 get reflected from the drawing board at point O, and then enter the other tube T2. We put our ear to the outer end of tube T2 and try to hear the ticking sound of the clock. We adjust the angle of tube T2 with respect to the drawing board till the sound of ticking of clock heard by us becomes the loudest.
Let us now draw a normal ON perpendicular to the reflecting surface (drawing board) and measure the angles AON and NOB. We will find that when the ticking sound of clock heard by our ear through the tube T2 is the loudest, then the angle of reflection of sound (NOB) is equal to the angle of incidence of sound (AON).
Moreover, the incident sound wave (AO), the reflected sound wave (OB) and the normal (ON) at the point of incidence, all lie in the same plane (which is the plane of the table). From this experiment we conclude that sound obeys the same laws of reflection as light.
Applications of Reflection of Sound
We will now discuss some of the applications of the reflection of sound. The reflection of sound is utilised in the working of devices such as : Megaphone, Bulb horn, Stethoscope, and Soundboard. All these devices involve multiple reflections of sound waves. We will describe all these devices in somewhat detail, one by one. Let us start with megaphone and bulb horn.
1. Megaphone and Bulb Horn
The devices like megaphone and bulb horn (and the musical instruments like trumpets and shehnai) are all designed to send sound in a particular direction, without spreading all around. All these devices and musical instruments have a funnel-shaped tube which reflects sound waves repeatedly so that most of the sound waves produced go in the forward direction (towards the audience). During successive reflections, the amplitude of sound waves adds up due to which the loudness of sound increases.
A megaphone is used to address a small gathering of people at places like tourist spots, fairs, market places and during demonstrations. One end of the megaphone tube is narrow and its other end is quite wide (see Figure). When a person speaks into the narrow end of the megaphone tube, the sound waves produced by his voice are prevented from spreading by successive reflections from the wider end of the megaphone tube.
Due to this the sound of the voice of the person can be heard over a longer distance. Thus, a megaphone works on the multiple reflection of sound.
A bulb horn is a cone-shaped wind instrument which is used for signalling in bicycles, cars, buses, trucks and boats, etc. Bulb horns are of different designs. One design of bulb horn is shown in Figure.
This bulb horn consists of a cone-shaped, bent metal-tube having a hollow rubber bulb at its narrow end. When the rubber bulb is pressed with hand, air is forced out from the tube making a loud sound. Just like a megaphone, a bulb horn also works on the multiple reflection of sound.
2. Stethoscope
Stethoscope is a medical instrument used by the doctors for listening to the sounds produced within the human body, mainly in the heart and the lungs (see Figure). A stethoscope consists of three parts :
(i) A chest piece (which carries a sensitive diaphragm at its bottom). The diaphragm amplifies the sound (of heartbeats, etc.).
(ii) Two ear-pieces (which are made of metal tubes). These are put by the doctor into his ears.
(iii) A rubber tube which joins the chest piece to the ear pieces. The rubber tube transmits the sound from the chest piece to the ear pieces.
The doctor puts the ear-pieces of stethoscope into his ears and places the chest-piece above the part of the patient’s body (such as heart, lungs, etc.) which is to be examined. The sound of heartbeats (or lungs) reaches the doctor’s ears by the multiple reflections of sound waves through the stethoscope tube [as shown in Figure(b)]. Thus, a stethoscope works on the principle of multiple reflection of sound.
3. Soundboard
In a big hall, sound can be absorbed by the walls, ceiling, floor, seats and even by the clothes of the people sitting inside. This leads to too much absorption of sound due to which the speech cannot be heard clearly. This problem is overcome by using a soundboard. The soundboard reflects sound and helps to spread sound evenly in the big hall.
The soundboard is a concave board (curved board) which is placed behind the speaker in large halls or auditoriums so that his speech can be heard easily even by the persons sitting at a considerable distance. The soundboard works as follows: The speaker is made to stand at the focus of the concave soundboard (see Figure).
The concave surface of the soundboard reflects the sound waves of the speaker towards the audience (and hence prevents the spreading of sound in various directions). Due to this, sound is distributed uniformly throughout the hall and even the persons sitting at the back of the hall can hear his speech easily. It is obvious that the soundboards work on the multiple reflection of sound.
The ceilings of concert halls, conference halls and cinema halls are made curved so that sound, after reflection from the ceiling, reaches all the parts of the hall. This is shown in Figure. A curved ceiling actually acts like a large concave soundboard and reflects sound down onto the audience sitting in the hall.
The reflection of sound produces echoes. Echo is called ‘pratidhvani’ or ‘goonj’ in Hindi. We will now discuss the formation of echoes.
Echo
If we stand in one corner of a big empty hall and shout the word ‘hello’, we will hear the word ‘hello’ coming from the empty hall in the form of an echo a little while later. It appears as if the hall is repeating our ‘hello’. This happens because the sound of our ‘hello’ is reflected from the walls of the hall and this reflected sound forms the echo (which we hear as ‘hello’ coming from the empty hall).
We can now say that: The repetition of sound caused by the reflection of sound waves is called an echo. When a person shouts in a big empty hall, we first hear his original sound. After a little while, we hear the reflected sound of shout. This ‘reflected sound’ is an ‘echo’. So, when we hear an echo, we are actually hearing a reflected sound, a short while after the. original sound.
Thus, an echo is simply a reflected sound. An echo is heard when sound is reflected from a hard surface such as a tall brick wall or a cliff. A soft surface tends to absorb sound, so there is no echo. We know that the speed of sound in air (at 20°C) is 344 metres per second.
So, if we shout at a wall from 344 metres away, the sound takes 1 second to reach the wall. The sound reflects from the wall, and takes another 1 second to return to us. So, we hear the echo 2 seconds after we have shouted. We will now calculate the minimum distance from a sound reflecting surface (like a wall) which is necessary to hear an echo clearly.
Calculation of Minimum Distance to Hear an Echo
It has been estimated by scientists that if two sounds reach our ears within an interval of \(\frac{1}{10}\)th of a second, then we cannot hear them as separate sounds, they appear to be just one sound. The human ear can hear two sounds separately only if there is a time interval (or time gap) of \(\frac{1}{10}\) th of a second (or more) between the two sounds.
This means that we can hear the original sound and the reflected sound (echo) separately only if there is a time-interval (or time gap) of at least \(\frac{1}{10}\)th of a second (or 0.1 second) between them. Now, knowing the minimum time interval required for an echo to be heard and the speed of sound in air, we can calculate the minimum distance from a sound reflecting surface (like a wall, etc.) which is necessary to hear an echo. These calculations are given below :
We know that : Speed = \(\frac{\text { Distance travelled }}{\text { Time taken }}\)
Here, Speed of sound = 344 m/s (at 20°C)
Distance travelled = ? (To be calculated)
And, Time taken = \(\frac{1}{10}\) s
= 0.1 s
Now, putting these values in the above formula, we get:
344 = \(\frac{\text { Distance travelled }}{0.1}\)
So, Distance travelled = 344 × 0.1
= 34.4 metres
Thus, the distance travelled by sound in \(\frac{1}{10}\)th of a second is 34.4 metres. But this distance is travelled by sound in going from us (the source of sound) to the sound reflecting surface (like a wall), and then coming back to us. So, our distance from the sound reflecting surface (like a wall, etc.) to hear an echo should be half of 34.4 metres which is \(\frac{34.4}{2}\) = 17.2 metres.
From this we conclude that the minimum distance from a sound reflecting surface (wall, etc.) to hear an echo is 17.2 metres (at 20°C). This means that in order to hear an echo of our shout, we should be at least 17.2 metres away from a sound reflecting surface like a wall. This has been shown clearly in Figure.
We can see from Figure 42 that though the sound reflecting surface (wall) is only 17.2 metres away from us, but the sound has to travel 17.2 + 17.2 = 34.4 metres to produce an echo (17.2 metres in going from us to the wall, and 17.2 metres in coming back from the wall to us, after reflection).
Please note that 17.2 metres from a sound reflecting surface is the minimum distance to hear an echo in air at a temperature of 20°C. This distance will change with the temperature of air. Actually, the speed of sound in air increases with increasing temperature.
So, the speed of sound in air will be more on a hot day (when the temperature is high) than on a cold day. Since the speed of sound is more on a hot day, therefore, an echo is heard sooner on a hot day (than on a cold day).
We have just said that 17.2 metres from a sound reflecting surface is the minimum distance to hear an echo. We will also hear an echo when the distance is more than 17.2 metres from a reflecting surface. But no echo can be heard when our distance from the sound reflecting surface is less than 17.2 metres.
When we are at a distance of less than 17.2 metres from a sound reflecting surface, then we will hear the original sound and the reflected sound as one, and no echo will be produced.
If there are several reflecting surfaces, then multiple reflections of sound take place and hence several echoes may be heard. For example, rolling of thunder is due to the multiple reflections of sound of thunder from a number of reflecting surfaces such as the clouds and the land.
Please note that the minimum distance from a sound reflecting surface is 17.2 metres to hear an echo when the sound travels in air. But when the sound travels in water, then the minimum distance for hearing an echo will be different (because the speed of sound in water is different).
If the speed of sound in water is taken as 1500 m/s, then the minimum distance to hear echo in water will be 75 metres. Thus, the minimum distance of a diver from an under-water rock to hear the echo of his own shout will be 75 metres.
The formation of echoes by the reflection of sound waves is used to measure the depth of sea (or ocean); to locate the under-water objects like the shoals of fish, shipwrecks, submarines, sea-rocks and hidden ice-bergs in the sea; and to investigate inside the human body.
In all these applications of echoes, we do not use ordinary sound waves. We use high frequency sound waves called ‘ultrasonic waves’ or ‘ultrasound’. We will discuss all this after a short while. At the moment we will solve a numerical problem based on echoes.
Example Problem.
A man claps his hands near a mountain and hears the echo after 4 seconds. If the speed of sound under these conditions be 330 m/s, calculate the distance of the mountain from the man.
Solution:
Here the time taken by the sound (of clap) to go from the man to the mountain, and return to the man (as echo) is 4 seconds. So, the time taken by the sound to go from the man to the mountain only will be half of this time, which is \(\frac{4}{2}\) = 2 seconds.
Now, knowing the speed of sound in air, we can calculate the distance travelled by sound in 2 seconds. This will give us the distance of the mountain from the man.
We know that: Speed = \(\frac{\text { Distance travelled }}{\text { Time taken }}\)
So, 330 = \(\frac{\text { Distance travelled }}{2}\)
And, Distance travelled = 330 × 2 metres
= 660 metres
Since sound travels a distance of 660 metres in going from the man to the mountain, therefore, the distance of mountain from the man is 660 metres.
Reverberation
If a sound is made in a big hall, the sound waves are reflected repeatedly from the walls, ceiling and floor of the hall, and produce many echoes. The echo time is, however, so short that the many echoes overlap with the original sound. Due to this the original sound seems to be prolonged and lasts for a longer time. In other words, a sound made in a big hall persists (or lasts) for a longer time.
The persistence of sound in a big hall due to repeated reflections from the walls, ceiling and floor of the hall is called reverberation. A short reverberation is desirable in a concert hall (where music is being played) because it gives ‘life’ to sound and boosts the sound level. But if the reverberation is too long, then the sound becomes blurred, distorted and confusing due to overlapping of different sounds. Modern concert halls are designed for the optimum amount of reverberation.
The excessive reverberations in big halls and auditoriums are reduced (or controlled) by using various types of sound-absorbing materials. Some of the methods used for reducing excessive reverberation in big halls and auditoriums are as follows :
- Panels made of sound-absorbing materials (like compressed fibreboard or felt) are put on the walls and ceiling of big halls and auditoriums to reduce reverberations.
- Carpets are put on the floor to absorb sound and reduce reverberations.
- Heavy curtains are put on doors and windows to absorb sound and reduce reverberations.
- The material having sound-absorbing properties is used for making the seats in a big hall or auditorium to reduce reverberations.
The soft and porous materials are bad reflectors of sound. The soft and porous materials are actually good absorbers of sound. For example, the materials like curtains (fabrics) and carpets, etc., are bad reflectors of sound but they are good absorbers of sound. The bad reflectors of sound do not give good echo of the sound falling on them.
They absorb the sound and hence muffle (or silence) the sound falling on them. We can hear more clearly in a room having curtains because curtains are bad reflectors of sound. The curtains absorb most of the sound falling on them, and hence do not produce echoes. On the other hand, in a room without curtains, there is a greater reflection of sound due to which some echoes are produced.
These echoes cause a hindrance to hearing. In addition to curtains, carpets and sofa-sets in our rooms also reduce the formation of echoes by absorbing sound waves. From this discussion we conclude that some of the sound-absorbing materials (or objects) which make our big rooms less echoey are curtains, carpets and sofa-sets.
The Frequency Range of Hearing In Humans
The sounds produced in our environment have many different frequencies. The sounds of all the frequencies cannot be heard by the human beings. For example, if the frequency of a sound is less than 20 hertz, it cannot be heard by human beings. And if the frequency of a sound is greater than 20,000 hertz, even then it cannot be heard by human beings.
Thus, a human ear cannot hear sounds of frequencies less than 20 hertz and more than 20,000 hertz. The human ear can hear sounds having frequencies of 20 hertz to 20,000 hertz. The range of frequency from 20 Hz to 20,000 Hz is known as the frequency range of hearing in humans. The sound which we are able to hear is called ‘audible’ sound. So, we can also say that : The audible range of sound frequencies for human ear is from 20 Hz to 20,000 Hz.
The sounds of frequencies lower than 20 hertz are known as ‘infrasonic sounds’ (or just ‘infrasound’). Thus, infrasonic sounds are very low-frequency sounds. Infrasonic sounds cannot be heard by human beings. Infrasonic sounds are produced by those objects which vibrate very slowly.
For example, a vibrating simple pendulum produces infrasonic sound. We cannot hear the sound of a vibrating simple pendulum because it vibrates with a frequency less than 20 hertz. Earthquakes, and some animals like whales, elephants and rhinoceroses also produce infrasonic sounds.
Rhinoceroses communicate with one another by using infrasonic sound having a frequency as low as 5 hertz. It is observed that some animals get disturbed and start running here and there just before the earthquakes occur. This is because, before the main shock waves, the earthquakes produce low-frequency infrasonic sounds which some animals can hear and get disturbed.
The sounds of frequencies higher than 20,000 hertz are known as ‘ultrasonic sounds’ (or just ‘ultrasound’). Thus, ultrasonic sounds are very high frequency sounds. Ultrasonic sounds cannot be heard by human beings. Though human beings cannot hear ultrasonic sounds but dogs can hear ultrasonic sounds of frequency up to 50,000 hertz.
This is the reason why dogs are used for detective work by the police. Bats, monkeys, deer, cats, dolphins, porpoises and leopard can also hear ultrasonic sounds. Bats can hear ultrasonic sounds having frequencies up to 1,20,000 hertz. In fact, bats can also produce ultrasonic sounds while screaming.
We cannot hear the screams of a bat because its screams consist of ultrasonic sound having a frequency much higher than 20,000 hertz (which is beyond our limit of hearing). In addition to bats, dolphins, porpoises and rats can also produce ultrasonic sounds as well as hear ultrasonic sounds. Children under the age of five years can hear ultrasonic sounds of frequency up to 25,000 hertz.
As people grow older, their ears become less sensitive to sounds of higher frequencies. Ultrasonic sound cannot be produced by ordinary vibrators like tuning forks. They are produced by special vibrators which can vibrate very, very rapidly. We will discuss this in detail in higher classes.