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Basic Physics Lesson - 17: Sound

Umesh is a freelance writer contributing his creative writings on varied subjects in various sites and portals in the internet.



Energy exists in many forms like light, heat, falling water, blowing wind, moving object etc. Sound is also a form of energy. We all are well aware of the various sounds which surround us. How do you feel when there is no sound around? We are accustomed to hearing so many sounds that it is interesting to find out what would we feel when there is no sound. It would definitely be something unusual if there is no sound around. However, some people like that situation called the pin drop silence. In general, we all will be hearing some sound feeble or loud everywhere. That is a part of our lives. In this article, we will learn about sound, its characteristics, how it travels, and things like that.


How do we hear

We all know and understand well that we hear with our ears. When we do not want to hear something being told by someone, we simply put our palms over the ears and press them tiight and no sound reaches our ears. It means until sound reaches inside our ears, we do not hear. Human ear is a great mechanism for enabling hearing and the first crucial part that any sound meets in our ear is the diaphragm which vibrates as per the frequency and loudness of the sound waves striking it and then through a complex network of small bones and nerves and other biological matter it is sent to the brain which interprets it in the precise ways and we come to know that what is that sound, how loud that is, and of course we can differentiate even whether it is a male voice or female voice or the voice of a child. Our brain is capable to distinguish the subtle changes in the nature of sound and those who are good music and song listeners might appreciate the tremendous detection power of our brains for sound recognition.

How sound is created

Whenever something moves or vibrates it creates a movement of the medium material and that movement or pressure in a wave form moves in all the direction affecting the adjacent material in the same way. So the initial disturbance created in the medium moves ahead and when it reaches our ears we hear it. This movement can also be visualised as a changing pressure in the medium which is starting from the point of origin to the receiver.

There are many mechanical devices which create sounds. Like a tuning fork which is a simple device with a metallic 'U' design with a handle at the base. When we stuck it at the arms, it vibrates and produces sound.

Likewise any moving part will create a sound, the pistons moving inside engines produce big sounds, scratching a rough surface creates sound etc. So, sound is generated whenever some movement or disturbance is there. Interesting thing is that sound produced in one action would be different than produced by some other action and that is because the frequency of vibration of that sound is different from case to case. Generally it would be a mix of frequencies around a characteristic sound frequency of that action and that together gives rise to that particular sound which just by closing our eyes we can tell as what type of action is going on.

When we speak, our throat, tongue, and vocal chord system make sound and depending upon what language we had learnt during our growing up, we speak them and create those sounds which are heard and understood by those who also know that language.

Animals do not have such developed vocal faculties and can produce only a limited number of sounds. Animals also produce sounds from their other body parts like bees produce humming sound from the vibrations of their wings.

When the medium like air are exposed to sudden change in pressure like in an explosions, a huge sound is created. These sounds reach quite far before they get attenuated.

Frequency of sound

Frequency of sound is its characteristics by which we feel it coarse or shrill. We can differentiate between a low frequency coarse sound and a high frequency shrill sound. Our ears are tuned to that. The audible range of frequencies for the human beings is approximately in the range of 20 Hz to 20 kHz. The notation Hz (Hertz) is actually used to represent cycle/second and is the unit of frequency. The sound frequencies below 20 Hz are known as infrasonic and above 20 kHz are called ultrasonic.

Children can detect sounds of slightly higher than 20 kHz also. When people get older their hearing powers are generally affected and for them the upper limit of 20 kHz comes down to 15-17 kHz range. Another problem in old age is inability to listen feeble (low amplitude sound) voices and one has to speak with them loudly or these old people have to use hearing aids which amplify the outside sound and then pass it to the ear diaphragm.

Sound is a wave propagation and every wave has a frequency and wavelength. The speed of the sound (v) and these two parameters that is frequency (f) and wavelength (λ) are related by the following formula -

v = f λ

Low frequency sounds are often termed as low pitch sounds while high frequency sounds are called as high pitch sound. A whistle gives a high pitch sound. A soft and gentle person may speak in a low pitch sound.

Pitch or frequency should not be confused with loudness or intensity of sound. When we increase the volume of an audio device the loudness or the intensity of sound increases. A loud sound can go to a much longer distance, as compared to the feeble one, before getting fully attenuated.

Sound of music

Who does not like listening to music? The beautiful melodies that generate the finest of sentiments in us.

What is music? It is a sequence of sounds (technically known as notes) that is pleasant to ears. The change in the frequency of that sound and its ascending and descending orders create that effect which we call music and by a different combinations of these sounds, different music pieces are created. When a lyric is sung under the modulations of that music it becomes so pleasant to hear and sometimes is even capable of mesmerising us.

Those who have interest in music or musical instruments might be familiar with the musical notes. For example in a piano the sound keys (each of the keys gives out a musical note) are arranged in a particular ascending sequence of frequency on the keyboard from left to right and when a key or combination keys are pressed it gives the sound of that particular frequency or mixed frequencies.

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How sound travels

When sound is created at one place near us, we hear it comfortably. How does this sound travel from that place to our ears? The answer is very simple. It travels through the medium. On the Earth's surface we talk with each other and the sound we create travels through the air and reaches the other person. Air is the medium through which it is travelling.

In our childhood, we used to make a hand telephone toy using the inside trays of the match boxes and connecting them through a thin thread. We were able to make a length of 20-30 meters easily and when we talk through this arrangement we could hear the clear voice of the other person which in fact came to us travelling through the thread. So, sound can travel through various mediums.

Sound is a wave energy and what it does is that it creates compression and expansion in the medium through which it travels and this compression and expansion only travels ahead keeping with it the characteristics of that sound. If the sound is loud the amplitude of these compressions and expansions will be more and if sound is faint the compressions and expansions will be small. What reaches our ears is these compressions and expansions of the medium (in this case air) only.

So, it is clear that sound needs a medium to propagate and without any medium it would not be able to move ahead.

This brings up a very important point for consideration that what would happen to the sound if there is no medium. For example there is no atmosphere on Moon and if two persons want to talk to each other on Moon then they will have to use some apparatus or wireless communication device like walkie-talkie for that!

When sound propagates in a medium then it is attenuated with the distance and after some distance its intensity becomes so feeble that one can not hear it and it goes unnoticed. Depending upon the medium this attenuation will be more or less. Anyway, in air the attenuation is negligible.

The speed of sound

The speed of sound in air is about 332 meters/second. The speed of sound in a medium depends on its stiffness as well as density. Stiffness means the extent to which the material is able to resist to the applied force or compression. In scientific notation the speed of sound (v) is related to the stiffness (Ks) and density (d) of the medium by the following relationship -

v = (Ks/d)1/2

So, the speed of sound will depend on stiffness as well as density of the medium.

Speed of sound in some of the common mediums is as under -

  • Water - 1496 m/s
  • Air - 332 m/s
  • Steel - 5940 m/s

Doppler effect

Sometimes, the source of the sound is moving and it has got a relative speed with respect to the observer or the person who is hearing it. Due to the movement of the source towards the person, the person gets more sound waves within a stipulated time which makes him to recognise the frequency of the sound at a slightly higher side. At the same time if the source is moving away from the person then the frequency seems to go down slightly. This effect is known as Doppler effect. Actually this effect takes place in other type of waves like light waves also but in case of sound as many of us have experienced it so it is in the knowledge of many people.

One example of this is the frequency of sound of siren of an approaching police car as compared to its sound when it is moving away from us.

Reflection and echo

Sound gets reflected as well as absorbed at the boundaries between the two different mediums. Some of us who have visited hilly areas might have observed the echo of our loud voices from the nearby hills. Some children make loud shrieks and then enjoy hearing their own echo returned back from the mighty hills.

This property of reflection of sound can be used to estimate distances by measuring the to and fro time of travel of sound from the person to the object from which it is reflecting back and heard like an echo by the person. For example, if we make a sound in a valley facing a hill and its echo reaches us after 1.5 seconds then using the speed of sound in air that is 332 metres/second, we can find the approximate distance of hill as 332 x 1.5 = 498 metres.

Some materials reflect the sound very efficiently while some have properties of reflecting less and absorbing more of the sound reaching them. Such materials are used for soundproofing of halls or rooms.

Measurement of sound energy

Human beings can hear from a feeble sound to quite a very loud one and the sound intensity from feeble to very loud will be covered in a very long linear range and will not be convenient to use. To mitigate this problem a decibel scale is devised to express the intensity or loudness of sound.

The intensity of sound is measured in watts per square metres and the base threshold hearing of human ear is approximately taken as 10-12 watt per square metre and that is called zero decibel point and any sound intensity which is 10 times more than the zero decibel value will become sound of 10 decibel intensity. From the level of this 10 decibel sound if we again go to a sound which is 10 times louder, then that will be a sound of 20 decibels. If we go on like that then we will reach a value of 110 decibel corresponding to 10-1 watt per square metre which is almost the intensity of sound of a loud music. The next 10 times louder sound is almost equal to that of a jet engine and is about 120 decibels. At this point we almost reach the loudness tolerance limit of our ears because any sound louder than that will start harming our ears. This decibel scale is very convenient and used for denoting sound intensities.

This decibel scale is mathematically represented by the following equation -

L = 10 log (I / Io)

L is the decibel value corresponding to the intensity of sound I.

Io is the base minimum sound intensity which human ears can hear and is taken as the sound intensity value of 10-12 watt per square metre.

Usage of sound energy in industries

Sound waves are used for determining the water depths of lakes or rivers. It is also used for medical diagnostic purposes like doing ultrasonography in hospitals for indirectly mapping and viewing the inside organs and their boundaries. This is a very popular method for locating the unwanted tumours or their growths etc inside the body at various places.

Some sound sources are also used for scientific experiments in laboratories and research centres.

Other lessons in basic Physics series

Lesson-1: Distance and Displacement.

Lesson-2: Speed and Velocity.

Lesson-3: Acceleration.

Lesson-4: Mass and Weight.

Lesson-5: Gravity.

Lesson-6: Volume and Density.

Lesson-7: Momentum.

Lesson-8: Force Work Done and Energy.

Lesson-9: Heat and Temperature.

Lesson-10: Circular Motion.

Lesson-11: Friction.

Lesson-12: Rotational Motion.

Lesson-13: Simple Harmonic Motion.

Lesson-14: Voltage and Current.

Lesson-15: Magnetism.

Lesson-16: Light.

Lesson-18: Electrical Resistance.

Lesson-19: Capacitance.

Lesson-20: Atomic Structure.

Lesson-21: Kinetic Energy.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Umesh Chandra Bhatt


Vanita Thakkar on June 23, 2021:

Good introduction to the science of Sound. Thanks for sharing.

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