What is SoundWhen someone tells us to do some thing then we hear the instructions through our ears. It means when a person speaks then from his mouth he is releasing some energy in the atmosphere which is reaching our ears and our brain is recognising that as sound of the words that he spoke to us. In other words he created some disturbance in the air in the atmosphere which reached our ears and we heard that and our brain understood that. This entity which is in form of energy and travelling through the medium (air) to us is actually the sound energy and whatever we hear is possible because so many sounds from so many diffrent sources are reaching us day and night and our brain analyses them accordingly. In this basic Physics lesson we will learn about this sound - what it is, how it is created, how it travels, and other scientific details about it.
Human ears are excellent hearing machinesHuman ears are the most advanced type of biological hearing machines and they can hear a variety of sounds coarse to fine as well as feeble to loud ones. There is a diaphragm in the ear which vibrates when some sound is falling on it and then those vibrations go to the inner ear through some small bone pieces and finally through the nerves to the brain where they are decoded and we get a feeling of hearing that sound. It is a very fast process and as soon as some sound falls on our ear diaphragm we hear it.
Source of soundThere are so many sources of sound and we get familiar to them while we grow up. We hear them and we also learn the meaning of all those sounds that is coming to us. We are habitual of hearing all sort of sounds coming to our ears and can distinguish many of them from each other. With our experiance we can most of the times identify the source also. During the rainy season who cannot recognise the thundering sound of clouds. likewise there are characteristic sounds pertaining to various sound sources.
It is interesting to note that there are many activities where sound is created. For example if a body moves on a surface then some sound is created. When two bodies collide or slide against each other then also some sound is created. so, some sort of sound is generally associated with activities.
In science experiments in the science laboratory we use tuning forks which are U shaped metal devices with a handle below and if we stuck it against our hand or some other body it vibrates and creates a sound.
How does sound travelWhen a sound is created then we hear it. How does it reach our ears? The answer is simple that it travels trough the air in the atmosphere. That raises another question that if there is no air in a place that is a condition of vacuum then what will happen? The answer is that sound needs a medium to travel and reach other plac and if air is not there then sound which is a wave energy does not propagate and then we would not hear it. For example there is no atmosphere in Moon as its gravity is not sufficient enough to hold it, the astronauts cannot talk to each other just like that. They have to use some walkie talkie or adequate communication system to communicate with each other.
Sound is a wave and it travels through the vibration of the medium particles through which it is passing. It could be air or water or any other material. We must understand that sound is an energy and it is obvious that it would attenuate while travelling and become weak as it moves furher and further from the source. This attenuation of the amplitude of the sound wave would depend on the type os the material through which it is travelling. A loud sound (high amplitude) will go to a long distance while a feeble sound (very low amplitude) will attenuate and die out near to the source itself.
Frequency and wavelength of sound wavesSound is characterised by its frequency and wavelength. We all have seen the musical instruments which create sounds of different frequencies. For example in a piano or harmonium we have low frequency sounds starting from the left side and these frequencies go on increasing as we move to the right side of the keyboard and maximum frequency is created at the extreme right end. Human ears can distinguish them as the change of that frequency is heard by our ears and understood by our brains.
Frequency of sound is measured in vibrations per second or cycles per second and in scientific notations it is denoted by Hertz having a short form as Hz (1 Hertz = 1 cycle/second).
Human ears are capable of hearing sound in the range of approximately 20 Hz to 20 KHz (20000 Hz). This range may slightly vary from individual to individual but that is an average figure. The sound frequencies also exist below 20 Hz and are known as infrasonic while the ones above 20 kHz are called ultrasonic.
Human ears hearing capacity deteriorates with aging and most of the elders have difficulties in hearing and they have to use a hearing aid to listen to others.
The speed of sound in air is about 332 m/s (metres per second). Speed, wavelength, and frequency are related to each other by the following equation -
Speed (v) = Frequency (f) x Wavelength (w)
Sound can travel through other mediums like water, metals etc but its speed would change as per the material through which it is travelling and to find the speed of sound in that material we will have to know their densities and stiffness. Stiffness is 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)
Speed of sound in some of the common mediums is as under -
Water - 1496 m/s
Air - 332 m/s
Steel - 5940 m/s
Sound echoSound is a wave and when it strikes or falls on a body then some part of it is reflected while other part is absorbed by the body and may travel through it till it attenuates fully. When reflected part is of good amplitude then it becomes an echo. For example in hilly areas if we make loud sunds standing on one hill then the sound goes forward to another hill nearby and after a little while we listen an echo of sound. With some simple calculations we can even find the distance between two hills. Suppose we hear the echo say after 1.5 seconds of making a loud sound then knowing the velocity of sound in air as 330 m/s we can easily find the distance as (332 x 1.5)/2 = 249 metres.
Measurement of sound energyHumans are capable of hearing sounds of different intensities ranging from feeble sound to louder ones. As the range of these intensities is quite large, a logarithmic scale is devised to understand that. This scale is known as decibel scale. Our ears on an average start hearing from a sound of intensity about 10 to 12 watt/square meter. This point is taken as 0 (zero) decibel. Now any sound which is 10 times more intense then this would be called as 10 decibel. Any sound that is again 10 times louder than this would be called as 20 decibel and so on. For example the sound intensity of loud music is around 100 to 110 decibel while a jet engine sound is around 120 decibel.
We have to note that our ears can tolerate sounds upto a certain loudness and after that the high decibel sound can harm our ears by damaging it. We have to avoid such environment or wear ear plugs to protect ourselves from that loud sound.
Applications of sound energySound waves are used for various purposes. They are used for determining the water depths of lakes or rivers. One of the important use of sound waves is for medical diagnostic purposes like doing ultrasonography in hospitals for indirectly locating and viewing the inside organs and their boundaries. Sonography is a very popular method in hospitals for locating the tumours or their growths or any extra growth inside the body. Sound waves are also used for scientific experiments in laboratories and research centres.
The author has explained the science behind sound and it is good for the students who are learning Physics. Sound is also an energy like light. Sound is mechanical energy and it travels as a wave. Even though light and sound are energies, there are many differences, Sound is a mechanical wave whereas light is electromagnetic. Sound is a longitudinal wave whereas light is a transverse wave. The speed of sound is much less when compared to light. The distance a sound can travel is quite large. But the intensity of the same will decrease as it travels and become inaudible to human beings.
Yes, there are many differences between the ways that sound and light travel and behave. The interesting thing about propagation of light is that it doesn't require medium and that is the reason that why we are able to see the far distant stars and galaxies just by the light that is coming to us through the vast vacuum in the universal empty space. Sound is a mechanical energy wave while light is an electromagnetic energy wave.
One interesting observation about the difference between the speed of sound and light is that when the clouds thunder then we see the light produced much earlier than the sound of thundering.
For example if the thundering clouds are about 600 to 700 m far in the sky from our location then their light will be seen by us instantaneously because of very high speed of light waves but sound will reach us only after about 2 seconds and that is the gap between the two things that is so obvious and conspicuous.