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difference between echo and reflection of sound

What is sound?

“Sound is a form of energy that is passed from one point to another as a wave.”Sound is an example of a longitudinal wave.One major reason for learning about sound is that we experience it every moment of our lives in the form of music,noise and communication.From the large vocabulary we have of sounds (e.g clank,ping,bang,pop,hiss) and the different adjectives that we have to describe sound (e.g high,low,soft,loud,mellow,pleasant,sharp),we certainly have a great variety of sounds.We will be learning how sound is produced,how sound can be classified ,how sound travels through various media and other topics.The more we understand about sound,the better we can use and control it.

Sound is produced by vibrating sources placed in a medium.The medium is usually air;but it can be any gas,liquid or solid.A vibrating object in a medium,such as air,produces sound waves through the shifting of layers of air particles.This is similar to the longitudinal waves produced when a slinky spring is vibrated parallel to its length.

A common vibrating  source of sound is the tuning fork.tuning forkA tuning fork is an instrument with two hard steel prongs.It produces a musical note when you hit it.The longitudinal nature of sound waves produced by a vibrating tuning fork.

For sound waves in air ,compressions and rarefactions can be thought of as changes in air pressure. Compressions are places where air pressure is slightly higher than the surroundings air pressure. Rarefaction’s are places where air pressure is slightly lower than the surrounding air pressure.

The transmission of sound

  • The mechanical nature of sound:

Sound waves are mechanical in nature in that they require a medium through which to move.Unlike light waves which are electromagnetic in nature ,sound waves cannot pass through a vacuum.

Characteristics of sound:

Sounds of different objects can be distinguished on the basis of different characteristics as described below:

Loudness:

“Loudness is the characteristic of sound by which loud and faint sounds can be distinguished.”When we talk to our friends,our voice is low,but when we address a public gathering our voice is loud.Loudness of a sound depends upon a number of factors.Some of them are discussed below:

Amplitude of the vibrating body:amplitude of sound waves

The loudness of the sound varies directly with the amplitude of the vibrating body.

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The sound produced by a sitar will be loud if we pluck its wires more violently.Similarly,when we beat a drum forcefully,the amplitude of its membrane increases and we hear a loud sound.

Area of the vibrating body:

The loudness of the sound also depends upon the area of the vibrating body.For example,sound produced by a large drum is louder than that by small one because of its large vibrating area.If we strike a tuning fork on a rubber pad,a feeble sound will be heard.But if the vibrating tuning fork is placed vertically on the surface of a bench,we will hear a louder sound.From this,we can conclude that the loudness increases with the area of the vibrating body and vice versa.

Thin walled glass goblets can vibrate when hit by sound waves.This is due to a phenomenon of sound known as resonance.Some singers can produce a loud note of particular frequency such that it vibrates the glass so much that it shatters.

Distance from the vibrating body:

Loudness of sound also depends upon the distance of the vibrating body from listener.It is caused by the decrease in amplitude due to increase in distance.

Loudness also depends upon the physical condition of the ears of the listener.A sound appears louder to a person with sensitive ears than to a person with defective ears.However,there is a characteristic of sound which does not depend upon the sensitivity of the ear of the listener and it is called intensity of sound.

Pitch:pitch of sound

“Pitch is the characteristic of sound by which we can distinguish between a shrill and a grave sound.”It depends upon the frequency.A higher pitch means a higher frequency and vice versa.The frequency of the voice of ladies and children is higher than that of men.Therefore,the voice of ladies and children is shrill and of high pitch.The relationship between frequency and pitch is illustrated.

Quality:

“The characteristic of sound by which we can distinguish between two sounds of same loudness and pitch is called quality.”While standing outside a room,we can distinguish between the notes of  a piano and a flute being played inside the room.This is due to the difference in the quality of these notes.

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Intensity:

The sound waves transfer energy from the sounding body to the listener.The intensity of sound depends on the amplitude of sound wave and is defined as:

“Sound energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves is called intensity of sound.”Intensity is a physical quantity and can be measured accurately.The unit of intensity of sound is watt per square meter (Wm-2).

Sound intensity level:

The human ear responds to the intensities ranging from 10-12Wm-2 to more than 1 Wm-2 (which is loud enough to be painful).Because the range is so wide,intensities are scaled by factors of ten.The barely audible and the faintest intensity of sound i.e,10-12Wm-2 is taken as reference intensity,called zero bel (a unit named after Alexander Graham Bell).

The loudness of a sound depends not only on the intensity of sound but also on the physical conditions of the ear.The human ear is more sensitive to some frequencies rather than the others.The loudness (L) of a sound is directly proportional to the logarithm of intensity i.e.,

L ∝ log I

L =K log I …..(1)

Where k is a constant of proportionality.

Let Lº be the loudness of the faintest audible sound of intensity Iº and L be the loudness of an unknown sound of intensity I,then by equation (1),we can write:

Lº = k log Iº ……(2)

Subtracting equation (2)  from (1),we get:

L – Lº =K (log I – log Iº)

=K log I/Iº

This difference( L -L),between the loudness L of an unknown sound and the loudness Lº is called the intensity level of the unknown sound,Therefore,the intensity level of an unknown sound is given by:

Intensity level = k log I/Iº            ……..(3)

The value of k depends not only on the units of I and Iº but also on the unit of the intensity level.If intensity I of any unknown sound is 10 times greater than the intensity Iº of the faintest audible sound i.e., I = 10 Iº and the intensity level of such a sound is taken as unit,called bel,the value of k becomes 1.Therefore ,using k =1, equation (3) becomes:

Intensity level =log I/Iº (bel) …..(4)

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Bel is a very large unit of intensity of a sound.Generally,a smaller unit called decibel is used.Decibel is abbreviated as (dB).It must be remembered that 1 bel is equal to 10 dB.if the intensity level is measured in decibels then equation (4) becomes:

Intensity level =10 log I/Iº (dB) ……(5)

By using equation (5) ,we can construct a scale for measuring the intensity level of sound.Such scale is known as “decibel scale”.The decibel scale is a logarithmic measure of the amplitude of sound waves.In a logarithmic scale,equal intervals correspond to multiplying by 10 instead of adding equal amounts.The intensity level of different sounds in decibels is given in table below:

Sources of sound Intensity (W m-2) Intensity level(dB)
  Nearby jet airplane  103 150
Fast train 101 130
Siren 100 120
Lawn mover 10-2 100
Vacuum cleaner 10-5 70
Mosquito buzzing 10-8 40
Whisper 10-9 30
Rusting of leaves 10-11 10
Faintest audible aound 10-12

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Reflection of sound:

When we clap or shout near a reflecting surface such as a tall building or a mountain,we hear the same sound again a little later.What causes this?This sound which we hear is called an echo and is a result of reflection of sound from the surface.It is defined as:”When sound is incident on the surface of a medium it bounces back into the first medium.This phenomenon is called echo or reflection of sound.”

The sensation of sound persists in our brain for about 0.1 s.To hear a clear echo,the time interval between our sound and the reflected sound must be at least 0.1 s.If we consider speed of sound to be a 340 ms-1 at a normal temperature in air,we will hear the echo after 0.1 s.The total distance covered by the sound from the point of generation to the reflecting surface and back should be at least 340 ms-1 ×0.1 s =34.0 m.Thus,for hearing distinct echoes,the minimum distance of the substance from the source of sound must be half of this distance,i.e.,17 m.Echoes may be heard more than once due to successive or multiply reflections.

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