The Nature of Sound Wave

7/29/2019 The Nature of Sound Wave

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THE NATURE OF SOUND WAVE

SOUND:

We live in a world of sound. Anywhere we go and

whatever we do, we hear sound. Some of these are familiar to

us, a friends voice, the chirping of birds, the ticking of the

clock, the barking of the dogs, the beating of your heart.

Certain sounds like music are pleasant to the ears; others are

not. We call the latter noise. Different sounds have different

effects. For instance, music soothes and relaxes. The crashing

sound of explosives can make us feel nervous.

Sound is defined as a mechanical wave that is an oscillating of pressure transmitted

through a liquid, solid or gas, composed of frequencies within the range of hearing.

Sound is restricted to the frequency range of 20 Hz to 20, 000 Hz to which the human

ear is sensitive. Waves with frequencies below this audible range (1-20 Hz) are called

infrasounds and those above (>20, 000Hz) are referred to as ultrasounds.

Sound waves are longitudinal waves. They are produced by a

series of vibrations parallel to the direction of travel of the waves.

When you pluck the strings of a guitar, the strings look hazy as they

vibrate. Touch your throat while talking and you will feel the vibrations

of your vocal cords. In each of these examples, the source of the

sound is a vibrating object.

PROPAGATION OF SOUND WAVES:

Like water waves, sound waves need a medium to spread in. sound waves can travel in

air, water, and solids. But they can even travel through narrow openings and around corners,

but not in an empty space or vacuum.

Sound can travel through solids.

In earlier days, doctors used stethoscopes consisting of

thin wooden rods with broadened ends. By placing one end to

his ear and placing the other end on the patient's chest, he

could hear the sound of the heart beats transmitted through the

wood. Motor mechanics sometimes, use wooden rods as

stethoscopes to assist in tracing the source of the knocking


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noises in engines. Cotton, wool and felt are poor conductors of sound. A piece of thread does

not conduct sound when slack, but will conduct it well when stretched.

Sound can travel through liquids.

We know that water transmits sound. This can be shown by clapping two pieces of stone

or metal against each other under water, when the sound of the clapping can be heard above

the water. In 1654, Otto Von Guericke found that fish were attracted by the sound of a ringing

bell underwater and therefore, concluded that sound could travel through water as well as air.

Sound travels with a finite velocity depending on the medium.

The following examples show that sound takes an appreciable

time to travel from one place to another:

A. Though lightning and thunder are produced simultaneously,

the flash of the lightning is seen much before the sound of the

thunder.

B. When a gun is fired at some distance, the flash is seen before

the sound is heard.

C. The puff of steam issuing from the whistle of a distant locomotive engine is seen before

the sound is heard.

D. In a cricket match, the striking of the ball by the batsman is seen before hearing the

sound.

Sound is produced by the initiation of a succession of compressive and rareactive

disturbances in a medium capable of transmitting these vibrational disturbances. Particles of the

medium acquire energy from the vibrating source and enter the vibrational mode themselves.

The wave energy is passed along to adjacent particles as the periodic waves travel through the

medium.Vibrating elements like reeds (clarinet, saxophone) strings (guitar, vocal chords),

membranes (drum, loudspeaker), and air columns (pipe organ, flute) initiate sound waves.

Sound waves are transmitted outward from their source by the surrounding air. When they enter

the ear, they produce the sensation of sound

During propagation, waves can be reflected (change in direction of a wavefront at

an interface between two different media), refracted (change in direction of a wave due to a

change in its medium), or attenuated (the gradual loss in intensity of any kind of flux through a

medium) by the medium.

The behavior of sound propagation is generally affected by three things:


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*A relationship between density and pressure.

*The motion of the medium itself.

*The viscosity of the medium.

Consider a vibrating tuning fork. As the prongs of the fork move back and forth, theydisturb air molecules close to them creating a back and forth movement of the air parallel to the

direction of the waves. These air molecules likewise transfer their motion to the neighboring

particles and to the other molecules. The air molecules then strike your eardrum, making it

vibrate.

Nearly all sounds reach you, with air as the transmitting medium. Dense gases are

better transmitters of sounds that rare gases. As you climb a mountain, you must speak a littlelouder to be heard. Air on mountain is less dense than in the lowlands. It does not transmit

sound so readily.

SPEED OF SOUND:

During a thunderstorm, a distant lightning flash can

be seen several seconds before the accompanying thunder is

heard. The timer at the finish line during a track meet may see the

smoke from the starters gun before he hears the report. Over

short distances, light travels practically instantaneously.

Therefore, the time that elapses between a lightning flash being

seen and the thunder being heard or between a gun being fired

and the report being heard must be the time required for the

sound to travel from its source to the listener.


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The speed of sound can be calculated us ing the basic wave equation v = f where isthe wavelength and f is the frequency of the wave.

On the other hand any temperature in degrees Celsius, the speed of sound in air is

determined by the equation v=330 m/s + [(0.6 m/s)/ C](T)

The speed of sound in air is 331.3 m/s at 0 C. This speed increases with temperature

about (0.6 m/s)/ C.

The speed of sound in water is about four times the speed in air. In water at 25 C sound

travels about 1,500 m/s. In some solids, the speed of sound is even greater like the steel rod

which travels approximately 5, 000 m/s about 15 times the speed of air. In general the speed

of sound varies with the temperature of the transmitting medium.

Table I. Speed of Sound (Gas STP)

Substance Density (g/L) Velocity (m/s) v/T (m/s C)

air, dry 1.293 331.35 0.59

carbon dioxide 1.977 259 0.4

helium 0.178 965 0.8

hydrogen 0.0899 1284 2.2

nitrogen 1.251 334 0.6

oxygen 1.429 316 0.56

Table II. Speed of Sound (Liquid 25 C)

Substance Density (g/cm3) Velocity (m/s)

acetone 0.79 1174

alcohol, ethyl 0.79 11207


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carbon tetrachloride 1.595 926

glycerol 1.26 1904

kerosene 0.81 1324

water, distilled 0.998 1497

water, sea 1.025 1531

Table III. Speed of Sound (SolidThin Rod)

Substance Density (g/cm3) Velocity (m/s)

aluminum 2.7 5000

brass 8.6 3480

brick 1.8 3650

copper 8.93 3810

cork 0.25 500

glass, crown 2.24 4540

iron 7.85 5200

lucite 1.18 4110

steel 7.85 5200

SOUND TRANSMISSION:

To produce sound waves, we must have a source that initiates a mechanical disturbance

and an elastic medium through which the disturbance can be transmitted. Most sounds come to


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us through the air that acts as the transmitting medium. At low

altitudes, we usually have little difficulty hearing sounds. At higher

altitudes, where the density of air is lower, less energy mar be

transferred from the source to the air. Dense air is more efficient

transmitter of sounds than the rarefied air.

So therefore, sound does not travel through a vacuum; it is

transmitted only through a material medium.

RECEIVING SOUNDS:

First, the outer ear collects sound waves which pass

through the ear canal.

Second, as they reach the eardrum at the end of canal, the

eardrum vibrates.

Third, the vibration moves on to the three bones of the

middle ear connected to the eardrum, collectively known as

ossicles,and then to the liquid of the coiled shape cochlea of the

inner ear.

Forth, hair cells in the organ of Corti in the cochlea then vibrate.

Fifth, the nerves at the hair cell pass on the message to the hearing center of the brain.

And lastly, the brains sound memory center stores message and identifies the sounds

received

Sources: *Physics Textbook *Modern Physics *Wikipedia.com

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The Nature of Sound Wave