The Nature Of Sound Wave 295p4l

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 friend’s 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

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 ed 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:

*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, they disturb 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 little louder 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 starter’s 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.

The speed of sound can be calculated us ing the basic wave equation v = λf where λ is the 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)

Δv/ΔT (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

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 (Solid Thin 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

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 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 on the message to the hearing center of the brain. And lastly, the brain’s sound memory center stores message and identifies the sounds received

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