Nazarsir.blogspot.com: Study Of Sound

Sound is a form of energy which creates the sensation of hearing in our ears. This energy is in the form of waves.
A medium is necessary for the propagation of sound waves.
Sound waves give rise to a chain of compression (place of higher density) and rarefaction (place of lower density) in the medium.
The particles of the medium oscillate about their central or mean positions, in a direction parallel to the propagation of the wave. Such waves are called longitudinal waves.
The Waves created by dropping a stone in still water, the particles of water oscillate up and down. These oscillations are perpendicular to the direction of propagation of the wave, such waves are called transverse waves.

At any moment during the propagation of a sound wave we would find alternate bands of compression and rarefaction of the medium i.e. bands of greater and lesser density.

The wavelength of sound waves is indicated by the Greek letter lambda (λ), while its frequency is indicated by nu (ν).
The amplitude, which is the maximum value of pressure or density, is indicated by A.
The time taken for one oscillation of pressure or density at a point in the medium is called the time period and is indicated by T.
The value of frequency determines the pitch (high or low) of the sound while the value of the amplitude determines its strength or loudness.

Velocity of sound :

The distance covered by a point on the wave (for example the point of highest density or lowest density) in unit time is the velocity of the sound wave.
Velocity = distance / time
Velocity = Wavelength / time
velocity of sound = wavelength x frequency.

v = λ/T

In any medium at fixed physical conditions the velocity of sound of different frequencies is very nearly the same.
The velocity is highest in solids and least in gases. It increases with an increase in the temperature of the medium.
The Italian physicists Borelli and Viviani, in the 1660s, first calculated the speed of sound.
Their value of 350 m/s is very close to the value of 346 m/s which is accepted today

Velocity of sound in a gaseous medium:

The velocity of sound in a gaseous medium depends on the physical conditions i.e. the temperature, density of the gas and its molecular weight.

Temperature (T):

The velocity of sound is directly proportional to the square root of the temperature of the medium. This means that increasing the temperature four times doubles the velocity.

Density (ρ):

The velocity of sound is inversely proportional to the square root of density. Thus, increasing the density four times, reduces the velocity to half its value.

Molecular weight (M):

The velocity sound is inversely proportional to the square root of molecular weight of the gas. Thus, increasing the molecular weight four times, reduces the velocity to half its value.
The molecular weight of oxygen gas (O2 ) is 32 while that of hydrogen gas (H2 ) is 2. Prove that under the same physical conditions, the velocity of sound in hydrogen is four times that in oxygen.
For a fixed temperature, the velocity of sound does not depend on the pressure of the gas.

Audible, infra- and ultra-sound

The limits of hearing of the human ear are 20 Hz to 20,000 Hz. That is, the human ear can hear sounds of frequencies in this range. These sounds are called audible sounds.
Our ears cannot hear sounds of frequencies lower than 20 Hz and higher than 20,000 Hz (20 kHz). Sound with frequency smaller than 20 Hz is called infra sound.
The sound produced by a pendulum and the sound generated by the vibrations of the earth’s crust just before an earthquake are examples of such sounds.
Sound waves with frequency greater than 20 kHz are called ultrasound.
The dog, mouse, bat, dolphin etc have a special ability to hear infra sounds. Thus, they can sense some noise which are inaudible to us.
Children under 5 years of age and some creatures and insects can hear waves with frequency up to 25 kHz.
Bats, mice, dolphins, etc, can also produce ultrasound.

Mechanism in Bat :

The Italian scientist Spallanzani was the first to discover a special mechanism present in bats.
He thus discovered that their ability to fly in the dark depends on their ears and not eyes.
The ultrasonic sound produced by bats, gets reflected on hitting an obstacle.
This reflected sound is received by their ears and they can locate the obstacle and estimate its distance even in the dark.

Uses of ultrasonic sound

For communication between ships at sea.
To join plastic surfaces together.
To sterilize liquids like milk by killing the bacteria in it so that the milk keeps for a longer duration.
Echocardiaography which studies heartbeats, is based on ultrasonic waves (Sonography technology).
To obtain images of internal organs in a human body.
In industry, to clean intricate parts of machines where hands cannot reach.
To locate the cracks and faults in metals blocks.

Reflection of sound :

Like light waves, sound waves, too, get reflected from a solid or a liquid surface.
These waves also follow the laws of reflection.
A smooth or a rough surface is needed for the reflection of sound.
The direction of the incident sound wave and reflected sound wave make equal angles with the perpendicular to the surface and all these three lie in the same plane.

Good and bad reflectors of sound

How much of the incident sound gets reflected decides whether a reflector is a good or a bad reflector.
A hard and flat surface is a good reflector while clothes, paper, curtains, carpet, furniture, etc. absorb sound instead of reflecting it and, therefore are called bad reflectors.

Echo :

An echo is the repetition of the original sound because of reflection by some surface.
At 22⁰C, the velocity of sound in air is 344 m/s. Our brain retains a sound for 0.1 s.
Thus, for us to be able to hear a distinct echo, the sound should take more than 0.1 s after starting from the source to get reflected and come back to us.
Distance = speed x time

= 344 m/s ´ 0.1 s

= 34.4 m

Thus, to be able to hear a distinct echo, the reflecting surface should be at a minimum distance of half of the above i.e. 17.2 m.
As the velocity of sound depends on the temperature of air, this distance depends on the temperature.

Reverberation :

Sound waves get reflected from the walls and roof of a room multiple times. This causes a single sound to be heard not once but continously. This is called reverberation.
The time between successive reflections of a particular soundwave reaching us becomes smaller and the reflected sounds get mixed up and produce a continuous sound of increased loudness which cannot be deciphered clearly.
This is the reason why some auditoriums or some particular seats in an auditorium have inferior sound reception.

SONAR :

SONAR is the short form for Sound Navigation and Ranging.
It is used to determine the direction, distance and speed of an underwater object with the help of ultrasonic sound waves.
SONAR has a transmitter and a receiver, which are fitted on ships or boats.
The transmitter produces and transmits ultrasonic sound waves. These waves travel through water, strike underwater objects and get reflected by them. The reflected waves are received by the receiver on the ship.
The receiver converts the ultrasonic sound into electrical signals and these signals are properly interpreted.
SONAR is used to determine the depth of the sea.
SONAR is also used to search underwater hills, valleys, submarines, icebergs, sunken ships etc.

Sonography :

Sonography technology uses ultrasonic sound waves to generate images of internal organs of the human body.
This is useful in finding out the cause of swelling, infection, pain, etc.
The condition of the heart, the state of the heart after a heart attack as well as the growth of foetus inside the womb of a pregnant woman are studied using this technique.
This technique makes use of a probe and a gel.
The gel is used to make proper contact between the skin and the probe so that the full capacity of the ultrasound can be utilized.
The gel is applied to the skin outside the internal organ to be studied.
High frequency ultrasound is transmitted inside the body with the help of the probe.
The sound reflected from the internal organ is again collected by the probe and fed to a computer which generates the images of the internal organ.
Female foeticide is now a cognizable offence under the PNDT Act.

Human ear :

The ear is an important organ of the human body. We hear sounds because of our ear.
When sound waves fall on the eardrum, it vibrates. These vibrations are converted into electrical signals which travel to the brain through nerves.
The ear can be divided into three parts:

1. Outer ear
2. Middle ear
3. Inner ear.

Outer ear or Pinna :

The outer ear collects the sound waves and passes them through a tube to a cavity in the middle ear.
Its peculiar funnel like shape helps to collect and pass sounds into the middle ear.

Middle ear :

There is a thin membrane in the cavity of the middle ear called the eardrum.
When a compression in a sound wave reaches the eardrum, the pressure outside it increases and it gets pushed inwards.
The opposite happens when a rarefaction reaches there.
The pressure outside decreases and the membrane gets pulled outwards.
Thus, sound waves cause vibrations of the membrane.

Inner ear :

The auditory nerve connects the inner ear to the brain.
The inner ear has a structure resembling the shell of a snail. It is called the cochlea.
The cochlea receives the vibrations coming from the membrane and converts them into electrical signals which are sent to the brain through the nerve.
The brain analyses these signals.