Notes-Class 9-Science-Chapter-12-Study of Sound-Maharashtra Board

Production of sound : Sound is produced due to vibrations of an object such as a stretched membrane or a string under tension.

It propagates in the form of waves. It requires a medium (substance in the form of solid/liquid/ gas) for propagation, i.e., it cannot travel through a vacuum.

Sound propagates in the form of longitudinal waves. As sound propagates through a medium, there is a change in the density and pressure of the medium.

• Compressions are regions of highest pressure as well as highest density.
• Rarefactions are regions of lowest pressure as well as lowest density.

A sound wave can be shown in the form of a graph. 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.

Figure A shows the changes in density while figure B shows the changes in pressure. The changes in density or pressure are shown in the form of a graph in figure C

Here, λ : Wavelength,

A : amplitude (maximum pressure or maximum density).

T: time taken for one oscillation of pressure or density at a point in the medium.

The frequency (ν) : is the number of oscillations per unit time.

ν = 1/ T.  ν determines the pitch and A determines the loudness of sound. '

Velocity of sound :

• Take your friend to a place where there are iron pipes e.g. the school verandah, a staircase or a fence.
• Stand near one end of the pipe and ask the friend to stand 20 feet to 25 feet away from you near the pipe.
• Ask the friend to gently tap the pipe with a stone. Press your ear to the pipe and listen to the sound coming through the pipe.
• The same sound coming through the air can also be heard. But which sound reaches you first?

From the above activity you can see that you hear the sound through the iron pipe much before you hear it coming through air. This shows that sound travels faster in iron than in air.

Characteristics of sound waves : The distance between consecutive compressions or consecutive rarefactions is called wavelength

• Its SI unit is the metre (m) and CGS unit is the centimetre (cm).
• One oscillation (vibration) corresponds to a change of density of the medium from the maximum to the minimum value and again to the maximum value. The number of oscillations (vibrations) per unit time is called the frequency (ν) of the wave. Its SI unit is the hertz (Hz).
• The time taken by consecutive compressions or rarefactions to cross a fixed point is called the time period (T) of the wave. Its SI unit (as well as CGS unit) is the second. The relation between frequency and period is  ν = 1/ T.
• The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude (A) of the wave. In the case of a sound wave, the unit of amplitude is that of density or pressure.
• The amount of sound energy passing per unit time through unit area is called the intensity 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 (v).

v = \(\frac{λ}{T}\) =  νλ

• It is almost the same for all frequencies in a given medium under the same physical conditions. It depends on the medium and increases with temperature.
• The higher the frequency of sound the higher is the pitch. It depends upon how the brain interprets the frequency of sound.
• Loudness of sound depends on the physiological perception of intensity of sound and increases with intensity.
• The quality or timber of sound is the characteristic of sound that enables us to distinguish one sound from another having the same pitch and loudness.

Uses of Ultrasounds :

• to establish ship to ship communication
• for welding plastic surfaces
• to kill bacteria in liquids like milk to preserve them
• in echocardiography
• for imaging internal organs of the human body (sonography)
• in industry to clean parts of a machine
• to detect cracks and flaws in metal blocks.

In reverberation, the intensity of sound and the time interval between the successive arrivals of the same sound go on decreasing with time. Excessive reverberation is not desirable as the speech of a speaker or the pitch of music cannot be heard clearly due to the mixing of reflected waves and direct waves produced after the production of earlier waves.

To avoid these problems, reverberation should be reduced/controlled. For this the roof and walls of the auditorium are covered with sound absorbent materials such as compressed fibre boards, rough plaster or curtains. Even the material of the seat is chosen such that it absorbs sound waves to the desired extent.

• 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. The time difference between transmission and reception is noted.
• This time and the velocity of sound in water give the distance from the ship, of the object which reflects the waves.
• SONAR is used to determine the depth of the sea. SONAR is also used to search underwater hills, valleys, submarines, icebergs, sunken ships etc.

• A gel is applied to the skin outside the internal organ to be examined. The gel provides proper contact between the skin and the probe used to transmit ultrasounds inside the body.
• The waves reflected from the internal organ are collected by the probe and fed to a computer.
• The computer produces the images of the organs.
• This method is painless. It is used in determining the cause of swelling, infection, pain, the condition of the heart, the growth of the foetus inside the womb of a pregnant woman, etc.

Outer ear or Pinna

The outer ear collects the sound waves and passes them through a tube to a cavity in 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.