Study of Sound Class 9 Science Notes Maharashtra State Board
Sound is a form of energy that 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. On the other hand, in 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.
A sound wave can be shown in the form 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 12.1 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.
The wavelength of sound waves is indicated by the Greek letter lambda (λ), while its frequency is indicated by nu (υ). 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 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 =\(\frac{\text { Distance }}{\text { Time }}\)
Any point on the sound wave covers a distance equal to λ (wavelength) in time T (time). Thus the velocity of sound is given by v = uλ because \(\frac{1}{T}\) = υ
Velocity = \(\frac{\text { Wavelength }}{\text { Time period }}\)
v = \(\frac{\lambda}{\mathrm{T}}\)
Thus, the Velocity of Sound = Wavelength × Frequency.
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. They measured the time between seeing the flash of a gun and hearing its sound from a long distance. Their value of 350 m/s is very close to the value of 346 m/s which is accepted today.
Velocity of Sound in Various Mediums at 25°C
| State | Substance | Speed (m/s) |
| Solid | Aluminum | 5420 |
| Nickel | 6040 | |
| Steel | 5960 | |
| Iron | 5950 | |
| Brass | 4700 | |
| Glass | 3980 | |
| Liquid | Sea Water | 1531 |
| Pure Water | 1498 | |
| Ethanol | 1207 | |
| Methanol | 1103 | |
| Gas | Hydrogen | 1284 |
| Helium | 965 | |
| Air | 346 | |
| Oxygen | 316 | |
| Sulphur dioxide | 213 |
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.
v ∝ \(\sqrt{T}\)
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.
v ∝ \(\frac{1}{\sqrt{\rho}}\)
Molecular weight (M): The velocity sound is inversely proportional to the square root of the molecular weight of the gas. Thus, increasing the molecular weight four times reduces the velocity to half its value.
v ∝\(\frac{1}{\sqrt{\mathrm{M}}}\)
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 a frequency smaller than 20 Hz is called infrasound. 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 a frequency greater than 20 kHz are called ultrasound. Dogs, mice, bats, dolphins, etc have a special ability to hear ultrasonic sounds. Thus, they can sense some noise which is inaudible to us. Children under 5 years of age and some creatures and insects can hear waves with frequencies up to 25 kHz. Bats, mice, dolphins, etc, can also produce ultrasound.
The Italian scientist Spallanzani was the first to discover a special mechanism present in bats. He covered various organs of bats like the eyes, ears, nose, etc. one by one and allowed them to fly in the dark thereby discovering how they can fly easily in the dark. He found that the bats with their ears covered began to collide with whatever came in their path. Even though their eyes were open. They were of no use to the 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 metal blocks.
Reflection of Sound
Make two sufficiently long, identical tubes out of cardboard. Arrange them on a table, in front of a wall as shown in Figure. Place a clock near the end of one of the tubes and try to hear the ticking of the clock at the end of the other tube by placing your ear there. Adjust the angles of the two tubes till you can hear the sound very clearly. Measure the angle of incidence θ1 and the angle of reflection θ2. Try to see if they are related in any way.
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 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
You must have visited an echo point at a hill station. If you shout at such a point, you hear the same sound just after a little while. This sound is called an echo. An echo is the repetition of the original sound because of reflection by some surface. To be able to hear the original sound and its reflection distinctly, at 22°C, what must the minimum distance be between the source and the reflecting surface? At 22°C, 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 seconds after starting from the source to get reflected and come back to us. Thus we can determine the minimum distance between the source and the reflecting surface as follows:
Distance = speed × 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 the 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 continuously. 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. This is shown in the figure.
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. 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.
Sonography
Sonography technology uses ultrasonic sound waves to generate images of the 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 the fetus 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. As this method is painless, it is increasingly used in medical practice for correct diagnosis.
Developments in science have led to advances in technology which have helped us make much progress. However, every coin has two sides and the misuse of science has had serious consequences for human life. The sonography technique can help to follow the growth and well-being of an unborn baby. However, the increasing incidence of female foeticide as a result of the discrimination between the girl and the boy child is an example of the grave misuse of this technique. Female foeticide is now a cognizable offense under the PNDT Act.
Human Ear
The ear is an important organ of the human body. We hear sounds because of our ears. 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:
- Outer Ear
- Middle Ear
- 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.
The ear is an important sensory organ. Sticks or other pointed objects should never be inserted into the ear to clean it. Also, one should not hear very loud music using earphones. It may cause grave injury to the eardrum. The SONAR technology was first developed during World War I to detect enemy submarines. This technology can be used in the air also. Bats use this technique to detect obstacles in their path so that they can avoid them and fly freely even in the dark.
Detailed Maharashtra State Board Class 9 Science Notes Study of Sound is particularly useful for answering essay questions.