Unit # 11 Engr Umair Akhtar Class 10th : Physics
Unit # 11
SOUNDS
Q.1. (a) Define sound. What are the basic requirements for-its propagation?
(b) Give a simple experiment to show vibrating body is necessary for sound waves.
(c) Give a simple experiment to show sound waves are mechanical waves.
Ans.(a) SOUND:
Sound is a form of energy. It gives sensation of hearing. It travels in the form of waves. Sound is produced by a
vibrating body. Sound waves travel in medium from the point of vibrating body to another point. There are two
basic requirements for the propagation of sound waves.
ONLY VIBRATING BODY CAN PRODUCE SOUND
WAVES.
ii. Sound waves travel in the medium whose particles can
vibrate. Sound waves are mechanical waves that cannot
travel in vacuum. These waves can be produced by vibrating
source. We speak that our sound is produced by vibrating
our vocal chords.
(a) EXPERIMENT:
Sound Waves Need Vibrating Body:
Tuning fork is a standard device to produce sound waves in laboratory. When we strike it
with a rubber pad its prongs start vibrating. We can hear the sound bringing it near our ear.
We can also feel the vibrations by slightly touching its one of the prongs with a plastic ball
suspended from a thread as shown in figure.
The tuning fork will push the ball because of its vibrations. Now if we dip the vibrating
tuning fork into a glass of water, we will see a splash. The splash of water is made by the
vibrations of the tuning fork.
We can also feel the vibration a drum producing sound with the help of a stick. Similarly we can feel the vibration
of our vocal card by a finger while speaking.
(c) SOUND WAVES ARE MECHANICAL WAVES:
Unlike light waves which are electromagnetic in nature and can pass through vacuum,
sound waves require some medium for their propagation. So we say sound waves are
mechanical waves. This can be proved by bell jar apparatus.
EXPERIMENT:
Arrange the apparatus as shown in figure, where a bell jar is placed on the platform of
vacuum pump. An electric bell is suspended in the bell jar with the help of two wires connected to a power supply.
By setting on the power supply, electric bell will begin to ring. The sound of the bell can be heard. Now start
pumping, out air from the jar by means of vacuum pump. The sound of the bell starts becoming feeble and feeble
and eventually dies but despite that bell is still ringing. When we put the air back into the jar we can hear the
sound of the bell again. From this simple experiment we can conclude that sound wave can only propagate in the
presence of air (medium).
Q.2. (a) Explain the mechanism of the propagation of sound waves in air.
(b) What are factors on which propagation of sound wave depend?
Ans. (a): MECHANISM OF PROPAGATION OF SOUND WAVES:
Sound waves are mechanical waves which start from the point of vibrating body to another point in the -form of
"compressions" and "rarefactions".
Sound waves are called "longitudinal" in nature. Sound waves are longitudinal in nature
means the particles of the medium vibrate in the same direction as the direction of
propagation of sound waves.
Simple Demonstration:
To explain the propagation of sound waves, we consider a "tuning fork" (an instrument with
two hard steal prongs). It is tuned by striking it on a rubber pad. Its prongs start vibrating
Page 1 of 13 0346-5086014
, Unit # 11 Engr Umair Akhtar Class 10th : Physics
about point "0".
When prong moves from "0" to "B" then air near it is compressed and when it returns back to A then the air
expands and its molecules form rarefactions. This process is repeated periodically. The sound waves travel or
propagate along the direction of oscillating molecules of air. That is why sound waves are called "compressional
waves". At positions of compressions the density of air molecules is larger and at positions of rarefactions the
density of molecules is smaller with respect to the surrounding air.
(b) Factors on which propagation of sound wave depend
Following are the factors upon which sound waves depend.
i. The sound waves can be produced in a medium whose molecules can vibrate.
ii. The speed of sound wave depends upon the nature of medium in which it travels.
iii. The speed of sound waves is about fifteen times greater in solids than its value in air (gases).
iv. The sound waves change their speed (in air) due to change in temperature, density due to humidity.
Q.3.Explain the characteristics-of sound waves.
Ans. The following are some main characteristics of sound waves.
1. LOUDNESS
The magnitude of "auditory sensation" due to sound waves is called loudness. Faint and loud voice can be
distinguished by the magnitude of loudness.
(a) Amplitude
The greater is amplitude of sound wave the larger is loudness. The intensity of loudness is found to be directly
proportional to "square of amplitude of sound waves. The greater is amplitude of vibrating string of a sitar the
larger is loudness of sound produced by it.
(b) Area of vibrating body
The larger is area (size) of vibrating body the greater is loudness of sound produced by it. It is confirmed by
sounding drum of large size. If we strike a tuning fork, on a rubber pad, a feeble sound will be heard. But if the
vibrating tuning fork is placed vertically on the surface of a table, we will hear a louder sound. Hence, larger
surface area then loudness of sound is increased.
(c) Distance from vibrating body
If a listener is at larger distance from the vibrating body, then smaller is loudness of sound heard by it.
(d) Quality of ears
The sensitive ears feel larger loudness than the defective ears.
2. PITCH
The quality of sound due to which a shrill voice can be distinguished by another grave voice is called pitch of
sound waves. Its value depends upon frequency of sound waves. The voice of ladies and children has larger pitch
than men's voice. Therefore, pitch of sound in ladies is larger than men.
3. Quality of sound
The quality of sound waves by which two sound waves of same pitch and loudness can be distinguished is called
"quality of sound. Its value depends upon the shape of sound waves. The irregularity in the shape of sound waves
(of same pitch and loudness) measures the low quality of sound.
4. INTENSITY OF SOUND
5. The energy transferred by sound waves per unit area per second, when unit area is at 90° to direction of
propagation of sound waves, is called intensity of sound. Its units are "watt per square meter"
Q.4 Derive relation between "loudness and intensity" for the case of sound waves.
Ans. RELATION BETWEEN LOUDNESS AND INTENSITY.
The audible sound waves, for a normal human ear have intensity ranging from
10-12Wm-2 to 1Wm-2. It is a very large range; therefore it is scaled in powers of ten.
The lowest intensity to which ear can response is 10-12Wm-2. It is taken as reference value and is written as zero
bel. (a unit named after Alexander Graham Bell) Therefore, one bel intensity is 10-11Wm-2 or it is equal to 10db
(decibel).
Experimentally, it is confirmed that "loudness is directly proportional to "logarithm of intensity value, we can
write as
Page 2 of 13 0346-5086014
Unit # 11
SOUNDS
Q.1. (a) Define sound. What are the basic requirements for-its propagation?
(b) Give a simple experiment to show vibrating body is necessary for sound waves.
(c) Give a simple experiment to show sound waves are mechanical waves.
Ans.(a) SOUND:
Sound is a form of energy. It gives sensation of hearing. It travels in the form of waves. Sound is produced by a
vibrating body. Sound waves travel in medium from the point of vibrating body to another point. There are two
basic requirements for the propagation of sound waves.
ONLY VIBRATING BODY CAN PRODUCE SOUND
WAVES.
ii. Sound waves travel in the medium whose particles can
vibrate. Sound waves are mechanical waves that cannot
travel in vacuum. These waves can be produced by vibrating
source. We speak that our sound is produced by vibrating
our vocal chords.
(a) EXPERIMENT:
Sound Waves Need Vibrating Body:
Tuning fork is a standard device to produce sound waves in laboratory. When we strike it
with a rubber pad its prongs start vibrating. We can hear the sound bringing it near our ear.
We can also feel the vibrations by slightly touching its one of the prongs with a plastic ball
suspended from a thread as shown in figure.
The tuning fork will push the ball because of its vibrations. Now if we dip the vibrating
tuning fork into a glass of water, we will see a splash. The splash of water is made by the
vibrations of the tuning fork.
We can also feel the vibration a drum producing sound with the help of a stick. Similarly we can feel the vibration
of our vocal card by a finger while speaking.
(c) SOUND WAVES ARE MECHANICAL WAVES:
Unlike light waves which are electromagnetic in nature and can pass through vacuum,
sound waves require some medium for their propagation. So we say sound waves are
mechanical waves. This can be proved by bell jar apparatus.
EXPERIMENT:
Arrange the apparatus as shown in figure, where a bell jar is placed on the platform of
vacuum pump. An electric bell is suspended in the bell jar with the help of two wires connected to a power supply.
By setting on the power supply, electric bell will begin to ring. The sound of the bell can be heard. Now start
pumping, out air from the jar by means of vacuum pump. The sound of the bell starts becoming feeble and feeble
and eventually dies but despite that bell is still ringing. When we put the air back into the jar we can hear the
sound of the bell again. From this simple experiment we can conclude that sound wave can only propagate in the
presence of air (medium).
Q.2. (a) Explain the mechanism of the propagation of sound waves in air.
(b) What are factors on which propagation of sound wave depend?
Ans. (a): MECHANISM OF PROPAGATION OF SOUND WAVES:
Sound waves are mechanical waves which start from the point of vibrating body to another point in the -form of
"compressions" and "rarefactions".
Sound waves are called "longitudinal" in nature. Sound waves are longitudinal in nature
means the particles of the medium vibrate in the same direction as the direction of
propagation of sound waves.
Simple Demonstration:
To explain the propagation of sound waves, we consider a "tuning fork" (an instrument with
two hard steal prongs). It is tuned by striking it on a rubber pad. Its prongs start vibrating
Page 1 of 13 0346-5086014
, Unit # 11 Engr Umair Akhtar Class 10th : Physics
about point "0".
When prong moves from "0" to "B" then air near it is compressed and when it returns back to A then the air
expands and its molecules form rarefactions. This process is repeated periodically. The sound waves travel or
propagate along the direction of oscillating molecules of air. That is why sound waves are called "compressional
waves". At positions of compressions the density of air molecules is larger and at positions of rarefactions the
density of molecules is smaller with respect to the surrounding air.
(b) Factors on which propagation of sound wave depend
Following are the factors upon which sound waves depend.
i. The sound waves can be produced in a medium whose molecules can vibrate.
ii. The speed of sound wave depends upon the nature of medium in which it travels.
iii. The speed of sound waves is about fifteen times greater in solids than its value in air (gases).
iv. The sound waves change their speed (in air) due to change in temperature, density due to humidity.
Q.3.Explain the characteristics-of sound waves.
Ans. The following are some main characteristics of sound waves.
1. LOUDNESS
The magnitude of "auditory sensation" due to sound waves is called loudness. Faint and loud voice can be
distinguished by the magnitude of loudness.
(a) Amplitude
The greater is amplitude of sound wave the larger is loudness. The intensity of loudness is found to be directly
proportional to "square of amplitude of sound waves. The greater is amplitude of vibrating string of a sitar the
larger is loudness of sound produced by it.
(b) Area of vibrating body
The larger is area (size) of vibrating body the greater is loudness of sound produced by it. It is confirmed by
sounding drum of large size. If we strike a tuning fork, on a rubber pad, a feeble sound will be heard. But if the
vibrating tuning fork is placed vertically on the surface of a table, we will hear a louder sound. Hence, larger
surface area then loudness of sound is increased.
(c) Distance from vibrating body
If a listener is at larger distance from the vibrating body, then smaller is loudness of sound heard by it.
(d) Quality of ears
The sensitive ears feel larger loudness than the defective ears.
2. PITCH
The quality of sound due to which a shrill voice can be distinguished by another grave voice is called pitch of
sound waves. Its value depends upon frequency of sound waves. The voice of ladies and children has larger pitch
than men's voice. Therefore, pitch of sound in ladies is larger than men.
3. Quality of sound
The quality of sound waves by which two sound waves of same pitch and loudness can be distinguished is called
"quality of sound. Its value depends upon the shape of sound waves. The irregularity in the shape of sound waves
(of same pitch and loudness) measures the low quality of sound.
4. INTENSITY OF SOUND
5. The energy transferred by sound waves per unit area per second, when unit area is at 90° to direction of
propagation of sound waves, is called intensity of sound. Its units are "watt per square meter"
Q.4 Derive relation between "loudness and intensity" for the case of sound waves.
Ans. RELATION BETWEEN LOUDNESS AND INTENSITY.
The audible sound waves, for a normal human ear have intensity ranging from
10-12Wm-2 to 1Wm-2. It is a very large range; therefore it is scaled in powers of ten.
The lowest intensity to which ear can response is 10-12Wm-2. It is taken as reference value and is written as zero
bel. (a unit named after Alexander Graham Bell) Therefore, one bel intensity is 10-11Wm-2 or it is equal to 10db
(decibel).
Experimentally, it is confirmed that "loudness is directly proportional to "logarithm of intensity value, we can
write as
Page 2 of 13 0346-5086014
