Important Points and Formulas: Sound | Science Class 9 PDF Download

 Page 1


1 
 
SOUND 
 
 
 Sound 
• Sound is mechanical energy, which produces sensation of hearing. 
• Sound is produced due to vibration of different objects. 
• A material medium is essential for the propagation of sound as it cannot travel in 
vacuum. 
• A region of compressed air (increased density or pressure) is called a compression 
and that of rarefied air (decreased density or pressure) is called a rarefaction. 
 Longitudinal and Transverse Waves 
• A wave motion is a form of disturbance (a mode of momentum and energy transfer) 
which is due to repeated vibrations of the particles about their mean positions and the motion 
is transferred from one particle to the other without any net movement of the medium. A wave 
motion is of two types: (i) Longitudinal (ii) Transverse. 
• Sound waves are longitudinal waves. Light waves, on the other hand, are transverse 
waves. 
• Sound wave propagates as compressions and rarefactions (i.e., as variation in density 
or pressure) in the medium. 
• As sound propagates, it is the sound energy that travels in the medium and not the 
medium itself. 
  
• The change in density (or pressure) from the maximum value to the minimum value 
and again to the maximum value is called an oscillation. 
• The number of complete oscillations per second is called the frequency (n) of the 
sound wave. The unit of frequency is called hertz (Hz). 
• The time taken for one complete oscillation in density (or pressure) of the medium is 
called the time period (T) of the wave. 
• The distance between two consecutive compressions (or crests) or two consecutive 
rarefactions (or troughs) is called the wavelength. The unit of wavelength is meter (m). 
• The distance travelled by a sound wave in its periodic time is also called wavelength 
(l) of the wave. 
• The relation between frequency (n) and time period (T) is  or v T = 1. 
• The speed of sound depends mainly on its nature and the temperature of the medium 
through which its propagates. 
• The relation between speed of the sound wave (v), its frequency (n) and wavelength 
(l) is v = n l. 
• The sound wave is described by: (i) its speed, (ii) its frequency (or wavelength) and 
(iii) its amplitude. These are called the characteristics of a sound wave. 
• In general, speed of sound in solids > speed of sound in liquids > speed of sound in 
gases. However, this relation is not always valid. 
v
T
T
v
1
,
1
= =
Page 2


1 
 
SOUND 
 
 
 Sound 
• Sound is mechanical energy, which produces sensation of hearing. 
• Sound is produced due to vibration of different objects. 
• A material medium is essential for the propagation of sound as it cannot travel in 
vacuum. 
• A region of compressed air (increased density or pressure) is called a compression 
and that of rarefied air (decreased density or pressure) is called a rarefaction. 
 Longitudinal and Transverse Waves 
• A wave motion is a form of disturbance (a mode of momentum and energy transfer) 
which is due to repeated vibrations of the particles about their mean positions and the motion 
is transferred from one particle to the other without any net movement of the medium. A wave 
motion is of two types: (i) Longitudinal (ii) Transverse. 
• Sound waves are longitudinal waves. Light waves, on the other hand, are transverse 
waves. 
• Sound wave propagates as compressions and rarefactions (i.e., as variation in density 
or pressure) in the medium. 
• As sound propagates, it is the sound energy that travels in the medium and not the 
medium itself. 
  
• The change in density (or pressure) from the maximum value to the minimum value 
and again to the maximum value is called an oscillation. 
• The number of complete oscillations per second is called the frequency (n) of the 
sound wave. The unit of frequency is called hertz (Hz). 
• The time taken for one complete oscillation in density (or pressure) of the medium is 
called the time period (T) of the wave. 
• The distance between two consecutive compressions (or crests) or two consecutive 
rarefactions (or troughs) is called the wavelength. The unit of wavelength is meter (m). 
• The distance travelled by a sound wave in its periodic time is also called wavelength 
(l) of the wave. 
• The relation between frequency (n) and time period (T) is  or v T = 1. 
• The speed of sound depends mainly on its nature and the temperature of the medium 
through which its propagates. 
• The relation between speed of the sound wave (v), its frequency (n) and wavelength 
(l) is v = n l. 
• The sound wave is described by: (i) its speed, (ii) its frequency (or wavelength) and 
(iii) its amplitude. These are called the characteristics of a sound wave. 
• In general, speed of sound in solids > speed of sound in liquids > speed of sound in 
gases. However, this relation is not always valid. 
v
T
T
v
1
,
1
= =
2 
 
• Sources that move faster than the speed of sound are said to have supersonic 
speeds. Bullets, jet aircrafts etc. travel at supersonic speeds. 
• A shock wave is produced when sound producing source moves with a speed higher 
than the speed of sound. 
• It is not necessary for an object to be a vibrating source of sound to produce a shock 
wave. 
• A shock wave carries a large amount of energy. 
• Sonic boom is a very sharp and loud sound produced by pressure variation 
associated with a shock wave. 
 Echo 
• Like light waves, sound waves are also reflected from a surface on which they fall. The 
laws of reflection of sound are the same as those of light. 
• The echo is the phenomenon of repetition of sound of a source by reflection from an 
obstacle.  
• The time interval between the incident sound and the reflected sound for hearing a 
distinct echo is 0.1 s. This is due to the reason that the sensation of sound lasts in our brain 
for 0.1 s and this property is called persistence of hearing. 
• For hearing a distinct echo, the minimum distance of the obstacle from the source of 
sound should be 17.2 m. This distance changes with change of temperature. 
 Multiple Echo 
• Multiple echoes are heard when sound is repeatedly reflected from a number of 
obstacles at suitable distances. 
• Megaphone, stethoscope, ear trumpet, hearing aid etc. are based on the phenomenon 
of multiple reflection of sound. 
 Reverberation 
• Reverberation is the phenomenon of persistence or prolongation of audible sound 
after the source has stopped emitting sound. 
• Reverberation is reduced by (i) carpeting the floor (ii) upholstering furniture and (iii) 
creating false ceilings with a suitable sound absorbing material. 
• The ceilings of concert halls are curved to enable the sound in reaching all corners 
of the hall. 
• A sound board is used to evenly spread the sound throughout the width of the hall. 
• The audible range of hearing for average human beings is in the frequency range of 
20 Hz to 20 kHz. Children under the age of five can hear upto 25 kHz whereas aged people 
become less sensitive to higher frequencies. 
 Infrasound and Ultrasound 
• Infrasound (or Infrasonic) has a frequency below 20 Hz. 
• Ultrasound (or ultrasonic) has a frequency above 20 kHz. 
Page 3


1 
 
SOUND 
 
 
 Sound 
• Sound is mechanical energy, which produces sensation of hearing. 
• Sound is produced due to vibration of different objects. 
• A material medium is essential for the propagation of sound as it cannot travel in 
vacuum. 
• A region of compressed air (increased density or pressure) is called a compression 
and that of rarefied air (decreased density or pressure) is called a rarefaction. 
 Longitudinal and Transverse Waves 
• A wave motion is a form of disturbance (a mode of momentum and energy transfer) 
which is due to repeated vibrations of the particles about their mean positions and the motion 
is transferred from one particle to the other without any net movement of the medium. A wave 
motion is of two types: (i) Longitudinal (ii) Transverse. 
• Sound waves are longitudinal waves. Light waves, on the other hand, are transverse 
waves. 
• Sound wave propagates as compressions and rarefactions (i.e., as variation in density 
or pressure) in the medium. 
• As sound propagates, it is the sound energy that travels in the medium and not the 
medium itself. 
  
• The change in density (or pressure) from the maximum value to the minimum value 
and again to the maximum value is called an oscillation. 
• The number of complete oscillations per second is called the frequency (n) of the 
sound wave. The unit of frequency is called hertz (Hz). 
• The time taken for one complete oscillation in density (or pressure) of the medium is 
called the time period (T) of the wave. 
• The distance between two consecutive compressions (or crests) or two consecutive 
rarefactions (or troughs) is called the wavelength. The unit of wavelength is meter (m). 
• The distance travelled by a sound wave in its periodic time is also called wavelength 
(l) of the wave. 
• The relation between frequency (n) and time period (T) is  or v T = 1. 
• The speed of sound depends mainly on its nature and the temperature of the medium 
through which its propagates. 
• The relation between speed of the sound wave (v), its frequency (n) and wavelength 
(l) is v = n l. 
• The sound wave is described by: (i) its speed, (ii) its frequency (or wavelength) and 
(iii) its amplitude. These are called the characteristics of a sound wave. 
• In general, speed of sound in solids > speed of sound in liquids > speed of sound in 
gases. However, this relation is not always valid. 
v
T
T
v
1
,
1
= =
2 
 
• Sources that move faster than the speed of sound are said to have supersonic 
speeds. Bullets, jet aircrafts etc. travel at supersonic speeds. 
• A shock wave is produced when sound producing source moves with a speed higher 
than the speed of sound. 
• It is not necessary for an object to be a vibrating source of sound to produce a shock 
wave. 
• A shock wave carries a large amount of energy. 
• Sonic boom is a very sharp and loud sound produced by pressure variation 
associated with a shock wave. 
 Echo 
• Like light waves, sound waves are also reflected from a surface on which they fall. The 
laws of reflection of sound are the same as those of light. 
• The echo is the phenomenon of repetition of sound of a source by reflection from an 
obstacle.  
• The time interval between the incident sound and the reflected sound for hearing a 
distinct echo is 0.1 s. This is due to the reason that the sensation of sound lasts in our brain 
for 0.1 s and this property is called persistence of hearing. 
• For hearing a distinct echo, the minimum distance of the obstacle from the source of 
sound should be 17.2 m. This distance changes with change of temperature. 
 Multiple Echo 
• Multiple echoes are heard when sound is repeatedly reflected from a number of 
obstacles at suitable distances. 
• Megaphone, stethoscope, ear trumpet, hearing aid etc. are based on the phenomenon 
of multiple reflection of sound. 
 Reverberation 
• Reverberation is the phenomenon of persistence or prolongation of audible sound 
after the source has stopped emitting sound. 
• Reverberation is reduced by (i) carpeting the floor (ii) upholstering furniture and (iii) 
creating false ceilings with a suitable sound absorbing material. 
• The ceilings of concert halls are curved to enable the sound in reaching all corners 
of the hall. 
• A sound board is used to evenly spread the sound throughout the width of the hall. 
• The audible range of hearing for average human beings is in the frequency range of 
20 Hz to 20 kHz. Children under the age of five can hear upto 25 kHz whereas aged people 
become less sensitive to higher frequencies. 
 Infrasound and Ultrasound 
• Infrasound (or Infrasonic) has a frequency below 20 Hz. 
• Ultrasound (or ultrasonic) has a frequency above 20 kHz. 
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 Applications of ultrasound  
 (i)  Industry 
 (ii)  Medical science 
 (iii)  Communication 
 (iv)  SONAR. 
• In industry, ultrasound is used in (i) cleaning instruments and electronic equipments 
(ii) plastic welding (iii) detecting flaws and cracks in metal blocks used in constructing big 
structures. 
• In medical science, ultrasound is used in (i) echo-cardiography (ii) ultrasonography 
(iii) surgery (iv) therapeutics. 
• SONAR is Sound Navigation and Ranging and is used to measure distance, direction 
and speed of objects lying under sea. It is also used in ship-to-ship communication. 
 Human Ear 
• The human ear can be divided into three parts: (i) the outer ear which collects sound 
waves (ii) the middle ear which amplifies the sound waves about 60 times and (iii) the inner 
ear which converts the amplified sound energy into electrical energy and conveys to the brain 
as nerve impulses for interpretation.