Chapter 25: SOUND
is produced by vibrating objects. Vibrations cause
compressions [high pressure] and rarefactions [low
waves travel through a medium like this. Sound waves are
longitudinal i.e. individual particles of a medium move in
a direction parallel to the direction of propagation of
the disturbance. The particles don’t move physically but
oscillate back and forth.
humans, the voice box or larynx has two vocal cords that
produce sound through vibration when lungs force air
cords in males are 20mm, women 15mm and children even
1: Larynx and Physiology of ear
vibrates when sound waves hit it; these vibrations are
sent to the brain from the inner ear.
sound waves are collected by the pinna [outer
ear] these then pass through the auditory canal to the ear
drum which amplifies it. The middle ear [hammer,
anvil, and stirrup] then amplifies it and sends it to the
inner ear. In the inner ear [the cochlea] converts
it to electrical signals which are taken to the brain by
the auditory nerve. The brain interprets them as sound.
Properties of Sound
of sound is proportional to square of its amplitude.
is measured in decibels.
decides the loudness of the sound.
sound has higher energy so travels a longer
as it moves away from the source its amplitude keeps on
or shrillness is determined by its
or timber of sound decides the pleasantness of
of single frequency is a tone; sound which is a
mixture of multiple frequencies is a note.
Speed of Sound
of sound remains almost same for all frequencies in
a given medium under same physical conditions. Intensity
is the amount of sound energy passing through a unit
area it’s different than loudness. Two sounds of equal
intensity can be of different loudness.
of sounds is highest
in solids and least in gases.
increases with temperature of the medium. Sound travels
through solids, liquids, gases but not through vacuum.
a sound producing object moves faster than sound, it
creates shock waves that have high energy. Air pressure
variations due to these shockwaves produce a
large, sharp sound called sonic
obeys the laws of reflection same as light waves. The
sensation of sound persists in our
brain for 0.1s. If the obstacle from which sound is
reflected is at distance more than 17.2m then we hear
echo. Sound absorbent materials absorb sound and prevent
horns, trumpets, shehenais, auditoriums are designed to
take advantage of multiple reflections of sound.
ear can hear sounds only between 20 Hz to 20000 Hz i.e. 20
-20000 oscillations per second. Children below 5 can hear
infrasonic sounds i.e. below 20Hz. During earthquakes
infrasonic sound are produced which are heard by birds and
animals and they are disturbed earlier.
sounds above 20 kHz. They have many applications
like in cleaning of instruments, detecting cracks in
building, machines, ultrasound machines and echo
cardiograph, SONAR. All depend on detection of reflected
ultrasonic waves from obstacles, body parts etc.
Effect is observed when a moving object causes
change in frequency of EM waves. In military it’s used to
detect enemy aircrafts.
Fig 2: Doppler effect
astronomy it’s used to find speed of moving stars and in sonography
it’s used to study heart beats and blood flow. In echocardiography
it used to study heart beats.
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