This document discusses acoustics and the reflection of sound waves. It covers topics like reflection of sound waves from rigid and curved surfaces, reflection between different media, intensity of sound waves, reverberation time, echoes, and reducing noise in buildings. The key points are: - Sound waves reflect in a similar manner as light waves, changing direction when reflecting off surfaces. - Reflection depends on the properties of the reflecting surface and the media on either side. - Intensity of sound depends on factors like amplitude, frequency, area of sound source, and distance from the source. - Reverberation time is the time it takes for sound intensity to decrease by a million times after the source

1. The branch of physics which deals with
production of best sound effects in buildings
and rooms.

2.  Reflection of Sound Waves: When sound
waves travelling in a medium strike the
surface of rigid body they are sent back into
same medium.

3.  Reflection of Sound Wave from Rigid Surface:
When sound waves strike a rigid surface it suffers a
change in direction and phase of λ/2.
 Reflection of Sound Waves from Curved Surface:
When sound waves strike a curved surface they are
focused at principal focus of reflecting curved surface.
 Reflection of Sound Waves at a rarer medium: The
type of wave is changed i.e. a compression is reflected
as rarefaction and vice versa. There is no change in
direction and motion of particles.
 Reflection of Sound Waves at a denser medium:
When sound waves are reflected from denser medium
type of wave remains unchanged, but sign is changed
and direction of motion is opposite to incident wave.

5.  Sound Waves obey laws of reflection same as
that of light rays. For e.g. a similar sound image
is heard while train engine blows its whistle
passing a large wall. The whistle is heard from
behind the wall while it is blown front the wall

6.  The phenomenon of bending of sound waves
in passing from one medium to another.
 When sound waves pass from one medium
into another there is change in velocity &
direction according to same laws as in light.

8.  Average sound energy transported per second
per unit area perpendicular to direction of
propagation of wave.
 Intensity, I of sound depends on
I is directly proportional to square of amplitude
of source.
I is directly proportional to square of frequency
of source.
I is directly proportional to area of vibrating
body (like sound from bigger speaker is more
than small speaker)
I is inversely proportional to distance from
source of sound

10.  Minimum intensity of sound which normal human
can hear is equal to an energy of 10-12 Watt/m2.
This is called Threshold of hearing denoted by Io
 Maximum intensity of sound which human ear
can hear without sensation of pain is 1Watt/m2.
 SI unit of intensity of sound is Watt / m2.
 Direction of intensity is average direction in
which sound is flowing.
 Energy emitted by body vibrating with frequency
n is E = 1/2 mω2r2

11.  Quality of Sound: The characteristic of sound
which enables us distinguish between two
sounds of same pitch and loudness.
 With this we can recognize the person
without seeing him.
 Differentiate between sounds produced by
different musical instruments.

12.  The degree of hearing sensation produced in the
ear.

13.  Reverberation : The persistence of sound in
room or hall (because of multiple reflections
from surfaces) after the original sound has
ceased is called reverberation.
 Reverberation Time : Time for which the
sound persists in a room or hall. For best
sound effects this time should be minimum.
 Standard Reverberation Time : Time required
for intensity of sound to fall to one millionth
of its original intensity after the original
sound is cut off.

15.  t = 0.16V / ΣaS
 Where t = reverberation time in seconds
 V = volume of room
 S = Area of sound absorbing materials in
room
 a = Coefficient of absorption

16.  If reverberation time is very large hen we will
have overlapping of sound and we will not be
able to hear words clearly so we should take
steps to reduce it.
 Few open windows act as perfect absorbers.
 Sound absorbing materials such as card
boards, asbestos sheets, curtains, pictures
should be put in room.
 Good number of audience should be present
in room

17.  Cushioned seats act as good absorbers.
 Curved walls and corners in room should be
avoided.
 Optimum Value of reverberation time. If
reverberation time is large words will overlap
& if it is zero then words will produce dead
effect.
 For example if R.T = 0 & we said “WORD” we
will hear sound as “WOR” D sound will
become dead.
 Optimum value of RT for speech = 0.5-1 sec
and music is 1-1.5 sec (imp for MCQ)

18.  An echo is sound heard (other than original
sound) produced by reflection from the
surface of large obstacle situated at some
distance from the listener.
 For e.g. if source and obstacle are 1km apart
(v = 340m/sec) listener will hear echo after
2X 1000 / 340 = 5.88 seconds.
 Twice distance is sound will travel to obstacle
and come back
 Minimum distance for an echo to be heard is
17m

20.  The intensity of hearing sensation is called as
loudness.
 To achieve good acoustical effects following
points should be kept in mind while
designing a hall
 Length of hall should be greater than breadth
(because sound being a longitudinal wave
travels back and forth)
 Wall surface behind loud speaker should be a
large parabola

21.  Speaker should be placed at focus of
parabola for equal distribution of sound.
 Electrical amplification increases intensity of
sound.
 Seats should not be near the walls and gaps
should be left between seats.
 Ceiling should be curved such that sound
does not gets focused at one point.

22.  When sound is produced in room listener
receives sound waves in two ways:
 Waves directly from source of sound
 Waves after reflection from walls or ceiling.
 The source is continuously producing sound and
sound waves which on reflecting from walls or
ceiling overlap with each other.
 This overlapping may cause interference
(discussed ealier). Where constructive
interference occurs sound increases and where
destructive interference occurs silence zones
form.

24.  If walls or ceiling are curved or cylindrical the
sound waves get focused and produce
undesirable concentration of sound energy.
 Remedy: Avoid corners and curved walls.
Plane walls with good absorbing materials
should be used.

25.  When sharp sound is produced in front of
staircase a musical sound is produced due to
regular series of reflected sounds, each
consisting of simple echo of original sound.
 Remedy: Stair case can be covered with carpet
or passage can be provided between hall and
staircase.

26.  Sound which produces an unpleasant effect
on ears.
 They can be of two types: Air Borne &
Structure Borne
 Remedy: Air borne noise can be reduced by
using double doors & windows
 Structure borne noise can be reduced by
introducing discontinuities in pipes for water
and gas.

27.  Sound Waves having frequency greater than
20Khz are called Ultrasonic Waves.
 They cannot be heard by Humans but can be
heard by animals like dogs, bats etc.

28.  Ultrasonic Waves can be produced by :
 Magnetostriction Oscillator
 Piezo Electric Oscillator
 To be done from book
 (Important for long answer questions
Principle, Construction and Working of both
oscillator)

29.  To be done from book
 Briefly describe all application of Ultrasonic
waves.
 Important for Section B part of exam.