The document discusses the physics of sound. It defines sound as a pressure wave that travels faster through solids than liquids or gases. Sound waves are longitudinal waves that create compressions and rarefactions. The key characteristics of a sound wave are its frequency, amplitude, wavelength, and speed. Frequency is measured in Hertz and determines the pitch of a sound. Amplitude determines loudness. Wavelength is the distance between compressions or rarefactions. Sound waves can be reflected, transmitted, absorbed, or cause diffraction or reverberation when interacting with surfaces. The range of normal human hearing is between 20-20,000 Hz. Different materials are used to absorb sound like porous absorbers, panel absorbers, and resonators

1. PHYSICS OF SOUND
Acoustics
By:
Akshay Bhatia

2. Sound
▪ Sound is a pressure wave which is created by a vibrating
object.
▪ Travels faster through solids than liquids or gases.
▪ The sound wave is referred to as a longitudinal wave.
▪ The result of longitudinal waves is the creation of
compressions and rarefactions within the air.

3. Sound waves travel faster through solids because there are
more particles close together to transfer the energy.

4. Propagation of sound
▪ The particles of the medium do not move forward themselves,
but the disturbance is carried forward. This is what happens
during propagation of sound in a medium.

5. Sound waves
▪ Sound waves are longitudinal waves.
▪ The reasons where the coils become closer are called
compressions (C) and the regions where the coils are further
apart are called rarefactions (R).

6. CHARACTERISTICS OF A SOUND WAVE
We can describe a sound wave by its:
▪ Frequency
▪ Amplitude
▪ Speed

7. Frequency
▪ Frequency tells us how frequently an
event occurs. Suppose you are beating
a drum. How many times you are
beating the drum per unit time is called
the frequency of your beating the drum.
▪ The change in density from the
maximum value to the minimum value,
again to the maximum value, makes
one complete oscillation. The number
of such oscillations per unit time is the
frequency of the sound wave.
▪ Its SI unit is hertz (symbol, Hz).

8. Pitch
▪ How the brain interprets the
frequency of an emitted sound is
called its pitch
▪ The faster the vibration of the
source, the higher is the frequency
and the higher is the pitch.
▪ A high pitch sound corresponds to
more number of compressions and
rarefactions passing a fixed point per
unit time.

9. Wavelength
▪ The distance between two
consecutive compressions (C) or two
consecutive rarefactions (R) is called
the wavelength
▪ OR Wavelength is distance between
any two points with the same phase.
▪ The wavelength is usually
represented by λ (Greek letter
lambda).
▪ Its SI unit is meter (m).

10. Amplitude
▪ The magnitude of the maximum
disturbance in the medium on either
side of the mean value is called the
amplitude of the wave
▪ The amplitude of a wave determines
a volume. – Volume tells how loud
or soft a sound is
▪ The amplitude of the sound wave
depends upon the force with which
an object is made to vibrate.

11. Reflection of Sound
▪ Sound bounces off a solid or a liquid like a rubber ball bounces
off a wall.
▪ Like light, sound gets reflected at the surface of a solid or liquid
and follows the same laws of reflection as you have studied in
earlier classes.

12. Echo
▪ If we shout or clap near a
suitable reflecting object
such as a tall building or a
mountain, we will hear the
same sound again a little
later. This sound which we
hear is called an echo.

13. Reverberation
▪ When A Number Of Reflected Sound Reaches A Listener That
They Can’t Be Distinguished Properly, Then It Is Termed As
Reverberation.

14. Range of hearing
▪ The audible range of sound for human beings extends from
about 20 Hz to 20000 Hz
▪ one Hz = one cycle/s
▪ Children under the age of five and some animals, such as
dogs can hear up to 25000 Hz.

15. Noise
▪ It is the unwanted sound which may be hazardous to
health, interferes with communications or is disturbing.
▪ When the sound waves are non periodic & irregular, they
produce unpleasing effect, such a sound is known as
noise.
▪ Noise is measured in decibels (dB).
▪ Noise above 85 decibels is harmful for human ear.

16. Noise
▪ A noise problem generally consists of three inter-related
elements-
– the source,
– the receiver
– the transmission path.
▪ This transmission path is usually the atmosphere through which the sound is
propagated, but can include the structural materials of any building containing
the receiver.

17. Noise levels
Concert and opera halls, recording
studios, theaters, etc.
Very quite
Private bedrooms, live theaters, television
and radio studios, conference and lecture
rooms, cathedrals and large churches,
libraries, etc.
Private living rooms, board rooms,
conference and lecture rooms, hotel
bedrooms
Quiet
Public rooms in hotels, small offices
classrooms, courtrooms
Moderate noisy
Drawing offices, toilets, bathrooms,
reception areas, lobbies, corridors,
department stores, etc.
Noisy
Kitchens in hospitals and hotels, laundry
rooms, computer rooms, canteens,
supermarkets, office landscape, etc.

18. Noise levels according to NBC

19. Types of noise
▪ Indoor noise
▪ Outdoor noise

20. Indoor noise
Indoor noises starts from
▪ the occupants’ footsteps
▪ banging of doors
▪ shifting of the furniture
▪ operation of the cistern and water closet
▪ playing of radio
▪ television, music system
▪ cooling and ventilation machinery, etc.

21. Outdoor noise
Source of outdoor noise are:
▪ Nearby streets
▪ Automobile traffic on road
▪ Children playing
▪ Services deliveries
▪ Road repairs
▪ Loud-speakers
▪ Various types of moving machinery in the operations

22. Factors influencing increase of noise level
▪ Increased use of glass
▪ Increased road traffic.
▪ Heightened demand for domestic machines (A.C., pool pump
etc.)
▪ Close living through increased urban density.

23. Noise Reduction Coefficient
▪ A Noise Reduction Coefficient is an average rating of how
much sound an acoustic product can absorb.
▪ Like a sponge absorbs water, an acoustic product absorbs
sound and the NRC tells us just how much sound those
products can soak up.
▪ NRC ratings range from 0 to 1.
▪ An NRC of 0 means that the product absorbs no sound
▪ An NRC of 1 means that the product absorbs all sound

24. Noise Reduction Coefficient cont…
At 0 NRC At .5 NRC At 1 NRC,
there is no sound
absorption
only 50% of the sound is
being absorbed by the
acoustic product
100% of the sound is
being absorbed by the
acoustic product

25. BEHAVIOUR OF SOUND

26. Sound behavior
Consider a sound source situated within a bounded space.
Sound waves will propagate away from the source until they
encounter one of the room's boundaries where, some of energy
will be-
▪ Absorbed.
▪ Transmitted.
▪ The rest reflected back into the room.

27. Sound waves
There are two types of sound wave
▪ Direct sound waves- it travel directly from the sound source
to the receiving point itself. It does not depend on room
shape and material but dependent on distance between
source and receiver.
▪ Indirect sound waves- after the arrival of direct sound
reflection from room surface begins to arrive , these are
indirect sound waves. It independent of the source/receiver
distance but greatly dependent on room properties.

28. Behaviour of sound wave

29. Reflection
▪ When A Wave Reaches A Boundary, Some Portion Of It Gets
Reflected.
▪ A Hard Smooth Surface Reflects More Sound Than A Soft Rough
Surface.
▪ It Bounces Around Before It Decays Completely.

30. Reflection cont.…

31. Effect of shape or form
▪ In A Perfectly Square Room, Decay
Of Sound Is Shorter.
▪ A Room With Unequal Wall Length
Results In A Smoother Decay.
▪ Curved Surfaces With A Parabolic
Shape Has The Habit Of Focusing
Sound At A Point.
– A Faintest Whisper Of A Person Standing
Across The Room Can Be Heard On This
Focal Point.

32. Diffraction
▪ Diffraction describes how waves bend, or change direction, as
they travel around the edges of obstacles.

33. Transmission

34. Absorption
▪ It occurs when impressing sound energy converted to the heat
energy in the body of absorber due to surface friction
▪ It occurs at:
– Air
– Furniture
– Boundary surfaces – wall, ceiling, floor
– Audience – clothes & body

35. Acoustic materials

36. Absorbing materials
▪ Sound absorbers are generally porous, lightweight material.
▪ There are three basic categories of sound absorbers:
– Porous materials commonly formed of matted or spun fibres ;
– Panel (membrane) absorbers having an impervious surface mounted
over an airspace;
– Resonators created by holes or slots connected to an enclosed volume
of trapped air.

37. Porous absorbers
▪ Porous absorbers are the most
commonly used sound absorbing
materials.
▪ Common porous absorbers include
– Carpet
– Draperies (curtains),
– Spray-applied cellulose,
– aerated plaster,
– fibrous mineral wool and glass fiber,
– open-cell foam,
– felted or cast porous ceiling tile.

38. Porous absorbers cont…
▪ Thickness plays an important
role in sound absorption by
porous materials.
▪ All of these materials allow
air to flow into a cellular
structure where sound
energy is converted to heat.

39. Panel Absorbers
▪ Panel (or diaphragm) absorbers are
characterized by thin sheets of
impermeable material that are rigidly
fixed to a structure.
▪ Typical materials include plasterboard
or wood paneling fixed by battens to
rigid walls, timber floors, glazing units
and so on.
▪ Fixing them in this manner creates a
small air cavity behind the panel thus
allowing it to vibrate.
▪ Such materials generally work well at
low frequencies having higher
absorption coefficients at these
frequencies and not particularly well at
high frequencies.
panel
batte
n
walls

40. Resonators
▪ Resonators typically act to absorb
sound in a narrow frequency range.
▪ The classic example of a resonator is
the Helmholtz resonator, which has
the shape of a bottle. The resonant
frequency is governed by the size of
the opening, the length of the neck
and the volume of air trapped in the
chamber.

41. Bass traps (corner)
▪ Bass Traps are acoustic energy
absorbers which are designed to
damp low frequency sound energy
with the goal of attaining a flatter
low frequency (LF) room response
by reducing LF resonances in rooms
▪ Controlling is essential especially in
the corners where very long waves
are generated.

42. Bass traps (wall or ceiling)
▪ The Sound trap is a broadband absorber with increased absorbtion
at the lower frequency range. It can be applied to walls and ceilings.

43. Thankyou