Basics of Acoustics

1. ACOUSTICS
Prepared By
Dr.R.Vettumperumal
Assistant Professor
VVCOE
Arasoor
Tuticorin

2. cs
Vibrations transmitted through an elastic material or a solid,
liquid, or gas, with frequencies in the range of 20 to 20,000 hertz,
capable of being detected by human organs of hearing.
Transmitted vibrations of any frequency.
• All sounds originate in the
vibrations of material objects.
• The source of all sound waves is
vibration.

3. Examples of Sound generation

4. Hearing Mechanism of Human Ear

5. Characteristics of
Sounds• Sound is form of energy.
• It requires medium to propagate.
• Sounds requires definite interval of time to travel from one point to
another point in a medium.
• Its velocity smaller than the velocity of light.
• Velocity of sound is maximum in solids, which have high bulk modulus and
least in gas.

6. Classification of Sounds

7. Pitch (or) Frequency
The sensation
conveyed to our brain by
the sound waves falling
on our ears.
Quality (or) Timbre
Which is enable us to
distinguish between
musical notes emitted by
different instruments.
Intensity of Sound (or) Loudness
The amount of sound energy
flowing per unit area Situated
normally to the direction of the
propagation of sound waves.

8. Weber Fechner
Law
The loudness produced is directly
proportional to logarithm of
intensity
Decibel
The logarithmic ratio of intensity
of a sound to Standard intensity is
called decibel or relative intensity
of sound

10. Woman holding pain during ormal delivary = 57 bel

11. Reverberati
on
The existence of sound in
a room even after the source of
sound has stopped to emit the
sound is called reverberation.
Reverberation Time
The time duration for which
existence of sound even after the
source of sound has stopped to emit
the sound is called reverberation.
Standard Reverberation
Time The time taken by the
sound intensity level to reduce by 60
dB from its initial sound intensity level
after the source of sound has stopped
to emit the sound

12. Absorption Coefficient
It is defined as the ratio of
sound energy absorbed by the surface
to that of total sound energy incident
on the surface.
Hall
T1
Volume = V
Hall
T2
Volume = V
(1)
(2)
(3)
(4)

13. Factors Affecting the Acoustics of
Buildings
1. Un-optimised reverberation time
2. Very low and high Loudness
3. Improper focusing (or) Interference
4. Echoes (or) echelon effect
5. Resonance
6. Noise

14. 1. Un-optimised reverberation time
If the reverberation time is
high then it produces echoes.
If the reverberation time is
very low, the sound will not
be clearly heard by the
audience.
Therefore, we should maintain
optimum reverberation
The optimum reverberation time can be
achieved by the following steps
By having the full capacity of audience in
the auditorium.
By choosing absorbents like felt, fiber,
board, glass etc inside the auditorium and
even at the back of chairs.
Reverberation time can be optimized by
providing windows and ventilators at the
places wherever necessary and using
curtains with folds for the windows.
The reverberation time can also be
optimized by decorating the walls with
beautiful pictures.
Speech = 0.5 S
Music = 1 – 2 S
Theatres = 1.5 – 3 S

15. 2.
LoudnessLoudness is the degree of sensation produced on the ear. If the intensity
of sound is weak, loudness my go below the level of audiability.
Remedies
Maximum reflection of sound from stage is
desirable to achieve good loudness.
Large polished wooden reflecting surfaces
above the speakers are helpful.
Quality of loudspeakers is essential.
Low ceilings are helpful for better reflection of
sound.
The wall at speakers end should be given a
parabolic shape.

16. 3. Improper Focusing (or) Interference
In some places of a hall, the sound will not
be heard properly and that place is said to be
a dead space, which is due to presence of
convex or concave surfaces in the hall as
shown in the figure.
Sometimes the sound waves will have
interference pattern because of ceiling
surfaces which will create maximum intensity
of sound(due to constructive interference) in
some places and minimum intensity of
sound(due to destructive interference) at
some places.
Hence causing uneven distribution of
sound intensity in the hall and hence causing
uneven distribution of sound intensity in the
hall.
Remedies
1. By avoiding curved surfaces (or)
covering the curved surfaces by
suitable absorbents, the focusing can
be avoided.
2. By evenly polishing and decorating
with absorbents the interference
effects can be avoided.

17. 4. Echoes (or) Echelon effect
 In some halls, the walls of the halls will scatter the sound waves rather
than reflecting it, thus way create nuisance effect due to echoes.
 The echoes are formed when the time interval between the direct and
reflected sound waves are about 1/15 of a second. This effect occurs
due to the reason that the reflected sound waves reaches the observer
later than the direct sound.
 If there is a greater repetition of echoes of the original sound to the
observer then the effect is called as Echelon effect.
Remedies
The echo can be avoided by lining the surfaces with suitable sound absorbing materials
and by providing enough number of doors and windows.

18. 5.
ResonanceResonance occurs when a new sound note of frequency matches
with standard audio frequency.
Remedies
The resonance effect can be avoided by providing proper
ventilation and by adjusting the reverberation time to the
optimum level.
Nowadays the resonance is completely eliminated by air
conditioning the halls.

19. 6.
Noise
Noise is an unwanted sound produced due to heavy traffic outside
the hall which leads to displeasing effect on the ear. There are three
types of noises.
Air Borne noise
Structure Born Noise
Inside Noise

23. Thank You