Behavior of sound in an enclosed space

1. Ravi Sharma

2. Sound can be described as a disturbance or
turbulence which passes through a physical
medium in the form of longitudinal waves
from a source to a receiver causing a
sensation of hearing.
This medium could be solid, fluid or gas.
The speed of sound through these different
media differs due to their molecular
composition.

3. Wavelength of sound – This the distance between two pressure peaks or
valleys, measured in metres (m) and represented with the Greek alphabet ‘l’
(lambda).
Period – This is the time taken for on complete oscillation. This is measured
in seconds(s) and represented with the letter ‘T’.
Frequency – This is the number of oscillations per second. This is
represented with ‘f’ and measured in Hertz (Hz).
Velocity of sound – This is the rate at which a sound wave travels from a
source through a medium to the receiver. The unit is m/s.
Amplitude – This is the distance between a crest (the highest point) and a
valley (the lowest point)
Pitch – it is the highness or lowness of a tone determined by the rapidity of
the oscillations producing it.

4. An enclosed space is a room or area bounded on every of
its sides. The materials for enclosure may be classified
into two:
Those that allow sound rays to pass through and
Those that do not allow sound rays to pass through.
On encountering barriers posed by the enclosure, sound
waves are likely to behave in the following ways:
◦ Reflection
◦ Absorption
◦ Refraction
◦ Diffusion
◦ Diffraction
◦ Transmission

5. This occurs when the wavelength of a sound wave is smaller
than the surface of an obstacle. In the case of an enclosed
space, the sound waves hit every side of the enclosure
continuously until the sound energy reduces to zero. The
amount of waves reflected depends on the smoothness, size,
and softness of the materials of enclosure. The angle of
incidence of sound rays is equal to that of the reflected rays
only if the surface of the reflector is flat. But when it is
curved, the angles are different.

6. When sound waves hit the surface of an obstacle,
some of its energy is reflected while some are lost
through its transfer to the molecules of the barrier.
The lost sound energy is said to have been
absorbed by the barrier. The thickness and nature
of the material as regards its softness and
hardness influences the amount of sound energy
absorbed.

7. This is the bending of sound when it travels from
one medium into another medium. The difference
in the composition of the two different media
bends the sound i.e. the angle of incidence
changes into an angle of refraction as it travels into
the new medium.

8. This is the scattering of waves from a surface. It
occurs as a result of the texture and hardness of
the obstacle is comparable to the wavelength of the
sound. The direction of the incident ray changes
when it strikes the surface of the obstacle.
Satisfaction is achieved when sound is heard in all
direction at equal level.

9. When the wavelength of a sound wave is
smaller or equal to the size of the obstacle,
the sound rays tend to bend round the edge
of the obstacle thereby turning the edge to a
sound source.

10. In this phenomenon, sound wave is carried by
molecules of the obstacle through vibration
and re-emitted at the other side irrespective
of the medium. It can be structure borne, air
borne or impact sound.

11. Reduction in its intensity of sound – This can results due to the
distance between its source and the receiver.
Absorption of direct sound by the audience – The listeners of the
sound absorb some of the sound in the process of hearing.
Absorption of direct and reflected sound by surfaces – The walls,
ceiling and floor of the enclosure absorbs and reflect sound waves
thereby controlling the way the sounds behave.
Reflection of sounds from right-angled corners - Sound incident to
a right-angled corner of room will be reflected back towards source
if surfaces are acoustically reflective. This can in turn produce
echoes especially in large spaces.
Dispersion of the sides of an enclosure - Reflections can be
controlled by making one surface dispersive i.e. not at right angle to
each. This would have affected the reflection of the sound thereby
affecting its behaviour.

12. Edge diffraction of sound - Edge diffraction results in the curvature
of part of a sound wave around the edge of a barrier. This causes
the obstacles to scatter the sound waves making it behave like a
source of sound.
Sound shadow - Any barrier interrupting a sound wave will create a
shadow, synonymous to light rays. However, because of edge
diffraction some sound will creep into this but such penetration is
frequency dependent - high frequencies are less diffracted than low
frequencies. Such problems can occur in auditorium with balconies.
Primary reflection – This depends on the angle of incidence which is
equal to the angle of reflection. Also, the nature of sound reflector is
important.
Panel resonance - Sound waves can propagate "through" a solid
material by panel vibration. The sound does not actually penetrate
the material but rather causes this to vibrate and act as a sound
source itself. The panel will be vibrated by both direct and reflected
sound waves.