Basics of Architectural Acoustics

1. Acoustics
AROH THOMBRE

2. What is Acoustics?
Acoustics is defined as the scientific study of sound which includes the effect of reflection, refraction
absorption diffraction and interference. It also deals with the properties of the sound waves, their origin,
propagation and their action on obstacles.
What is Sound?
Sound is an alteration of pressure that propagates through an elastic medium such as air which produces and
auditory.
Why Do We need Acoustics?
Acoustics are fundamentally important to learning environments. Learning is intrinsically linked with
communication, and aural (sound) communication is acoustics. Similarly, learning is about concentration, and
external noise is a major distracting factor in education.
The importance of acoustics is not limited to classrooms. Noise in corridors and public spaces can soar if they
are too reverberant (too much echo), with voices raised louder and louder to overcome the background echo,
just like shouting conversations at a noisy cocktail party or restaurant. So to come over this problems of
sounds we need acoustics.

3. Terminologies related to Acoustics:
Airborne sound — Sound or noise radiated directly from a source, such as a loudspeaker or machine, into the
surrounding air.
Ambient noise — Total noise level in a specified environment.
Audible frequency range — The range of the sound frequencies normally heard by the human ear. The audible
range spans from 20 Hz to 20,000 Hz, but for most engineering investigations only frequencies between about
40 Hz and 11,000 Hz are considered.
Decibel (dB) — (1) Degree of loudness. (2) A unit for expressing the relative intensity of sounds on a scale from
zero for the average least perceptible sound to about 130 for the average pain level.
Diffraction — The process whereby an acoustic wave is disturbed and its energy redistributed in space as a
result of an obstacle in its path.
Direct sound — Sound that reaches a given location by direct, straight-line propagation from the sound source.
Frequency — Repetition rate of a cycle, the number of cycles per second.
Noise — Unwanted sound.
Noise Reduction (NR) — The difference in sound pressure level between any two points along the path of sound
propagation.

4. Terminologies related to Acoustics:
Reflection — Redirection of sound waves.
Refraction — Change in direction of sound waves caused by changes in the sound wave velocity.
Reverberant sound/reverberation — The sound in an enclosed space, which results from, repeated reflections
at the boundaries.
Sabin — Unit of acoustic sound absorption, equivalent to the absorption by one square meter of perfect
absorber.
Sound absorption — (1) The process by which sound energy is converted into heat, leading to the reduction in
sound pressure level. (2) The sensation perceived by the sense of hearing.
Sound insulating material — Material designed and used as partitions in order to minimize the transmission of
sound.
Sound insulation — The reduction or attenuation of sound by a solid partition between source and receiver.
This may include a building wall, floor, barrier wall or acoustic enclosure.
Sound intensity — The sound flowing per unit area, in a given direction, measured over an area perpendicular
to the direction of flow; units are W/m2.

5. How is Sound Measured?
Sound energy travels in waves and is measured in frequency and amplitude.
Amplitude measures how forceful the wave is. It is measured in decibels or dBA of sound pressure. 0 dBA is the
softest level that a person can hear. Normal speaking voices are around 65 dBA. A rock concert can be about 120
dBA.
Frequency is measured in the number of sound vibrations in one second. A healthy ear can hear sounds of very
low frequency, 20 Hertz (or 20 cycles per second), to a very high frequency of 20,000 Hertz. The lowest A key on
the piano is 27 Hertz. The middle C key on a piano creates a 262 Hertz tone. The highest key on the piano is
4186 Hertz.

6. How do we Hear Sound?
Sound waves travel into the ear canal until they reach the eardrum. The eardrum passes the vibrations through
the middle ear bones or ossicles into the inner ear. The inner ear is shaped like a snail and is also called the
cochlea. Inside the cochlea, there are thousands of tiny hair cells. Hair cells change the vibrations into electrical
signals that are sent to the brain through the hearing nerve. The brain tells you that you are hearing a sound
and what that sound is.

7. Good Acoustics Involve?
• Good distribution of sound to all the seats, which depends on proper shaping and finishes of all interior
surfaces.
• Natural sound diffusion and envelopment.
• A sense of intimacy for the audience and a sense of ensemble for both performers and audience.
• Proper reverberation times through out all frequencies, which depend on room’s volume and the total sound
absorption of all materials.
• Freedom for the acoustical faults of echoes, flutter, and focus.
Factors affecting architectural acoustics:
• Reverberation time
• Loudness
• Echelon effect
• Structure Borne sound
• Echo
• Focusing due to Walls and Ceilings

8. Factors affecting architectural Acoustics:
Reverberation time:
• When the reverberation time is too high, the sound produced by the speaker will persist for a long period
of time.
• Similarly ,when the reverberation time is low, sound dies quickly and becomes inaudible in a short amount
of time.
• In order to improve the sound, reverberation time of a hall should be increased to an optimum value.
Loudness:
• Reverberation time of a hall is directly proportional to loudness.
• Low loudness results in existence of sound for a shorter period while high loudness results in existence of
sound for a longer period.
• Therefore sound produced by the speaker should be within audible range.
Structure Borne sound:
• Sound waves generated inside a hall are known as structure-borne sound.
• They are produced due to apparent motion of benches & footsteps & propagated through walls and floors.

9. Factors affecting architectural acoustics:
Echelon effect:
• Unwanted sounds are produced when people walk on staircase or floors or hard paved paths due to poor
finishing of the floor surface, structural effects, etc.
• The above mentioned unwanted sound are termed as ‘echelon effect’ .
Echo:
• If the time interval between direct sound and reflected sound is less than 1/15 of a second, the reflected
sound is helpful in increasing loudness.
• But if the time interval is less than that, then the sound arrives later and will cause confusion .
Focusing due to Walls and Ceilings:
• Sound produced by speaker undergoes multiple reflections at ceilings and walls.
• Reflected sounds from ceilings and walls should not be focused on particular point, rather it should be
distributed throughout a hall.
• Generally a plane surface reflects sound uniformly but a curved surface does not. So reflection of sound
from a curved surface produces a harmful effect.

11. Acoustical Materials:
Sound Absorbers:
These sound absorbing acoustical panels and soundproofing materials are used to eliminate sound reflections
to improve speech intelligibility, reduce standing waves and prevent comb filtering. A wide variety of materials
can be applied to walls and ceilings depending on your application and environment. These materials vary in
thickness and in shape to achieve different absorption ratings depending on the specific sound requirements
TYPES :
• Acoustical foam panels
• White paintable acoustical wall panels
• Fabric wrapped panels
• Acoustical wall coverings
• Ceiling tiles
• Baffles and banners for ceiling
• Fiber glass blankets and roll

12. Acoustical Materials:
Sound Reflectors:
It is very important to provide as much natural reinforcement for the unamplified voice as possible. This
applies equally in smaller rooms, classrooms, meeting rooms etc, but is particularly important for larger spaces
where the distance between the speaker and the listener is greater.
Natural reinforcement is achieved by the strategic placement of reflective surfaces. For example in theatres it
is common to place reflectors above the stage, and to angle these to give useful reflections, particularly to the
back of the auditorium. Hard flat surfaces can be considered to reflect sound in a similar manner to they way
that a mirror reflects light (i.e the angle of incidence equals the angle of reflection.

13. Acoustical Materials:
Sound Diffusers:
These devices reduce the intensity of sound by scattering it over an expanded area, rather than eliminating the
sound reflections as an absorber would. Traditional spatial diffusers, such as the polycylindrical (barrel) shapes
also double as low frequency traps. Temporal diffusers, such as binary arrays and quadratics, scatter sound in a
manner similar to diffraction of light, where the timing of reflections from an uneven surface of varying depths
causes interference which spreads the sound.
TYPES:
• Quadra pyramid Diffuser
• Pyramidal Diffuser
• Double duty Diffuser
• Quadratic Diffuser