Lecture 02,03 Environmental Design 2

1. Arch 231
Environmental Design-II: Visual and Sonic Environment

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Noise in free field

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The inverse square law is applicable only to free field conditions
where there is no obstruction, no solid objects from which the sound could
be reflected. Open air conditions approximate the theoretical free field.
where l is intensity in W/m2
W is the source power in watts and
d is distance in m
“According to the inverse square law, every doubling of the distance
will decrease the intensity to one quarter.”

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Due to the logarithmic relationship between Sound Level (dB) and
Intensity (W/m2) , the sound level this will correspond to a reduction of
6 dB for every doubling of the distance, regardless of the magnitude of
intensity.
If a sound at 1 km from source: l' = 0.01 W/m2
at 2 km: l" = 0.0025 W/m2 = (l' /4)
2) The number of decibels (N) : N= 10 log (l/lo)
Where, l = the measured intensity ; lo = reference intensity: 10ˉ¹² W/m2 which is the lowest intensity
perceived as a sound
Reference : 1)

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or:
If speech at 2 m: l' = 10 ˄(−8) W/m2
at 4 m: l" = 25 × 10˄(−10)W/m2 =(l' /4)
2) The number of decibels (N) : N= 10 log (l/lo)
Where, l = the measured intensity ; lo = reference intensity: 10ˉ¹² W/m2 which is the lowest intensity
perceived as a sound
Reference : 1)
In this way 6 dB reduction of sound level occurs for every doubling of the distance.

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Distance also affects sound by the molecular absorption of energy
in the carrying medium. This molecular attenuation in air is only
significant for high frequency sounds. For every 300 m distance this
reduction is:
1 dB at 1000 Hz
40 dB at 9000 Hz
Loud noises from a great distance (e.g. thunder) are heard at a lower pitch
than from nearby.

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Effect of a wind velocity gradient
In moving air (wind) there is usually a velocity gradient , so the
spherical wave front is distorted. Figure explains how this will result
in an increased sound downwind and a decreased sound upwind.

8. As the velocity of sound increases with air temperature, a
temperature gradient will also distort the spherical wave front:
Figure shows how this will produce -
•an increased sound effect for a ground level observer at night
•a decreased sound effect during day-time temperature inversion.
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Effect of temperature gradients

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Acoustic shadow
Screening or barriers in the path of sound can create an 'acoustic shadow'–
if the sound is of a high frequency.
At low frequencies diffraction will occur at the edge of the barrier
-thus the 'shadow' effect will be blurred.

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Noise in enclosed spaces

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Behavior of sound in enclosed space
Sound incident on the
surface of a solid body
(e.g. a wall) is
- partly reflected,
- partly absorbed
(converted into heat)
and
- partly transmitted to
air on the opposite
side.

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The term 'absorption coefficient' is used normally to
indicate all the sound that is not reflected (that is, it
includes the part actually absorbed and that which is
transmitted). The absorption coefficient is denoted by a: it
is a decimal fraction - a non-dimensional quantity.
Absorption of sound

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r = reflected If source I =1
a = absorbed r + a+ t =1
t = transmitted
For room 1: 'absorption coefficient' = a +t (all that is not reflected: 1 − r)
For room 2: ‘transmission coefficient' = t (r +a is not transmitted)
Airborne sound transmission

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Absorption coefficient is a measurement of the efficiency of a surface or
material in absorbing sound.
For example, If 55% of the incident energy is absorbed by a material, the
absorption coefficient is 0.55. One square foot of this material gives 0.55
absorption units (sabins).
Absorption (A) is the product of the absorption coefficient (a) and of the
area of a given surface (s):
A = a × s
It is measured by the 'open window unit', which is the absorption of a 1
m2 opening having an absorption coefficient of 1 (i.e. zero reflectance)

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An open window- a perfect absorber, sound passing through it never returns
to the room, having absorption coefficient of 1.0. So, Ten square feet of open
window would give 10 sabins of absorbance.
The Absorption coefficient of a material varies with frequency and angle at
which the sound wave impinges upon the material.

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Acoustic ceiling tiles lay into a
suspended or dropped
ceiling grid. These tiles can be
made of various acoustical
materials like fiberglass, foam,
wood, polyester, and other
substrates.
Glass fiber acoustical panel are
manufactured from high density,
3rd generation glass wool. The
visible surface is a high quality
batch painted glass tissue in
white, black, yellow or any other
color and the back of the tile is
covered with glass tissue.
Acoustical wall panels are
composed of mineral wool or
foam that has been
compressed; These mini-
panels are a great solution for
reducing echo and loud
sounds in gymnasiums,
auditoriums, studios,
hallways, basements,
churches, offices, and more.

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When the sound is in an enclosed space, reflection will occur from
the bounding surfaces: the reflected part will reinforce the
sound within the space and the remainder will be lost for the
system.
Reflection of sound

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The essential mechanism of reflection from a flat
surface is simple.
Figure shows the reflection of point-source sound
waves from a rigid, plane wall surface. The
spherical wave fronts (solid lines) strike the wall
and the reflected wave fronts (broken lines) are
returned toward the source.
This is called a specular reflection and behaves the
same as light reflections from a mirror, described
by Snell’s law.
Reflection from a flat surface

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Reflection from a convex surface
Reflection of plane wavefronts of sound
from a solid convex surface scatters the
sound energy in many directions as shown
in Figure.
This reflecting sound returns and diffuses
the impinging sound.
More effective, if the size of reflector is
larger than the wavelength.

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Plane wavefronts striking a concave
surface tend to focus to a point as
illustrated in Figure.
The precision with which sound is
focused is determined by the shape
and relative size of the concave
surface.
Spherical concave surfaces are often
used to make a microphone highly
directional by placing it at the focal
point. Such microphones are
frequently used to pick up field
sounds at sporting events or record
animal sounds in nature.
More effective, if the size of reflector
is larger than the wavelength
Reflection from a concave surface

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A parabolic surface can focus sound
precisely at a focal point or, the
converse, a sound source placed at the
focal point can produce plane, parallel
wave fronts. In this case, the source is an
ultrasonic Galton whistle driven by
compressed air.
A parabola generated by the equation y =
x2 has the characteristic of focusing
sound precisely to a point.
A very “deep” parabolic surface, such as
that of Figure, exhibits far better
directional properties than a shallow one.
Reflection from a parabolic surface
Galton Whistle: In the mid-1800s, Sir Francis Galton was presented with a dilemma. He wanted to test hearing ability for higher
frequencies but did not have a piece of equipment to adequately measure them. Using some scientific ingenuity, he went about
creating an object to produce the sound frequencies he wanted to study. He ended up with a small brass tube with a slit at the
end of it. Air would be forced through the tube, coming out at the slit as an audible tone. Along the tube a siding piece could be
maneuvered up or down the tube to create different frequencies. The sliding plug was marked so that precise notes could be
recorded in research. It became known as the “Galton Whistle.”

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Whispering Gallery, St Paul`s Cathedral
Graphic example of a whispering gallery
showing symmetrical sound focusing
points.
A whisper directed tangentially to the
paraboloid-shaped surface is readily
heard by the receiver on the far side of
the room. More generally, concave
surfaces pose acoustical problems.

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Reverberation is the phenomenon of persistence of sound after it has been
stopped as a result of multiple reflections from surfaces such as furniture,
people, air, etc. within a closed surface. These reflections build up with each
reflection and decay gradually as they are absorbed by the surfaces of objects
in the space enclosed.
Reverberation of sound

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In an enclosed space, even from a single source, there will be a complex
pattern of inter-reflected sound, which is usually referred to as
'reverberant sound'.
The first reduces with the distance, but the second can be taken as constant
throughout the space.
Reverberant sound: sum of an infinite number of paths
a the direct component
Thus at any point in the space the total sound received will consist of two parts:
b the reverberant component.

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A reverberation is an echoing sound.
When you bang on a big piece of metal, you can hear the reverberation even
after you stop banging.
It is the same as the echo, but the distance between the source of the sound
and also the obstacle through which it gets reflected is less in case of this
reverberation.
The quantitative characterization of the reverberation is mainly done by using
the parameter called reverberation time.
Reverberation time is usually defined as the length of the time when the sound
decays by about 60 decibels starting from the initial level

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Reverberation chamber
Reverberation chamber is a room designed to create a diffuse or random incidence
sound field (i.e. one with a uniform distribution of acoustic energy and random
direction of sound incidence over a short time period).
Reverberation chambers tend to be large rooms (the resulting sound field becomes
more diffused with increased path length) and have very hard exposed surfaces.

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Anechoic chamber
An anechoic chamber (an-echoic meaning "non-reflective") is a room designed to stop
reflections of either sound or electromagnetic waves. They are also often isolated from
energy entering from their surroundings. This combination means that a person or
detector exclusively hears direct sounds (no reflected sounds), in effect simulating being
outside in a free field.
360-degree image of an acoustic anechoic chamber

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Transmission of sound

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•The Magnitude of the reverberant component depends on the absorbent
qualities room surfaces
•The more absorbent these surfaces are, the lesser the reverberant component
will be.
•A good rule of thumb is that every doubling of the total absorption in the space
will reduce the reverberant sound level by 3 dB.
If all surfaces in the space were perfect absorbers, conditions would be the same
as in a free field: zero reverberant component.
•When airborne sound impinges on a solid body, some of the energy of vibrating
air molecules will be transmitted to the solid material and induce a vibration of
its molecules. This vibration will spread in the body as 'structure borne sound'
and may be re-emitted to air on other surfaces.

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Figure shows some possible sound paths from a source in one room to a
listener in another room. Of the five paths 1 is airborne, 2, 3 and 4 are
structure borne. Path 5 is strictly speaking also structure borne, but for
practical purposes the transmission through a wall of a sound perpendicular to
its plane is considered as 'airborne sound transmission'.
Sound transmission paths T.L = Transmission loss

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The only way to reduce structure borne transmission is to prevent the spread of
vibrations by introducing structural discontinuity, i.e. a physical separation
or flexible connections only.
Sound transmission class
(STC) is a rating of sound
isolation of a building
wall assembly. The higher
the STC rating, the better
sound isolation the wall
assembly is to achieve.
STC is widely used to rate
interior partitions,
ceilings/floors, doors, and
windows.

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Fig. Staggered stud partition

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