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ALOBA TOWOJU E. ARC/09/7363 ASA’AH YVONNE O. ARC/09/7365€¦ · the behaviour of sound in...
Transcript of ALOBA TOWOJU E. ARC/09/7363 ASA’AH YVONNE O. ARC/09/7365€¦ · the behaviour of sound in...
A PAPER
ON
THE BEHAVIOUR OF SOUND IN NON-
RECTANGULAR HALLS
COMPILED BY
ALOBA TOWOJU E. ARC/09/7363
ASA’AH YVONNE O. ARC/09/7365
SUBMITTED TO:
THE DEPARTMENT OF ARCHITECTURE,
THE FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE.
IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE
AWARD OF
BACHELOR IN TECHNOLOGY (B.TECH) IN ARCHITECTURE.
COURSE LECTURER:
PROFESSOR O.O OGUNSOTE.
JULY 2014.
INTRODUCTION
Over the years, spaces such as
swiftly shifted from just being regular
interesting architectural forms, leavi
such as circles and ellipses, horseshoe
advancement in the form of this spaces coupled with the aesthetics
men but even elites, it is a turn off for acousticians because little or nothing is known about the
acoustic qualities of such shapes.
Conspicuously, not many experts have attempted to research into this aspect, this paper therefore
intends to ride on the research works of a few
behavior of sound in non-rectangular halls and relating them with sound behavior in rectangular
halls. The peculiar acoustics challenges will also be highlighted and possible recommendatio
will be suggested afterwards, therefore the design of hall spaces can be eventually ma
1.0 NATURE OF SOUND WAVES
Sound is the human ear’s response to pressure
Sound travels in space by a phen
travelling layers of compression and rarefaction of
fluctuations. The characteristics
characteristics of its surfaces and
level. It’s important to review the behavior of sound in
decay of sound in a room and its eff
Propagation of Sound Indoors:
Sound from an omnidirectiona
Inverse Square Law, and its intensity reduces a
Over the years, spaces such as community halls, religious auditoria, lecture the
from just being regular shapes such as square or rectangle into more fanciful, and
esting architectural forms, leaving architects with the option of using curvilinear shapes
such as circles and ellipses, horseshoe shapes, fan-shapes and polygons. Despite this notable
advancement in the form of this spaces coupled with the aesthetics they present not only to lay
, it is a turn off for acousticians because little or nothing is known about the
ic qualities of such shapes.
ot many experts have attempted to research into this aspect, this paper therefore
intends to ride on the research works of a few painstaking acousticians or architect, stating the
rectangular halls and relating them with sound behavior in rectangular
The peculiar acoustics challenges will also be highlighted and possible recommendatio
will be suggested afterwards, therefore the design of hall spaces can be eventually ma
RE OF SOUND WAVES
s response to pressure fluctuations in the air caused by vibrating
travels in space by a phenomenon called wave motion. This motion is created by outward
ssion and rarefaction of the air particles, that is, by pressure
fluctuations. The characteristics of room (room geometry, volume, and the absorption
acteristics of its surfaces and contents) greatly influence both the sound quality as well as its
review the behavior of sound in rooms as well as the phenomenon of the
room and its effect on speech communication and perception.
f Sound Indoors:
Sound from an omnidirectional source in a free space Fig (a) radiates in accordance wi
and its intensity reduces at a rate of 6dB per doubling of distance.
Fig. 1. Sound reflection
halls, religious auditoria, lecture theatres, e.t.c, have
shapes such as square or rectangle into more fanciful, and
ng architects with the option of using curvilinear shapes
and polygons. Despite this notable
they present not only to lay
, it is a turn off for acousticians because little or nothing is known about the
ot many experts have attempted to research into this aspect, this paper therefore
painstaking acousticians or architect, stating the
rectangular halls and relating them with sound behavior in rectangular
The peculiar acoustics challenges will also be highlighted and possible recommendations
will be suggested afterwards, therefore the design of hall spaces can be eventually made better.
in the air caused by vibrating objects.
omenon called wave motion. This motion is created by outward
the air particles, that is, by pressure
of room (room geometry, volume, and the absorption
th the sound quality as well as its
ms as well as the phenomenon of the
perception.
a) radiates in accordance with the
t a rate of 6dB per doubling of distance.
Elevating the audience Fig (b) provides both better
in intensity at listening position is limited to 3dB
radiation.
Adding reflecting surfaces will provide further strength to the
reinforcement can be obtained by more
in sound transitioning to indoor acoustics is,
extended Fig (e). At this point, t
continuously reflected by the enclosing
Fig. 2.Sound reflection from surrounding surfaces
2.0 ACOUSTICAL PHENOMENA IN ENCLOSURES
When a sound wave meets an obstacle, its behavior will
on its size relative to the wavele
be simply replaced by imaginary sound rays
traveling in straight lines in every direction
b) provides both better sight lines and hearing lines. The reduct
position is limited to 3dB as the result of the change to
Adding reflecting surfaces will provide further strength to the direct signal. Further
rcement can be obtained by more fully enclosing the sound source. Fig (
tioning to indoor acoustics is, when the enclosure on around
e). At this point, the radiated sound is completely contained and will be
enclosing boundaries until it dies away
Sound reflection from surrounding surfaces
ACOUSTICAL PHENOMENA IN ENCLOSURES
an obstacle, its behavior will depend on the nature of the obstacle and
relative to the wavelength of the sound. Studying the behavior of sound
imaginary sound rays perpendicular to the advancing
very direction within the space.
nd hearing lines. The reduction
as the result of the change to hemispherical
direct signal. Further
e sound source. Fig (c, d)The final step
when the enclosure on around the listening area is
he radiated sound is completely contained and will be
Sound reflection from surrounding surfaces
epend on the nature of the obstacle and
of sound in a room can
perpendicular to the advancing wave front,
2.1 Reflection:
Similar to the effect of light ray reflected from a mirror
produced by a new source when in a positio
behind the surface. This illustrates
angle of reflection in all cases equals the angle of incidence.
Fig. 3.Reflection of convex, plane and concave surfaces
Convex reflecting surfaces tend to
sound waves in the room.
2.2 Diffusion
If the sound pressure is equal in all parts of an
are traveling in all directions, the sound
diffusion or sound dispersion
promotes a uniform distribution of sound, accentuates
prevents the occurrence of undesirable acoustical defec
achieved, listeners will have the sensation of sound
Similar to the effect of light ray reflected from a mirror, the reflected sound to be as it was
source when in a position corresponding to the original source but situated
This illustrates the familiar law of reflection from which is seen that the
angle of reflection in all cases equals the angle of incidence.
Reflection of convex, plane and concave surfaces
Convex reflecting surfaces tend to disperse and concavesurfaces tend to concentrate the
ure is equal in all parts of an auditorium and it is probable that sound waves
in all directions, the sound field is said to be homogeneous, in other words,
sion prevails in the room. Adequate sound diffusion is necessary
tribution of sound, accentuates the natural qualities of speech, and
undesirable acoustical defects. When satisfactory diffusion
ill have the sensation of sound coming from all directions at equal levels.
Fig. 4.Diffusion of sounds
reflected sound to be as it was
n corresponding to the original source but situated
ion from which is seen that the
Reflection of convex, plane and concave surfaces
urfaces tend to concentrate the reflected
it is probable that sound waves
, in other words, sound
te sound diffusion is necessary as it
atural qualities of speech, and
ts. When satisfactory diffusion has been
directions at equal levels.
2.3 Diffraction
Diffraction involves a change in direction of waves as
barrier in their path which causes soun
corners, columns, walls and
Experience gives evidence that deep galleries cast
underneath, causinga noticeable loss in high frequency sound which doesn’t
protruding balcony edge. This condition
2.4 The Growth and Decay
As sound waves travel at about 344 meters/second, the
within an auditorium will generally reach a listene
seconds. Shortly after the arrival of the
surrounding surfaces will reach the lis
about 50 milliseconds. If the sound source is a
point will not suddenly disappear, but will
to die off and reflections get weaker
total and final sound level, the sligh
The rate of buildup of sound is much steeper t
is perceived by the ear as being instantaneo
sounds, and because sound decay is far
sound, not its buildup, that affects the acoustics of lecture halls
This gradual decay of sound energy is known
relationship between absorption and
sound pressure level (in dB) of a decaying rever
change in direction of waves as they pass through an opening or around a
path which causes sound waves to be bent or scattered around such obstacles as
corners, columns, walls and beams. This is more common for low frequency than for high.
that deep galleries cast an acoustical shadows on the audience
underneath, causinga noticeable loss in high frequency sound which doesn’t
ng balcony edge. This condition creates poor hearing conditions und
Decay of Sound
at about 344 meters/second, the sound coming direc
will generally reach a listener after a time of anywhere from
hortly after the arrival of the direct sound, a series of semi distinct reflections from
surrounding surfaces will reach the listener. These early reflections typically will occu
If the sound source is abruptly switched off, the sound
ot suddenly disappear, but will fade away gradually as the indirect sound field begi
off and reflections get weaker. As the direct sound level is often only a little less
the slight and slower stages of build-up is often passed unnoticed.
The rate of buildup of sound is much steeper than its decay. Because of its steepne
the ear as being instantaneous. Due to the transient nature
and because sound decay is far more perceptible than build-up[5], it’
not its buildup, that affects the acoustics of lecture halls
Fig. 5.Decaying of sound vs. time
This gradual decay of sound energy is known asreverberation and, as a result of this proportional
relationship between absorption and sound intensity, it is exponential as a function of ti
(in dB) of a decaying reverberant field is graphed against
through an opening or around a
d waves to be bent or scattered around such obstacles as
mmon for low frequency than for high.
hadows on the audience
underneath, causinga noticeable loss in high frequency sound which doesn’t bend around the
creates poor hearing conditions under the balcony.
sound coming directly from a source
r after a time of anywhere from 0.01 to 0.2
es of semi distinct reflections from
reflections typically will occur within
the sound intensity at any
fade away gradually as the indirect sound field begins
only a little less than the
up is often passed unnoticed.
decay. Because of its steepness, the buildup
us. Due to the transient nature of most practical
up[5], it’s the decay of
s a result of this proportional
exponential as a function of time. If the
berant field is graphed against time, the
reverberation curve renders as fairly straight,
factorsincluding the frequency spectrum of the sound and the
2.5 Reverberation
As a listener the first thing one hears is the direct sound
source. This is followed by a series of early reflections fro
sound has to travel further, so will
(unless some focusing of the reflection occurs).
conditions [1] because sound within this ti
listener’s brain.
Reverberation is a smooth decrease in the energycontent of successive reflections, so that the
reflections are not individually perceptible.
off is related to the amount of
in space in various directions and hits room surfaces,
By extensive and ingenious measurements in h
considerable amount of research in this area
volume of an auditorium, the amount o
called the Reverberation Time (RT).
Other phenomena of sound in enclosures include refraction, absorption, transmission,
and echo. These will not necessarily be explained
3.0 FACTORS AFFECTI
Sound intensity dies out over distance
Sound travels at constant speed and so delay
rve renders as fairly straight, although the exact form depends[15]
y spectrum of the sound and the shape of the room.
g one hears is the direct sound which travels in a straigh
followed by a series of early reflections from side walls, ceiling etc. Reflected
has to travel further, so will arrive later. It will not be as loud as the direct component [9]
sing of the reflection occurs). It should be less than 30ms for good listening
because sound within this time interval is perceived as one
Fig. 6.Sound level against time
decrease in the energycontent of successive reflections, so that the
not individually perceptible. The persistence of sound in a
off is related to the amount of absorption in the room. As soon as a sound is produced i
ections and hits room surfaces, from which it’s reflected and
ingenious measurements in halls of different types, Sabine
amount of research in this area and arrived at an empirical relationship between the
of an auditorium, the amount of absorptive material within it and a quantity which he
called the Reverberation Time (RT).
Other phenomena of sound in enclosures include refraction, absorption, transmission,
and echo. These will not necessarily be explained due to the scope of this paper.
FACTORS AFFECTING SOUND BEHAVIOR IN ENCLOSURES
Sound intensity dies out over distance
at constant speed and so delay depending on distance
ough the exact form depends[15] upon many
shape of the room.
ch travels in a straight line from the
m side walls, ceiling etc. Reflected
oud as the direct component [9]
ss than 30ms for good listening
me interval is perceived as one impression in a
decrease in the energycontent of successive reflections, so that the
The persistence of sound in a room after it’s turned
soon as a sound is produced it travels
s reflected and re-reflected.
alls of different types, Sabine carried out a
at an empirical relationship between the
and a quantity which he
Other phenomena of sound in enclosures include refraction, absorption, transmission, diffraction
due to the scope of this paper.
NG SOUND BEHAVIOR IN ENCLOSURES
depending on distance to perceiver.
The perceiver in a room will also hear the echo resulting from the walls, ceiling, and
floors, low frequency sounds tend to have lower efficiencies due to their harmonics of a
single sound travel differently.
Part of the sound wave curves around the edge of a barrier which is known as edge
diffraction.
Barriers that obstruct or prevent sound transmission cause sound shadow to occur.
Surface absorption of direct and reflected sound.
The angle of incidence is equal to the angle of reflection.
4.0 GEOMETRICAL ANALYSIS OF SOUND RAY
In analyzing the behavior of sound in any enclosed space, the concept of a sound ray and the
geometrical study ofsound ray paths play an important role, more so in the design of large rooms
and auditoria.
In tracing sound ray, the wave nature of sound is neglected, and the propagationof sound is
studied like the propagation of light rays. This assumption is valid when the wavelength of
sound is small compared to the area of the surfaces of the room, and large compared with their
roughness. All phenomena due to the wave nature, such as diffraction and interference, are
ignored, since propagation in straight lines is its main postulate.
Three major techniques will be discussed in the geometrical analysis of sound ray. They include;
1. Ray Diagram Graphical Techniques
2. Image Source Technique
3. Hybrid Techniques
4.1 Ray Diagram Graphical Techniques
Ray diagram analysis is used to study the effect of roomshape on the distribution of sound and to
identify surfaceswhich may produce echoes. It’s based on simulating a point source emitting a
large number of rays. The rays are followed on their way through the room either through a
sufficiently long time span or until they hit a surface.[19] Therefore the sound is believed to be
like a particle where certain particles are emanating from the speaker and then they are bouncing
back and forth following the specular reflection rule[20](angle of incidence equals angle of
reflection).
Fig. 7.
4.2 Image Source Technique
Image source or mirror source method is commonly usedf
properties of enclosures. It’s
sound contributions from images of the real
all transmission paths betwee
possible sound reflection paths should be discovere
method. It can determine to a high degree of
direction, the early reflections
feeling of listeners.
4.3 Hybrid Techniques
Computerized prediction techniques continue to be anactive research area. Also,
prediction of room acoustical responses still rely
As an example, the two leading c
CATT Acoustic and Odeon; both
computer programs for room acoustic
being hybrid, they comprise
Fig. 7.Sound ray tracing Technique
Image Source Technique
Image source or mirror source method is commonly usedfor the analysis of the acoustic
s based on regarding all reflections from the boundary surfaces as
butions from images of the real source. The strength of this method is that it covers
transmission paths between source and receiver. To model an ideal impulse response al
paths should be discovered and this is guaranteed
n determine to a high degree of accuracy in terms of level, arrival time and
early reflections, which are the most important in the subjective hearing and
Fig 8.Image Method Technique
Computerized prediction techniques continue to be anactive research area. Also,
acoustical responses still rely on the ray tracing technique to a large extent.
ading commercial software that are used by practitioners today, i.e.
Acoustic and Odeon; both use variants of the ray tracing technique
uter programs for room acoustic predictions are based on models that we characterize as
e elements from ray tracing method as well as from im
or the analysis of the acoustic
l reflections from the boundary surfaces as
is method is that it covers
an ideal impulse response all the
d and this is guaranteed by the image
erms of level, arrival time and
subjective hearing and
Computerized prediction techniques continue to be anactive research area. Also, computerized
on the ray tracing technique to a large extent.
ed by practitioners today, i.e.
use variants of the ray tracing technique A number of
models that we characterize as
method as well as from image source
method. An important aspect when developing such programs is to reduce computing time. A
common practice is based on initially finding available image sources by following ray
trajectories, and thereby one is testing whether these reflection sequence will contribute to the
energy in a given receiver position in the same manner as when using image source method.
However, application of such technique is limited by time and lack of availability. This research
investigating the most common simple shapes in lecture halls, so I’ve used both Ray tracing and
Image source method which are reliable techniques in such case.
5.0 RESEARCH INTO SOUND BEHAVIOUR IN DIFFERENT SHAPES OF SPACES
A thesis report was done by Nada Mohamed Ibrahim Yousif, in The Sudan University of science
and technology. It was his final Master’s degree thesis presentation, July 2011. The following
information were gathered from the thesis as he researched into the acoustics in various shapes
of halls that could possibly be designed.
The primary purpose of the research was to investigatethe relationship between speech
intelligibility and architectural features in lecture halls. Therefore different possible lecture hall
shapes: square, rectangle, rhombus, fan-shaped, horseshoe and polygons were examined, in
addition to evaluating quantitatively their differences acoustically. The architectural features of
these shapes were also simplified for the computational analysis. The hall seating capacity was
adjusted in order to provide the same condition in each shape, because seating capacity has a
major effect on the absorption area which in turn influences the RT, and so there will be
adequate sound level throughout the hall without sound reinforcement systems. Generally the
hall capacity less than 500 seats, speech will be well heard in the rear part of the hall [5].
Therefore, seating capacity about 400 seats was considered in this study.
5.1 Measurement Method:
Measurements were taken for various acoustical parameters along a pre-arranged grid
comprising nine spaced points in each shape to uniformly represent the entire lecture hall. Each
shape was composed of three position rows: front, middle, and rear, and each row were divided
by three parts: right, center and left side. ArchiCADv12 was used to design all of the shapes. In
this research, the sound source was placed 1.20m set back from the edge of the stage, and raised
1.10m above the stage floor. Level of normal speech from the speaker is taken as equal to 65dB.
Acoustical analysis was applied at horizontal and cross sections of typical listening positions
using both Ray diagram and Image source method as clearly seen in the appendices. Each shape
is examined for diffusion, loudness and associated acoustic defects, through the calculation of
possible reflection paths to typical listener positions, giving a predictable data that will stand up
to testing and verification.
5.2 Shapes Configuration:
The following abbreviations are used for each model:
FNL : Fan shape with linear background wall
FNC : Fan shape with curved background wall
HS : Horse shoe shape
FR : Flat rectangle shape
DR : Deep rectangle shape
SQ : Square shape
RH : Rhombus shape
PN : Pentagon shape
HX : Hexagon shape
OC : Octagon shape
The acoustic parameters used were:
1. Path length
2. Total internal surface
3. Absorption area
4. Volume
Fig. 9. values of volume per seat for each shape (m³). Source: Madan Mehta- Architectural Acoustics, page 238
5.3 Regular shapes, fan-shape and horseshoe shape
For the sake of the scope of this paper, little or no emphasis will be given to the rectangular
shaped halls observed in this experiment; nevertheless it will be referred to in comparisms to be
made.
Sound pressure level mean values in four shapes werecompared in Figure (10). The mean values
in each shape ranged from 47.9 to 50.5dB. The flat rectangle and rhombus had the highest mean
value of 50.5 dB & 49.3dB respectively, probably due to the short path length of the direct
sound. However, the SPL values at different positions varied in deep rectangle and square
shapes.
Fig. 10.Mean values of sound pressure level for flat rectangle, square, deep
rectangle and rhombus
Fig. 11 Sound pressure levels at front, middle and back seats for Flat rectangle, square, deep rectangle and rhombus
Table 1 Descriptions of Fan shapes & Horsethe average of
Fig 12 Mean values of sound pressure level for FNL, FNC and HS
When the sound pressure level values were compared
value of 52.9dB was found at front pos
shaped with linear back wall and fanshaped
51.8 dB respectively. Fan-shaped ge
according to the small amount of
Fig 13 comparison of normalized values of
Descriptions of Fan shapes & Horse-shoe shape [RT mid givesthe average of reverberation time at 500Hz and 200Hz]
Mean values of sound pressure level for FNL, FNC and HS
When the sound pressure level values were compared at front, middle and back, the highest
52.9dB was found at front position of the horse shoe shaped, at this position,
linear back wall and fanshapedwith curved back wall had the values of 52.2 and
shaped geometrical features provide more
l amount of absorption provided and the small amount of volume/seat.
comparison of normalized values of the mean free path in FNL, FNC
shoe shape [RT mid gives reverberation time at 500Hz and 200Hz]
Mean values of sound pressure level for FNL, FNC and HS
at front, middle and back, the highest
ition of the horse shoe shaped, at this position, the fan-
ack wall had the values of 52.2 and
ometrical features provide more reverberant energy
and the small amount of volume/seat.
the mean free path in FNL, FNC and HS.
Fig 14.Comparison of the absorption area needed in FNL, FNC and HS
First reflections of sound come to the audience fromdifferent dir
source, due to the short mean free paths
and reflected sound leads to differen
sensation” or spatial impression.
shoe, acoustical defects are more likely to occur, which suggest
used and less reverberant energ
back wall were 4.4, 3.6 respectively,
wall, again it suggests more reverberant energy in fan
5.4 Polygon shapes
Table 2.Descriptions of Polygon shapes [RT mid gives the average ofreverberation time at 500Hz and 200Hz]
Sound pressure level mean values in three shapes were
from 48.7 to 49.9dB. However the three shapes have similar SPL values
Comparison of the absorption area needed in FNL, FNC and HS
reflections of sound come to the audience fromdifferent directions, also directly from the
short mean free paths in such a way the perception at the sa
sound leads to different stimulations to the two ears, creating the so
or spatial impression. While in the fan-shaped hall with curved back wall and horse
more likely to occur, which suggest more absorptio
energy. The volume/seat in horseshoe and fan
back wall were 4.4, 3.6 respectively, while it was only 2.5 in the fan-shaped with linear back
everberant energy in fan-shaped [with linear back wall].
Descriptions of Polygon shapes [RT mid gives the average ofreverberation time at 500Hz and 200Hz]
Sound pressure level mean values in three shapes were. The mean values in each shape
9.9dB. However the three shapes have similar SPL values
Comparison of the absorption area needed in FNL, FNC and HS
ections, also directly from the
the same time of direct
creating the so-called “spatial
shaped hall with curved back wall and horse
more absorption materials to be
seat in horseshoe and fan-shaped with curved
shaped with linear back
[with linear back wall].
Descriptions of Polygon shapes [RT mid gives the average of
The mean values in each shape ranged
9.9dB. However the three shapes have similar SPL values with slightly higher
level in hexagonwith a value of 49.9dB due to the less attenuation of the direct sound. This
suggests high sound level in hexagon.
Fig. 15.Comparison of sound pressure level in Pentagon, Hexagon and Octagon
Sound pressure level mean values in three shapes werecompared. The mean values in each shape
ranged from 48.7 to 49.9dB. However the three shapes have similar SPL values with slightly
higher level in hexagonwith a value of 49.9dB due to the less attenuation of the direct sound
Fig 15.Comparism in sound pressure level Fig 16 sound pressure level at different positions.
CONCLUSION
It is an undeniable truth that the best types of space shapes for auditoriums are rectangles, however with
the findings that this paper has been able to present, it is possible to have other shapes, yet with good
acoustic values.
REFERENCES
Evaluations of geometric configurations on the acoustic quality of auditoria, Nada Mohamed Ibrahim Yousif, 2011. W.C. Sabine, Collected Papers on Acoustics, New York: Dover Publications, 1921.
O.O. Ogunsote, Lecture Notes, Arc 507: Environmental Control III (Acoustics and Noise Control), Federal University of Technology, Akure, 2007