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A
Write-up
On the
Reverberation in lecture theatres
FEDERAL UNIVERSITY OF TECHNOLOGY AKURE,
AKURE, ONDO STATE
NIGERIA.
By
ARC/09/7358 AKINWALE OYINLOLUWA B
ARC/09/7361 OBONGIFREKE AKPAN
Submitted to
THE DEPARTMENT OF ARCHITECTURE
SCHOOL OF ENVIRONMENTAL TECHNOLOGY,
FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE, ONDO STATE.
IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF BACHELOR OF TECHNOLOGY IN
ARCHITECTURE
Supervised by
PROF. OLU OLA OGUNSOTE.
JULY 2014
2
TABLE OF CONTENTS
COVER PAGE
TABLE OF CONTENTS
ABSTRACT
CHAPTER ONE
Broad –spectrum knowledge of reverberation
1.0 INTRODUCTION
1.1 reverberation and the art of architectural acoustics
1.2 reverberation time
CHAPTER TWO
2.0 REVERBERATION IN LECTURE THEATRES
2.1 reverberation in small lecture theatre, FUTA.
2.2 Reverberation in big lecture theatre, FUTA
3.0 SUMMARY
4.0 REFERENCE
3
ABSTRACT
This paper describes our experience with lecture halls/ theatres with regard to the
reverberation within the halls, sound reinforcement and audio frequency induction loops for
persons with hearing impairment.
The optimum reverberation time and geometrical conditions for lecture rooms are well
known. Recent research basically confirms this knowledge but indicates the need for shorter
reverberation times. However, many existing lecture halls do not fulfill these requirements.
Furthermore, too many existing reinforcement systems are deficient in a number of aspects.
This write-up makes a comparison of the level of reverberation in three lecture theatres,
precisely, SMALL LECTURE THEATRE AND BIG LECTURE THEATRE, Federal University
of Technology, Akure, Ondo State, as well as the in finishes and construction materials, furniture
type and arrangements, in other to give an overall analysis of the perceived acoustic properties
of the hall.
In addition, the write up is expected to propose ways of improving the acoustic properties
of the hall with respect to the outcome of the analysis carried out in other to create a long
reverberation time free zone and an acoustic friendly environment for the users.
4
CHAPTER ONE
Broad – spectrum knowledge of reverberation
1.0 INTRODUCTION
The question of the reverberation in lecture rooms has been researched internationally in
recent years. The influence of long reverberation time on the students and teachers, for example,
was investigated in an important study by Wallace Sabine (1868-1919).
In various countries the latest knowledge is being incorporated into the respective
standards and recommendations. Of course, good acoustics are also important in lecture halls.
The background noise must be minimized and the room form and materials must be designed so
as to support the acoustics in order to provide high speech intelligibility.
Since the number of seats in a lecture hall is generally higher than in a small classroom
the achievement of good speech acoustics is more complex. In addition, a sound reinforcement
system fulfilling demanding criteria is usually required. Actually, the basics of good acoustics
for lecture halls are well known.
Nevertheless noisy lecture halls with excessive reverberation, echoes or flutter echoes
are common. The speech intelligibility is correspondingly poor. Although high quality
components are generally employed for the sound system, the results, unfortunately, are still
unsatisfactory. Often the specification and reinforcement concept do not adequately consider the
interplay between room and electro acoustics.
5
1.1 REVERBERATION AND THE ART OF ARCHITECTURAL ACOUSTICS
In 1895 Harvard University, that school situated on the Charles River, opened its Fogg
Art Museum. This wonderful building, still much used today, contained an equally wonderful,
large lecture hall. Audiences flocked in, eager to experience intellectual enlightenment in
beautiful new surroundings. But there was a large, ugly fly in this intellectual ointment: No one
could understand any of the lectures that were given in Fogg lecture hall. And it wasn't because
the lectures were too esoteric, or because the lecturers mumbled into their beards, or because the
students were too distracted. The problem was acoustics and hearing. For help, Harvard turned to
one of its own, ( , then a 27 year-old Assistant Professor of Physics. To that point in his career,
Sabine had distinguished himself primarily through his teaching; research productivity was not
his strongest suit. Even though Sabine had not previously worked with this sort of problem, his
writings show that he was a remarkably quick study and a truly dedicated scientist. His published
work also provides a model of good, clear scientific prose. After diagnosing and then solving
Harvard's problem with the Fogg lecture room, Sabine devoted the rest of his career to the new
field that he had virtually created in the process: architectural acoustics.
First, Sabine noted that when anyone spoke in the Fogg Museum lecture room, even at
normal conversational levels, the sound. of his or her voice remained audible in the room for
several seconds thereafter. You can imagine how difficult this made it to comprehend what the
speaker was saying. The speaker's words first reached the listener directly. Then several,
successive acoustic "copies" of the speaker's words reached the listener. These "copies" were
produced by strong sound reflections from various hard surfaces in the room. If enough surfaces
produced strong, audible reflections, listeners would be bombarded by several versions of
lecture, each version only slightly delayed in time relative to its predecessors. Sabine found that
when a lecturer spoke at a normal conversational level and then stopped, his or her last words
remained audible for about 5.5 seconds. During these 5.5 seconds, if the speaker had not stopped,
he or she might have uttered an
additional 12-15 words. A listener, then, had to contend with a real jumble: a mixture of i)what
the speaker was saying right now, and ii)everything else that the speaker had said during the
previous 5.5 seconds.
6
This was a definitely low-tech operation. With just a stop watch, highly trained auditors,
and a portable pipe organ pipe for
equipment, Sabine used compressed air to create sounds of any steady level. Upon shutting off
the compressed air flowing
through the organ pipe, he measured how long it took for the sound to become audible. He found
first that it didn't make
much difference where the auditor stood, or where in the hall the sound source was. As a result,
his findings had considerable generality. We can translate Sabine's basic finding into modern
units. Recall that normal conversational-level speech remained audible for
5.5 seconds. Since normal conversational level is about 1,000,000 times above threshold
intensity, Sabine's measurements
meant that it took 5.5 seconds for the sound level to drop by a factor of 1,000,000 (60 db).
Sabine made thousands of measurements in Fogg Hall. And he made similar numbers of
measurements in other rooms atHarvard. He found that in different rooms, sound took different
amounts of time to die away --to drop by 60 db. Sabine used the term to signify the time that is
needed for a sound's intensity to drop by 60 db. Sabine discovered that reverberation time
increased with a hall's volume; he also found that a hall's reverberation time could be drastically
changed by the hall's contents and building materials.
1.2 REVERBERATION TIME
In October 29, 1898, Sabine derived the expression that relates reverbation time to two key
variables. In his expression, T is
the time required for the residual sound to decay below audible intensity, starting from a
1,000,000 times higher initial intensity:
T = 0.161 V/A
where V is the room's volume in cubic meters, and A is the total absorption in square meters.
More than a century later,
Sabine's equation remains a foundation of architectural acoustics.
Studying various rooms that had been judged to be acoustically good, Sabine discovered that
good halls tended to have reverberation times of 2-2.25 seconds .
7
CHAPTER TWO
2.0 REVERBERATION IN LECTURE THEATRES
Reverberation is simply the persistence of sound in an enclosed space in an enclosed
space after the sound source has been removed or the sound has stopped while reverberation
time is the time required for a loud sound to be inaudible after turning off the sound source.
Reverberation is dependent on the total room absorption (a) by the seats and audience as
well as the volume of the room (v). unanimously, the reverberation time (t) is directly
proportional to the volume of the room.
That is
t=0.05v/a
The volume of the lecture theatre accessed by the ceiling height determines the
reverberation time of sounds, which in turns determines speech intelligibility.
Figure 1: chart showing the relationship between reverberation time and volume of space.
However, to expediently have a broad-spectrum of reverberation in lecture theatres, a
comparison of the level of reverberation in various lecture theatres must be carried out.
8
2.1 REVERBERATION IN SMALL LECTURE THEATRE, FUTA.
The Small LT. is designed as a bungalow; it functions presently as a lecture hall for all
students according to their fixed lecture hours, accommodating ideally at most 200 students at a
time.
The building consists of the Main hall, main entrance porch, preparation space (back
stage), stores and toilets. The internal dimension of the main hall is 9mx12m long; its headroom
is approximately 7m, it design is such that the seating area slopes downwards towards the stage
or lecture front, giving the hall a theatrical effect.
The stage is raised 0.4m above floor level. The hall is accessible from all four sides, its
main entrance located at the northern part (behind the seats), two side entrances located near the
stage and an entrance to back stage. The roof is in form of a simple gable shape covered with
parapet wall slab.
Plate 1: Right Side Elevation Plate 2: Rear Facade
9
Plate 3: 600mm X 600mm Cellotex Plate 4: Leather Finish Cushion Chairs
Ceiling Finish
Plate 5: Heavy Carpet on Concrete Platform
Finishing materials
Floor finish Terrazzo on concrete
Ceiling finish 600mm x 600mm Acoustic ceiling boards
Wall finish Painted Plastered sandcrete block wall
Podium floor finish Heavy carpet on concrete
Windows Natural anodized frame with 6mm clear float
glass
Figure 2: Table Showing Finishing Materials Used In Small Lecture Theatre.
10
Figure 3: Floor Plan of Small Lecture Theatre
Figure 4: reflected ceiling plan of small lecture theatre.
11
Unanimously, the finishes used in the small lecture theatre goes a long way in
influencing the reverberation time in the lecture hall due to the absorption coefficient of the
materials at various frequencies.
Volume of small lecture theater= length x width x height =
16.1m x 15.1m x 6.0m = 1458.66 cube meters
Reverberation time in small LT= 0.05v/a
Where v= the volume of the lecture hall= 1458.66 cube meters
And a= the total absorption in square meters = 86 square meters
(0.05x 1458.66)/ 86= 0.85seconds
2.2 REVERBERATION BIG LECTURE THEATRE
The building is designed as a lecture theatre with a maximum capacity of 450 persons
seated. It has a rectangular geometry with the longer side parallel to the small lt. On the interior
space, a podium is provided at the front of the theatre for lecture purpose. The theatre is divided
into two rows by middle and two side aisles which provide for easy circulation. There is a
150mm riser demarcating each pews in the sitting area .these is done for direct vision for each
seating arrangement of the students along a row of seats, and also for direct line of auditory
perception for the listeners.
The space has two escape routes at the back of the seating for easy evacuation of users in
case of emergency.
The front area of the building has a space used as a mini canteen and the business centre. While
the back area of the building has business centres at the lower and upper floor connected with a
geometrical staircase.
The building is properly landscaped with the planting of trees, shrubs, provision of
ramped walkways at the sides i.e. the proper use of hard and soft landscape elements. The
building covers an area of 522450000sq metres. The internal dimensions for the lecture space are
23025m by 17550m: the external dimensions of the building are 29025m by18000m.
12
Plate 6: picture showing the acoustic ceiling finish plate 7: heavy carpet on podium
Plate 8: clear float glass as window finish plate 9: Students In The Lecture Theater
13
Figure 5: Floor Plan of the Big Lecture Theatre
Volume of BIG LT= length x width x height
= 23.1m x 17.6m x 7.2m= 2927.23 cube meters
Reverberation time = 0.05v/a
= ( 0.05 x 2927.23) /130= 1.12 seconds
Hence, the inference from the study carried out is that, the reverberation time in most lecture
theatres is quite too long and the recommended reverberation time for lecture halls is 0.2 seconds
for the purpose of achieving speech intelligibility.
The chief means of achieving the short reverberation time is to reduce the head room of the
lecture room, and also taking cognizance of the finishes used because the total absorption
coefficient of the materials go a long way in counter – balancing long reverberation time in
lecture halls with large volume.
14
3.0 SUMMARY
Lecture halls should be designed according to the latest knowledge concerning acoustical
requirements. This can be accomplished with reasonable constructive and instrumental
expenditure.
In the case of renovations, for various reasons the room acoustics of lecture halls often do
not meet the requirements. The reverberation time is much too long. The sound system must then
be especially carefully designed. For most of the seats a fairly good speech intelligibility
can be achieved. However if the acoustical design of the room is poor then today’s requirements
for speech intelligibility often cannot be met for all the seats, even with a very sound good
system.
The problems of hearing impaired persons should be considered routinely by employing
assistive listening technology. For lecture halls, however, this requires careful planning.
15
4.0 REFERENCES
D. MacKenzie, S. Airey, ’Classroom Acoustics. A research project. Summery report.’ Heriot-
Watt University, Edinburgh, 1999
J. Seidel, L. Weber, P. Leistner, ’Acoustic properties in German class rooms and their effect
on the cognitive performance of primary school pupils’, ForumAcusticum 2005, Budapest
J. M. Klatte, M. Wegner, J. Hellbrück, ’Noise in the school environment and cognitive
performance in elementary school children. Part B - Cognitive psychological studies’, Forum
Acusticum 2005, Budapest Acoustical Society of America, ASA. ’Classroom acoustics. A
resource for creating learning environments with desirable listening conditions.’. Acoustical
Society of America, 2 Huntington Quadrangle Melville, NY 11747, 2000
DIN 18041:2004-05, ’Hörsamkeit von kleinen und mittleren Räumen. (Acoustical quality in
small to medium-sized rooms.)’, Beuth Verlag G