Federal University of Technology Akure

A

Write-up

On the

Reverberation in lecture theatres

FEDERAL UNIVERSITY OF TECHNOLOGY AKURE,

AKURE, ONDO STATE

NIGERIA.

By

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

TABLE OF CONTENTS

COVER PAGE

TABLE OF CONTENTS

ABSTRACT

CHAPTER ONE

Broad –spectrum knowledge of reverberation

1.0INTRODUCTION

1.1reverberation and the art of architectural acoustics

1.2reverberation time

CHAPTER TWO

2.0REVERBERATION IN LECTURE THEATRES

2.1reverberation in small lecture theatre, FUTA.

2.2 Reverberation in big lecture theatre, FUTA

3.0SUMMARY

4.0REFERENCE

ABSTRACT

This paper describes our experience with lecture halls/ theatres with regard to the reverberation within the halls, sound reinforcementand audio frequency induction loops for persons with hearing impairment.

The optimum reverberationtime and geometrical conditions for lecture rooms are well known. Recent research basically confirmsthis knowledge but indicates the need for shorter reverberation times. However, many existing lecture hallsdo not fulfill these requirements. Furthermore, too many existing reinforcement systems are deficient in anumber 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.

CHAPTER ONE

Broad – spectrum knowledge of reverberation

1.0INTRODUCTION

The question of the reverberation in lecture rooms has been researchedinternationally in recent years. The influence oflong 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 beingincorporated into the respective standards and recommendations. Of course, good acoustics are also important in lecturehalls. The background noise must be minimized and theroom form and materials must be designed so as to supportthe acoustics in order to provide high speech intelligibility.

Since the number of seats in a lecture hall is generallyhigher than in a small classroom the achievementof good speech acoustics is more complex. In addition, asound reinforcement system fulfilling demanding criteriais usually required.Actually, the basics of good acoustics for lecture halls arewell known.

Nevertheless noisy lecture halls with excessivereverberation, echoes or flutter echoes are common.The speech intelligibility is correspondingly poor. Althoughhigh quality components are generally employedfor the sound system, the results, unfortunately, are stillunsatisfactory. Often the specification and reinforcementconcept do not adequately consider the interplay betweenroom and electro acoustics.

1.1REVERBERATION AND THE ART OF ARCHITECTURAL ACOUSTICS

In 1895 Harvard University, that school situated on the Charles River, opened its Fogg Art Museum. This wonderfulbuilding, still much used today, contained an equally wonderful, large lecture hall. Audiences flocked in, eager to experienceintellectual 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 inFogg lecture hall. And it wasn't because the lectures were too esoteric, or because the lecturers mumbled into their beards, orbecause 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 ofPhysics. To that point in his career, Sabine had distinguished himself primarily through his teaching; research productivity wasnot his strongest suit. Even though Sabine had not previously worked with this sort of problem, his writings show that he wasa remarkably quick study and a truly dedicated scientist. His published work also provides a model of good, clear scientificprose. After diagnosing and then solving Harvard's problem with the Fogg lecture room, Sabine devoted the rest of his careerto 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 tocomprehend what the speaker was saying. The speaker's words first reached the listener directly. Then several, successiveacoustic "copies" of the speaker's words reached the listener. These "copies" were produced by strong sound reflections fromvarious hard surfaces in the room. If enough surfaces produced strong, audible reflections, listeners would be bombarded byseveral 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 remainedaudible 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 rightnow, and ii)everything else that the speaker had said during the previous 5.5 seconds.

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 considerablegenerality.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 roomsatHarvard. He found that in different rooms, sound took different amounts of time to die away --to drop by 60 db. Sabine usedthe term to signify the time that is needed for a sound's intensity to drop by 60 db. Sabine discovered thatreverberation time increased with a hall's volume; he also found that a hall's reverberation time could be drastically changed bythe 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 havereverberation times of 2-2.25 seconds .

CHAPTER TWO

2.0REVERBERATION 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 offthe 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.

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

Plate 3: 600mm X 600mm Cellotex Plate 4: Leather Finish Cushion Chairs

Ceiling Finish

Plate 5: Heavy Carpet on Concrete Platform

Finishing materials

Figure 2: Table Showing Finishing Materials Used In Small Lecture Theatre.

Figure 3: Floor Plan of Small Lecture Theatre

Figure 4: reflected ceiling plan of small lecture theatre.

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. Thebuilding 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.

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

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.

3.0 SUMMARY

Lecture halls should be designed according to the latestknowledge concerning acoustical requirements. This canbe accomplished with reasonable constructive and instrumental

expenditure.

In the case of renovations, for various reasons the roomacoustics of lecture halls often do not meet the requirements.The reverberation time is much too long. Thesound 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 theroom is poor then today’s requirements for speech intelligibilityoften cannot be met for all the seats, even witha very sound good system.

The problems of hearing impaired persons should beconsidered routinely by employing assistive listeningtechnology. For lecture halls, however, this requires carefulplanning.

4.0 REFERENCES

D. MacKenzie, S. Airey, ’Classroom Acoustics. A researchproject.Summery report.’Heriot-Watt University,Edinburgh, 1999

J. Seidel, L. Weber, P. Leistner, ’Acoustic propertiesin German class rooms and their effect on the cognitiveperformance of primary school pupils’, ForumAcusticum 2005, Budapest

J. M. Klatte, M. Wegner, J. Hellbrück, ’Noise in theschool environment and cognitive performance in elementaryschool children. Part B - Cognitive psychologicalstudies’, Forum Acusticum 2005, Budapest Acoustical Society of America, ASA. ’Classroomacoustics. A resource for creating learning environmentswith desirable listening conditions.’. Acoustical

Society of America, 2 Huntington QuadrangleMelville, NY 11747, 2000

DIN 18041:2004-05, ’Hörsamkeit von kleinen undmittlerenRäumen. (Acoustical quality in small tomedium-sized rooms.)’, BeuthVerlag G

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