TABLE OF CONTENTS 1 LIST OF FIGURES 1 LIST OF TABLES 2 INTRODUCTION 2 REVERBERATION AS PHENOMENON 2 TYPES OF REVERBERATION 5 REVERBERATION CHAMBERS 5 SPRING REVERBERATION 5 PLATE REVERBERATION 5 DIGITAL REVERBERATORS 5 DIGITAL PROCESSING TECHNIQUES 6 GATED REVERBERATION 6 REVERSE REVERBERATION 7 REVERBERATION TIME 7 THE SABINE EQUATION 8 MEASUREMENT OF REVERBERATION TIME 9 PROCEDURES FOR CALCULATING REVERBERATION TIME 9 ABSORPTION COEFFICIENT 9 CASE STUDY I 10 CASE STUDY II 10 SOME EXAMPLES OF REVERBERANT SPACES 12 DIRECT AND REVERBERANT SOUND 12 DIRECT AND REVERBERANT SOUND FIELDS 13 ARTIFICIAL REVERBERATION 13 CONCLUSION 14 REFERENCES 15 LIST OF FIGURES REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE O.M./AWOSOLA A.A. 1
Transcript
TABLE OF CONTENTS 1LIST OF FIGURES 1LIST OF TABLES 2INTRODUCTION 2REVERBERATION AS PHENOMENON
THE SABINE EQUATION 8MEASUREMENT OF REVERBERATION TIME
9PROCEDURES FOR CALCULATING REVERBERATION TIME 9ABSORPTION COEFFICIENT
9CASE STUDY I 10CASE STUDY II 10SOME EXAMPLES OF REVERBERANT SPACES 12DIRECT AND REVERBERANT SOUND 12DIRECT AND REVERBERANT SOUND FIELDS 13ARTIFICIAL REVERBERATION 13CONCLUSION 14REFERENCES 15LIST OF FIGURESFigure.1: Sound waves travel many different paths before reaching your ears.
3Figure. 2: Multiple reflections of a sound impulse as heard by a listener.
3Figure. 3: Impulse response of a room.
4
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
7Figure. 8: Effect of surfaces on reverberation time. 9Figure.9: A room impulse response with the pre-delay parameters labeled. 13LIST OF PLATEPlate 1: Showing exterior (shape) and interior (space) of ETF Lecture Theatre FUTAkurePlate 2: Showing Wes Brook 100 Lecture Theatre, UBC- Vancouver, BCLIST OF TABLESTable 1: Examples of Reverberant reverberation
INTRODUCTIONReverberation (reverb for short) is almost certainly one of the most
heavily used effects in lecture theatre, many will immediately think of a signal processor or the reverb knob on their amplifier. However, many people do not realize how important reverberation is, and that we actually hear reverberation every day, without any special processors. Reverberation is simply the persistence of sound in an enclosed space because of repeated reflection or scattering, after the sound source has stopped.
The word Reverberation comes from the Latin word reverberate (meaning to “beat back”) and has been observed although only in about 1900 was a quantitative method achieved for measuring and predicting this. The ability to quantify reverberation comes from conceiving sound as energy that is 'soaked up' by absorbent surfaces or escapes through openings.
Hard surfaces reflect sounds back. Open windows let them escape. From a listener's perspective, with acute and spatially accurate hearing, the sound that comes back to you is a major way of understanding the nature and properties of inhabited space.
It is tempting to define reverberation as a series of echoes, but this is not correct. 'Echo' generally implies a distinct, delayed version of a sound, as you would hear with a delay more than one or two-tenths of a second. With reverberation, each delayed sound wave arrives in such a short period of time that each reflection is not perceived as a reproduction of the original sound. Even though not every reflection can be discerned, the effect exhibited by the entire series of reflections is still heard. However minimal, the reverberant quality of any space, whether enclosed or not, helps to define the way in which it is perceived. Although it may not be realized consciously, reverberation is one
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
of many cues used by a listener for orientation in a given space. If so many reflections arrive at a listener that he is unable to distinguish between them, the proper term is reverberation.
REVERBERATION AS PHENOMENOMArchitecturally, Reverberation is the persistence of sound in an enclosed
space (such as a room or auditorium) after a source of sound has stopped. Reverberation is the repetition of sound via reflection from a plane. It is the persistence of sound in a particular space after the original sound is removed. When sound is produced in a space, a large number of echoes build up and then slowly decay as the sound is absorbed by the walls and air, creating reverberation, or reverb. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude, until they can no longer be heard. Large chambers, especially such as cathedrals, gymnasiums, indoor swimming pools, large caves, etc., are examples of spaces where the reverberation time is long and can obviously be heard. Different types of music tend to sound best with reverberation times appropriate to their characteristics.
Reverberation is a result of multiple reflections. A sound wave in an enclosed or semi-enclosed environment will be broken up as it is bounced back and forth among the reflecting surfaces. Reverberation is, in effect, a multiplicity of echoes whose speed of repetition is too quick for them to be perceived as separate from one another. Reverberation will also increase the ambient noise level and apparent loudness of sounds within a space, an important factor to consider in the acoustic design of classrooms, daycare areas, office and industrial spaces. Reverberation will also make speech indistinct by masking the onset transients, but with many types of music, particularly symphonic, reverberation adds to the blend of the individual sounds when the reverberation time is 1 - 2 seconds. Longer times tend to blur the sounds and require slower tempo to avoid indistinctness. Reverberation times of less than a second are necessary for speech comprehension.
Architectural acousticians stress the importance of early reflections (arriving within the first 80 ms) which reinforce the direct sound as long as the angle of reflection is not too wide. Reflections arriving after 80 ms add reverberant energy, which is often described as giving the sound spaciousness, warmth and envelopment. The acoustic design of such spaces usually involves creating a balance between clarity and definition on the one hand, and spaciousness on the other. Listeners often have different preferences as to this balance.
Figure.1: Sound waves travel many different paths before reaching your ears
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
Figure. 2: Multiple reflections of a sound impulse as heard by a listener.
In reverberation, for a short period after the direct sound, there is generally a set of well-defined and directional reflections that are directly related to the shape and size of the room, as well as the position of the source and listener in the room. These are the early reflections. After the early reflections, the rate of the arriving reflections increases greatly, as these are more random and difficult to relate to the physical characteristics of the room. This is called the diffuse reverberation, or the late reflections. It is believed that the diffuse reverberation is the primary factor establishing a room's 'size', and it decays exponentially in good concert halls. A simple delay with feedback will only simulate reflections with a fixed time interval between reflections. An example impulse response for a room, which is depicted in Figure. 3.
Figure. 3: Impulse response of a room.
A further significant cue for the perception of depth and distance is the ratio of direct to reverberated sound. In larger spaces, the intensity of the direct sound falls off more sharply with distance than that of the reverberated sound, and thus the ratio shifts in favour of the latter. In an enclosed space, the reverberation time is proportional to the volume of the space and inversely proportional to the sum of each surface area multiplied by its absorption coefficient.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
Another very important characteristic of reverberation is the correlation of the signals that reaches the ears. In order to give a listener a real feeling of the 'spaciousness' of a big room, the sounds at each ear should be somewhat incoherent. This is partly why concert halls have such high ceilings. With a low ceiling, the first reflections to reach the listener would have bounced off the ceiling and reach both ears at equal time. By using a very high ceiling, the first reflections to reach the listener would generally be from the walls of the concert hall, and since the walls are generally of different distances away, the sound arriving at each ear is different. This characteristic is important for stereo reverberation design. However, excessive reverberation can ruin the acoustical properties of an otherwise well-designed room. A typical record representing the sound-pressure level at a given point in a room plotted against time, after a sound source has been turned off, is given in the decay curve shown in the illustration below. The rate of sound decay is not uniform but fluctuates about an average slope.
REVERBERATION CHAMBERSReverberation Chambers are the latest development in EM testing, for
performing radiated immunity, radiated emission and shielding effectiveness testing. A Reverberation Chamber is much like a large microwave oven, which “cooks” the test object all over. Reverberation Chamber development will produce a test technique that is robust (i.e. highly repeatable, highly reproducible, and technically thorough). An increasing number of Reverberation Chambers are being built around the world, with general agreement on test standards and procedures.
SPRING REVERBERATIONSpring reverberations provide a relatively uncomplicated and inexpensive
method for creating reverberation effects. In amplifiers, the spring reverberations are usually enclosed in a metal box, called the reverberation pan, which is attached to the bottom of the amplifier. The pan takes an audio signal and produces a reverberated version, which is then mixed into the dry signal.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
O.M./AWOSOLA A.A.5
The operation of a spring reverberation is quite simple - the audio signal is coupled to one end of the spring by a transducer, (which is a device that converts energy in one form to another, in this case, electrical and mechanical energy. Some other familiar transducers are the pickups on a guitar, microphones, and speakers). This creates waves that travel through the spring. At the other end of the spring, there is another transducer that converts some of the motion in the string into an electrical signal, which is then added to the dry sound. When a wave arrives at an end of the spring, part of the wave's energy is reflected and stays in the spring. It is these reflections that create the characteristic reverberation sound.
PLATE REVERBERATION
Plate reverberations are not used widely outside of the studio since they are expensive and rather bulky. The setup is similar to a spring reverberation, but instead of being connected to the springs, the two (or more) transducers are connected to different points on a metal plate. These transducers send vibration waves throughout the plate, and reflections occur each time a wave reaches the edge of the plate. The reverberation can be controlled by adjusting the damping of the plate and the location of the transducers.
DIGITAL REVERBERATORS Digital Reverberators use various signal processing algorithms in order to
create the reverberation effect. Since reverberation is essentially caused by a very large number of echoes, simple DSPs use multiple feedback delay circuits to create a large, decaying series of echoes that die out over time. More advanced digital reverberation generators can simulate the time and frequency domain responses of real rooms based upon room dimensions, absorption and other properties. In real music halls, the direct sound always arrives at the listeners’ ear first because it follows the shortest path. Shortly after the direct sound, the reverberant sound arrives. The time between the two is called the 'arrival time gap'. This gap is important in recorded music because it is the cue that gives the ear information on the size of the hall, better digital reverberations can incorporate this arrival time gap and hence sound more realistic. Digital reverberation systems are commonly implemented as software plug-in.
DIGITAL PROCESSING TECHNIQUESReverberation lends to the world of digital computers, Implementations
can be broken down into efficient circular buffers and operations on delay lines. The advances in digital hardware have made reverberation processors available at inexpensive prices that are portable, and quite flexible. Early digital reverberation algorithms tried to imitate a room reverberation by using primarily two types of infinite impulse response (IIR) filters, so that the output would gradually decay. One such filter is the comb filter, which gets its name from the comb-like notches in the frequency response. The other primary filter is the All-pass filter. The All-pass filter has the nice property that all frequencies are passed equally, reducing a coloration of the sound but this only really applies over a long period of time the All-pass filter does affect the phase
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
O.M./AWOSOLA A.A.6
of the signal, so it can shape transients, and also exhibit ringing with abrupt inputs. Schroeder did much of the early work on digital reverberation, and one of his well-known reverberator designs uses four comb filters and two All-pass filters, as shown in figure 8. This design does not create the increasing arrival rate of reflections, and is rather primitive when compared to current algorithms.
Figure. 5: Schroeder’s Reverberator.
GATED REVERBERATIONA gated reverberation is created by simply truncating the impulse
response of a reverberator by changing the IIR filters to FIR, or on the other hand, only allowing a sound to make a certain number of reflections. The amount of time before the response is cutoff is called the gate time, as shown in figure 9. Some reverberation units may allow a more gradual decay of the sound, rather than a sudden silence. Gated reverberations are most commonly implemented with digital processing, and are commonly used on drums.
Figure. 6: Impulse response of a gated reverberation.
REVERSE REVERBERATIONThe reverse reverberation puts a
little twist on reverberation responses as seen above. Instead of simulating reflections that become quieter and gradually fade away, the reverse reverberation simulates reflections where the sound gets louder over time, and then abruptly cuts off. The length of time the sound builds up is often referred to as the reverse time, or the gate time, as it resembles a gated reverberation simply reversed in time. Figure 10 shows an example of what a reverse reverberation impulse response is.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
At times, a reverse reverberation may sound very much like a slap back delay because it ends suddenly, but if you listen closely, you can hear the sound building up.
REVERBERATION TIMEA measure that is used to characterize the reverberation in a room is the
reverberation time. Reverberation Time The reverberant sound in an auditorium dies away with time as the sound
energy is absorbed by multiple interactions with the surfaces of the room. A more reflective surface assists sound to take longer period before dying away and the enclosed space is said to acoustically “live”. A very absorbent room takes shorter period for the sound generated to decay and the room is “dead”. Reverberation time depends on initial loudness of sound and hearing ability of the observer.
Reverberation time is the time for the sound to decay by 60dB (become effectively inaudible) after the power source is shut off.
Reverberation time = time to drop 60dB below original level (Hunt 1978). T = 0.3log10V, (3) where T is the optimum reverberation time in seconds, for speech and V is
the room volume in cubic meters (McGuiness, et al. 1980). The surfaces of the room determine how much energy is lost in each
reflection. Highly reflective materials, such as a concrete or tile floor, brick walls, and windows, will increase the reverberation time, as they are very rigid. Absorptive materials, such as curtains, heavy carpet, and people, reduce the reverberation time and the absorptive nature of most materials generally varies with frequency. One may be able to notice difference on a performance. During the sound check, the room will sound 'bigger', but during the actual performance, the room may not sound as empty. People tend to absorb quite a bit of energy, reducing the reverberation time.
Bigger rooms tend to have longer reverberation times since, on the average; the sound waves travel a longer distance between reflections. The air in the room itself will also attenuate the sound waves, reducing the reverberation time. This attenuation varies with the humidity and temperature, and high frequencies are affected most. Because of this, many reverberation processors incorporate low pass filters.
THE SABINE EQUATIONIn the late 19th century (1890s), Wallace Clement Sabine started
experiments at Harvard University to investigate the impact of absorption on the reverberation time. Using a portable wind chest and organ pipes as a sound source, a stopwatch and a clean pair of ears he measured the time from interruption of the source to inaudibility (roughly 60db). This time varies directly with the dimensions of room but inversely as the absorption present. The Sabine equation predicts the reverberation time as:
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
t (reverberation time) = 0.16v (volume in meters) /a (sum of all absorptions).He established a relationship between the RT60 of a room, its volume, and
its total adsorption (in Sabins). This is given by the equation:
Where c is a mathematical constant valued to be 0.161, V is the volume of the room in m3, S is total surface area of room in m2, a is the average adsorption coefficient of room surfaces, and Sa is the total adsorption in Sabins.
It is worth noting that the total absorption in Sabin generally changes depending on frequency and that the equation does not take into account room shape, dimensions, or losses from the sound traveling through the air. In general, most rooms absorb less in the lower frequencies, causing a longer decay time.
The reverberation time RT60 and the volume V of the room have great influence on the critical distance dc (conditional equation):
Where critical distance rH is measured in meters, volume V is measured in m3, and reverberation time RT60 is measured in seconds.
If the total absorption is not more than about a tenth of the reverberation time i.e. in 'normal' circumstances, it takes no account of the shape of the room although it can be calculated for different wavelengths of sound since materials will absorb different sized sounds at different rates. In general, the reverberation time of a room is proportional to the volume of the room and inversely proportional to the sum of all absorptions.
In reality, every place within a room has its own specific reverberation and is determined by the size and geometry of the space boundaries and the sound being made as well as the acoustic qualities of the materials of enclosure. An example of this is a gothic cathedral, where each bay of the vault is faceted, hard surfaces, which create local pockets of aural focus. So reverberation time is a valuable basic measurement - which often throws up a fundamental issue - it should be seen as one step towards understanding a room's actual acoustics.
MEASUREMENT OF REVERBERATION TIMEReverberation time could be measured using a level recorder (a device
that plots a graph of noise level against time on a ribbon of moving paper). A loud noise is produced, and as the sound dies away, the trace on the level recorder will show a distinct slope. The analysis of this slope shows the measured reverberation time. Modern digital sound level meters carry out this analysis automatically, on digital data.There are two basic methods for measuring the reverberation time of a large room:
Impulsive noise sources such as a blank pistol shot, or balloon burst may be used to measure the impulse response of a room.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
A random noise signal such as pink noise or white noise may be generated through a loudspeaker, and then turned off. This is known as the interrupted method, and the measured result is known as the interrupted response. The sound level in the room is measured as a function on a frequency
analyzer or on a chart recorder. The slope of the sound level as a function of time may then be used to determine the reverberation time, which is often given as a measurement of decay time. Decay time is the time it takes the signal to diminish 60 dB below the original sound
Figure. 8: Effect of surfaces on reverberation time.
The optimum reverberation time for an auditorium or room depends upon its intended use. Around 2 seconds is desirable for a medium-sized, general-purpose auditorium that is to be used for both speech and music. A classroom should be much shorter, less than a second, and a recording studio should minimize reverberation time in most cases for clarity of recording.
PROCEDURES FOR CALCULATING REVERBERATION TIME Measure approximately the total surface area S and volume V of the
room. Identify the individual surfaces making up the total surface area. Look up the absorption coefficients for these surfaces in the octave bands
centered at 125, 250, 500, 1000, 2000, and 4000 Hz. Determine the average absorption coefficient for the room in each of
these octave bands. Calculate the approximate reverberation time from \Eq{RT-60} for each of
these octave bands. Measure the reverberation time in each of these octave bands using the
interrupted noise method.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
O.M./AWOSOLA A.A.10
Measure the reverberation time in each of these octave bands using the impulse method.
ABSORPTION COEFFICIENTThe absorption coefficient of a material is a number between zero (0) and
one (1), which indicates the proportion of sound that is absorbed by the surface compared to the proportion, which is reflected back into the room. A large, fully open window would offer no reflection as any sound reaching it would pass straight out and no sound would be reflected. This would have an absorption coefficient of one (1). On the contrary, a thick, smooth painted concrete ceiling would be the acoustic equivalent of a mirror, and would have an absorption coefficient very close to zero (0).
The reverberation time is strongly influenced by the absorption coefficients of the surfaces as suggested in the illustration in figure 6, but it also depends upon the volume of the room as shown in the Sabine formula.
CASE STUDY 1: BIG LECTURE THEATRE, FUTAKURE
Plaate 1: Showing exterior (shape) and interior (space) of the ETF Lecture Theatre
ANALYSIS………. The ETF lecture theatre indicated echo and reverberation problem at a magnified manner as it could be seen from Fig 9 since the chairs, desks and the windows are not made of materials that can absorb sound substantially, therefore the generated sound energy propagating and hitting them will be reflected and may eventually increase echo and reverberation. The ceilings are made of cellotex materials which is capable of absorbing part of the sound. But the building shape does not permit even distribution of sound. The cellotex is a good sound absorber that reduces reflection and sound waves reverberated rooms for speech require a shorter reverberation time than for music. A longer reverberation time can make it difficult to understand speech. If the reverberation time from one syllable overlaps the next syllable, it may be difficult to identify the words that sound alike, if on the other hand, the reverberation time is too short, tonal balance and loudness may suffer. Reverberation effects are often used in studios to "smooth" sounds; the effect is commonly used on vocals to help remove inconsistencies in pitch.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
CASE STUDY II Wes brook 100 Lecture TheatreUBC - Vancouver, BC
Plate 2: Show Wes brook100 Lecture Theatre, UBC-Vancouer, BCANALYSIS……………..
This Wes brook 100 lecture theatres was planned to be renovated due to its reverberation effect. Planning for the renovation of the 280 seat Wesbrook 100 lecture theatre began in the summer of 1996, with construction slated for summer of 1997. The intent was to develop Wesbrook 100 into a 325 seat lecture theatre that would be suitable for the Sciences faculty plus outside rentals and seminars, utilizing modern video display systems, an improved audio system and a simplified control system using a touch screen.
The original room configuration had problems with excess noise and excessively long reverb time which resulted in poor speech intelligibility and difficulty for the students communicating across the room. The noise control design was done by BKL Consultants, and developed the design for both the acoustical treatment and the system design. The excessive noise was reduced by 20dB to bring it in line with background noise criteria for lecture rooms. The acoustical treatment reduced the reverb time significantly to improve the speech intelligibility of unamplified speech, as well as reinforced speech, indicating very good speech performance, especially critical for listeners.
Below is the table of before and after reverb time values for the lecture room...
The ceiling height is over 7 feet at the back of the room, and 15 feet at the front of the room, which resulted in a substantial variation in sound level with the 60 foot deep room and the original column speakers. The new sound system was changed to a distributed ceiling speaker system using digital signal delays
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
O.M./AWOSOLA A.A.12
and high quality 6.5" loudspeaker devices. A pair of image speakers flanks the projection screens, and the ceiling speakers are delayed to give the image speakers precedence for playback. The sound system is quite transparent in its speech reinforcement, with the signal delays making the instructor the apparent source of the speech.
SOME EXAMPLES OF REVERBERANT SPACES
Basic factors that affect a room's reverberation time include the size and shape of the enclosure as well as the materials used in the construction of the room. Every object placed within the enclosure can also affect this reverberation time, including people and their belongings. Examples of Reverberation TimeReverberation time of some of the most famous halls in the world include
Vienna, Musikvereinsaal : 2.05 seconds Boston, Symphony Hall: 1.8 seconds New York, Carnegie Hall: 1.7 seconds
However, the overall average reverberation time does not tell the whole story. The variation of reverberation time with frequency is also importantRooms for speech require a shorter reverberation time than for music. A longer reverberation time can make it difficult to understand speech. If the reverberation time from one syllable overlaps the next syllable, it may be difficult to identify the words that sound alike, if on the other hand, the reverberation time is too short, tonal balance and loudness may suffer. Reverberation effects are often used in studios to "smooth" sounds; the effect is commonly used on vocals to help remove inconsistencies in pitch.
DIRECT AND REVERBERANT SOUND
In a room with a sound source emitting a constant sound power, the reverberant background sound builds up to a constant level. The equilibrium sound level occurs when the total sound absorption by the boundaries of the enclosure
equals the rate at which sound energy is being injected into the room by the source. Clearly, a room with high absorptive coefficient “A” will have a lower reverberant sound level. A simple formula that can be used to calculate the background reverberant sound level is
Whe re W is the sound power in watts emitted by the sources in the room. W0 is the reference power level of 10-12 Watts. LW is a decibel value.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
O.M./AWOSOLA A.A.13
DIRECT AND REVERBERANT SOUND FIELDS
Direct and reverberant sound fields in a room are important in acoustics. If the direct sound from a source that reaches you is louder than the reflections, you are in the direct field. If, on the other hand, the sound pressure due to the reflected sounds is greater than the direct sound, you are in the reverberant field. The point at which the direct field and reverberant field intensity are the same is called the critical distance. The reverberant field is also important for music. Firstly, it helps one to hear all the instruments in an ensemble, even though some of the performers are farther away from others. In addition, many instruments, such as the violin, do not radiate all frequencies equally in all directions. In the direct field alone, the violin will sound quite different and even unpleasant as you move with respect to the violin. The reverberant field in the room helps to spread out the energy the instrument makes so it can reach the ears. The sketch below depicts the sound received by a single listener as a function of time because of a sharp sound pulse some distance away.
Figure. 9: A room impulse response with the pre-delay parameters labeled.
ARTIFICIAL REVERBERATION
Artificial reverberation is produced by means of a reverberation chamber or echo chamber, multiple tape echo, or more commonly, by exciting a metal spring or plate at one end, and picking up the delayed signal at another point. However, these units tend to have very uneven frequency response, falling off sharply at high frequencies, with the result that the sound is characteristically coloured or blurred. In addition, the echo density (i.e. the number of reflected repetitions per second) is often not high enough to avoid a 'fluttering' of the sound, particularly with very short percussive sounds. However, digital processing devices and computer techniques such as the Schroeder model have been developed in recent years that allow a good simulation of naturally produced reverberation. These systems allow for a variable ratio of direct to reflect sound, and some such as chowning's at Stanford University include both global reverberation (i.e. Reflected sound from all directions) and local reverberation (i.e. that coming from the direction of the sound source). Others allow the frequency spectrum of the reverberation to be
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
O.M./AWOSOLA A.A.14
controlled (e.g. to simulate 'bright' or 'dark' rooms with greater or lesser high frequencies, respectively), or the reverberation to be gated (i.e. Attenuated at the end of the direct sound) or even reversed.
CONCLUSIONIn view of the fact that reverberation determines the characteristic
features of a space, it determines the level of acoustic excellence. This is truly the secret of great acoustics. Awareness of this secret in the early stages of design brings excellent acoustics for new critical-listening space within our society,
UK regulations, Building Bulletin 93 (BB93), states that in lecture rooms (fewer than 50 people) an unoccupied mid frequency (Tmf) reverberation time of <0.8 seconds must be achieved. In lecture rooms (more than 50 people) an unoccupied mid frequency (Tmf) reverberation time of <1.0 seconds must be achieved. In assembly halls and multipurpose halls, even lecture theatre. BB93 states an unoccupied mid frequency reverberation time of between 0.8 - 1.2 seconds must be achieved. It also states "For very large halls and auditoria, and for halls designed primarily for unamplified music rather than speech, designing solely in terms of reverberation time may not be appropriate and specialist advice should be sought."
Another very important characteristic of reverberation is the correlation of the signals that reaches the ears. In order to give a listener a real feeling of the 'spaciousness' of a big room. However, excessive reverberation can ruin the acoustical properties of a well-designed room.
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
O.M./AWOSOLA A.A.15
REFERENCES Effion, W. 1997. McGrawHill Encyclopedia of Science and Technology.
Acoustical Noise Publication. 8th ed., Vol. 1, pp. 76-87, McGraw Hill, New York, NY, USA.
Hunt, R. 1978. Origin of Acoustics. Yale Univ. Press, New York, NY, USA. McGuiness, W.J.; Stein, B; and Reynold, J. 1980. Mechanical and
Electrical Equipment for Building. Fundamentals of Architectural Acoustics (Chapter 26) and Building Noise Reduction (Chapter 27). 6th ed., John Willey and Sons Inc., New York, NY, USA.
Wilson Krieger, (1994): Noise Control: Measurement, Analysis, and Control of Sound and Vibration,,
Knudsen and Harris, (1958): Acoustical Designing in Architecture, reprinted by the Acoustical Society of America, 1978.
Rettinger, (1977): Acoustic Design and Noise Control, Vol.\ I, (Chemical Publishing Co.)
Noise and Vibration Control, (1988): Revised Edition, Edited by Beranek, (Institute of Noise Control Engineering), Chapters 8, 9.
Noise Reduction, Edited by Beranek, (reprinted by Peninsula Publishing, 1988), Chapters 10, 11.
Orfanidis, Sophocles, (1996): Introduction to Signal Processing. New Jersey, Prentice Hall. (Gardner, William. The Virtual Acoustic Room. M.S. MIT, 1992.
Moorer, James. (1979): "About This Reverberation Business". Computer Music Journal, Volume 3, Number 2 13-28.
M.R. Schroeder, (1962):"Natural Sounding Artificial Reverberation," Journal of the Audio Engineering Society, vol. 10, no. 3, pp. 219-223;
John Chowning, (Jan. 1971):"The Simulation of Moving Sound Sources," Journal of the Audio Engineering Society, vol. 19, no. 1, pp. 1-6.
Kinsler, Frey, Coppens, and Sanders, (1982): Fundamentals of Acoustics, Third Edition, (John Wiley & Sons) Chapter 13.
Davis and Davis, (1989) Sound System Engineering,, Second Edition, (Howard W. Sams & Co.)Chapters 7-9.
www.sdngnet.com www.reverberation.com
REPORT ON REVERBERATION IN LECTURE THEATRES PRAPARED BY AKINTUNDE
![FUTA on CHOGM | 08-11-2013 sinhala [Part 01]](https://static.documents.pub/doc/80x56/55cf9b17550346d033a4b0fe/futa-on-chogm-08-11-2013-sinhala-part-01.jpg)
