Architectural Acoustics

by Vern O. Knudsen November 1963

 

Sound is as much a part of man's man-made environment as heat or light. It can now be' effectively managed, notably in rooms where music is heard, by applying the principles of acoustical physics

 

 

          The opening of a large concert hall these days is almost inevitably followed by a spate of reports, reviews, criticisms and opinions about its acoustical qualities. Amateurs and com­petent critics alike try to compare the music heard in the new hall with their recollection of the same or similar music heard in concert halls of acknowledged acoustical excellence. This exercise in auditory memory is not easy, and it gives rise to many pretentious statements. Yet the fact remains that, as in wine tasting, the subjective evaluation of experts is the court of last appeal. For this reason architectural acoustics is an art as well as a science. If a new concert hall shows palpable deficiencies, as sometimes hap­pens even today, the impression is strengthened that the science of acous­tics has failed, or at least has been found wanting. Such a judgment is much too harsh. What usually happens in such cases is that the available knowledge, for a whole complex of reasons, has not been adequately applied. For example, critics reported serious deficiencies in the acoustics of Philharmonic Hall in New York, which opened a year ago. The original acoustical engineers and an in­dependent team of consultants have now established these deficiencies by objec­tive methods, and a number of changes have been made in the auditorium's de­sign. The results of these changes, how­ever, are still subject to critical evalua­tion, and it would be premature to dis­cuss them here.

The purpose of this article is to de­scribe the objective acoustical elements that have led to the design of many fine music halls and auditoriums. The appli­cation of acoustical knowledge to ar­chitecture dates back barely 60 years. Until about 1900 the design of a suc­cessful music room was almost entirely a matter of luck. Today the design can be based on well-established principles of physics and engineering.

Acoustics is one of the oldest branches of physics. It originated in the study of music, which probably began with Py­thagoras more than 2,500 years ago. By means of a single stretched string he showed that consonant intervals in mu­sic can be expressed by ratios of simple whole numbers. Acoustics has come a long way since then, both as an inde­pendent branch of physics (physical

acoustics) and in association with other sciences and arts. In the second category are psychoacoustics and physiological acoustics, which deal broadly with the nature of speech and hearing; com­munication acoustics, which deals with the auditory aspects of telephony, radio and sound reproduction; musical acous­tics, which deals with the acoustics of the human voice and musical instru­ments, and architectural acoustics.

REFLECTION AND DIFFRACTION of sound waves can be studied by photographing the wave patterns created by an electric spark. This sequence shows the waves generated by a spark in a model of Royce Auditorium at the University of California at Los Angeles. The spark wave originates from a point on the stage and travels to the rear of the auditorium,

 

 

Acoustics first became associated with architecture when men began to assem­ble in groups to hear speeches, listen to music and see and hear plays. To create a favorable setting for such ac­tivities the Greek and Roman open-air theaters and forums evolved, and many of them have survived to this day. The typical open-air amphitheater consists of steeply banked benches arranged in a semicircle( in front of a platform. With the passage of time the platform evolved into a stage with massive rear and side walls of masonry (and sometimes a ceil­ing) that served the acoustical purpose of reflecting, directing and thereby re­inforcing the sound intended for the audience. Vitruvius, the first-century Roman architect and engineer, wrote that large vases tuned as resonators were often located in the seating area to re­inforce certain sounds. Whether or not such vases were actually used is uncer­tain, but in any case they could only have absorbed sound, not reinforced it.

 

The Greeks did, however, develop one acoustical device of considerable value: the masks worn by actors. In addition to providing exaggerated facial expres­sions appropriate to the various roles, the masks served as megaphones that im­proved the mechanical coupling between the voice-generating mechanism and the surrounding air. A megaphone does not amplify the voice, but it does enable more of the available vocal energy to emerge in the form of sound waves than would emerge without the aid of the megaphone.

The principal defect of the Greek and Roman theaters is that the semicircular tiers of seats act as reflectors that tend to focus sounds from the stage back to a point on or near the stage. Moreover, the echoes from concentric tiers are re­inforced at certain frequencies and di­minished at others. The reason is that the vertical risers, which form the backs of the benches, create an echelon of uni­formly spaced reflecting surfaces. The re­flected waves are in phase and reinforce each other when the distance between risers is equal to one, two, three or any other whole number of half-wave­lengths. When the distance between ris­ers is one, three, five or any other odd number of quarter-wavelengths, the re­flected waves meet in contrary phase and thus tend to cancel each other [see illustration on page 82]. For example, risers that have a spacing of 2.5 feet will constructively reinforce a series of sounds that have wavelengths in feet of 5, 2.5, 1.67, 1.25, 1 and so on, corre­sponding to tones that have frequencies in cycles per second of 225, 450, 675, 900, 1,125 and so forth. These frequen­cies constitute a harmonic series. The same riser spacing of 2.5 feet leads to wave cancellation in a series of odd-num­bered harmonics with frequencies of 112.5, 337.5, 562.5, 787.5 and so on.

The effect of such wave reinforcement and cancellation can readily be demon­strated by speaking, singing or clapping hands on the 'stage of a typical Greek or Roman open-air theater. The sound reflected from . the tiers of benches produces a sustained echo whose character­istic pitch is determined by the distance separating adjacent risers. As a result, when speech or music is heard in an open-air theatre or in a room or audi­torium in which there are parallel and uniformly placed reflecting surfaces­the reflected sound may suffer a serious distortion in frequency. Fortunately in an open-air theater these frequency-de­pendent reflections generally pass over the heads of the audience, but since the reflections come to a focus on the stage they can be extremely disturbing to per­formers rehearsing in an empty theater. The problem largely disappears, how­

shown in plan view. (The black semicircle is not part of the right). This produces an echo on the stage and in the front rows of plan but a mask to keep the bright spark from fogging the film.) seats. The echo was reduced by treating the rear wall with ab• The wave front begins as a simple arc (left) and becomes almost sorptive material. Note the complex patterns created by the prosce­atraight as it is reflected from the concave rear wall (middle and right. The photographs are by L. P. Delsasso and the author.

 

                                                              

 

 

SIMPLE PLOTTING OF WAVE REFLECTIONS, called ray acoustics, has only limited value for predicting the acoustics of an auditorium. Several rays are shown superimposed on a spark photograph of Royce Auditorium. Note that rays A and B on reflection [A', B'] fail to predict the complex diffraction patterns from the proscenium at left of stage. Rays C and D, however, represent reasonably well the reflections from a straight wall.

New acoustical problems arose when civilization and culture spread north­ward and it became necessary to provide enclosed buildings for theaters, churches and other auditoriums. In these build­ings sound echoed from walls and ceil­ings, and when the enclosed space was finished with hard and sound-reflecting materials such as marble, stone and concrete, the architect encountered a vexing problem: excessive reverbera­tion. This phenomenon is merely sus­tained echoing, and for the most part it was accepted as an inevitable result of building large enclosures of durable materials. Indeed, much of the majesty of a cathedral derives from the sonorous reverberations it imparts to voices and musical sounds. Although such rever­berations may be acceptable, and even desirable, for church music and services, they must be held to strict limits in de­signing a lecture room, an auditorium, a concert hall or an opera house.

 

Acoustics of Closed Spaces

 

In order to handle the acoustical prob­lems of enclosed spaces architects, and more recently acoustical engineers, have developed two basic procedures. The earliest and simplest utilizes ray, or geometrical, acoustics; the more recent and comprehensive procedure requires a detailed analysis of how waves of dif­ferent frequencies actually interact with reflecting and absorbing surfaces of vari­ous shapes and dimensions.

Ray acoustics assumes that sound waves travel in straight lines and that when they encounters a new medium, such as the wall of a room or any substance whose density or elasticity differs from that in which the sound originated, the waves are reflected, refracted and transmitted in a fashion that is uniform for all wavelengths. It is assumed, for example, that sound waves are reflected from surfaces in the same way a billiard ball without spin rebounds from a cushion; in other words, that the angle of reflection equals the angle of incidence. In employing ray acoustics architects superimpose on their two-dimensional plans and sections families of straight lines that represent incident and reflected sound waves. This simple technique is useful for uncovering gross acoustical faults such as focusing effects from con­cave surfaces and for determining the shape of enclosures that will give opti­mum distribution of sound.

 

 

MODEL OF LONGITUDINAL SECTION [lower part of upper fig] of Royce Auditorium produced this complex wave pattern in a spark photograph. Diffusive reflections from the coffered ceiling cannot be predicted by ray acoustics. Analysis of such patterns is the objective of wave acoustics.

Ray acoustics is valid, however, for wavelengths that are small compared with the dimensions of the reflecting surfaces. The wavelength of a sound wave that has a frequency of 1,000 cycles per second is about 1.1 feet. For such a sound wave to be reflected in the simple manner predicted by ray acous­tics, the dimensions of the reflecting surface must be at least two or three times the wavelength, or two or three feet. For wavelengths that are not short compared with the dimensions of the reflecting surfaces, ray acoustics fails. Thus sound that has a frequency of 100 cycles per second and a wavelength of 11.25 feet will not be reflected in a sim­ple manner from a surface that measures two or three feet across. The limitations of ray acoustics become apparent when one considers that the wavelength of audible sound varies from about 56 feet at the low-pitch end of the range to less

than an inch at the high-pitch end. Vir­tually every architectural detail in an auditorium or music room will be large compared with the shortest wavelengths and small compared with the longest.

In order to overcome the limitation of ray acoustics one must employ wave, or physical, acoustics, which is based on the physical theory of waves. Only wave theory can cope with the real be­havior of sound in rooms. The preceding discussion of reinforcement and cancel­lation in sound waves reflected from the tiers of benches in an open-air theater gives an example of wave acoustics. An analysis of an open-air theater by means of ray acoustics would show only that sound reflected from the curved tiers of benches would come to a focus at the stage. Such acoustical phenomena as room resonance, reverberation at low frequencies, interference, diffraction and the reflection and transmission character­istics of openings in a room or of systems of suspended panels can be understood and subjected to control only by the rigorous application of wave acoustics.

Singing in the Shower

Perhaps the most familiar example of room resonance is that produced by someone singing in a tiled shower. The singer, who may be impressed by his vocal power, is not hearing his true sing­ing voice; he is primarily exciting, or activating, the resonance, or natural, fre­quencies of a highly resonant chamber. The resonance frequencies for a shower stall, or any rectangular room, are de­termined by its dimensions and those of its occupant. For simplicity in calcu­lating the resonance frequencies I shall assume that a glass door completely closes the entrance to the shower and ignore the presence of the occupant.

 

 

 

THEATER AT EPIDAURUS is widely regarded as the most beauti- seat is intact. The regular spacing of the risers behind the seats ful in Greece. Although the stage no longer exists, nearly every creates unusual sound reflections as illustrated on the next page.

 

My shower is three feet square and eight feet high, and I have demonstrated to my own satisfaction that my presence in it does not appreciably alter the low­frequency resonances. One can regard the enclosure as a kind of organ pipe, eight feet long and closed at both ends. The fundamental tone, or lowest fre­quency of vibration, of such an organ pipe is a tone whose wavelength is twice the length of the pipe, or 16 feet. To find the frequency of a sound of this wavelength one divides 1,125 feet (the speed of sound in air at 68 degrees Fahrenheit) by 16 feet, which yields al­most exactly 70 cycles per second. As every music student knows, such an eight-foot organ pipe also generates a whole series of harmonic overtones that have frequencies of two, three, four, five and so on times the fundamental fre­quency, corresponding to 140, 210, 280, 350 and so forth cycles per second.

These are not, however, the only res­onance frequencies in a three-dimension­al shower; there are also transverse modes of vibration with their appropriate resonances and harmonics. It turns out that there is a triply infinite series of res­onance frequencies. They are determined by the dimensions of the shower, the ve­locity of sound and the appropriate as­signment of integral numbers in groups of three, corresponding to the three di­mensions of the shower. The first seven members of the triplet series are 0, 0, 1; 0, 0, 2; 1, 1, 0; 0, 2, 0; 2, 0, 0; 1, 1,1 and 1, 1, 2. The three digits in each triplet are integers, two of which may be zero, and each of the three may increase (theoreticallv) to infinity.

The first four resonance frequencies for my shower, calculated by the ap­propriate formulas, are 70, 140, 187 and 264 cycles per second. These are only the first four of the triply infinite series of resonance frequencies for this simple enclosure. At higher frequencies the separate modes of the resonant vi­brations come closer and closer together and ultimately can no longer be resolved either by ear or by instrument. Therefore at sufficiently high frequencies this enclosure (or any rectangular room) has a frequency response that is essentially "flat," which means that it responds to all frequencies alike.

The prominence of resonance frequen­cies in a room is dependent on the reflec­tive properties of its walls. Bathroom tile, for instance, reflects about 98 per cent of the sound energy that strikes it.

 

WAVE REFLECTION from regularly spaced risers, as in Greek a half-wavelength (top pair of photographs), the reflected waves open-air theaters, can be demonstrated with water waves in a ripple are in phase. When the setback is equal to odd multiples of a quarter-wavelength. When setback of risers is equal to integral multiples of quarter-wavelength (bottom), the reflected waves are out of phase.

 

 WAVE DIFFRACTION occurs when waves pass around an object fairly sharp "shadow" (top left) and the waves tend to emerge or through an opening. When the wavelength is small compared from the hole in a "beam" (top right). As the wavelength is in­with the size of the object or hole, the object tends to create a creased (bottom), the waves tend to spread more in both cases.

 

Consequently the resonance frequencies in a tiled shower are very prominent; moreover, the small dimensions of the shower give rise to resonances that have fre­quencies well within the audible range. In contrast, the prominent resonances in large rooms occur at frequencies that are usually below that range. Resonance frequencies can readily be suppressed by placing sound-absorptive materials on the wall surfaces of a resonant room. With nothing more than three large terry-cloth towels one can reduce the resonances in a tiled shower to the point where they are barely noticeable. This is done by placing one towel on the floor and centering the other two on ad­jacent walls.

 

More than 30 years ago I investigated the resonances in a special experimental room eight feet square-and 9.5 feet high. The room had concrete walls 10 inches thick and contained only one opening, which was sealed by a steel door. The first four resonance frequencies for this room, calculated for a temperature of 70 degrees F., were 59.2, 70.3, 92.9 and 99.7 cycles per second. These calculated values agreed with the experimentally determined frequencies quite precisely.

            

 

RESONANCE PEAKS [upper fig] were determined by the author for a massive between 50 and 185 cycles per second. The resonance peaks agree concrete room eight feet square and 9.5 feet high. The curve with those calculated. The peaks are proportional to the linear shows how the room responds to sounds that have a frequency range deflection of an oscillograph, therefore indicate sound pressure.

 

DECAY OF TONES [lower fig] in the room described above is shown in these of other frequencies do not. When the room is stimulated with a oscillograms. Sounds of 92.9 and 99.7 cycles per second, which tone not a resonance frequency, the tone stimulates two or more coincide with room resonance frequencies, decay smoothly. Tones resonance frequencies, which decay together and give rise to beats.

 

The top illustration on the next page shows how this room responds to sounds that have a frequency range between 50 and 185 cycles per second. It shows resonance peaks not only at the four frequencies listed above but also at nine higher frequencies, all of which agree with calculated values. The amplitudes of the peaks are proportional to the linear deflection of an oscillograph; hence they are more prominent than they would be if they were converted to decibels, which are based on a logarithmic scale. (On the decibel scale each interval of 10 units corresponds to a tenfold variation in sound energy. Thus a 100-decibel sound contains a million times more energy than a 40-decibel sound. To the ear, which hears a 10-decibel increase is an approximate doubling in strength, the 100 decibel sound is about 26 [2 at power6], or 64, times louder than the 40-decibel sound. )

 

The bottom illustration on the opposite page shows how pure tones in the frequency range between 90 and 100 cycles per second die away in the same experimental room. In this range there are two prominent room resonances, at 92.9 and 99.7 cycles per second. Inspec­tion of the seven oscillograms shows that sounds of these two frequencies decay smoothlv, whereas sounds that do not coincide with resonance frequencies de­cay irregularly. Analysis of the five ir­regular decay oscillograms reveals that all are made up of two or more of the resonance frequencies of the room. For example, the fourth oscillogram consists of the decay of the two resonance fre­quencies of 92.9 and 99.7 cycles per sec­ond. It is apparent that when the room is stimulated with a tone that has a frequency about halfway between these two resonance frequencies, both frequencies are stimulated and decay to­gether. In the process they give rise to "beats" with a frequency of 6.8 beats per second, which is precisely 'the frequency difference between the two resonance frequencies (99.7 - 92.9 = 6.8) .

These tone-decay oscillograms demon­strate convincingly that room reverbera­tion is made up of the free decay of the room's resonance frequencies, or nat­ural modes of vibration. Such a phenom­enon is readily understood in terms of wave acoustics but is wholly unpredict­able by ray methods. For lack of understanding of wave acoustics many archi­tects believe that optimum reverberatory properties of a room can be obtained by treating one surface of the room, usually the ceiling, with an absorptive acoustical material ("acoustic tile"). Although such a treatment is usually beneficial in large public rooms such as restaurants and offices, where the prime objective is to reduce the general noise level, it is oftenharmful in music rooms, and it is almost never sufficient measure for obtaining good acoustical performance.

 

SOUND ABSORPTION IN AIR [ curve below] varies with frequency, humidity and temperature. Black curves show sound attenuation at typical concert-hall humidity, 60 per cent, and at 70 degrees Fahrenheit. Colored curves show attenuation at humidities for which sound absorption is highest. at 70 degrees F. Each 20-decibel drop rep resents a decrease by a factor of 100 in sound energy. Thus music heard outdoors on a very dry, warm night will sound deficient in high frequencies, particularly at distances over a few hundred feet.

             

              

ROYAL ALBERT HALL, opened in 1871, was originally plagued by echoes reflected from the great dome. The colored lines show direct reflections of equal travel time. A listener in the front of the auditorium would hear an echo nearly a fifth of a second behind the direct sound.  Echoes and reverberation were much reduced by installation of a velarium, or heavy fabric awning velarium, or heavy fabric awning [broken lines]

 

 

Interference and Diffraction

 

We have seen in the discussion of open-air theaters how regularly spaced reflecting surfaces can lead to deleteri­ous reinforcement and cancellation of harmonic series of sound frequencies. Similar but usually less serious interfer­ence effects also take place indoors when sound waves encounter the boundaries of a room. The interference between the direct and reflected waves is aggravated if the room has prominent modes of res­onant vibration. To minimize such dif­ficulties the architect must avoid large, smooth reflecting surfaces and either judiciously introduce irregularities in the boundary contours of the room (for in­stance window frames, pilasters and niches for art objects) or install ran­domly placed panels of sound-absorptive materials on the large reflecting surfaces. The purpose is to attain a high degree of diffusion of the reflected sound so that everywhere in the room there will be a multitude of reflected sound waves com­ing from all directions and meeting in random phases.

Even more subtle than the effects produced by interference are those pro­duced by diffraction of sound waves. Everyone has noticed how sound waves bend around corners. An automobile concealed by a building can be heard even when it cannot be seen. To shut out noise from a room one must close a door or window completely. The clos­ing of the last half-inch often excludes more noise than was excluded by the entire closure up to that point.

Sound waves, like light waves, bend or spread around an obstacle when the dimensions of the obstacle are compara­ble to the wavelength. The waves pass around the obstacle and unite in various combinations of phases, yielding the familiar diffraction patterns. The same kinds of patterns arise when waves pass through a small hole. The sound energy emerging through such a hole is often much more than would be indicated by multiplying the incident sound energy per unit area (a wavelength or more in front of the hole) by the area of the hole.

Because audible sound waves cover such a broad spectrum in size, the dif­fraction patterns produced by a given obstacle (or hole) can be exceedingly complex. Moreover, a hard (reflective) obstacle that is, say, two feet across will almost totally reflect high-frequency

sounds, which have wavelengths meas­ured in inches, and, if it is adjacent to an opening of comparable size, the obsta­cle and opening (acting together) will be almost totally transparent to low­ frequency sounds, which have wave­lengths measured in tens of feet.

One of the architect's problems is that in trying to correct one kind of acoustical defect he may introduce oth­ers. This is particularly true of the use of suspended ceiling panels, which are useful for overcoming such acoustical defects as echoes and long-delayed re­flections. Suspended panels have been used to good advantage m several im­portant concert halls and auditoriums, among them the Stockholm Concert Hall and the Tanglewood Music Shed in Lenox, Mass. There have also been less successful examples. The difficulty is that sounds of short wavelength are almost completely reflected by the panels, whereas sounds of long wave­length pass almost completely around the panels, to be reflected later by the ceiling above. For most arrays of panels the transmission, reflection, diffraction and scattering effects are greatly de­pendent on the wavelength of the sound. Precise calculation of these effects by the methods of wave acoustics is possible only for regularly spaced circular or rec­tangular panels, and even such simple arrays require formidable calculations. Where such calculations cannot be made it is imperative that the acoustical effects be studied in three-dimensional models using sound waves whose wavelengths are in the same ratio to the model as actual sound waves are to the full-sized room. Thus if the model is one twenty­fourth actual size, the experimental sound waves must be reduced in length correspondingly. In such a model one would use sound waves with a frequency of 24,000 cycles per second to simulate the effect of 1,000-cycle-per-second waves in a full-sized room.

The behavior of these high-frequency waves, which are inaudible, can be studied in a number of ways. One meth­od is to photograph the waves produced by an electric spark [see illustration on pages 78 and 791. Another method, re­cently developed in Germany, is to re­cord in the model the high-frequency sounds on magnetic tape and play them back at reduced speed so that they can be heard as they would sound in a full­sized version of the room.

 

ANECHOIC CHAMBER is employed by Manfred R. Schroeder of Bell Telephone Labora­tories to evaluate stereophonic playback of a sound recording that has been modified by a computer to simulate the acoustics of a newly designed music room or concert hall.

 

Manfred R. Schroeder of the Bell Telephone Laboratories has developed a promising method of using an elec­tronic computer to simulate the acoustics of planned music rooms. He has devised computer programs that will modify a tape recording of music so that the music sounds as if it were being played in the room under study. For appraisal the tape is played back through multiple loudspeakers in a special anechoic, or echoless, chamber [see illustration on preceding page]. Whatever method one uses, it is essential that the performance of suspended panels and all other acous­tical innovations be fully explored in advance of installation.

When the architect faces the job of designing a new concert hall or other music room, he must pay primary atten­tion to three things: the shape of the total enclosure, the design of the stage and music shell and the reverberation time. A felicitous shape is a requirement of the highest priority. Unfortunately many architects believe that faulty shapes can be corrected by covering the offend­ing surfaces with highly absorptive ma­terials and by adjusting the reverberation time. Thus deluded, they adopt a fashionable construction method, such as the concrete shell, and produce a build­ing that is an acoustical perversion. A bad shape is a permanent liability.

 

MORMON TABERNACLE, built in Salt Lake City about 100 years phone used to make the upper decay curve on the opposite page. ago, has generally fine acoustics, particularly when the audience is C and D show pistol and microphone locations used in making the about 2,500, a number that provides optimum reverberation time. lower decay curve. The colored line in the sectional view (bottom) In the plan view A and B show the locations of the pistol and micro- indicates where the builders used plaster containing cattle hair.

 

 

It is not difficult to arrive at acoustically satisfactory shapes using the methods of ray and wave acoustics. Experience and ratings by competent listeners indicate that for generally rec­tangular rooms that have volumes be­tween 15,000 and 500,000 cubic feet a favorable ratio of length to width is about four to three, and a good ceiling height is about .6 times the cube root of the volume. Therefore for a chamber ­ music room seating about 200 people the favorable dimensions would be about 52 by 40 by 20 feet. It is usually desirable to make the side walls diverge slightly and to incline the floor so that it is not parallel to the ceiling. The reason is that parallel surfaces have a tendency to produce flutter echoes. If opposing surfaces must be kept parallel for some reason, flutter echoes can be suppressed by the use of diffusive or absorptive panels. Whenever possible, particularly in large music rooms, the design should be subjected to a thorough wave-acoustics analysis to ensure that the room will not be impaired by ill ef­fects of resonance, interference and dif­fraction.

 

Reflection and Reverberation

 

An important consideration in music rooms is the time delays in the successive reflections reaching listeners seated in various parts of the room. For rooms that have volumes between 150,000 and 400,000 cubic feet the first reflections should be delayed not more than about 30 to 35 milliseconds beyond the arrival of the direct sound, and these first re­flections; should be followed by a suc­cession of reflections, coming from all directions, that will "envelop" the lis­teners with a relatively smooth but slightly undulating reverberation (much like a vibrato) of the optimum duration and frequency characteristic. For larger rooms the first reflections should be de­layed not more than about 45 millisec­onds; the reflections should be diffuse and should come in good proportions from the side walls, the rear wall and the ceiling.

The acoustical design of the stage en­closure must meet two general require­ments. First, the reverberation character­istic of the stage space, with its normal hangings and equipment, should not dif­fer appreciably from that of the audience space. Second, there must be a properly

 

ECHOES IN MORMON TABERNACLE, recorded by Harvey Fletcher, William L. Woolf and the author, were produced by pistol shots at two different locations. The top curve shows the flutter echo when the pistol is fired in one balcony and recorded in an opposite balcony (see upper illustration on opposite page). A smoothly decaying reverberation (bottom) re­sults when a pistol is fired on the rostrum and recorded part-way back in the Tabernacle.

designed music shell that will enable all the members of an orchestra to hear each other clearly and distinctly, blend and unify the sound of the entire ensemble and reflect a large portion of this en­hanced sound to the audience.

The subject of reverberation time, the third major aspect of music-room design, has been given extensive study by acousticians, beginning more than 60 years ago with the pioneer work of Wallace C. Sabine of Harvard Univer­sity. As we have seen, reverberation in a room is the persistence of the natural modes of vibration, or resonance fre­quencies, after the source of sound in the room has been stopped. For acous­tical purposes reverberation is defined as the time required for the persistent sound to decay, or diminish, by 60 decibels, which is an energy factor of a million.

 

 

 

Experience has shown that the op­timum reverberation times for sound of 1,000 cycles per second are as follows: .5 second for small practice rooms (vol­ume about 500 cubic feet); .8 to one second for rehearsal rooms (volumes up to 15,000 cubic feet); 1.1 to 1.4 seconds for chamber-music rooms (volumes be­tween 35,000 and 75,000 cubic feet); 1.7 to two seconds for large concert halls (volumes between 350,000 and 700,000 cubic feet), and about two to 2.2 seconds in very large halls used for organ music and choral works. In Europe habit and experience give preference to reverberation times about 10 per cent longer than these.

Acousticians have also given much thought to the problem of how the reverberation time should vary with frequency. Should it be the same for all frequencies? Should it be based on the frequency distribution of sound energy in music, so that on the average all components will die away to inaudibility in the same length of time? Or should it be such that the rate of growth or decay of loudness level will be the same for all frequency components? Fortunately the last two criteria lead to about the same reverberation characteristics for frequencies below 1,000 cycles per sec­ond: a gently rising one in which the reverberation time is about 50 per cent longer at 62 cycles per second than it is at 1,000 cycles. Thus if the optimum re­verberation time for 1,000 cycles is two seconds, that for 62 cycles is three seconds.

In order to obtain the desired reverberation times the designer of a music room has available a wide selection of building and decorative materials: stone, brick, wood, plaster, natural and syn­thetic fabrics and a great variety of acoustic tiles and composition panels. Their sound-absorption coefficients have been determined by careful measure­ments in reverberation chambers. By constructing the interior surfaces of a music room with judicious combinations of materials it is possible to obtain op­timum reverberation times for all im­portant frequencies throughout the audi­ble range. Two recent examples of music rooms designed to meet these criteria are Hertz Hall on the Berkeley campus of the University of California and the Seattle Opera House.

 

Two Older Music Halls

 

I shall mention briefly two famous structures that were designed before acoustical knowledge was applied to architectural design: the Royal Albert Hall in London and the Mormon Tabernacle in Salt Lake City. The former, opened in 1871, is interesting because it exhibits nearly A the acoustical de­fects that should be avoided in the design of concert halls. The bottom illustration on page 86 shows several of the long­ delayed echoes that result from the high, domed ceiling. Sound reflected from the ceiling can be delayed nearly a fifth of a second behind the direct sound and, because of the focusing effect of the ceiling, it can be nearly as loud. These extremely disturbing echoes have been greatly re­duced by suspending a convex velarium, or canopy, below the ceiling, which also helps to reduce another defect of the hall: excessive reverberation.

 

HERTZ HALL of the University of California at Berkeley is an chestra. Absorptive panels on aide walls provide good diffusion and example of an auditorium that meets high acoustical standards. proper reverberation. The architect was Gardner A. Dailey and Associates. The acoustical consultants were Delsasso and the author.

 

The forward stage can be elevated to take an entire symphony or

 

In contrast, the Mormon Tabernacle, completed in 1867, is famous for its good acoustics. Its virtues are somewhat surprising considering that its floor plan is approximately elliptical and the high, domed ceiling is elliptical in its trans­verse section [see illustration on page 88]. Fortunately the convergent reflections from the ceiling are disturbing at only a few locations. For the most part the concave surfaces around and above the organ and choir area sustain and blend the instrumental and vocal sounds and project them most effectively throughout the auditorium.

A recent study I have made in association with Harvey Fletcher and Wil­liam L. Woolf has shown that the ellip­tical ceiling gives rise to a very prominent flutter echo when a sharp sound is made along the upper side balconies. The echo, set off by a pistol shot, is shown in the illustration on page 89. Fortunately this flutter echo is not activated appreciably by sounds originating in the organ and choir area.

Except for the floors, which are wood, the interior surfaces of the Tabernacle are mostly lime plaster on wood lath. Today such construction would produce an excessively long reverberation time, but at the time the Tabernacle was built it was the custom to mix large amounts of cattle hair with the plaster. This made the plaster considerably more absorptive than-it would have been otherwise. Even so, when there is no audience, the reverberation time is about four seconds at 1,000 cycles per second. The optimum time for the Tabernacle, when music is being played, would be about 2.2 seconds.

It happens that the optimum rever­beration time is obtained almost exactly when there is an audience of some 2,500. With an audience of about 6,500 the reverberation time drops to slightly below 1.5 seconds at 1,000 cycles per sec­ond. The Tabernacle management and radio station KSL frequently get letters of praise for the quality of music broad­cast when the Tabernacle contains an audience of approximately 2,500. Com­plaints about the acoustics are often received, however, when the audience numbers 6,000 or more.

Compared with designing a music room, the job of designing a room used primarily for speech (for example lecture rooms and legislative rooms) is relatively simple. Like a music room, a speech room should be free from external sources of noise, from resonances and from echoes and sound-focusing effects. The reverberation time should be about a second (slightly less for small rooms and slightly more for large rooms) . Finally, speech must be clearly heard throughout the room. This means that unless the room is small an amplifying system must be provided.

The general need for amplification can be demonstrated by using speech­articulation tests of the type developed by telephone engineers to determine the intelligibility of transmitted speech. The results are expressed in terms of Percentage Speech Articulation (PA) . The PA is determined by having typical speakers call out speech sounds that occur in English words and having a panel of listeners record what they think they hear. If they hear correctly 75 per cent of the speech sounds, the PA is 75 per cent, which is the minimum value acceptable for satisfactory hearing.

 

SEATTLE OPERA HOUSE has many acoustical features. Rear expose absorptive chambers that change reverberation time. The walls at all levels are inclined forward to prevent echoes and reflect architects were B. Marcus Priteca and Lames 1. Chiarelli. The sound beneficially. Wood panels on side walls can be opened to acoustical consultants were Paul S. Veneklasen and the author.)

 

The illustration at the top of page 85 shows in a family of curves how the hearing of the unamplified speech of the average speaker depends on the time of reverberation and room size. It will be seen that except for the smallest rooms with ideal reverberation times the PA values are below 75 per cent. The conclusion is that amplification is almost always necessary.

The illustration at the bottom of page 85 shows the PA for 14 speakers of widely different vocal strengths in an auditorium of 400,000 cubic feet. The curves indicate clearly why some speak­ers are heard much better than others and why some are not heard at all, and they demonstrate once again why it is generally advantageous to amplify speech.

 

The Acoustics of Homes [ in the paper we are in 1960…]

 

Perhaps the biggest failure of U.S. architects (and acousticians) is in not doing something constructive about the acoustical environment of the home, particularly the apartment dwellings that have been built in the past 20 years. An easily attained objective would be to shorten the reverberation time of small rooms to about .5 second and that of large living rooms used for music as well as speech to no more than one second. Typical values in U.S. homes with conventional plaster walls and ceiling, and with scant carpeting, are from 50 to 100 per cent higher. The more urgent and difficult problem is to screen out unwanted noise, whether it is of external or internal origin. (The flushing of a toilet makes a racket that often carries through every room of the house.)

 

U.S. cities should adopt as construc­tion standards the many commendable and necessary acoustical features that are to be found in virtually all new apartments in both eastern and western Europe. Apartments are carefully planned so that rooms in which the loud­est noises are likely to originate are the farthest from those in which the most quiet is desired. There is, for example, maximum separation between the bedroom of one apartment and the living room of the adjacent one. There is a heavy wall between adjacent bathrooms; the entrance hall is used as a sound lock between the living room and the bedroom; entrance doors are of solid-panel construction, well fitted in their frames so that threshold cracks are eliminated. Floors above ground level are usually floating concrete slabs, so that impact sounds as well as air-borne ones are thoroughly insulated.

 

The effective control of noise in these modern European buildings is no accident. It is required by the high stand­ards of building codes. Sweden, for instance, requires that there be enough sound insulation between rooms in resi­dential buildings to reduce air-borne sounds by 48 decibels. Even higher standards are required in hospitals and certain other buildings. In the U.S., in contrast, sound insulation is completely ignored by practically all building codes. (It is a hopeful sign that a new code being considered for New York City may include noise-level specifications for the first time.)

The provision of quiet buildings, par­ticularly those in which people live, learn and recover from illness, is an es­sential objective of good community planning. A nation that prides itself on . its high technology and high standard of living should be willing to pay the additional 5 or 10 per cent that would be entailed in creating buildings with satisfactory acoustics.

I am sure it can be demonstrated that good acoustics is good business, but far more important, the providing of quiet buildings is indispensable for good health and the growth of culture. Home should be our refuge from a noisy world, where taut nerves may find rest from and refreshment for the strains of high-pressure living.

 

 

HOLLYWOOD BOWL, noted for its acoustics, was opened in innovation, since much copied. The architect was Lloyd Wright, 1922. The shell over the stage was an architectural and acoustical son of Frank Lloyd Wright. The author was acoustical consultant.

 

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