development of architectural acoustics

Sigma Xi, The Scientific Research Society

The Development of Architectural Acoustics: The application of the science of acoustics toarchitectural designs has produced greatly improved halls for both speech and musicAuthor(s): Robert S. ShanklandSource: American Scientist, Vol. 60, No. 2 (March-April 1972), pp. 201-209Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/27843021 .

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The Development of Architectural Acoustics

The application of the science of acoustics to architectural designs has produced greatly improved halls for both speech and music

Robert S. Shankland

From the earliest days of man's com

munication with his fellows, there has been a steady evolution in the use of

spaces for speech communication and

for music and the related arts. The first notable developments were in the classical Greek theaters. These evolved for centuries from Hellenic into Hellenistic times and were then

modified by the Romans, often in ways not beneficial for acoustics. Their acoustical properties have been studied since ancient times and often praised in superlative terms. Scientific analy sis reveals several important factors

contributing to their acoustics. A most vital circumstance was the ex

tremely low background noise at the sites of Greek theaters. There were no industrial or airplane noises or street

traffic, the sites were far enough from the sea to eliminate surf noise, and they

were sheltered from wind.

Another prime factor was that the actors were highly trained for speak ing and singing and exerted great physical effort in performance. There were some rudimentary acoustical

Robert S. Shankland received the B.S. degree in

physics in 1929 and the M.S. in 1933 from Case School of Applied Science, and the Ph.D. degree from the University of Chicago in 1935. He has conducted research and served as a consultant in architectural acoustics for forty years. During World War II he was Director of the Under water Sound Reference Laboratories at Mountain

Lakes, New Jersey, and Orlando, Florida, op erated by the OSRD fcr underwater sound re search and the development of Sonar. He has been a member of the physics faculty of Case and Case Western Reserve University since 1930, serving as department chairman from 1939 until

1958, and is now Ambrose Swasey Professor of Physics. This article is based on the authors

spring 1971 Sigma Xi National Lectures at

Waterloo, McGill, and Yale Universities, and the Air Force Cambridge Research Center. The

drawings are by Mrs. Shankland. Address:

Rockefeller Physics Building, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106.

aids, such as mask mouthpieces to

help direct sound to the audience, and

resonating sound boxes to stand on, but for the most part, loudness de

pended on the skill of the performers. Greek audiences were not uniformly quiet, and hucksters cried food and

drink, but for key moments in the per formances people were very attentive,

especially as the plays had religious or political significance.

Figure 1 is a drawing of the ancient

(about 420 b.c.) Greek theater at

Syracuse in Sicily. A most essential feature was that seats were arranged

approximately in semicircles on a steep conical slope. Thus, the sound propa gating to the audience was confined to a smaller solid angle than on level

ground, with a corresponding increase in intensity. Determinations of speech articulation in classical theaters have shown that speech intelligibility im

proves with increased seating angle. A

specific comparison of this effect on

speech articulation was made in the theater at Syracuse. The seats of this theater are cut in the native rock and, as seen in Figure 1, their angle at the rear of the theater is steeper on the left than on the right side. The speech articulation scores dropped from 50 to 40 percent between these two loca

tions. At Epidaurus the upper sections of seats are set at a steeper angle than

the lower sections to improve hearing conditions.

Sound-reflecting surfaces were located

near the performers. In the Greek theater a circular stone "orchestra"

floor was built near the focus of the

seating. This sound-reflecting surface

increased the sound going to the audience. Further developments intro

duced a raised stage (proskenion) be hind the orchestra, which gave better

angles for direct sound from actor

to audience and also for sound re flected from the orchestra surface toward the seats. The stage house

(skene) gave additional sound-reflect

ing surfaces to increase the projection of sound from performers to listeners.

Certain vertical areas in the skene in

corporated wooden panels (pinakes) as sound reflectors ; they also resonated and produced the equivalent of a

weak reverberation. The effect was

small but had psychological impor tance for the actors. Vitruvius dis

cusses in great detail the use of reso

nating bronze vases {echeia) placed in the audience area to improve the acoustics. These were located in

recesses along the center cross-aisle of

the theater. A weak sympathetic reso nance might be produced in these tuned vases, but they could not have had a marked acoustical effect, for

they could hardly have received sufficient sound energy from the stage to be strongly activated.

Figure 2 shows speech articulation re sults with standard word lists made in several ancient theaters in Italy and

Sicily. The contours show the aver

age percentage of words heard cor

rectly at various locations in the audi

toria. The results prove that hearing conditions are better in the theaters than at the same distances on level

ground. The measurements also show

that the greatest improvement occurs near the center of the theaters, where both the direct and reflected sound are most effective and where the maxi mum useful sound is scattered from the sides toward the center of the theater. Sound scattered by people differs from that diffracted by empty seats, but the effect on loudness and

speech articulation is similar. High up in the theaters, listening conditions are poorer, and in the top third of

large theaters satisfactory hearing, as

1972 March-April 201



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Figure 1. Greek theater at Syracuse, Sicily. Seats are cut in the native rock, and the

angle of slope varies considerably at the rear. Half of the circular orchestra was

covered by the enlarged Roman stage and stage house, which have entirely dis

appeared. Articulation tests prove that

hearing conditions in the rear seating sections are unsatisfactory.

Figure 2. Speech articulation determinations in ancient theaters at Syracuse, Taormina, and Segesta in Sicily, and at Ostia Antica, Fiesole, and Pompeii in Italy. Hearing conditions are better than at corresponding distances on level ground (colored lines), but hearing conditions at the rear in the

larger theaters would not meet accepted standards for speech intelligibility.

judged by modern standards (85 percent articulation), was hardly pos sible. It should be noted that hearing conditions near the front of the theaters (90 percent curves in Fig. 2) are poorer than on level ground. This acoustical defect in the front seats has been noted even in the most perfectly preserved theater, that at Epidaurus. It is due to sound scattered back with a time delay from the auditorium, which blurs the direct sound from the

stage.

Wind has a pronounced negative ef fect on hearing conditions, especially for speech. Tests in the restored Roman theater at Ostia Antica proved that speech articulation scores in the rear seats would drop from 75 to 45

250 250 - Distance from speaker in feet

percent with moderate winds. Greek theater sites were chosen to be shielded from wind, but the Romans often chose sites which failed to meet this requirement, but the defect was

compensated for in part by the shield

ing of large stage houses and over head awnings to give an almost en

closed space.

In geometrically perfect theaters a

distinct echo tone can be heard at the focal point owing to constructive inter ference of sound scattered by the seats. The half-wavelength of the fundamental tone equals the spacing between adjacent rows of seats. The harmonics are also reinforced, and thus the echo tone from an impulsive sound is very harsh in quality. The effect falls off rapidly with distance

away from the focal point of the auditorium and is absent in theaters such as those at Syracuse and Segesta, where the seating geometry is not

perfect. It also disappears with an

audience.

The Romans modified the Greek theater in several important respects that resulted in less satisfactory acous

tics. The sound-reflecting "orchestra"

was reduced to a semicircle by widen

ing the stage, and the remainder was

preempted as a seating place for dig nitaries and enlarged by four wide circular steps for chairs. Consequently,

202 American Scientist, Volume 60



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Figure 3. Blossom Music Center, outside

Cleveland, showing the audience lawn area, similar to the site of a Greek theater, which is served by a time-delayed loud-speaker

system. The pavilion has reverberation to

provide concert hall acoustics for 4,500

people, and the large orchestral shell directs

many early reflected sounds to the audience.

(Architects: Flynn, Dal ton, van Dijk and Partners. )

acoustical properties of the orchestra were lost. The stage was also lowered for the benefit of the front-seat occu

pants, causing sound to travel to the main auditorium at a less effective

angle, and the deeper stage gave poorer rear-wall reflections. Roman

designs greatly increased the com

plexity and height of the stage house, which increased the reflected and dif fused sound going from actor to audi ence, but the higher stage structure

produced delayed sound and even echoes that seriously reduced speech intelligibility. The awnings to shield from the sun produced reflected sound and reverberation that further im

paired the acoustics.

In recent years there has been a re newed interest in outdoor theaters, and principles learned from the Greeks are useful in their design. The site for the Blossom Music Center near Cleve land (Fig. 3) was chosen in a natural

amphitheater reminiscent of a Greek theater. Its location was also selected for low background noise. Present

day outdoor theaters have a stage shell of greater efficiency than the classical stage house to project sound to the audience, and a significant por tion of the listeners are now seated in a roofed and partially enclosed pavilion. In addition, electronic sound amplifi cation is now essential for large out door theaters.

The development of the Greek odeion

paralleled that of the theater and

played a major acoustical role in the

change from open-air to enclosed

spaces for plays and music. Odeions were moderate-sized square structures

with steeply stepped curved seating and stepped wood roofs. They varied in capacity from 200 seats to the 2,000 accommodated in the Odeion of Herodes Atticus at Athens. The en closed space confined the sound, gave increased loudness, and produced useful reverberation that favored in

struments like the harp and flute and led to musical forms utilizing har

mony. Reverberation in the larger odeions was controlled by a heavy drape vela that was suspended from the

ceiling.

A much later development, with its roots in the classical theater and

odeion, was the Italian opera house. A notable building in this evolution was the small roofed theater at Pom

peii. Its interior was rectangular, but the seating and stage evolved directly from the Roman modifications of the Greek theater described above. It

gradually became clear that speech articulation is improved when seat

ing is arranged on steep angles to pro vide more effective sound paths. The Italian opera house is the logical ex tension of this principle, where the horseshoe of boxes can be considered as the limiting shape for a steep sta dium. The main-floor acoustics were

inferior, and for years this space was

used primarily for the opera ballet and the standing audience.

In the Greek theater the acoustical

requirements for speech and music were almost identical, but when per formances went increasingly indoors as civilization moved to Sicily, Italy, Northern Europe, and England, the acoustical problems became more

specialized. The first major changes in indoor acoustics since the time of the

Greek odeion occurred in the Chris tian churches. As these became larger they emphasized acoustical problems that had been of only minimal con cern in earlier times. The hard sur faces of the interior wall, ceiling, and floor areas gave increased prominence to reflected sound, which produced serious disturbing echoes and rever beration. These difficulties stimulated a long architectural development of room shapes that ultimately led to a

gradual improvement in acoustics.

The first Christian churches were

rectangular in shape and closely pat terned after the Roman basilicas.

The columns, pilasters, piers, sculp ture, and decorations in these churches

helped the acoustics, but as the size of the sanctuaries increased, the acoustical conditions steadily de teriorated. Another favored shape for

early churches derived from the Roman Pantheon. These were circu

lar and, when small, as in S. Con stanza at Rome, the acoustics were




satisfactory, but larger circular churches, like S. Steffano Rotundo, were distinctly inferior. This led to a return to the rectangular shape of the

Romanesque church that continued into modern times. But today, im

proved structural technology and ver satile sound amplification systems have made many new and novel

shapes for churches possible, but not always to their acoustical advantage.

There was a long interval after the Roman Empire before rooms having acceptable acoustics were available. The failure to solve acoustical prob lems greatly delayed the evolution of art forms with speech and music for both secular and religious purposes. Poor acoustics was so prevalent in

large churches that it was a prime in fluence in the development of the form of religious service in the Romanesque, Gothic, Renaissance, and Baroque periods. It was essential for music to be played and sung slowly, and speak ing gave way to chanting to give reasonable intelligibility. Thus, Medi eval church music was limited to

Gregorian forms and slow choral works. At best the acoustical results were hardly satisfactory, and the services more and more became visual

spectacles rather than means for com

munication by sound. As a result, the next important advance in acoustics

occurred elsewhere.

A major influence for this advance, during the period before and after 1600 in Italy and Sicily, involved the small chapels or "oratorios" built adjacent to large churches. Figure 4 shows the interior of an oratorio and

emphasizes the essential acoustical elements in the architecture and decorations. These rooms were used

for worship by aristocratic groups that had gained political and social distinction for service to church and state. The oratorios were of moderate

size, rectangular, not too wide, and

with high ceilings. It is now accepted that this is the ideal room shape for

good acoustics. Another extremely im

portant feature of the oratorios is that

they were decorated with a wealth of

sculptural detail, especially on wall surfaces, which produced much sound diffusion at all audible frequencies.

It has gradually become clear that the acoustical excellence of a room for

music, as well as for speech, is dis

tinctly improved by increasing the diffusion of sound. The chief effect of

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Figure 4. Drawing of the oratorio of San

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gular rooms of this type with high ceilings and much sculptural wall ornament were

the forerunners of Italian concert halls. Acoustical conditions were ideal for the

development of music.

diffusion from the front and side walls in the oratorio is to provide many early reflected sounds from source to listener. Diffusion of sound at the rear wall and ceiling eliminates strong reflections that could produce echoes. The lower walls were covered with

heavy, soft fabrics that reduced re flections and helped control reverbera tion. The excellent acoustics of the

oratorios, coupled with the support of talented musicians, made possible a sustained artistic development of sing ing and instrumental music to a de

gree of sophistication that could never have occurred in large churches with their poor acoustics.

The oratorios, because of their mod erate size, had short reverberation

times, which made rapid and com

plex music, both singing and instru

mental, possible. Soloists, choral

groups, and instrumentalists found the acoustical conditions highly favor able for the progress of their art.

Organs of high quality became avail able for accompaniment and, of even

greater interest, woodwind and

stringed instruments that would have been wholly impractical in large

churches gained favor in the oratorios. The violin, which had hitherto been

only an instrument for street music, became socially acceptable, and the flute and harp experienced a revival of

popularity they had not known since the time of the Greeks. This inevitably led to the development of new forms of musical composition. The "ora

torio" as a recognized musical form took its name from the room in Rome

where St. Philip Neri conducted musical groups and produced his

compositions. It is also of interest to note that Handel wrote much of his music in a small church at Canon's Park at Edgeware, England, and it was often performed in the Holywell Music Room at Oxford, both rooms

having acoustical properties similar to the Italian oratorios.

In northern Italy, and especially in

Venice, the palace ballroom, which had acoustical properties similar to the oratorio, provided another ex cellent environment for musical de

velopments. The secular palace ball room permitted an even more com

plete break with early church music than the oratorio could allow and also




4- A St. Thomas Church, Leipzig B Troy, N.Y., Concert Hall C Severance Hall, Cleveland

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Figure 5. Reverberation time-frequency characteristics for the Troy, New York, concert hall; Severance Hall, in Cleveland; and St. Thomas Church, in Leipzig. The first two are excellent concert halls, and St.

Thomas, where Bach composed and per formed his greatest compositions, has re verberation conditions suitable for a concert hall. (All the halls were occupied when the reverberation times were recorded.)

gave increased prominence to instru

mental music independent of the

organ. The new freedoms led to con

tinual change in the design of larger halls and in the forms of musical

composition and ushered in the era of classical music as a secular art.

The building of Italian-type concert halls spread throughout northern

Europe as music developed during the two centuries after 1650. Italian architects were often employed in

Germany, Russia, and elsewhere in

Europe, and noted English architects,

including Inigo Jones, studied the

buildings designed by famous Italian architects, especially Palladio. Jones's

designs, especially the Banqueting Hall in Whitehall Palace, were of major importance for later concert

halls in England. One of these with

exceptionally fine acoustics is Covent Garden in London, equally fine for both opera and orchestral concerts.

Thus, concert hall evolution into the late nineteenth century was largely based on Italian models initiated by the oratorios. The culmination of acoustical excellence in the rectangu lar Italian concert hall is best illus trated by the Gewandhaus in Leipzig (destroyed in World War II), the

Musikvereinssaal at Vienna, and Bos

ton Symphony Hall, generally con

sidered to be the best concert halls in the world.

Acoustical principles

During the continuous evolution of rooms for various uses, a parallel

growth in theoretical understanding of the behavior of sound in enclosures has gradually influenced architectural

design. One of the most important principles for room acoustics is the limit of perceptibility or precedence effect, discovered by Joseph Henry in 1854 while designing a new lecture hall for the Smithsonian Institution. This effect makes delayed sounds reflected from wall and ceiling surfaces (and today from suitably time-delayed directional loud speakers) blend with the direct sound to create a single acoustical image in the listener's consciousness.

Professor Henry conducted outdoor

experiments near a large vertical wall

of the original Smithsonian building and found that sound reflected from this wall failed to give a distinct echo

when the time delay behind the initial sound was less than 50 milli seconds. He concluded that reflections from wall and ceiling surfaces in an auditorium with time delays that did not exceed this limit would blend with

the direct sound to give increased loudness. He designed the wall and

ceiling surfaces of the Smithsonian Lecture Hall to give useful early re flected sounds in accordance with this

principle. All delayed reflections heard within the 50 millisecond interval, provided these delayed sounds are not too loud compared with the direct

sound, will be heard as a single signal of increased loudness and identified as coming from the direction of the first sound perceived. Although the

delayed sounds are not individually detected, they nevertheless form an acoustical image quite different from that produced by a direct source alone. This "precedence effect," or

"intimacy," is of major importance for music and is a prime objective in concert hall design.

The direction, intensity, quality, and time delay of a reflected or amplified sound are all critical to achieve the full effect of this phenomenon. The best directions for delayed sounds are near the plane defined by the source and the listener's ears, and at angles not greater than 80? from the line of

propagation from the source. Thus, delayed sounds from stage shell and side walls are more desirable than those from the ceiling or rear wall.

The intensity of a delayed sound should not exceed that of the direct sound by more than about 10 decibels, and the distribution of time delays should be uniform in the allowed 0 to 50 millisecond interval. The spectral character of the early reflected sound should be nearly the same as that of the direct sound for the precedence effect to be fully operative. This is the basic defect in the use of suspended ceiling panels to produce early re

flections, as they are inefficient scat

terers of low-frequency sound. The

precedence effect is especially im

portant for large churches where directional loud speakers are mounted

along the nave in circuits to give proper time delays, depending on the distance from the pulpit, and for sound amplification for large out door audiences.

A great advance in architectural acoustics was made by Wallace C.

Sabine, of Harvard University, in the years just before 1900. He made the first quantitative study of rever

beration, the phenomenon of sound

continuing to be audible in a room for an appreciable time after the source ceases to radiate. Sabine defined




reverberation time as the interval for

a million to one (60 decibel) decrease in sound-pressure level after a sound

source is stopped. He found the rela

tion of reverberation time to the air

volume of a room and the total sound

absorption by surfaces, furnishings, and the audience. Air absorption is

also important at high frequencies.

Acceptable reverberation times for

rooms vary within reasonable limits, but experienced listeners agree that

the optimum values are near 1.0

seconds for speech, 1.5 seconds for

opera, 2.0 seconds for symphonic

music, and somewhat longer for

organ. Long reverberation times ad

versely affect speech intelligibility and clarity of music, while too short a time reduces loudness and causes an

undesirable sensation called "dry ness."

Not only should a room have the

proper average reverberation time

but the variation of reverberation

time with frequency throughout the audible range is also critical. For the

acoustics of a hall to preserve the

natural tone of musical instruments, there should be a nearly constant

reverberation time through the middle

frequency range, from 300 cycles to

1,500 cycles. At higher frequencies the reverberation times usually be

come progressively shorter because

of air absorption. Too short a rever

beration at high frequencies robs music of its "brilliance." Reverbera

tion times below 300 cycles should

progressively increase to the lowest audible frequencies. Prominent low

frequency reverberation gives "full

ness of tone," which is especially de

sirable for music. Figure 5 shows

typical reverberation time-frequency characteristics for two concert halls that have excellent acoustics, and also for Bach's St. Thomas Church in Leipzig. Note that the reverberation in this church is essentially the same as for a concert hall.

T he diffusion or scattering of sound is a highly important factor for the acous tics of rooms. For effective diffusion, sound must strike architectural fea tures, furniture, people, or other objects having dimensions approximat ing the wavelength of the sound. For low-frequency sounds, objects must be several feet across, while for the highest frequencies objects of only a few inches in dimension will be effec

the sound intensity throughout a room

and eliminate troublesome dead spots,

focusing, and echoes. The most im

portant aspect of diffusion, probably, is that the scattered sound provides

many early reflections. This is likely the "secret" of many fine old halls in

Europe.

Large chandeliers are also effective

sound diffusers. However, the sound

scattered by a chandelier is pre dominantly high frequency, for low

frequency sound travels through a

chandelier without much deviation.

Thus, the scattered sound has a very

different frequency spectrum from the incident, so is not as useful for the

precedence effect as that diffracted from a wall diffuser, where all wave

lengths are scattered through large angles. Large chandeliers are also

effective in breaking up focusing and

high-frequency standing waves that cause undesirable intensity variations

in the high harmonics of musical instruments.

The amount of sound diffusion that is

optimum is not completely under

stood. In modern construction there

is little danger of excessive sound dif fusion because the installation of

architectural features or decorations

for this purpose is expensive, and with

present-day building skills it is ex

tremely difficult to achieve. Best

listening conditions seem to result when the sound field preserves some direc

tionality and is not completely ran domized. There is a limit (seldom attained) to the degree of desirable diffusion.

The binaural hearing sense enables a

listener to identify and concentrate

attention on desirable sounds and ignore much unwanted sound; it is especially important for discriminat ing against noise. In architectural acoustics the binaural effect helps a listener establish the location of a sound source and also gives him a

feeling for the size and shape of a room.

Examples of acoustical

designs Concert halls. Experience has proved that the most desirable shape for a concert hall is rectangular. The room should not be too wide; the

ceiling should be high, to insure

adequate reverberation; and the or chestra stage should be located in the hall itself aned not in a recesserd space

as in the legitimate theater. There

should be as much architectural

detail as possible for sound diffusion,

especially on the side walls and near

the orchestra. These conditions pro

duce early reflected sounds that come

predominantly from the side walls and the vicinity of the stage rather than from the rear wall and ceiling; they thus have the best time-delay sequence and are near the horizontal plane. The surfaces adjacent to the stage and in the orchestra shell should give adequate mixing of sound and also

provide reflections so that the players can hear each other.

The reverberation time versus fre

quency characteristic of a concert hall, as shown in Figure 5, should rise at low frequencies and slope off at high frequencies. However, it should be

emphasized that correct reverbera

tion conditions alone will not insure an excellent concert hall.

A concert hall design that both au

diences and performers praise is

essentially rectangular, with a high ceiling, with a balcony or balconies that do not extend too far over seats

below, and with articulated box or

balcony fronts, to give considerable diffusion. An example of a fine stage design for this type of hall is shown in

Figure 6. In this hall (in Troy, New

York) early reflected sounds reach the listener with a wide spread in time

delays but with the shorter time

delays emphasized. As is evident in

Figure 6, the stage rear wall, especially the curved portion, has architectural

detail for effective diffusion of sound. The wooden main floor is closely coupled structurally to the stage, and

it is possible that this feature con tributes to the acoustical quality of the hall.

The development of concert halls of

rectangular shape is one of the most

important factors in architectural

acoustics, and halls which depart markedly from this shape often have inferior acoustics. On the other hand, halls that adhere to the Italian oratorio-concert hall todition, such as the new Kennedy Arts Center concert hall in Washington, almost without

exception have excellent acoustics.

Departure from the rectangular shape, however, need not be disastrous for the acoustics. For example, in the fan

shaped Severance Hall in Cleveland, seats in the dress circle and balcony




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Figure 6. Stage details of the concert hall in Troy, New York. The rear wall and curved

extension, side boxes, and exposed organ

pipes provide much sound diffusion and give many early reflections to both audience and orchestra.

receive excellent sound by many early reflections from ceiling and side walls in a masterful design by the late

Dayton C. Miller, and the original lack of early reflected sound on. the

main floor has been corrected by a

special stage shell with effective sound

reflecting and diffusing properties.

The most successful opera houses all have the general features of the classic golden horseshoe. Examples are La Scala in Milan, San Carlo at

Naples, the Teatro Massimi at Pa lermo, and the opera houses in Rome and New York. Since the intelligibility of speech as well as the quality of music is important for opera, the reverberation times of an opera house

should be shorter (1.5 seconds) than ihose in a concert hall (2 seconds).

Legitimate theater. The acoustical re

quirements for the legitimate theater are closely related to those for the

opera house, but with important differences. Both must provide ade

quate loudness, low noise levels, and

clarity of articulation. The prime need

in the theater for intelligibility of

speech, however, is in part accom

plished by a shorter reverberation time (1 second). But it must not be too short, for reverberation adds

"body" to speech, and, furthermore, too short a reverberation would mean excessive sound absorption, with con

sequent loss in loudness, and a sense

that the performance was staged out doors. The best conditions are achieved with the lower frequencies less emphasized than for music.

It is most effective to provide the early reflected sounds in a theater from

wall and scenery surfaces on or near

the stage. This is because the actors turn at various angles from the au

dience, and only by having efficient sound reflectors at the rear and sides of the stage as well as a hard floor can

adequate loudness of speech be in sured. Another factor that makes it desirable for early sound reflections to

originate on or near the stage is that theaters are usually wider than con

cert halls, to bring the audiences near the performance, and thus side wall

reflections would have too long a time

delay.

Ceiling reflections in a theater are

important primarily for the balcony seats. It is also essential to provide properly designed soffits under the balconies, with hard surfaces sloping upward toward the stage so that sound will be reflected to the seats under the

balcony.

The recent trend toward providing halls of multipurpose use for con

certs, opera, and theater, to save

building costs, has put great demands on acoustical designs, since the op timum conditions for concert hall and theater differ considerably. This situation has been met by two general approaches. One is to have a system of variable sound absorbers in the auditorium so that the reverberation time conditions can be altered

throughout the necessary range. The other is to have complex mechanisms that adjust the ceiling height and

balcony openings to change the air volume and hence the reverberation, as needed for play, opera, or concert.

Elaborate engineering is required for these installations, and some have

proved successful-for example, in the Jesse Jones Theater at Houston, Texas, and at the new Arts Center in

Ottawa, Canada. However, both the

building and maintenance costs for these complex features are great, and

the question is moot as to whether it is better to provide one hall with variable acoustics or to provide sep arate halls for different types of

performances, as has been done at

Lincoln Center in New York and the

Kennedy Center in Washington.

Churches. The acoustics of churches has had a longer and more complex history than that of any other building form, and it is no less complicated today when many new large churches of novel and imaginative architecture are

being constructed. A church sanctuary should provide excellent acoustical conditions for speech, solo and choral

singing, and organ music. Since the acoustical criteria for these differ con

siderably, a design that will be satis

factory for them all is difficult to achieve. Speech from pulpit and lectern requires acoustical conditions

like those of the legitimate theater. For organ music longer reverberation

times than those considered optimum for speech and other forms of music are usually desired, but this require




ment is often exaggerated. In a large church the necessity for adequate loudness adds complex demands on

both architectural acoustics and sound

amplification.

For proper speech intelligibility with out a carefully designed directional and time-delayed sound system, the

reverberation time with a normal

congregation should not exceed 2.5

seconds. In small churches this limit can hardly be exceeded. For large churches extensive sound-absorbing treatments are often needed to control

reverberation. Sound-absorbing seat

cushions help to make the reverbera

tion time less dependent on congrega tion size.

In large churches a compromise must

be made between the reverberation

time desirable for speech intelligibility and that needed for music, especially organ. St. Thomas Church in Leipzig, where Bach was choirmaster and

organist, has an average reverberation

time (with a large congregation) of 2.2 seconds. The rapid and intricate

organ music of Bach had great clarity with this short (for churches) reverber ation, while in large reverberant

cathedrals the details of his com

positions are lost in the prolonged rumble.

In the long evolution of church arch itecture the acoustical conditions have

varied greatly. In the early Christian era two types were used: the rec

tangular and the circular. The first evolved from the Roman basilica, and the second derived from the

large circular mausoleum, such as the

Pantheon and the Tomb of Augustus. Circular forms were gradually aban

doned because of the superior acoustics

of the rectangular form.

Early Romanesque basilica-type churches, such as S. Cl?mente, S.

Sabina, and S. Maria in Cosmedin in

Rome, are moderate in size and have

excellent acoustics both for music and the spoken homily. These early churches originally had elaborate coffered wood ceilings, which both absorbed and diffused sound. As churches increased in size to the great Romanesque and Gothic cathedrals of Medieval times their acoustics became progressively poorer. Loudness

and intelligibility of speech and clarity in music were lost, and the service

was forced to emphasize visual com

munication and chants and slow

choral works. The open vowels of Latin were intoned to increase loud

ness and to minimize the effects of excessive reverberation, and great skill was developed in using building resonances to reinforce the sound.

Churches of great size and poor acoustics reached their limit in the Gothic cathedrals. In these structures

the acoustical coupling between nave,

side aisles, and other spaces is strong, and the entire edifice acts nearly as a

single air volume to give long reverber ation. The modern Gothic cathedral, e.g. St. John the Divine in New York

City, has excessive reverberation, which only slightly improves with a

large congregation because of the enormous air volume and the large areas of stone and glass.

It was only with the Reformation and the reemphasis on preaching that church acoustics were improved, es

pecially in the Lutheran Church in

Germany and the Presbyterian Church in Scotland. Hard stone surfaces were covered with wood, and

seating was supplied for larger con

gregations. Smaller churches were

built, and large cathedrals often were divided into several areas of worship. All these changes reduced reverbera

tion and improved the acoustics.

The Counter-Reformation also led to

improved acoustics in churches

through the development of Baroque decoration and architecture. This

employs a wealth of architectural detail and ornament and weakly

coupled side aisles and chapels, and the resulting increase in sound dif fusion and absorption produced acous

tical conditions superior to those in

Romanesque and Gothic churches of

comparable size.

Today it is possible to produce artic ulate speech in large churches by sound amplification even with long reverberation. This requires the use

of directional column-type loud

speakers so that most of the sound is directed toward the congregation and

only a small fraction strikes hard wall or ceiling surfaces. This reduces sound

energy in the reverberant field in

comparison to useful direct sound. It is essential that the loud speakers be operated with suitable time delays to make use of the precedence effect.

Many new churches are built with

circular, hexagonal, or octagonal floor

plans to bring the congregation closer to the service. These changes from the ideal rectangular shape greatly com

plicate the acoustics as sound must be

projected to the congregation through an angle of 180? or more. This puts great strain on the preacher and also

imposes severe requirements for the sound amplification system. Modern

materials and newer construction

methods permit great flexibility in church design, with the imaginative use of large areas of glass and other sound

reflectors, and, unless carefully planned, the acoustics in these churches can be disastrous for both speech intelligibility and music. No rules cover all cases, but the fundamental

requirements to achieve adequate loudness and clarity must be met, and excessive reverberation, echoes,

and focusing must be avoided. Sound diffusion is of the utmost importance in churches of this form, but modern

building costs make it extremely difficult to fulfill this basic require

ment. Rows of columns, side chapels,

adjacent spaces with openings to the main sanctuary, recessed windows

and doors, connecting narthex areas,

choir lofts, and similar features all make for improved acoustical con

ditions and should be used whenever

possible. In large churches the total air volume should be divided into

separate weakly coupled spaces to the

greatest extent compatible with visual

requirements.

Trends in architectural acoustics

There are many possibilities for future

developments in architectural acous

tics. The immense variety of new

building materials and methods of construction have greatly increased

the range of novel and unconventional room shapes that architects can create.

This flexibility in design has permitted many deviations from the traditional forms formerly considered acceptable in rooms for speech and music.

Many new structures are advanta

geous acoustically, but others have

proved to be ineffective. As a result acoustical designing for architecture has become increasingly challenging.

A factor of major importance today is the design of acoustically coupled spaces, which means that the total air volume is divided into several connected regions. Close acoustic cou

pling permits sound energy to propa gate readily back and forth between the




several volumes. Conversely, with

weak acoustical coupling the sound

energy leaving one space does not

easily return. Coupled air spaces will be of the greatest importance in the

design of large rooms, where weak

acoustical coupling is needed.

The new architectural freedom should result in designs that provide more

adequate early reflected sound and more efficient sound-diffusing ele ments. It will also increase the possible methods of reverberation control,

including satisfactory variable ele ments. Great care must be exercised,

however, to insure that the imagina tive designs made possible by ad vances in structural engineering ex

clude undesirable echoing and strong reflections, focusing, disturbing dif fraction effects, dead spaces, and

unequal sound distribution. A wide

range of new room shapes will be

evolved, and for these to function

properly there must be ever closer

collaboration between acoustical

science and architectural design.

It has long been realized that acous tical problems become progressively

more difficult with increasing room size. The continual growth in au

dience size and the evolution of more

sophisticated concerts, operas, plays, and church services will place great demands on architectural acoustics.

While the acoustical requirements for small rooms are essentially the same

for both speech and music, these re

quirements diverge greatly for large spaces and present difficult conflicts in design. At the same time artistic creations for large audiences will become more specialized and varied, making the related acoustical factors ever more critical. The skillful use of coupled air spaces will be helpful as will better use of absorptive, re flective, and sound diffusing architec tural elements. Large halls will also require greatly improved sound am plification systems, which will re quire major advances in loud speaker and microphone design and asso ciated electronic circuits. The pre cedence effect must be employed in much more quantitative fashion than in its present applications. The re

quirements for improved electronics pose challenges that must be solved in close collaboration with architectural acoustics. It will not be satisfactory simply to install loud speakers when a building is completed; instead, an entire system must be desig-ned from

the outset to be compatible with all basic features of the architectural

design.

A steady increase in noise is a growing hazard in rooms for speech and music.

It is essential that this problem be considered as a prime factor in the

design and construction of buildings. Here the technology is well under

stood, and it is largely a matter of cost and attitudes for acceptable con

ditions to be achieved.

Architectural acoustics in the future will be closely linked with the evolu tion of new art forms. They will react with each other. New art forms

will demand new facilities from archi tectural acoustics, and advances in

acoustics will continually open new

possibilities that cannot be realized in the present environments of theater,

concert hall, or opera house. One

of the chief factors that will stimulate this collaboration will be larger and

larger audiences, which will have a

profound influence on the character

of artistic performances. A strong

positive approach will be needed to insure excellence. This will not mean

simply making conventional types of

performances available for larger au

diences, but will require an imagina tive evolution of the art forms them

selves, whether these be music, plays, church services, or special events.

Changes in art forms will inevitably demand changes in room shape. The

central stage may be supplemented by many auxiliary performance areas,

and here acoustical developments for

the design of coupled rooms will come to the fore. A beginning has been

made, and the future holds great

promise.

9p

4'4

'A N

'Just don't think about it. We've always been carnivorous, and we always will be carnivorous.''




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