Reprint_Krokstad1_v2

The Hundred Years Cycle in Room AcousticResearch and Designby Asbjrn KrokstadReprint fromReflections on soundIn honour of Professor EmeritusAsbjrn KrokstadEdited by Peter SvenssonTrondheim, June 2008NTNU

Norwegian University of Science and Technology

Faculty of Information Technology, Mathematics and Electrical Engineering

Department of Electronics and Telecommunications

Acoustics Research Centre

ISBN 978-82-995422-3-4

THE HUNDRED YEARS CYCLE IN ROOM ACOUSTIC RESEARCH AND DESIGNAsbjrn Krokstad, NTNU, TrondheimAbout the paper.The paper was originally presented at Technische Universitt Berlin as an invited paper at the Lothar Cremer 100-Year Symposium on November 25, 2005, honoring one of the former giants in room acoustics research and consulting, (and even in violin physics) at his100-year birthday. A shorter version has earlier been presented at the50-year celebration of the Acoustical Society of Norway in Oslo on April 1, 2005, and in a popularized form at the Krokstad 75-year Symposium NTNU on May 19, 2006.The verbal presentations may have deviated from the manuscriptpresented here due to the fact that I never have been able to give a lecture by reading a manuscript. The oral presentation was of course illustrated by a great number of slides, especially of existing halls, illustrating the principles presented. These pictures would have filled a book, so only a few are included in this paper. Nearly all may be found in the two books of Beranek: Music, Acoustics & Architecture Wiley, 1962, and Concert Halls and Opera Houses Springer Verlag,2004 (abbreviated BeMAA and BeCHOH). Even better: the halls discussed may still be visited, preferably at a performance, so both visual and auditive impressions may be obtained.Derivations of equations, and the background and the theoretical treatments of the concepts discussed, may be found in three very important books:Lothar Cremer & Helmut A. Mller: Die wissenschaftlichen Grundlagen der Raumakustik, Hirzel Verlag 1978 (Band 1 and Band 2). Trying to write in English, I am only referring to the translation of these comprehensive books by Theodore J. Schultz published as:

Lothar Cremer & Helmut A. Mller, translated by Theodore J. Schultz: Principles and Applications of Room Acoustics, Applied Science 1982 (Vol 1 and Vol 2) (abbreviated C,M&S: Vol. I, Vol. II).Wallace Clement Sabine: Collected Papers on Acoustics, Dover 1964 (abbreviated WCS).

Heinrich Kuttruff: Room Acoustics, Applied Science 2nd ed.1979 (abbreviated HK).These books have a very complete list of references to original scientific papers and to scientists who have made contributions to the field of room acoustics. The few mentioned in this paper are only those who have had a great influence on applications: design, methods of design, measurements and evaluation of halls.The term hundred years cycle refers to the fact that today, andfor the last 10 years, nearly all concert halls, opera houses and theatres built or planned are basically copies of halls that are more than a hundred years old. After experimenting with halls differing greatly from the preferred shapes of older halls, one has realized that the empirical knowledge behind the old designs is not easily replaced by theories and calculations.Controlling the complicated physics of sound in closed spaces is difficult, even when using computers. But the main problem is a lack of a complete set of proven criteria to be used when designing halls. Halls for concerts, opera or theatre not only have an integrated influence on the subjective impression of the music and speech produced, but the halls are both visually, auditive and corporally a part of the total experience of a concert, opera performance or play for both performers and audience.During the last century, empirical criteria were gradually replaced by room acoustical criteria based on research. Empirical criteria, which may also be called architecturally formed criteria, use successful halls, or elements of halls, as models. The benefit is that the criteria are complete and proven, and may easily be implemented by the architects. But the models used must be copied very exactly, leaving few possibilities for innovations. And it is a great problem to adapt new halls to changing demands. It is easy to understand that both architects and acousticians were eager to implement room acoustical criteria defining only the important aspect of the sound field produced, allowing a multitude of different designs.

The change from architectural to room acoustical criteria have notbeen without difficulties. A gruk by the Danish scientist, philosopher

and author Piet Hein may serve as a subtitle of the paper:To know what you dont know, is also a way of knowing all. Or inverted:If you dont know what you dont know, the usefulness even of your knowledge is at chance. Not all experts in room acoustics have openly admitted, even to themselves, that not all aspects of design for music or speech communication are based on proven knowledge. Both acousticians and architects underestimated the complexity of designing rooms for optimal live communication, not only of the information aspects of speech or music, but even the emotional and artistical communication between the stage and an audience. Acoustical innovations and design principles were used uncritically, resulting in a dramatical decline in room acoustics qualities during the period 1930-1970. Especially halls for music suffered.During the last 60 years room acoustical research has greatly supplemented our knowledge, and improved both criteria and design methods. And even more importantly, taught us to be more careful when using incomplete and unproven criteria. In practice this means more conservative designs greatly based on proven empirical knowledge.To describe changes in acoustical concepts and designs, the century is divided in eras, named by a dominating new feature in practical design. The eras are greatly overlapping, and practical implementations were often delayed for several years relative to the research results which are the basis.The pre-acoustician era.A challenge for all architects and acousticians: all halls that are widely famous for fine acoustics were built before room acoustics was established as a profession.

GrosserSaalMusikvereininVienna(BeCHOHp.173), completed in 1870, is still considered to be the best concert hall for

classical music of the world, not matched, and certainly not exceeded, by any newer halls.

Semperoper Dresden, restored after being destroyed during the war, does, perhaps in competition with Teatro Coln Buenos Aires,Staatsoper Vienna and Staatsoper Prague, serve as a model for most newer opera halls.

Even the horse-shoe shaped theatres from the 19th century are still acoustically preferred to differently shaped newer halls.Refined empirical knowledge thus seems to be a more trustworthy basis for design than the scientifically based room acoustical criteria developed the last 100 years. The comparisons are not quite fair. The fine acoustics of several older halls are mostly a result of building halls which are impressive visually. Halls, the arenas for cultural achievements, and architecture were fields of competitions between members of the upper class of cities, countries or empires. The cost level of older halls would be so high that realization would have been impossible today when culture is in competition with sports, health care and education.Musikverein Grosse Saal, known from the world wide TV broadcasting of The New Year Concerts with Vienna Philharmonic Orchestra, is of parallelepipedic shape (shoebox) with balconies both on side walls, back wall and behind the stage, see Fig. 1. The ceiling height is more than 17 m. This basic design is found in a lot of acoustically excellent older halls: Smetana Hall Prague, Gewandhaus Leipzig, Concertgebouw Amsterdam, Symphonic Hall, Boston.The Freemason Movement used their international contacts to provide excellent concert halls to several cities in Europe during the19th century, halls using the famous halls as model.It was perhaps not acoustics which was the dominating

Figure 1 The Grosse Saal at Musikverein, Vienna.motivation behind the design of these famous halls. Selecting the rectangular basic form seems natural because it is the shape which is easiest to build and to use. Great ceiling height makes halls and buildings impressive, which certainly is one reason for the great ceiling height also in churches in the Christian part of the world. The balconies may have been introduced to secure the best possible seats to the prominent part of the society. And competition between kings, emperors or members of the aristocracy was perhaps the explanation for the use of the best class materials and rich decorations of concert halls, opera halls, theatres, and even churches.Grosse Saal has two rows of statues, ten magnificent crystal chandeliers, visible beams and a gilded pattern of relieves in the ceiling, see Fig. 1. The back wall of the stage is dominated by a pipe organ. Building materials used are stone, wood, plaster and great areas of glass. Wood and plaster are painted with oil based paint, or gilded (The Golden Saal).

The acoustical success may be explained in room acousticalterms.Normal angels (/2) between planes, as in rooms with a rectangular based shape, is the only angle that does not produce focal areas. Sound is reflected back from a corner in the opposite direction to the direction of incidence (RADAR reflectors).Dominating axial standing waves, which may be a problem insmall rooms with parallel surfaces, is not a problem in large halls. Flutter echoes are heard in halls with only two parallel reflecting planes, seldom in a concert hall.A ceiling height of about 17 m is necessary to obtain reverberation times recommended for concert halls.By using several types of building materials in floor, walls andceiling, possible resonant absorption at the same frequencies for large areas is avoided.Oil painted or gilded surfaces reduced absorption due to open

surface pores of most building materials, so only the unavoidable thermal and viscous surface absorption is left. Grosse Saal has a reverberation time that exceeds 2 sec. even at 4 kHz (BeCHOH p.

594).

Glassisanexcellentreflectorifresonantabsorptionand coincidence is avoided. Composite windows of several small glass areas with lead gratings are thus preferable to the large glass areas used in modern buildings.

Balconies are directing sound in the incident direction (vertically) by combined reflections from the wall and the floor underneath the balconies. Horizontally the sound is directed backwards. The side balconies are thus giving important contributions to early lateral sound, enhancing the spatial impression (Barron, Marshall, C,M&S Vol. I p. 541).In halls without balconies, sound incident on side walls above the height of the source must propagate upwards to the ceiling and down before reaching the audience. For a height of the ceiling of 14 m or more, most of the side wall reflections are delayed by more than the limit of 80 ms for early sound.Making the halls rather narrow secures early lateral sound even tothe central and frontal part of the audience, which often is a problem due to the fact that the delay is relative to the direct sound, which has a small propagation distance to the first rows of seats. At rows in the back of a hall, time delays of reflected sound are relatively less due to a greater delay of the direct sound. In narrow halls an increasing amount of early reflections towards the back seats thus compensates for reduced levels of the direct sound.The magnificent decorations are extremely important as diffusion structures. Diffusion effectively removes audible effects of coloration and flutter echoes, and will improve the uniformity of the spatial distribution of sound, both for early reflected sound and during the late decay. Diffusion from geometrical structures depends on dimensions measured in wavelength. Elements of moderate linear dimensions are diffusing high frequency sound, but not low, and will produce geometrical dispersion: that different frequency ranges of complex sound will arrive at the listeners ears with different time delays. The influence on perceived sound is not known, even if some experiments have been performed. Moderate deviations of the phase from a linear frequency dependency are not detected by the ear, but large degrees of phase distortion are clearly observed.

Three types of structures in Grosse Saal are important for

obtaining sound diffusion at all frequency ranges:

The pipes of the organ are exited at their resonance frequencies by the incident waves, collecting, as all resonant absorbers, incident energy over a much greater area than the cross-section of the pipe, and re-radiating most of the energy as spherical waves. The organ covers, of course, nearly the total frequency range of music. (The organ thus contributes passively to the excellent acoustics of nearly all churches).

Sculptures in heavy materials have a spectrum of dimensions, from the whole body volume, working even at low frequencies, to details as hands and fingers.The magnificent chandeliers are clusters of small reflecting crystal elements and light bulbs: elements which alone have very small diffusing effects at low frequencies, but adding coherently large clusters covers a broad frequency range.It has to be underlined that the magnificent visual impression of many of the halls from the 19th century greatly contribute to the total impression and joy of attending a live performance.As an example of theatres from the 19th century, I will use the oldTrndelag Theatre in my home town Trondheim, built in 1850 and still in use, see Fig. 2. The personal experience from more than 600 evenings in this theatre, as audience, as musician and conductor, in combination with acoustical measurements, makes it possible to combine subjective experience and objective data.The hall is of a traditional horseshoe shape, with two, partly three balconies, seating 350. Materials are brick in the outer walls, wood in floors, balconies and inner walls. The chairs are space saving and moderately upholstered. Besides the upholstered chairs, and curtains at the stage, no absorbing materials are found. The orchestra pit may accommodate 30 musicians.

Figure 2 Den gamle scene at Trndelag theatre, Trondheim.The reverberation times in the empty hall are 0.45 sec. in the octave bands 1000-4000 Hz, increasing to 0.55 sec. at 100 Hz.The speech intelligibility from stage to several audience positions has been tested and is found to be remarkably high (>0.9) at all seats. This is confirmed by measuring the Speech Transmission Index (STI).Even more remarkably: the hall is quite acceptable for music, both from the listeners and the performers point of view.The horseshoe shape with several balconies, as found both inolder theatres and opera halls, is no doubt chosen to secure the best possible contact between the audience and the stage, first of all visually. But, as a result most of the audience is also hearing excellent- ly, being covered well by the direct sound, and thus able to localize and discriminate between sound sources. Short reverberation times give high definition. First-order reflections from below balconies are contributing to increasing the definition at seats that are not in the first row of the balconies. The wooden railings in front of all balconies are reflecting sound which otherwise would be absorbed by the audience.Rich decoration by relieves, chandeliers and beams in the front railing are contributing to partly diffuse first-order reflections both back to the stage and to the rest of the hall.The volume of the hall is so small that no added absorption isnecessary to keep the reverberation times so short. The design thus favours direct sound due to short distances and a distributed audience, and first-order reflections by a lot of reflecting areas in the ceiling, in front of, and underneath, balconies, while higher order reflections (reverberation) are to a high degree absorbed by audience areas.Reflections from the walls of the orchestra pit, from the ceiling above and first-order reflections from the rest of the hall seems to be sufficient for musicians both for hearing themselves, singers at the stage and the rest of the orchestra.Concentration, contact and visibility may be the characterizing

terms for the traditional theatres and opera halls.

Opera is a very expensive art, involving several groups of highly skilled performers. The opera halls therefore are normally greater than theatres to allow a greater audience. The reverberation times in Italian opera halls (La Scala, Milano) were moderate, possibly to facilitate the understanding of the text, which mostly is in Italian. Opera halls outside Italy, such as Semperoper Dresden, used more volume per seat to improve the conditions for music (the Italian text was not understood anyhow).

In the Christian part of the world, churches have been, and are, among the best concert halls, and are of course found even in smaller communities. The volume per seat is large, building materials are mostly stone, brick, timber or thick wooden panels. The reverberation time is long even at low frequencies, commonly longer than 2 sec.Pipe organs are important also as diffusing elements, and the pulpit, altar, organ gallery, chandeliers and decorations all contribute to a high degree of broadband diffusion. Excellent room acoustics has no doubt been a great source of inspiration for the many composers and performers who have made church music an important part of our musical heritage.Several medium size halls suitable as ballrooms, and even forrecitals and chamber music, complete the picture of halls built before1900.Characteristic for the era: halls were design for a specialized function. The economic system did not request profitability, so it was not needed to sacrifice the best solutions to find compromises between diverging conditions of use.SabinePure admiration is my excuse for using more than a page on one of many who have contributed extensively to our knowledge in room acoustics. It is justified to consider Sabine the founder of room acoustics as a science, not only the physics but also the development of the basic criteria for adapting acoustics of the room to applications as lecture rooms or concert halls. And he even established the tradition of using acousticians as consultants for the design of greater halls.A little more than one hundred years ago, Wallace ClementSabine, assistant professor of Mathematics and Natural Philosophy, was ordered by the head of his faculty at Harvard University to find a method of improving the speech intelligibility in some rooms to be used as lecture rooms. After working in his spare time, evenings and nights,formorethan

twoyears

studying,

experimentallyand theoretically, sound fields in closed rooms, he formulated his well known lawsintroducingtheconcept

ofreverberationand reverberation time. He demonstrated its practical importance for the listeningenvironmentby usingtestgroups

andvarying

the reverberation time by varying the number of similar cushions in the room to obtain subjectively founded optimal reverberation times for music and for speech. A reprint of his papers describing his ingenious experiments and his analysis is collected and commented in WCS.Sabine expressed his findings in a very elegant and concentrated form, demonstrating the great benefit of formulating physical laws as mathematical formulas. In a modern form using SI units Sabines formula is well known:T = 0.163 V , A = $ "i # SiAiT - Reverberation time, sV - Volume of the room, m3A - Room absorption, m2 - Absorption factor, no dimension(using the European factorinstead of coefficient) S - Room surface area, m2Sabine is characterizing the acoustics of a room by its reverberation time in seconds, a quantity which may be measured with a watch. Imagine the importance at a time when measuring the sound pressure was nearly impossible. (Even today it is a great benefit to measure parameters which are not dependent on calibrated pressure measurements). Sabine used a well defined source: organ pipes driven by compressed air, and his own threshold of hearing was used to define the end of the decay. (We now of course define reverberation time as the time for a 60 dB logarithmic decay). The reverberation time depends on only two quantities:1) Volume of the room, the only quantity which is clearly defined in all types of rooms, regardless of shape.2) Room absorption, defined by Sabine as equivalent area open window, an area given in m2.The constant (0.163) of course depends on how reverberation time is defined, and on the units used, but it also depends on the speed of sound, and thus on the air temperature in the room. Sabines formula indirectly states that the reverberation time is not dependent on:shape of the room, placement of absorption, source position, measuring position.

The reverberation time is a global parameter for a room, while most room-acoustical parameters introduced later are local parameters varying from seat to seat. It is often stated that Sabines equation isbased on a diffuse sound field, i.e. a sound field where the energy density is uniform and the intensity vector is independent of direction. Experience shows that Sabines formula gives a reasonably precise prediction of the decay rate in most practical situations, even when the conditions of a diffuse field are not satisfied.Through the years several attempts have been presented for refining Sabines equation, especially for removing the obvious problem that for an absorption factor of 1 on all room surfaces (anechoic room), the reverberation time should be zero, while Sabines equation still gives a greater numerical value.Eyring (C,M&S pp. 227,231) solved the problem with a formula which may be expressed in a power series expansion of the mean absorption factor for all room surfaces. Sabines equation has only the linear (first order) term, which is dominating for small values of the mean absorption factor, while more terms are needed when the mean factor is greater than about 0.5. (Three terms increase the range to about 0.8). A thorough treatment of reverberation and formulas is given both in C,M&S and in HK.Sabine was engaged as a consultant for the design of a new concert hall, Boston Symphonic Hall, which was completed in 1900. He thus became a pioneer even in room acoustic consulting. Sabine realised that reverberation time was not the only parameter of importance for concert halls. He recommended the use of proven knowledge and old Gewandhaus in Leipzig, well known for excellent acoustics, was used as a model for the Music Hall. The number of seats had to be increased from 1500 in Leipzig to 2600 in Boston, which was taken into account by adjusting the reverberation times. Boston Symphonic Hall is rated among the best in the world.Problems with mediocre concert halls from the 20th century couldhave been avoided if Sabines humble admission of limitation in his theoretical knowledge had been followed up by all consultants.

The Absorption eraA very rapid expansion of two media: motion picture and radio, during the first half of the 20th century, promoted the applications of Sabines theories strongly. Most halls built in this period, both municipal and private, were cinemas or multi-use halls where cinema was an important function. The interest in room acoustics increased with the introduction of sound film about 1930. The acoustics is on the sound track and contributions from the hall should be avoided, was the dominating point of view. A great interest in absorption and absorbingmaterials started. A new industry developed, producing and marketing absorbing materials for different applications, and acoustics consul- tants and architects were cooperative.Broadcasting studios and film studios were also designed asanechoic as possible. If reverberation was wanted, controlled amounts of reverberant sound from empty rooms with a long reverberation time were mixed in live or on a sound track.Up to about 1970 large areas of perforated panels were assumedto indicate high acoustical quality rooms. Dead studios may be argued by the need of high definition for monophonic transmission and a bandwidth of 4.5 kHz. In a cinema the reduced sound levels due to absorption may be compensated by an efficient loudspeaker system. Speech intelligibility and noise reduction are of course reasonable arguments for using large amounts of absorption. Absorption was therefore used in auditoria, theatres and even in churches and concert halls. (Several hundred square meters of absorbing areas has had to be removed from Oslo Concert Hall, built in 1975).The music and the musicians suffered.Unnecessary use of absorbing materials is still a problem, perhaps due to the use of reverberation time as the only design parameter. The reverberation time is only defining the ratio between volume and room absorption area, not the quantities by themselves.Research in the period.A centre for fundamental research in acoustics in the 1930s and1940s was Bell Telephone Laboratories in USA. The main applications were of course in telephone and radio, both audio and transmission. But fundamental research was done also in hearing, speech intelligibility, and statistics of speech and music as signals, topics of great interest even for room acoustics.The Fan Shape EraLarge cinemas were made fan shaped to limit the sight angle for the audience to a maximum of 30 from the normal to the screen. The fact that the centre of gravity for a fan shaped audience area is rather far from the screen, may in a cinema be compensated for by using a large screenandapowerfulsoundsystem.Architectsandbuilders uncritically copied the fan shape even for theatres and concert halls for several decennia. The architects were perhaps also inspired the old Greek and Roman theatres, such as Epithaurus. The differences between a two dimensional picture of a motive, and the motive itself

(a)

(b)

(c)Figure 3 Illustration of (a) a two dimensional picture as seen normal to the screen, (b) the same picture as seen from an angle of 450 and (c) a picture of the real motif from 450.(live) were not quite realized. The three pictures in fig. 3 are showing a well known motif: a picture as seen normal to the screen, the same picture as seen at 45 from the normal, and last is a picture of the real motif at 45. There is no need for limiting the sight angle at live performances: plays and concerts. On the contrary, intimacy, good contact between an audience and the stage, both visually and auditive, is of great importance, leading to a benefit of placing the audience close to the stage, even if it means to the sides of, or behind, the stage.

The fan shape era lasted to about 1980. The winning proposal of

the architect competitions for the concert halls in the largest cities of

Norway: Oslo Concert Hall, and Grieg Memorial Hall, Bergen, were both fan shaped. A protest from me resulted in a modification of Oslo Concert Hall. In Bergen we had to compensate the difficult basic shape with a large area of specially designed reflecting areas on walls and in the ceiling.Problems with fan shaped concert halls are:the centre of gravity of the audience is a long distance from the stage (many seats in the back row),the side walls are poorly illuminated from the stage, and are not covering the audience area with lateral early sound, resulting in a lack of spatial impression,halls will be wide and in the midst of the audience, it is difficult to be reached by any lateral early sound at all,a large back wall, often combined with the ceiling, may give echoes at the stage and/or for the audience.The Deutlichkeit (Definition) Era.Thiele (C,M&S p. 430) introduced in 1953 a parameter he calledDeutlichkeit (eng. Definition or Distinctness),

50ms"p2dtD = 0

" p2dt50msp is the measured or calculated sound pressure in a listenersposition as caused by an impulsive and non-directive source.The ratio between early energy (direct sound and reflections up to a delay of 50 ms re. the direct sound) and the total energy of the pressure impulse response is the first of several parameters which in different ways try to describe the characteristics of the transfer impulse response in a room which is of great importance phychoacoustically.Haas (C,M&S p. 474) had some years earlier revitalized Theprecedence effect showing the dominating influence of the first arriving wave front, and even confirmed that about 50 ms is the delay limit for hearing echoes with speech signals.

Lochner & Burger (C,M&S p. 432) expressed definition as Signal to Noise/Ratio using two weighting functions: one defining early sound, which is including direct sound and is enhancing speech intelligibility, and one defining late sound, which was considered as noise, reducing intelligibility.

Allformulasaregivenasratiosbetweensoundenergycontributions, and are thus not dependent on calibrated measurement of sound pressure, and may as well be expressed by the impulsetransfer function h(t) or H(). Only the time scale must be calibrated. (Exactly as when measuring the reverberation time).Thieles formula may give some problems due to the discontinuityat 50 ms: a strong reflection delayed by 49 ms is numerically increasing the Deutlichkeit, while a delay of 51 ms is decreasing it. In most large halls a lot of first order reflections are arriving delayed by about 50 ms, so the measured Deutlichkeit may be varying from seat to seat while listeners impressions are nearly the same.Cremer and Krer (C,M&S p. 434) proposed using the centre of gravity (the first moment) of the squared impulse response as a measure of the relative strength of early and late arriving sound. This centre time has no discontinuity (a weighting function

linearly increasing with delay time) and show variations that are well correlated with perception.In spite of this obvious problem, Thieles Deutlichkeit when presented became an important supplement to the reverberation time, and recommended values for different types of halls are available. Deutlichkeit is, however, a local parameter, varying from seat to seat, while the reverberation time in most halls shows an astonishly small range of variation, so even in practise it is very nearly a global parameter.Deutlichkeit had for a period a great direct influence on design. In most halls for speech, and even in some for music, the audience area, walls and ceiling were formed to keep the Deutlichkeit at a near optimal value, also in back rows of seats, by compensating the reduced direct sound with increasing levels of early reflections. Examples may be found in BeMAA pp. 101 and 259.Leo BeranekAs a basis for the design of the new concert hall and opera hall at Lincoln Centre, New York, Beranek organised a survey of 54 halls, bothbymeasurements,architecturaldescriptions,andsubjective evaluations by selected listeners and musicians.

The survey, as presented in the book BeMAA, represents a very

important step forward in room acoustics, first of all by collecting the very extensive information, and especially for introducing subjective evaluations, allowing both musicians and listeners to express their experiences.

Strangely, of all important attributes discussed by Beranek, only the dubious parameter initial gap, the gap between the direct sound and the onset of reflections, was lifted to the top level as supplement to

reverberation time when characterising a hall. This unfortunate choice had a great influence on the design of The Lincoln Centre Symphonic Hall. The original hall was partly fan shaped. A cloud of adjustable reflectors was designed and adjusted to keep the initial gap low for all seats. We all know that the hall was not a success acoustically. We may say that Beranek did not know what he did not know, even if the information was buried in his collected data. In retrospect it is easy to see that the lack of lateral early reflections was one of the negative aspects of the original hall.Beraneks ratings of the 54 halls, which were not published in the book, but were available on demand, showed that nearly all halls rated as excellent were shoe-box shaped halls with balconies. The survey has thus had a very large influence on design and is probably the main reason for the change back to the shoebox shaped concert halls.My protest against the choice of a fan shaped hall in Oslo and in Bergen was to a great extent based on a paper given by Beranek at the International Congress on Acoustics in Copenhagen 1962.Nearly at the same time as the survey, Berlin, with Cremer as consultant, realized the real great experiment, the 360 Philharmonie Hall, a hall with a stage surrounded by audience. The idea is wonderful: to secure as intimate contact as possible between the audience and the musicians. The problems with a large concert hall with the audience surrounding the stage are:it is difficult to supply early lateral sound to every part of the audience area, and to the musicians on stage,a large surround hall must be rather wide, and the mean propagation time between reflections is long.These problems must be solved by using reflecting surfaces distributed among the audience areas, and bodies mounted or hanging under the ceiling, giving partially horizontal reflections. The reflecting surfaces must be rather large to reduce geometrical dispersion, and must of course be illuminated frommanysources onstage. Toobig reflecting areas will reduce the audience area, and the main idea: intimacy, is reduced. (It must be underlined that these problems do not exist in a small surround hall).

Simplified: in a concert hall the solid angle covered by absorbing audience as seen from sources on stage must be smaller than the angle

covering reflecting surfaces. The ratio of direct sound energy mostly absorbed by the audience to the energy supplied to reflections, a partof which may be the reverberant field, may be estimated from these solid angles.The successful design of a surround concert hall thus depends on room acoustics criteria which are complete and proven. The sound distributions must be carefully calculated to find a design that fulfills the criteria for most of the audience area and the stage. First order reflections may be traced reasonably well, but higher order reflections, and the influence of diffraction and diffusion are nearly impossible to calculate by hand. Berlin thus underlined the need for computer assisted design in room acoustics.Beraneks survey and Cremers ideas resulted in a very active period in room acoustics research, both on criteria and on calculations. Universities in both West and East Germany, and in Great Britain, were centres of research, and a remarkable number of bright and dedicated young scientists fulfilled their PhD degree on a topic within room acoustics. Not only universities were contributing. Broadcasting companies in Great Britain, Germany, Netherlands, France, USA as well as in Eastern Europe and in Scandinavian countries were active in applied room acoustics, and several consulting firms all over the world gave contributions of great value.New methods such as Factor Analysis and Multi DimensionalScaling were introduced in subjective testing, and new tools such as Kunstkopf (Artificial Head), TRADIS, Electroacoustical simulations became commonly used. Perhaps most important, the computers became an ordinary tool. PCs with benchmarks like

previous supercomputers

became

common tools,

and

new

series of supercomputers reducing the CPU time from hours to seconds removed hardware limitations.Design tools.Scalemodels

haveofcoursebeenusedforalong timefor visualization of design ideas. Scale models with sound, both for measuring and auralization (listen to music or speech in a model of a hall before realization in full scale) became possible in the late fifties due to extended frequency ranges of loudspeakers and microphones, and by using tape recorders with selectable tape speed to transform the frequency in proportion with the scale factor. The most elaborate use of auralization in a physical model was perhaps done by acousticians at BBC by designing a new studio for symphonic music in Manchester. Barron has used measurements in small scale physical models to a large extent both in research and design.Computer simulations in room acoustics.Manfred Schroeder (Gttingen and Bell Labs) and Heinrich Kuttruff (Gttingen and Aachen) were the pioneers in the application of computers in room acoustics. The acoustics group at NTNU (former NTH) and SINTEF in Trondheim was perhaps the first to publish a paper demonstrating the use of computers as a design tool (CM&S p.118).Being employed as a lecturer at NTH, I joined the first group to fulfil a course in computer programming, and got training in using the first computer acquired by NTH, a Danish GIER computer. The first application in room acoustics was to test the absorbing properties of a new anechoic room, especially to find the lower limiting frequency for free field measurements. Ordinarily, this had been tested by finding deviations from the distance law: that the sound pressure amplitude is decreasing linearly with the distance from the source. My idea was to simulate reflections by single image sources. The strengths and positions of the image sources were found by computer calculation of frequency responses in several positions and to adapt calculated measured responses. The knowledge of the first order image sources could then be used to correct measurements by cancelling contributions from reflections. The work was accepted as my Ph.D. thesis, but the cancelling method was never used in real measurements.When NTH and SINTEF acquired a Univac 1100 computer, ayoung student and specialist (Orakel) in programming, Svein Srsdal, was hired to extend the image source method to concert halls. The problem of visibility of image sources in rooms which are more complicated than a parallelepiped, forced us to change to the use of ray tracing. At the University of Oslo, professor Wilhelm Lchster had formulated a master project using a computer to study the problems of measuring absorption in a reverberation room, and a student, Svein Strm, developed Fortran routines for these calculations using ray tracing.

After completing their master thesis both Svein Strm and Svein

Srsdal were employed by the Acoustics Laboratory, a group of SINTEF under my leadership, to continue the project of room acoustics simulation using computers. Srsdal did most of the work on a system level, and also made a specially developed program for controlled storing of intermediate data, Strm developed most of the routines and I was responsible for the acoustics. We had a workingprogram in 1967, and the paper demonstrating the possibilities of the method was published in 1968 (CM&S p. 118).We were quite aware of the limitations of the ray tracing method. The method was characterised by us as a sampling of the geometry of the room by spherically diverging rays, gradually changing from deterministic to statistical hits of the surfaces when going from first order reflections to higher orders.By a two dimensional Fourier transform of boundary surfaces, atleast two hitting rays per wavelength are necessary to avoid aliasing (which here means that a small detail may be treated as a main dimension). By increasing the number of rays emitted by the source, precision may be improved, but surfaces parallel to the direction of rays may never be hit. We therefore limited our calculations to reflections within delay time intervals up to 200 ms, using only energy summation. The direct sound was normally calculated using the distance law, and the reverberant tail of the impulse response was calculated by Eyrings or Sabines equations.The program was organized as a set of routines. For several of these routines, alternatives may be used to cover different situations. (Statistical or geometrical reflection). The history of each ray: surfaces hit, angles of incident, and propagation times were calculated and stored. After completing the basic ray tracing, the energy of each ray could be modified to describe directivity of the source, angle- dependent absorption at the surfaces which were hit, and finally: directivity of the microphone. The ray history really gave a possibility to determine the sound pressure amplitude and phase (seldom used).The program was suitable also for designing sound reinforcement systems adapted to the room. Three types of microphones were used:The whole audience and stage area.Selected small plane areas (ordinarily selected parts of the audience or stage areas).

Spherical surfaces which could be placed in points of interest. The two last types of microphones were used to calculate room

acoustical parameters: definition, centre time, intensity and lateralfactor.

Simulating diffuse reflections was possible by reflecting rays randomly in direction from a surface, but the number of rays had to be increased, giving problems even for large computers in the first decennium.Our main interest was concert halls. We could then simplify the calculations as most surfaces are nearly totally reflecting, except the audience areas and the stage which are absorbing more than 80%, and were in some calculations considered as totally absorbing. A ray was reflected and kept its initial energy, and its lifetime ended when hitting the audience or stage area. The three types of output were (see an example in Fig. 4):Graphical plots of hit-points over the audience and stage areas, sorted in delay time intervals relative to the direct sound. The incident directions for the rays were plotted as tails in the lateral direction with their length given by the sine of the vertical angle.Plots of echograms based on energy input to rectangular or spherical microphones.Numerical values of room acoustical parameters.The program was used both for research and in consulting. The research projects were performed and reported by Svein Strm. The first research project was a comparison between halls described by Beranek, some in the group with highest and some with lowest ratings. The geometry was simplified, using only about 100 plane surfaces to simulate the halls (really half of a symmetrical hall).The example presented in Fig. 4 demonstrates a shoebox showing the space and time distribution of reflected sound. The next project was a systematic study of shapes, starting with a shoebox and modifying to fan shape of different opening angels, inverse fan, partly fan etc. while keeping the volume and the audience area constant (and thus the reverberation time).The first computer assisted designed concert hall in the world is Hjertnes in Sandefjord, Norway. Griegs Memorial Hall in Bergen was a more difficult task due to the fact that the concrete body of the building had been completed 7 years earlier, and was fan shaped. Using computer simulation greatly improved the communication with the architect and with the responsible building committee. Most people trust a computer more than an acoustical consultant. Measurements in the completed hall verified the calculated parameters.

Several computer programs for room simulations are now on the marked, two of them from Scandinavia. Calculation of diffracted

sound has been included rather recently.

Figure 4 Example of output from the ray tracing program. From Investigation on room shapes by S. Strm, A. Krokstad, S. Srsdal, and presented at the 7th ICA in Budapest 1971.Calculated binaural impulse responses may be realised as digital filters. Anechoic recordings of music (and speech) may be played through such filters. By listening using headphones, or cross-cancelled loudspeakers, a fairly good impression of the listening conditions in different seats may be tested in the design stage.

The Spatial Impression Era.Marshall and Barron (CMS p 541) found by experiments with electro- acoustically simulated sound fields that early lateral reflections contributed greatly to spatial impression, or envelopment by sound. Early was found to be up to 80 ms delay for music (compare to 50 ms for speech). Barron defined a new parameter: Lateral factor (or lateral efficiency)80ms$

( p " sin #) dtl = 25ms

80ms$0ms

p2dt = 0 in the direction of the sourceThis definition is the realistic form, as (p sin ) may be measured by a gradient microphone (Figure 8-microphone) with its axis normal to the direction to the source. The direct sound is excluded in the numerator by integrating from a delay of 25 ms, and by using a directional microphone. The lateral factor is thus a measure of the fraction of early energy of lateral reflections to the total energy of the early part of the impulse response. It is of course a local parameter.The directional weighting has maxima to each side, and, strangely enough, the same weighting for forward and backward reflections (a concert hall doesnt need a back wall (Tanglewood Music Shed BeMAA p. 139). (Inter aural cross correlation factor is used as an alternative).Marshall and Barron showed that the cross section of a concert hall to a large degree determines the value of lateral efficiency, and that a narrow shoe-box shaped hall with balconies is giving a great spatial impression. Marshall, an architect by education, designed two halls in his native country, New Zealand, with great values of lateral efficiency combined with intimacy (BeCHOH p. 433).Conditions for the performers.Comments on the acoustics from singers, musicians and conductors have been considered as important, but Anders Gade (Copenhagen) was the first scientist who succeeded in quantifying in sound field parameters the experience of active performers. The work of Jrgen Meyer, Braunschweig was a great inspiration for the work of Gade, and also in the acoustics group in Trondheim have several experiments on stage acoustics been completed.Most of our work is published as master reports only, but some projects were presented at a Meeting of the Nordic Acoustical Societies in Trondheim. Perhaps the most original of our experiments was a test to find the preferred ceiling height in outdoor music sheds (pavilions). We made a ceiling of wooden panel (40 m2), made diffusely reflecting by 30 buckets glued to the panels on one side, while the other side was a reflecting plane. The rather large building element was lifted by a mobile crane to form a ceiling over the voluntary test band, which was the military band of 26 professional musicians in Trondheim.The test arena was the midst of a football field, far from reflecting surfaces. The ceiling was lifted in steps of 0.50 m from 4 to 8 metres, and in each position turned around between the diffuse and the plane surface. In all positions the band played one quick movement and one slow movement from Suite Ansienne by Johan Halvorsen. The judgements of the musicians were given separately for each movement in each position, both an evaluation of the ease of playing and a comparison with the previous condition. The test was then repeated in a random order of ceiling elevation and diffusion.The preferences were quite clear: the height of 4.50 meters above ground for the plane ceiling, 4 meters for the diffusely reflecting ceiling. The individual differences between the musicians were small, as was the difference between the two widely different tempi of music.Based on simulations and interviews Gade defined two stage parameters for music execution:100ms"

p2dtSupport, ST = 10 log 10 ms dB5ms"0ms

2

direct80ms"

p2dtHearing each other, EEL = 10 log 0 ms dB5msas measured with an impulse source

"0ms

2

directSupport indicates a soloists concern about reaching out in the hall. The energy of received reflected sound, delayed more than 10 ms to exclude the direct sound, and up to 100 ms (the early sound and a little more), is compared to the direct sound at a distance 1 m from the instrument.

Hearing each other When playing or singing in an ensemble, it may be difficult to hear each other due to masking by his own produced sound, so the ratio between early energy from each of the others are compared with the level produced by himself describes this problem.The multi-purpose or multi-use era.(The distinction between multi-purpose and multi-use is of course that purpose is the plan. The use may deviate from the planned, after some years the difference may be astonishing). The arguments for multi-purpose halls are of course purely economical, but are a consequence of the attempt to include smaller towns and people living in the countryside in cultural activities. It seems economically convincing that a town using the concert hall for concerts only each fortnight should use the hall for other cultural activities in between.From about 1960 halls even in large cities have been planned to

cover several types of activities, in the first place cultural activities, such as concerts, theatre, cinema; but also congresses, indoor sports, dance, bingo etc. Acousticians had experienced the problems, but started eagerly, especially in Scandinavian countries, research and development to solve the problems. Three type of solutions to the acoustics in multi-purpose halls are used:giving priority to one application and accept problems for the rest,choose compromises,adjust the acoustics to the purpose by designing a technology for varying the acoustical properties of a hall.If the responsible builder is able to make a definite and clear priority for one application from a list of all the planned ones, it is possible to make the hall function well at least in one application. Choosing compromises may result in a hall not really useful to any application.

The stage is perhaps the greatest problem. The stage of a theatre or opera hall has side scenes and a tower and a lot of carpets used by

changing scenes; and several bridges across the hall for spotlights. A choir or an orchestra is dependent on reflecting environments, not only to secure early reflections between members of an ensemble, but also

to connect the stage to the reverberant field of the main hall. To get rid

of all carpets and to close all openings to side scenes and the stagetower is a great job, and need qualified people. Button controlled solutions are very expensive and are difficult to keep updated.The acoustics of the hall may be varied in different ways:Movable building elements or reflectors may be used to divide one big hall into two small ones, or to reduce the effective ceiling height of the hall, and thus the reverberation time. The architect and acoustician George Izenour, USA, has designed some impressive multi-purpose halls using movable reflecting elements.Absorbing elements/carpets may be unfolded to cover reflecting areas, or elements which have one reflecting side and one absorbing may be turned around. Variable absorption will change the reverberation time, but may also modify the direction and distribution of reflected sound.Electroacoustical reverberation enhancement, also called active methods. We may include ordinary sound reinforcement systems in the group. A sound reinforcement system with highly directional loudspeakers may make a concert hall useful for speech by reinforcing only the direct speech sound.Three types of reverberation enhancement systems are in use:Multi channel narrow band systems, used in Royal Festival Hall, London.MCR (Multi Channel Reverberation, broad band channels reinforcing reverberant sound (Pioneered by Philips).Channels with electronic processors, picking up mostly direct sound and radiating reverberant sound.When marketing systems, differences in details are often claimed as differences in principles. The general problem with all types of system is acoustic feedback, making the systems potentially unstable. Too small margins of stability result in coloration, long reverberation for one or a few pure tones where the loop gain is at a maximum. Active systems have a lot of drawbacks compared with the best passive solutions, and are therefore recommended by me only as a last resort.

Experience show that multi-purpose solutions should be avoided.Different halls for different activities, with excellent conditions for the audience and for performers, should be found, if not in every community, so within reach by modern communication by cars. Not only the acoustics is a problem by multi-use halls. Qualified personnel

is necessary for changing the hall to the next and different user. The cost for managing a multi-use hall over its life time may thus be greater than building two specialized halls.Today?Most responsible builders, architects and acoustical consultants today choose the secure solution, making near copies of the old halls. One important difference is common: modern halls are not so richly decorated with relieves, statues, chandeliers etc as older halls. Economics prevents building halls as richly decorated as Musikverein Vienna or the opera hall in Praha. Lack of diffusion and the use of materials of a lower surface quality are mostly the reasons for modern halls still to be inferior to the old ones.Two important questions:Is it possible to build halls that function even better than the old? Must changes in cultural activities and technical possibilities bereflected stronger in the planning of the next generation ofarenas for live performances?Today people have access to top artists on CD, DVD, radio and TV at home, and even outdoors, by headphones. Is an upbringing with a near to permanent envelopment of technically transferred music or speech changing the listening ability of people? As an example: recorded music has normally a higher definition than in a concert hall. Listening to a violin concerto in a concert hall may be a disappointment due to the fact that the soloists part is not so clear and dominating as on a recording.Some new concert halls try to satisfy the modern listener by increasing both the definition and the spatial impression, and at the same time making the halls even more intimate. (BeCHOH, p. 219). Using three balconies with enhanced RADAR reflectors under the floors increase contributions to early lateral reflections. The ceiling height is increased to 23 m, keeping the reverberation times close to 2 sec. Audience on several balconies, even behind the stage, reduces the length of the hall and the distance from back seats to the stage. The visibility is improved and, as in an opera hall, the level and quality of the direct sound is high.

Most opera houses today have, or plan to install, screens at eachseat to present the running text. Reverberation time may then be increased to enhance the music. The intelligibility of speech or singing is no longer important. It must be realised that halls are no longer theonly key to cultural experience. The social aspects, the undisturbed concentration, and the two-way contact between stage and audience are of importance for making people leave their comfortable chair in front of the TV screen.Sound reinforcement is an integrated part of modern dance- and popular music. Even in theatres it is commonly used. It may be said that sound reinforcement is making the acoustics of halls obsolete.Classical symphonic concerts (Hollywood Bowl), plays(Stiklestad) and operas may move outdoors, and acoustics are no longer giving the limitation on size of the audience. Weather conditions may make halls preferrable, but only for shelter, not for acoustics.Strangely, acoustically transmitted speech and music has survived so far, and people with an upbringing in pure acoustics will always consider electroacoustically transmitted signals a step down in the experience of music and speech.

What about the generations with an electro-acousticalupbringing?#

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