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.c.COUSTICS OF TfIE ROY.AL FES'rIVAL Rill

D.G. Beadle C.L.S. Gilford A~L. l'Jewme!n T. Somerville F.L. Ward

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. C.L.S. Gilford T. Someriille

Report no~ B.OliB Serial No. 1951/15

'.

~J"yATE AND OONFIDENTIAL

Reseorch Department -September, 1951

Figs. Nos. B.048.1\ to B.048.3'

AOOUSTIOS OF THE ROytuJ FESTIVAL HALL .-.._4 ____________ _

SUMf'ElillY

This report is a review of the desi~1 and physicalcheracte­ristics of the Royal Festival Hall based on experience with other' concert h811s. An attempt is made to shOw.how these characteristics are'related to opinions on the acoustics of the hall orrived at by listening to orchestr8's, though ,the lack of any agreed consensus <;>f opinion mal~es such·8 corrEi­lotion difficult.

IlJTRODUCTION

As' the only permanent structur~ in the South Bank Exhibition, the ROY81 Festival fT.1.811 was designed with great Care to ensure that the sound tnsulation and internal acoustics should leave nothing to be· desired. 'I'he acoustic design was carried out by Mr. Hope Hagenal in consultation with the Building Research 'Station and,' in the pro-11minary planning, advice Was sought from .~eric8n and Continental acousticians.

A survey ofBri tish concert hi311s we s also undertaken by Building Research Station 2nd here 8gein the techniques an'd methods of assessment fevoured by other wor\::ors, such as Irigorslov, Furrer . and Bolt were used. The conclusions from the survey were briefly: -

(1) R~verberation time was t'he most important single cri terian.

(2) Stoady-stcte measurements wer~' of most value for comparing sound levols in different parts of the auditorium.

(3) Pulse techniquo, ;/liveness ll concepts arid ide8s of diffusion wore not sufficiently devoloped to be used in connection with new designs.

The desigIlw8s based on the' following considerotions:-

(I) Tho hall should S08t the moximum numbor of people consistent wi th good acoustics.

The total volume is approximately 7~0,OOO cu.ft., the auditorium,seating 3,500 people.

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(2) Insulation against external air- and structure-borne noises, 'especially from the trains on Hungerford bridge, should be adequate.

This has been achieved satisfa,ctorilY; trains on Hungerford bridc;e cannot be heard when there is an

'audience present ~ and the underground 'trains are only' just perceptible.

(3) Opposite surfaces should be 50 or more out of parallel to eliminate standing Waves.

This was not done in the final design. The walls are parallel, but ~here are splays bes~de the orchest~a.

(4) Reverberation time should be as long 8S possible, bea~ing in mind that it has been found difficult t~ sustain a long

. enough reverberation time in large mode~D halls. To this end the ceiling should be 0 good reflector, thore should be no carpet on the floor, and unnecessary porous absorp­tion should be avoided.

In the final de:-sign, carpet vie s laid' on some of' the gangitJ8Ys for roasons unconnected with acoustics. - , Porous absorbents wore used in .3 few places, e.g. at,the back of the h~ll and in the cornice region to eliminate the possibility of echoes. ,Large

. areas of rockwool were also used .at the sides of the hall, covored vd thwooden slats of· such a section that they appear to overlap, yet allo'w free paSsage for s9und waves. The rock~ool could however be removed from these areas for the purpose ofadj,ustment of ,the reverberation time.

(5) The auditorium and platform should be steeply raked to .give good IV sight lines;! and to reduce high frequency attenuation at the" rear seats due to the sound waves passing 8t grazing inCidence mrer the heads of the ~iudience.

(6) Jl lorge reflector should be suspended' over the orchestra, deflectinc sound towards the aUdienco.

Originally desi~led to bo hung fairly low down, the. reflector was finally ,raised to within a fow foet of the coili11g to avoid masking the organ.

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Acoustic 'measuremen'ts were made on iebruarY' 27th and again on April 16th and 17th when the hall was in its final acoustic condition, except for the installation of some of ·the seats~' Some of the tests were made with Build'ine; Research Station el1;gineers present "viththeir apparatus, and results were Com­pared to make sure that- therew8s no disagreement on'the results of physical measurement.-Agreement "vas satisfactory but it

I should be noted thDt their values for"other h[1lls Tneasured in de­pendentlytond to be higher. than ours.

Messurements on February 27th,usinC 'warble tone, gavo the curve of Fig. 1(8) for the empty hall without either carpet or seats. Fig •. l(b) ;s a cl1.rvefor the full hall derived from a Building Research Station recordillB of' pistol shots during the first test concert on Febn18ry 14th~ octave filters being used for analysis. The use of these comparatively wide filters has the effect of increasing the apparent revorberation time above 4000 c/s and this should be token into co~siderGtion in 011

.curves for tne hall full.

The first test-concert had shown that the hall was too deild, (1.6 secs. at 500 c/ s) Clnd mOdifications were already .in progress when the· curve of Fig •. l(a) was ob.tained. Areas of 'Wood-wool in) the cornice region, intended for <echo-s1.lppression, wero being plastered ovJ1r and the rockw.ool was being abstracted from: the back of the wood slats on the f18nks of the hall and from behind the .' J wood panellIng generally.

Figs. 1 (d and 1 (d) show the result of these changos, modi~ fied also by the presence of t.he carpet and soats. Fig. l(c) was measured on April 16th, while Fig. l(d). Was derived from chords. recorded during G concert. It will be seen thotFig., 1(0.) is even lower then FiS. l(b) abov6'3QO c/s. . This may 1:;le explained by ~he fact that the £neas. of sound .BbsorbingIl1steri81s removed,.or covered were not very great nnd therefore the 8cldi tionof the carpet waS sufficient to nullify the effec~ 'of tho chanses.

A..'1 tL.11explained feature of the reverberation curves is the wide differe.::J.ce hetween Figs. l(c) cmd l(d) below 100 c/s. There appmns to be considerable absorption :i,.n this region' in' the empty hall which is nullified by. the presence of the 8.udience, It may be that this absorption is caused by the reinforcod concrete floor structure and is therefore varied by the weight of the audience.

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Fig. 2 shows the, final curve of 1 (d) compared with corres­ponding curves of three Bri tish concert halls having goodreputa-' tion~'(l)TheLi~erpool.Philharmonic Hall curve was measured by Parklll . of the Buildll1g Research Station.,

Curve2(e), is a second' curve for the Royal Festival Hall measured from chords occurring in 8 broadcast performance of Mozartf s· overture to \VIl Seraglio;;. The microphone used for the, brOadc3st wDsabout 40 feet nearer to the orchestra th811'that used lor the'recordinG from which Fig~ 1 (d) was obtained. ,The only Sig..l1ificaut difference between thes'e two curves is at the upper 'frequency. end, a "difference which corresponds to 8' sub..­jective difference in brir';htness between bro8dcast transmissions ?nd the recording. The microphone position chosen fbr the recording 1\]88 arranged to Give the best possible representation of binaurDl lis.tening in the hall.

According.to an extlapolation of Research Department data for studios a:q.d concert halls up to 600,000 eu.ft. the, optimum reverberationtim~for this size should be 1.95 . secs. The average value ',is abou.t "1.6 secs.

~-; ClLIDJf..l1i.OTOGRA,PIJ.CJ. '

PUlsed glide photographs were taken on April 16th and 17th. The information in these displays cannot yet be expressed quanti tati velybut certain conclusions can be , drawn. (See Res€larch Report No. B.04G.) The "opening-out of the successive traces to be seen ut 100 c/s in Fig. 3(a) is typical of colouration occurring as a result of large structural areas vibrating at one frequency. A similar cause is probably responsible for the long docays in the' 80 - 95 c/s region in Figs. 3(b) and (c). On listening to 'the pulses a slight ring was no~icec1 at 85 c/s. This could not be traced to the accessible panelling (see bolow), ond the organ cav,1ty wlth', it's plywood" covering was suspected as a " possible "source. Time Was inSufficient for a detailed inves-tigation~ Many of the decays at ()ther frequencies ore irregular in the mannercornmon in large" concert halls and' are therefore not, reproduced in this report.,

(l)P. H. Parkin'IISome Objective Measuroments in British" Concert fL:ills" Building, Research Str tion note No. D97.

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BEIIJ,VIOUR OF THE P.'U'TELLTITG -----,,~-. - ......... ~--. _._,-----

The panels covering the sides of the hall weTe examined with the contact microphone to determine the natural frequencies and the corresponding decay factors. These tests were made during the April visit after the rockwool damping had been removed. Resonance. frequencies were found to be 100 cia for the larger' and 150 cls for the smaller panels, both types being' fairly uniform., The decay . times were of the order of 1.3 secs., suggesting that there might be colouration, but as no colourations at those frequencies have beep. reported it is clear that the decay times ere sufficiently far below the reverberation time of the hall. It is interesting to note, however, th2t the lower resonance frequency corresponds with the. indication of colour0tion alrJady noted on one of the pulsed glide photo~r8phs, Fig. 3(a), The position of the microphone for this pulsed glide Was nearer the side of the hall, and therefore nearer the panelling, than the other microphone positions, to which

'Figs. 3(b) and 3(c) refer, These photoc;rnphs do not show the colourotion in question.

ECHOES ---No echoes could be found having amplitudes large enough for

them to be traced by means of' the 'directional microphone.

SUBJECTIVE IMPRESSIONS ---~=.;:;.;;.;,;;;;,

For a balanced assessment of the acoustics of a hall it·is necessary to consider the opinions of the general musical publ~c and of· speCialists professionally concerned vd th music in one Way or ~:mother. Itis too early at present, for two reasons,to di!3-cover the general public reaction. F~rst, London concert-goers tend to base their ideas of £\coustics on the /i,ueen! s I-Iall which was llgood;l, and the'Royal Albert It31iwhich is"l1badi1 • The latter is notorious for its 10ng-p8th reflections from largecoJ;lc8ve. surfaces, and consequently I1badacoustics 01 is' usually iqentified' with this one aspect. Judged in accordance with this criterion, a hall without Itechoes il is a good one ,and the' comments of many. music critics suggest that. nothing more need be said. In the second place the sre8t publicitY,before and at the time of the ope~ing concert has inevitably crea~ed prejudices and slliJrpened differencos of opinion so that Cl baitmcer view will only be possible ufter the hall has been in use for some time. . . . -

Most newspaper critics have commented favourably on the acoustics of the' hall, but [] ilUmber' of them wri to of ha rsh tone and' masking by the'louder instru-!Uents, and El few more consio.er the definition to be bad. This view seems to be shared by some B.B.C. 'staff resIX'nsible

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:ror the quality of broadcasting from the hall, for in their opinion it has not been easy to make satisfactory broadcasts, from the point of view of either definition or tone. 'It has also been found necessary to 8ttenu3t~ the extreme bass, presumobly on account of the upward rise of the, reverberation 'time forfrequenc'ies below 100 c/s indicated, by the curves of Figs.,2(d) glnd 2(e) ~

Individuals in ReseDrch Depa;rtment' also vary in tl1eir stated vievJs. Some of those who were invited to attend the experimental concerts thought the hall was good, but most agree tl1at it is ildeadi1 and that definition is poor in loud passages. Some consider that def'inition and tone are both bad and there is a generel feeling that there is too little bass, an impression which cannot easily be reconciled with the'

, shape of the reverboration characteristic.

In the April 15th test concert colouration was noticed by one 'of the writers on tympani tuned to lower F (87 c/s)., This observation Was cOnlmunicoted to Building Research Station in a letter dated"Aprll 16th, but the colouration was stated to be Itnot serious~ In frequency it coincides both with tho ring hoard whon pulsing the hall and with the featuros of Fig. 3(b) and (c) "on the pulsed glides. There are no com­plaints of echoes.

VUi th this review of public and ,expert opinion inter­preted in the light of personal observation it is possible to 'hazard a summary of the main characteristics of the hall as they are most likely to be judged by.posterity •..

(l) Reverberation time is too low. This may be almost unavoidable in a hall designed to seat the maximum audience in the most comfortable conditions. The· .'effect of dcadness .is enhance,d by the direct nature of the sound paths to all parts of the auditorium.

(2) Tone quality tends to be hJJrd.

(3) Defini tion is not as good. as it is in some other halls. It is Good for quiet playinG but loud passages sound confused .•

(4) There are no serious colourations and no disturbing echoes to be heard.

(5) 'Tympani and bras's tend, to ovorpowor the rest of the ()rchestra.

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DISCtJ3SION OF RESUL'lll .-..._---------" Research Department views on the design of concert halls are

presented in detoil below and it is felt that there is no reason to reject them as a result of experience with the~oyal Festival Hall now that it has, been completed. -

. The ma'in criticisms of the design concerned the questions

of diffusion and direction of sound. Many recent concert hall designs, starting with the Salle Pleyel in Paris, have a fan­shaped- plan with a reflector round the orchestra and a cei~ing so shaped as to throw strong reflections from, the stage on to ' the audience. The back wall is heavily damped to I preven t echoes. The intention is to throw 8 bean composed of direct' sound and first reflections as uniformly as possible over the auditorium r so that little scattered sound remains. Such a design mainyains the sound level in the rear seats. In the exPerience of Research D<?partment, 'halls of this type'give an extremely hard quality, u.sually without good definition, certainly without the richness of tone to be found in severql of the'older halls in which the sound is oll6wed, to find its way by nUmerous diverse paths to the ,audience.

The Royal Festival Hall is to some ex'tent a compromise, , tending tQwards the i1directed sound l1 type. The sido walls are parallel but the splayed sections near tho platform, the, reflector, alid the 'shape, of the ceiling, V1direct" the sound to the audience. '

In the initial. form there were no irr~gularitios which could Cl ssist materi811y in producing '.~ di:t;fhse sound field except the li.ghttinG rolls stretchin6 t:r;'?nsve:rsely ~~ross the c'eiling. These could cause 'some scattering but only" in '0 vertical 'plane. The bias of thecompromj.se, w~lile initially towards directed sound" changed during the progress of the desi:;n in favour of sCattered sound. The flying hO,xes v~ere ther:efore added to break up the side wall'surfaceS; but these cannot affect sound reflected from the ceiling. Moreover the padded cavities thus formed, actinG like the rows of boxes in Continental epera house's, contri­bute unwanted absorption.

Research Department opinion favours a shape designed '1;0

produce the Greatest possible r8ndomness in the sound field, with consequentuniformi ty of energy distribution, rather than to form the sound into' a beam directed on the audience. It is thought that only in this way is it possible to achieve a smooth decay 01' sound with good definition and blending of the instrumonts.

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That this may be expected is clear from an examination of t:p.e pehaviour of, sound coming from difforen~ parts of, an extended ~otina sourc~such as an orqhestra. In the "directedn type of hall, the sound coming from a particular Brollp of instruments, say the double-basses, will be fonned"by 'V'arious large reflec-ting surfaces into a number of beams. Honce the intensity of sound from this group of instruments will vary widely from ono part of the :t:loor to another, and the intensity pattern may change quite appreciably with frequency. According to the fre­quency dependence of the pattern, two effects may be noticeable' when :Listenin; to the orchestra. Ei ther the, group of instruments' will be inaudible or abnormally audible, giving rise to bad definition, or the.ir contribution will vary as their notes rise and fall, giving a sensation of poor blend. O~ly by arranging a perfectly diffuse sound, field cc~ one pe sure of hearing"all the instruments in their correct relationship all the time. Lumpy 3nd irregular decay of sound will also result from over-- , direction of sound.

Defini tion is also "affected by the st8ge construction. }J[asking of many instruments durine; loud P8SS8gCS by the tympani or heavy brass may be serious if the latter sections arc placed in a position such that they obtain reinforcement from nearby reflectors. Wi t"1 a conventional orchestral layout, masking is often reduced by the presence of s' choir at the back, and it' is for' this' purpose that in there-trc3tment of Maida Vale Studio No. 1 permanent padded choir Goats will be installed. In the Royal Festival Ho, 11 the piywood barrier' behind the orchestra ,and the reflector above are eX8mp~es of nearby reflectors. '

CONCLUSIONS - ---This discussion makes it possible to describe the sub-, '

jectivolmpressions pfthe Royal Festival Hrill in tenns of ,its physical structure.

The IIdeadness ll is caused by Q low reverberation time aggraVated by the hishlevels of direct sound and" first reflections reldive to that of later reflections. ,The hardness of tone'is also partly associ@ted with deadness; and possibly with insufficient diffusion. The reflector, smooth splayed side walls and absorbing back wall counteract the effect of any diffusing irregu12rities which rr~ybe present elsewhere and the trend is· to' degrade definition and blendinG' causinG: irregulordoc8Ys. The same design featureshawever are'us~ful in preventing .seriouS long..:.term

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echoes since most of the sound is absorbed by the audience and the back wall.

Both the hard reflecting barrier separating t~e choir seats -from the or~hestra, and the overhead reflector, cause reinforcement of those instruments, particularly tympani and heavy' brass, which are normally placed near them.

There are no serious colourations in the bass, since the elm panelling, even wit~out the intended'rock:wool backing, is suffiCiently damped not to cause trouble. 'J:lhere are signs however that this condition is only. just satisfied, the approximate coincidence of the resonance frequencies of similar panels being a potential danger. A slight colouration at' about 90 c/s may disappear when the permanent organ is installed.

Many 'concert hall's, both old and modern, ar'e inferior to the Royal Festival I-fall, but it has, nevertheless, on account of the' "directed sound ll theories vjhich have influenced its design, several undesirable features which have been the subjects of critiCism. In the opinion of Research Department such theories should now be abandoned, though it is perhaps too late to alter the design of the new Q,ueen' sHall. Thi's hall will resemble the Royal Festival lfull in many respects, but wi'll be fan-shaped, the principle of IIdirected sound li

having been taken to its logical conclusion. The Q,ueen's lfull is therefore likely to have, in greater degree" all the faults which have here been discussed.

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FIG.3

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