..

RESEARCH DEPA.R.TME.'NT

AOOUSTIC 11/lE.ASlJRJ!lvlEl\,TS '.iN BELFAST STUDIOS

Report No. B.038 Serial No. 1949/6

Resgs'roh Department. January 1949.

Surumarz

REPORT NO. B.038 Serial No. 1949Z6

ACOtBTIC Iv.::EASUREiL:BNTS IN BELFAST STUDIOS

Figs. Nos. B.038.1 to B.038.4

This report desoribes acoustic measurements in Studios Nos. 2, 3 1

5 and 8, Belfast. It shows that the new t:reatment in Hos. ~"i, 5, and 8 gives an aesthetic inwrovement over tho original temporary treatment, and that tho acoustic results aro greatly improvod. Thore is, however, too much absorption at middlo frcqucnoios and too little in the bass.

General.

The Belfast studios were oompleted after the outbreak of war and a temporary acoustic treatment, consisting of Cabot's quilt and felt, was used. Recently it was deoided to re-tre~t some of the studios at a minimum cost with more permanent cnd less unsightly materials. The troatment adopted wns the sarw for studios Nos. 3, 5 a.nd 8. It con­sists of c.. 6' de.do of flecked blue linoleum fastened on 7/8t! battons spaced 3' e.part, with crorun distempered hessi!m over throe lc.yors of carpot felt covering the rest of tho wr'.lls. The fc.lso ccil'ings of thin white muslin, nprroximntely 3i' bolow the Llain COiling, ero acoustically tr~Dspcront and tho ventilation ducting abQvo thom pre~ dUQOS such a high cJllbiont souna. lovel in the studio that tho plc.nt has to bo shut down during transmission.

Studio No. 2 has not yet been modified and measurements VIere bnly made to obtain data.

Studi~

This is a small talks studio of :3,100 cu. ft. (17%' x 16' x 11' high). It still has its tempo:rary treatment and measurements were made with a view to supplying data on whioh to base a tr6atlilent using permanGnt L'"Jaterials. On roturning to London it was found that plans have now boon made for this studio to bocome a corltrol cubicle, and in.

,consequence the results givon below are of academic interest only • . '

Thocoiling of this studio is roupJ1 concrote and tho bare briol: walls have boon distempGrod and dloapod with a 3' friozo of Cabot' oS

quilt. Four hardboa.rd panols (2'10" xlll) and five Cabot's quUt pan61s of tho salilC, size arc distributoc. around tho rJalls to completo

.~ .• !

the acoustic treatment. Considerable additional absorption must be expected from the furniture which consists of a settee, two easy chairs and eight ordinary chairs, though it is probable that this varies according to the programme. The sound insulation is poor and it is necessary to close an adjacent lavatory during trwlsmission.

The tone pulseI' showed that tho studio was poor ovor the region 230 ... 1,7E<; cls and Fig. 1 gives somo typical photographs. Tho raggod nature of tho docays is clear from Figs. l(a), (0) and (d), and dead­noss is most marked i~ Figs. l(b) and (8).

The hardboard panels had their main resonance at 175 c/s with subsidiaries at 125 and 220 c/s, but the decay time of the main mode was less than that of the stUdio so that no colouration was noticed from this sourco.

The averago reverberation time of five miorophono positions is giv(;n in Fig. 2(a). The studio is too doacl at high froquoncies and a little too livo at low, tho optimum time for this sizo of talks studio bOing 0.52 seconds.

studios 1:os. 3 ~

Thoso studios form part of a dramatic sui te. They era both of . fl 1 .'

the same size (28¥' x 212 ' x 14' high) and havo the sruno acoustic treatment.

The tone pulsar gavo p00r transient response in the rogions of 75 c/s, 550 cls fu~d 1,100 c/s in both studios and characteristic photo­graphs arc shown in Fig. 3. Those. photographs illustrate the dcadness of the studio at. most frequencies, only those at the lowest frequencios showing recognisablo exponontial dacays,

The oontact lnicrophono was used to find tho mnin resonQ~c6s of the linoleum and distemporod Lessian panels. As one would expect, these are similar ill both studiOS, though slight variations were found from panel to panel. The linoleum resonated at 250 and 500 cls and the hsesial1 showed several resonances botwoon350 and 600 c/s. The decays of all panels woro short so that·noparticular colouration would bo oxpGctod.

The reverb~ration time was moasU):'od for six poai tions in oach studio and tho average curves are plotted in Figs. 2(b) and 2(c). The optimum. time given by Dagcnal and ',wood for speoch in a studio of 8,700 cu.ft. is 0.6 seconds, although for dramatic productions a somowhat lower figure would be acceptable, Although the curves f8X

both studies have the sanc general shapc thcro are ccmsiderublo divergoncios at low ruld hiv~ frequencies. It is possible that the

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spaoes above the false oeilings are different, one glvlng more bass absorption than the other, but this oould not be asoertained, The thiokness of the distemper on the hessian appeared to be greater in No. 5 and the high frequencies would therefore be reflected to a. greater Bxtont than in No. 3. There were also some differen.es in the furnishings of the two studios, whi.h were not sufficient to cause the Variance.

Studio No. 8

This is a medium sized general purposo studio having a volumo of 30,000 CUI ft. (45~t x 29' X 33' high). It has u carpet at onc cnd IilO that a partial ,tlive-end dcad-Emd lf affoct is obtained. The control cubicle looks into the doad ond e.nd a mirror has to' bo used to sce the live cnd.

The ionc pulsar indicated thut the studio ~s reasonable abovG 2,000 c/s, but below this frequoncy several rGgions of poor trnnsient responso wore found, notably sround 130 c/s, 400 c/s, 960 c/s and 1,750 c/s. The ropresentative photographs as shown in Fig. 4 havo charactoristics similar to those of studios 3 wid 5.

Contact microphono mcasurenY~nts showed that the linoleUITl panels i had several rG80nanCcs, tho mnin ones boing around 250 c/s, 400 c/s and 600 c/ s, bu.t as tb.e decay times were short compared with those of the studio there is no colouration. Calculations based on Rssearch Report B.032 gave 500 c/s as the resonanC6 frequency of linoleuxn in conjunction with a 7/8" air space, a value which VTas in fair agreoment with the observed behaviour.

The reverberation timo was measured for eight positions of the microphone and the average curve is given in Fig. 2(d). This curvo confirms that the linoleum .panels are absorbing in the region of 500 cls and indicates tho lack of bass absorption. The optimum rovorboration time for a general purpose studio of 30,000 cu. :ft. is 0.8 seconds, and it is clear that a reverberation tm10 of over 2 seconds in the bass is far from satisfactory.

Conclusion§

The 6xp6rimonts have shown reasonably consistent results for' the type of troatmont used in Studios Nos. 3, 5 and 8. The appoarance of the studios has bGen improved but tho acoustics, though better, are still not good. No bass absorption has beon provided and the middle froquency region is teo dead. Bettor use could he.vo been made of tho linoleum by varying tho spacing from the walls to decroase the absorption at middle frequencios while increasing it in tho bass.

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Studio No. 2 is a bad talks s-tudios, suffering from low and middle frequency colouration. When the new studio is constructed the opportunity will be taken to improve the aooustic treatment.

DD?

Investigation by ,

Report by

DIG. Beadlo C .L.S. Gilford .T. SOillorvillc

D.G. Beadlo C.1.S. Gilford.

{H. L. Kirks)

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APPENDIX

g~nstruction and Treatment of the Studio~

No. 2 Nos. 3 and 5 No. 8 r-~'--" ---.. --~- "-~~--.-------,------' ---.- -.-.--.-~--.-..,

, I

Length Width Height Volume Floor

Walls

Coilit.ce

(ft. ) (ft. ) (ft. ) (cu.ft.)

Approx. absorbine arElas (sq.ft.)

, Carpot Hardboard Cabot's quilt Brickwork Wood

Lino1ci1.1li'l", 7/8" space

Hessian OV6r fc1t

"---.~-~---

l7i 16 11

3,100 Carpet

Bare brickwork except ten Cabot's quilts and f·our hard-" board panels in wood frames covered with netting.

I

2B-I 2113 14

8,700 Carpet

4~ "'2 29 23

30,000 27 ft. of length carpet. Rsrnainder

, st:rip floor.

I,IicroPI.:one skirt ing. 6' dado of li;;'oleum on 7/8" battens. Distempered hessian on same battens above dado, over 3 thicknesses carpet fa1t. Battens at 3' era. 2 ~eta double doors oxcept No. 5, 1

I sat.

Jj'also coiling of rilUs1in. Concreto abovo.

285 93

324 187

21

516

355'

620

1'76 (No.3) 138 (No.5) 558

355

785

225

810

1,950

I I

I I I

CP CP

n.

-

TH 5 P>-j' ,-OGAAPH 5 T~1E pp :)PE Y

THE BRITlS!-l BR DCA'" "le Ct. Cj011ATI AND ~A) >.jO- Si: "fPR ul'CED OR CLOSE:) Tv A T..,IRD PAR -, " ,vn 'I"Y ITHOl T .... t ." G. ..,. ". PERMISSION 0" THE C''')''i'ORAT'O~.

f : 242

r,------------------ ---

(e)

.f ::. 950 +:1800

NOTE :-

; = FREQUENCY IN CYCLES PER SECONQ

PULSE LENGTH': 100 MILLISECONDS.

TIME BASE = , SECOND.

FIG. I. PULSE PHOTOGRAPHS STUDIO 2.

- -m

- --- --- ----'------

(e)

+,. 620

0

Thit drawong/ speclflcatlon IS the property of the Briti sh Broadcasting Corporation and may not be reproduced or dIsclosed to a thIrd party in any form without the wrrtten permIssIon of the Corporation

I\l W ~ 111 Cl '" c:o (I) -0 0 0 0 0 0 o 0 g 1\1

0 0

'" J> 111 Cl " (1J 0 0 0 c 8 I) 0 C 0 0 0

Freq0ercy .., -ye' p,.r

FI G. 2. REVERBERAT ION

to- N U -J' .!" Cl ;-J

" 0 Cl 0 0 -0 0 °8 8 0 0 0 0 0

0 0 0 0 0

second

CURVES.

B BC ACOUSTIC MEASUREMENTS IN BELFASTSTUDllJ~~rr-1 l i p

(l) (1)- III W

b 0 9 0 0 0 00 (:) Cl 0 08 0 0

0 0

4 SHEETS

o::J o::J ()

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m O m C r~ ~n ~ ~ rn

~~ OC -JJ 0'" Ul~

rn Z

cri

THIS PHOTOGRAPH IS THE PROPU THE BRITISH BROADCASTING CO"POAA AND MAY NOT BE REPRODUCED 0" CLOSED TO A THIRD "ARTY IN ANY WITHOUT THE WRITTEN PERMtSSION THE CORPOAA TlON.

(Q)

f • 85 P.L . a 100

(e)

f = 1150

P.L. : 100

(= 550

P.L.· to

(b)

f wo 178

P.L. &100

(fl

f • 1700

P.L . -lOO

(J)

f 11: 440

P.L.- 100

NOTE : -

(C)

f'"' <400

P .L.: 100

(9)

f : 60

P. L.= 100

(k)

f. 980

P.L. = 100

f • FREQUENCY IN CYCLES PER SECOND.

P. L. = PULSE LENGTH IN MILUSECONDS.

TIME BASE - 2 SECONDS.

FrG. 3. PULSE PHOTOGRAPHS STUDIOS 3 (a -0 & 5 (9 - L)

•

(d)

f % 650

P.L.= 100

( <: 75

P. L . =IOO

(l)

f - 3600

P.L =100

= I Vi

'" IU-C I m ..

<0

THIS PHOTOGRAPH IS THE DD 1''\1><''. TV

THE BRITISH BROADCASTING CORPORA AND MAY NOT BE REPRODUCED OR CLOSED TO A THIRD PARTY IN ANY FO WITHOUT THE WIUlTEN PERMISSION OF THE CORPORATION.

(a)

f -130

P.L..'" 270

f· 960 P.L. . · 270

f .. 130

P.L. "" 27

f '" 960 P.L.=: 27

NOTE:-f • FREQUENCV CVCl.! P.L.. PULSE L.£NGTH t TIME BASE. 2 SECOND

~tG. 4.

f· 400

P.L.- 270

f .1750

P.L. e 270

P.L.- 27

PER SECOND.

MILLISECONDS.

PUI-SE PHOTOGRAPHS STUDIO 8.

(d)

(. <400

P.L.- 27

f. 17S0 .

P.L- 27