RESEARCH DEPARTIvIENT

GLASGOW CONTINUITY SUITE

RGPort No. B.037 Serial No. 1949/4

Research Department

Sumr:lary

REPORT NO. B.03? §.erio.l No. 1949/4

GLASGOVi CO~TTnmITY SUITE

Ja.nuary, 1949

Figs. Nos •. B.037.1 to B.037.3

This report doscribos a.coustic moo.surenents in Glasgon Continuity Studio. It is shor:n that the results aro

·unsatisfa.ctorYj the studio is too dead and suffers from colouration in the bass and rr~dd16 froquencies. Excessivo I;libila.nce, very disturbing in the studiO, is not so serious over tho microphone.

General

The Continuity SUite, consisting of a studio and control room, has been formed frora part of the large listening hall. The con­struction and treatment are based on a design of 1946, except for a change in position of the Acousti-celotex tiles, A later design preferred by the Acoustics Committee was abandoned on grounds of expediency.

The studiO, which has a volmne of 1,200 cu.ft., is nearly cubical (ll~' x l~' x 10' high), with parallel four-layer walls as described in Report B .036 (IICai:nden walling!!), inside a briCk outer ahell o A departure has been made from tho normal construction of the four-layer wall by spacing the inside plaster board 7/8 t1 from the building board to accommodate the signal light wiring etc. with­out spoiling the sound insulation. The 7/8" battens carrying the plaster board are' spaced l' apart. A suspended ceiling is ·used, consisting of 51! x 2" joists covered Vii th p8rforated hardboard with 311 rockwool and a 1'9" air space above. The '!iood floor is laid on concrete and Carpeted. Additional absorption is provided by a 3' deep band of 'Acousti-colotex tiles, the lm,er edge of which is 2' 6" from the floor. Full details of the treatment are given in the AppendiX.

E2£}2erimental

The studio Was tested on November 18th and 19th 1948, after it had been complct od by the builders. A microphone was set up in the position proposed for the final layout, and listening tests on male and female speech were carriod out. Speoch was coloured in the bass and middlo frequencies and had an unpleasant hardness. The announcers said that the studio was oppressively dead and unpleasantly sibilant.

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They also complained that the cream Celotex tiles ~ore very ~iring to the eyes and suggested that they should be painted a darker colour to mako the holos less visible.

Pulses of tone sho-,-IGd peculiar transient response at some frequencies confined to the regions of 500 - 600 c/s and 1,500 -2,500 0/s. Photographs sho71ing'tho raggedness of the decays ca.n be seen in Fig. 1. The ~orst region was aroUnd 2,000 c/s and is tho most likely cause of the sibilance of speech in the studio. Impul­sive sounds such as tapping or the noise of tho acoustic pulse gener­ator shoTved a distinct ring at 2,000 C/Se The normal position for the microphone is level rlith the band of Celotex tiles so that the sibilance is considorably reduced over the microphone.

Contact miorophone measurements on the vialls showed that they \701"0 resonating at soveral frequencies bot-;;een 90 o.nd 300 c/ s, and iJere consoquently providing most of the bass absorption.

Tho reverberation time "as moasul"od, for five microphone positions and thec.verage curve is shovm in Fig. 2, togethor ~7ith a curve calculated from aroo.sand kn07ffi absorption coefficients. The port ion belou 700 cl s is shorm dotted' i1here it approaches 0.2 soeonds, ,lhich is the shortest time that can be measured by the Noumann lovel recorder. The decay curves bC310,7 this frequency and above 4,000 c/s are oxponential, but bcti70en 1,000 and 4,000 o/s double slopes -;7ere oncounterod. Fig. 3 sh07:s typical decay rocords over this range, and iliustrates tho importance of tho 10':Jer slope .at 2,000 and 2,800 c/s. Calculations of reverberation t.iroe based on the initial and finai decay slopes are sho~n in Figs. 2(a) and 2(c) respectivoly. It i1ill bo S06n that tho calculatod curve, corresponds fairly closely to the curve for the initial decay slopes (Fig. 2 (a) ) • Fig. 2 (c) sho';;s the rosonance at 2,000 c/ sand indi cutes the presence of undnmpod resonant structures causing the colouration '.7hich '.7as previously noted at 2,000 c/s. It nas thought that the largo areas of plo.storod Gyproc, soparated from the building board, might be responsible for this colouration, and this has sinco boon confirmod by Qxperiments in, the reverboration room.

The reverberation time of 0.2 - Oe3 seconds is too short, the preferrod time given by Bagcmul and Wood for 0. studio of 1,200 CUI ft. boing 0.48 seconds, and in order to try tho ?ffect of increasing tho timo somo areas of Celotox :mre temporo.ri'ly covered m th ply7JOod. Listening tosts under these conditions shoi7ed that an improvoment could bo obtainod, though no attempt ~as made to find the optimum cop.ditions.

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The floor is extremely s~ueaky and noise from the remainder of the building is conducted into the studio by the radiator pipes. The radiator itself can be excited by tone from the loudspeaker, but it is unlikely that this will be troublesome on transmission.

Conclusions

This studio is unpleasant to te.lk in, mainly because of sibilanQo. Speech over the microphone is coloured in the bass and middle fre~uen­cies but, owing to tho positioning of the Colotex tiles, the sibilance noticeablo in the studio is not obtrusive. Eyo strain is caused by tho holos in the Celotox tiles inrraodiatoly in front of the speaker and ma~' lead,to complaints.

It is considerod, that the studio is too dead in tho bass and middle fre~uoncios, and that better use oould be made of the Celotex tiles by redistributing thom into panels. The Gyprocplastor board should be in contact with the building board.

DIP

Investigation by:

:H.oport . by :

D.G. Beadle C.L.S. Gilford T. Somerv illo

D.G. Beadle.

. j I I . r 111 !"k~ V2';:'

(H. L. Kirke)

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APPENDIX

Oonstruction and Troatment of the studio.

Dimonsions

Volumo

:rloor

Wall Oonstruction

Wall Treatment

Q.9iling

ll~' X 10!' x lOt high.

1,235 cu. fto

Existing doal floorboards on battens fixed to concroto~ Oarpet and felt undorlay. (Fblt underlay alone used for tests.)

"Gamden" typo. Plaster board ovor building board on timber frame. 7/$" spaco betWeen plastor board and building board. {Spacing battens at l' contres.}

Skim coat of hard plaster over plaster board, oxcept for 3' frie~6 of Ac6usti-colotex tiles, lowor odge 2'6" from floor. IIIicrophono skirting, single door and control window.

False ceiling of porforated hardboard (3/16 11

dia. holos, i" spacing), fixed to joists. Backed by 3" rock"ilool and l! 911 air space.

Ap'Proxim.a to absorb ing aroaE!

Hard plaster 290 sq.ft. Acousti-celotox tilos 106 " " Oar pet 106 " " Oei'ling 120 11 " Glass 13 tI " Wood 70 " H

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THIS PHOTOGAAPH IS THE THE BRITISH BROADCASTING COJlPORA AND MAY NOT BE "EPRODUCED OR ClOSEt) TO A THIRD PARTY IN ANY WITHOUT THE WRm£N PERMISSION THE CORPORATION.

NOTE:- . t - FREOUENCY I CYCLES.

PER S~COND.

P.L. ;: PULSE LENGTH IN MfLLfSECONDS.

TIME BASE - 400 MI1..LISECONDS.

P.L.-S

(l)

( -6000

P.L .-SO

(c)

f =500 P.L .- ·50

.~ ,

f .. 2.00 P.L .: 50

v

~ I '.

. f -600 0

P:L =S

FIG. I.

.

t---

• f .2100

P.l. .-S

" TYPICAL PULSE PHOTOGRAPHS .

o

This drawing/speCification IS the property of the British Broadcasting Corporation and may not be reproducA or disclosed to a third party In any fon1l without t_ wntten permiSSion of the Corporation

N o w

o N o o '" o

o

~rerz.u~ C.) in c ,cles per second

FIG. 2.

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REVERBERATION CHARACTERISTIC S.

TICS OF GLASGOW CONTINUITY SEARCH

REPORT

CA)

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I C • m .0

B. 037 · 2 3 SHEETS.

ISSUE I

3 - 2 - 49

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BBC OSj1/0C

1400c/s.

4000c/s.

,

( -'V

I I __ I __ I __ I I

.----------,55

f-----------ISO

A f\ 45

I V \ 40 I 35

~--------+--~30 - -- I 25 db ---- 20 ,

IOOOc/~· f-

fV ---- - I IS

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0° o I TIME IN SECONDS .

~50

.L v 45 I -- 40

I 35 f-----------~--~30

) 25 db ." f--------

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/ 15

2000c/&.

TIME IN SECONDS.

50 f\ "-..!. 45 I \ 1 40

1 2800 c/'&. 1 35 I 30

1 } 25 db

I f 20 rI I 15 / r 10

I 1 5 J /

0 I 0° TIME IN SECONDS.

FIG. 3. TYPICAL DECAYS IN THE REGION 1000-4000 e/s.

ACOUSTICS OF GLASGOW CONTINUITY STUDIO.

RESEARCH DEPT IDR'N [~ REPORT

[CH'p, B. 037 · 3 AP~p ,c.y<.. 3 SH E E TS