RESEARCH DEPARTMENT
SOUND INSULATION AND ACOUSTICS
IN THE BELFAST STUDIO CENTRE
Report No. B. 059
Serial No. 1954/38
C. L. S. Gilford, M. Sc, A.M. I.E. E. , F.Inst. P.
W. K. E. Geddes, M. A. Syf0 uc/ o* A-M*** '
M.W. Greenway, M. Sc. (Eng. ) ■ T
A. L. Newman, A. R.P.S.
F.L. Ward, B. Sc. , A.M. I.E. E. (*• Proctor Wilson)
Report No. B.059
S0UKTD INSULATION" AND ACOUSTICS
IN" THE BELFAST STUDIO CENTRE
Section Title Page
SUMMARY 1
1 INTRODUCTION 1
2 SOUND INSULATION BETWEEN ' STUDIOS 2
2.1. Insulation Between Studio No. 1 and Adjacent Studios 2
2.2. Insulation Between Studios Nos. 3 and 5 3
2.3. Insulation of the Continuity Studios 3
2.4. Insulation Between the Control Room and Studios Nos. 3 and 5 . 3
2.5. Insulation of Studios Nos. 2 and 6 3
2.6. Studio-to— Cubicle Insulation 4
2.7. Discussion of Inter-Studio Insulation 4
3 GENERAL ACOUSTICS 5
3.1. Studio No. 1 (75,000 ft 3 ) Orchestral Studio 5
3.2. Studio No. 2 (4,000 ft 3 ) Talks Studio 5
3.3. Studios Nos. 3 and 5 (6,300, 6,700 ft 3 ) Drama Studios 6
3.4. Studio No. 4 (4,100 ft 3 ) Effects Studio . . 7
3.5. Studio No. 6 (1,500 ft 3 ) Talks Studio 7
3.6. Studio No. 8 (30,000 ft 3 ) General Purpose Studio 7
3.7. Continuity Studio (2,750 ft 3 ) 9
3.8. Echo Rooms 9
3.9. 4th Floor Listening Room 10
4 CONCLUSIONS 10
4.1. Sound Insulation ....' 10
4.2. Acoustics of Talks and General-Purpose Studios . 10
4.3. Music Studios 10
5 REFERENCES . 11
APPENDIX 16
CONFIDENTIAL
Report No. B.059
September 1954
Serial No. 1954/18
SOUND INSULATION AND ACOUSTICS
IN THE BELFAST STUDID CENTRE
SUMMARY
During the years 1952/3, all the studios in Broadcasting House, Belfast,
except Studios Nos. 1 and 4, were given permanent acoustic treatment. This report
summarises the results of acoustic measurements in all the studios, describes certain
modifications which were made during the building work, and discusses conclusions of a
general nature which resulted from the work.
1. INTRODUCTION. •
Broadcasting House, Belfast, was completed structurally at about the time of
the outbreak of war, in 1939, but the studios remained unfinished, the acoustic
treatment consisting of strips of carpet felt hung from the bare brick walls. In
1947 Studios Nos. 3, 5 and 8 were given an inexpensive treatment of rockwool covered
with hessian, in conjunction with linoleum membrane bass absorbers. Acoustic tests
described in Research Report B.038 , showed that these studios, though improved, were
too reverberant in the bass and had excessive absorption at middle frequencies. The
present report describes the new permanent treatment of Studios Nos. 2, 3, 5, 6, 8 and
the Continuity Studio, carried out in the years 1952 and 1953, and gives the results
of acoustic measurements in these studios and in No. 1 which has new temporary
treatment.
Particular attention was paid to the defects in sound insulation, due to bad
studio layout, which have been a source of difficulty in operation. Some of these
defects cannot be overcome without major structural alterations, and their effects can
therefore be reduced only by careful planning of rehearsal and transmission bookings.
It is desirable, however, to examine the causes of these faults so that they may be
avoided in future.
The building contains one orchestral studio (No. 1), a general— purpose
studio (No. 8), two smaller drama or music studios (Nos. 3 and 5), two talks studios
(Nos. 2 and 6) and a continuity studio. Studio No. 4 is an effects studio and store,
still untreated. There are two echo rooms and an acoustically treated listening room.
In this report, sound insulation is considered first, followed by details of the
acoustics of individual studios. Table 1, in the Appendix, summarises the dimensions,
volumes and locations of the studios.
2. SOUND INSULATION BETWEEN STUDIOS.
All the studios except Studio No. 2 are accommodated in one wing of the
steel-framed building, as indicated in Pig. 1. It is evident from the diagram that
the possibility of sound leakage exists between Studio No. 8 and Continuity Studio,
Studio No. 5 and Continuity Studio, Studios Nos. 5 and 3, and between Studio No. 1 and
the two drama studios Nos. 3 and 5. The effects of long— path transmissions through
the steel frame have also to be considered. For example, it is found possible to
hear in Studio No. 1, the sounds of an orchestra playing in Studio No. 8, sound
transmission taking place through the walls or frame of the building. Impact sounds
originating on the solid floor of Studio No. 1 can be heard throughout the wing.
The worst potentialities of this studio lay— out were anticipated in the
original designs by providing a false ceiling to Studio No. 1, thus reducing trans-
mission to Studios Nos. 3 and 5, and by building inner shells for the two latter
studios, isolated from the main structure, to reduce the leakage between them and also
leakage from the floors above. The Continuity Studio also had a false ceiling to
avoid leakage from Studio No, 8 above. Nevertheless several minor difficulties arose,
so that modifications of detail were found necessary as the building progressed.
This section of the report gives an account of the insulation measurements made in the
course of the investigation, a summary of the results being given in Table 2 of the
Appendix.
2. 1. Insulation Between Studio No. 1 and Adjacent Studios.
The first measurements of sound insulation were carried out in June 1952
during the construction of the inner shells of Studios Nos, 3 and 5. It was found
that there was very considerable leakage from Studio No. 1 into Studios Nos. 3 and 5,
particularly the former, through the ventilation ducts. A Tannoy "Phonmeter" in
Studio No. 3 gave scale readings of 65-75 during a rehearsal of the Northern Ireland
Light Orchestra in Studio No. 1, from which it will be understood that the leakage was
sufficient for the music to be heard in considerable detail.
It was found that the leakage occurred through spaces surrounding the
ducting and through the walls of the ducting itself where this was not lagged.
Correction of these faults resulted in a considerable improvement, making Studios Nos.
3 and 5 usablej the subsequent addition of a false ceiling to Studio No.l made
further improvement and the sound insulation is now considered to be satisfactory.
The measurements in these last two stages are shown in the first and third columns of
experimental results in Table 2.
"Footsteps— machine" noise was measured from Studios Nos. 3 and 5 to Studio
No. 1, and loudness levels, summed from octave band measurements by the method of
Mintz and Tyzzer , being 64 and 69 phons* respectively. These figures are outside the
limit of 58 phons tentatively fixed by Research Department for small studios, but it
was decided that no building alterations were necessary for the following reasons:
The phon, as defined by the B,S.L , has no relevance to measurements In the field. In this
report the unit is used loosely, for want of a practically realisable unit, to mean a single
figure compounded from a series of o et av e- b and width measurements and related as accurately
as possible to subjective loudness.
a. Studio No. 1 would be used mainly for orchestral music, for which a higher
background noise is acceptable.
b. The false ceiling was not yet built.
c. Much of the floor area in Studios Nos. 3 and 5 would be carpeted.,
2.2. Insulation Between Studios Nos. 3 and 5.
Studios Nos. 3 and 5, lying side by side, are intended as drama studios of
equal size, Studio No. 3 being acoustically dead, and Studio No. 5 live. Studio No. 5
is used also for solo musical programmes and small dance bands, and hence good
insulation is required between the two studios to prevent programmes of this type
interfering with drama in Studio No. 3. However, the sound insulation between the
two studios is unsatisfactory, despite their inner-shell construction. The path of
the leakage could not be ascertained with certainty, but appeared to be through the
ceilings rather than through the three brick walls separating the studios. No
improvement could be expected without considerable building work, supplemented by
further experiments.
2.3. Insulation of the Continuity Studios.
The most likely source of interference to the Continuity Studio is Studio
No. 8, immediately above, which was used mainly for the Northern Ireland Light
Orchestra before the completion of Studio No. 1. In practice owing to the floating
floor construction of Studio No. 8 and a 3 ft space above the ceiling of the Continuity
Studio, insulation is almost adequate. The "footsteps— machine" figure is 61 phons
and the insulation for airborne sound is better than 45 dB at all frequencies above
70 c/s. Both types of interference can be heard in the studio, though they are not
very intrusive over the microphone circuit.
2.4. Insulation Between the Control Room and Studios Nos. 3 and 5.
The new control room lies above Studios Nos. 3 and 5, which must, therefore,
be protected from ordinary impact sounds and airborne sounds at loudspeaker level.
The airborne sound insulation is adequate, being better than 45 dB at all frequencies
above- 70 c/s. The footsteps noise, however, was unsatisfactory as found, the loudness
level being 62 phons. A layer of hardboard on the control room floor reduced this to
52 phons, from which it was estimated that a Korkoid covering would be satisfactory.
No measurements have yet been made in the final condition but the impact and airborne
noise insulation appears to be adequate in service.
2.5. Insulation of Studios Nos. 2 and 6.
These studios are more favourably sited than the rest, as they do not lie
adjacent to any other studios either horizontally or vertically, and it is not
necessary therefore to resort to arranging bookings in such a manner as to avoid
inter-studio interference. No insulation measurements were made, though in neither
case is the insulation completely satisfactory. From a microphone in Studio No. 2 it
is possible to hear a loudspeaker in the floor below, footsteps in two corridors, a
compressor-pump outside the building and a piano in a neighbouring practice room.
Studio No. 6 lies adjacent to a corridor and an attendant has to be stationed
outside during transmissions to hush staff walking past.
There is little which can be done about either studio except to reduce the
interfering noises; acoustic treatment of the vestibule of Studio No. 2 has, however,
reduced the transmission of footstep noise from the corridor outside.
2.6. Studio-to-Cubicle Insulation.
The measurements of insulation between studios and their cubicles are
tabulated in the Appendix, Table 3. Where further comment in the case of particular
studios is necessary, it will be included in Section 3 dealing with the acoustics of
the individual studios. "Howl— round" figures are given in Table 4.
2.7. Discussion of Inter-Studio Insulation.
The principal difficulties with regard to sound insulation between the
studios arises from the layout of the building, all the music studios having common
boundaries with studios intended for talks or drama. The possible disadvantages of
the steel frame have been avoided by double-shell construction of the studios where
necessary. The common boundaries could have been eliminated entirely, however, by a
different layout of the available space. Interference of one programme by another
can only be avoided by a complete separation of studios used for music from those used
for talks or drama.
Fig.1
Layout of studios in north wing of B.H.,Beltast.
(Not to scale)
3. GENERAL ACOUSTICS.
The acoustic treatment of the studios follows current B.B. C. practice, using
membrane absorbers for low frequency absorption and glasswool or rockwool covered with
perforated hardboard or Tygan fabric. The results of measurements are summarised in
tables and graphs at the end of the report. The reverberation characteristics of the
studios are shown in Figs. 2 and 3 and those of the cubicles in Figs. 4 and 5.
3.1. Studio No. 1 (75,000 ft 3 ) Orchestral Studio.
This studio is only partly completed. A permanent false ceiling has been
built, but the walls remain as distempered brickwork and the floor is concrete, partly
carpeted and having a central square of linoleum.
The only acoustic treatment consists of some Cabot's quilt and 45 membrane
absorbers taking the form of boxes closed with linoleum or roofing— felt membranes
covered with cartridge paper. The reverberation time is shown by Fig. 2(a).
The optimum reverberation time for a studio of this size is 1°2 sec, and
the studio consequently sounds rather dead at high frequencies. This will be
corrected by covering some of the Cabot's quilting with paper and by painting the
brickwork with Snowcem, a finish which is less porous than distemper. As a temporary
measure, the loudspeaker circuit in Echo Room B has been given a rising characteristic
and is used to supplement the high frequency reverberation for certain programmes.
The listening cubicle to Studio No. 1 is virtually untreated acoustically
and has too high a reverberation time at low frequencies, Fig. 4(a). Temporary
measures are being taken to correct this fault.
3.2. Studio No. 2 (4,000 ft 3 ) Talks Studio.
Studio No. 2 and its cubicle are formed within two rooms of identical dimen-
sions, the cubicle being reduced in width to allow space for a vestibule insulating
both entrances from the corridor. The studio has a ceiling of Colterro lath and
plaster on wood joists, and additional bass absorption is provided by 3 in. double
membrane absorbers on the walls and beneath a mahogany shelf running along one side of
the studio. The remainder of the acoustic treatment consists of 2 in. thick rockwool
covered with perforated hardboard. The entire floor is covered with a heavy carpet
absorbing strongly over the entire middle and upper frequency range.
With this treatment the reverberation curve was found to be not very
different from the final condition shown in Fig. 2(b); subjectively there was a
colouration at 90 c/s and there was a lack of middle frequency reverberation compared
with the rest of the range, the curve falling to 0°25 sec. at 2 kc/s. This condition
was corrected by replacing some of the 20$ slotted hardboard by imperforated hardboard,
bringing the 2 kc/s figure up to 0°4 sec. and reducing the low-frequency reverberation
by about 0° 1 sec. After about a year's experience of the studio in service, however,
it was felt that the middle and upper frequency reverberation could be reduced with
advantage. Some of the unperforated hardboard was therefore replaced by Tygan
fabric. Fig. 2(b) shows the reverberation characteristic for the final condition,
which is now regarded as satisfactory.
The ventilation noise level is too high, giving readings of 34r-40 dB on the
Dawe Sound Level Meter, with a 40 dB A. S.A. weighting network, compared with the
accepted maximum of 30.
In the initial state the double observation window was fitted with two panes
of glass of -| in. and J in. thickness respectively. The studio and cubicle were very
prone to acoustic feedback resulting in a howl if a high monitoring level was used,
the maximum permissible gain from the microphone in the studio to a normal listening
position in the cubicle being only 5 dB for some microphone positions. The frequen-
cies of howl were found to be 32 c/s or 64 c/s corresponding with the common first and
second lengthwise modes of the studio and of the cubicle (17-| ft}. It is clear that
the unusually low stability was due to the fact that pressure maxima occurring in the
cubicle along the partition wall at 32 c/s and its harmonics were able to excite the
same modal frequencies in the studio. The window was found to be the principal
leakage path at -these frequencies and the thicknesses of the two panes were therefore
increased to -| in. and -| in. respectively. This modification greatly improved the
howl— round stability, though it did not appreciably affect the average sound insulation
figure which was already satisfactory.
Pig. 4(b) shows the reverberation characteristics of the cubicle.
3.3. Studios Nos, 3 and 5 (6,300, 6,700 ft 3 ) Drama Studios.
Studios- Hqs.. 3 and 5 will be considered together since they form a pair of
drama studios and the problems of their design and treatment were in many respects
similar. They are rather low for their horizontal dimensions, and the original
designs did not include any absorbing material on the ceilings. Experience with the
studios in 1 and 1A Portland Place suggested that severe rings and flutters would be
expected. Ceiling absorbers as used in Portland Place were therefore added, consist-
ing of glasswool blankets enclosed in Tygan— covered frames, suspended at varying
distances up to 18 in. from the ceiling. Fig. 6 which is a photograph of Studio No. 5
shows the appearance of the ceiling absorbers.
Each studio is built in the form of a complete brick shell isolated from the
rest of the building by cavities around the walls and ceiling and by a layer of
glasswool quilt beneath the concrete floor. The ceiling, consisting of Colterro lath
and plaster, is suspended from the main structure by felt pads, and the inner walls
stand on two layers of bituminous roof ing- felt . The inner walls were originally
intended to be of 3 in. clinker block which would have provided a great deal of
absorption below 250 c/s, but 4-|in. brickwork was substituted in the interests of
improved insulation. The bass absorption thus lost was difficult to recover and it
was eventually necessary to increase very considerably the number of low-frequency
membrane absorbers in each studio.
When first tested, both studios were excessively reverberant in the bass and
there was a dip in the reverberation characteristic at 1,000 c/s due to the use of
slotted (20$ open area) hardboard for covering the porous absorbers on the walls.
Studio No. 3 has an average reverberation time of 0°4 sec. above 175 c/s and
is found to be good for drama productions requiring dead acoustics (Fig. 2(c)). It
is equipped with an effects staircase. The reverberation time of Studio No. 5
averages 0'52 sec. over the same frequency range (Fig. 2(d)). This value is found
to he satisfactory for musical programmes, but there are colourations at 130 c/s and
190 c/s. A sufficient range of acoustic conditions for drama, making use of both
studios can be obtained by the use of screens.
The reverberation characteristics of the control cubicles are shown in
Figs. 4(c) and 4(d). Neither cubicle is good, that of Studio No. 3 being generally
too reverberant while that of Studio No. 5 is described as "boomy".
3.4. Studio No. 4 (4,100 ft 3 ) Effects Studio.
This studio still has a temporary treatment of carpet felt hung on the bare
brick walls. It is used as an effects store and no tests were made.
3.5. Studio No. 6 (1,550 ft 3 ) Talks Studio.
Studio No. 6 intended for use as a recording studio, has the longer walls
parallel and the shorter walls inclined at an angle of about 3° to each other. Its
cubicle is similarly shaped but narrower in plan. The acoustic treatment of the
studio has no unusual features.
There are colourations at 150 c/s and 430 c/s but speech quality is never-
theless fairly satisfactory. The reverberation curve is shown in Fig. 3(a). Listen-
ing tests were made more difficult by a ring in the cubicle at about 165 c/s cau"- a T
the sheet metal casing of the Marconi control desk. A ribbon microphone is used with
slight bass correction to reduce the effect of excessive reverberation below 120 c/s.
Sound insulation to the cubicle is just adequate and though the "hovl-round" figure is
not good, no trouble has been experienced.
The cubicle is boomy, the reverberation cur/e (Fig. 5(a)) rising steeply in
the bass.
3.6. Studio No. 8 (30,000 ft 3 ) General Purpose Studio.
In its original state, as described in Research Report B.038, this studio
was characterised by excessive bass reverberation and by extreme deadness m the
region of 500-1,000 c/s. In 1950 a wood strip floor, was laid and there was a
noticeable improvement. The middle frequency region still remained dead, however,
owing to the action of the linoleum membrane absorbers which forced a dado round the
studio. At the beginning of 1952 the permanent re-treatment of the studio was
finished; a polished wood reflector covered a large central are>a of the wall behind
the orchestra and sufficient absorption to bring the mean reverberation time to
1-2 sec. was dispersed about the walls and ceiling. Fig. 7 shows the reverberation
characteristics measured in the course of these changes. Fig. 7 U) is the original
curve measured in 1948, and Fig. 7(b) is an estimated curve after the wood strip floor
had been laid. Fig. 7(c) was measured by S. S.E.H. S.B. ' s department in April 195^,
after the permanent treatment had been completed. In this condition, the studio was
considered to be too reverberant for normal use with the Northern Ireland Light
Orchestra and further modifications were made. Additional absorbing materials were
fixed to the ceiling and the reflector was slotted and backed with rockwool to prevent
unwanted reinforcement of the tympani and brass. Fig. 7(d) shows the reverberation
characteristic thus obtained; the mean time is about 09 sec;, and although this is
approximately equal to the previously accepted B.B.C. optimum time, the studio was now
rather too dead for a satisfactory distant microphone balance, The definition had
been improved, however, and colourations previously present had been eliminated. A
further slight adjustment was made to lift the reverberation time in the region of
1,000 c/s, glass fibre being removed from behind slotted hardboard panels over a total
area of 100 ft 2 . This change resulted in an appreciable improvement, though the
studio remains rather too dead for music. The placing of bass instruments is very
critical and definition is poor with a single microphone. The results obtained with
drama and with the multi-microphone balances normally used by the N.I.L.O. are
acceptable.
Fig. 7(e) is an estimated reverberation curve for the final condition. The
experiments confirm previous experience in showing that the achievement of good
definition and balance with an orchestra of 25 players in a studio as small as
30,000 ft 3 may be difficult if an adequate reverberation time for good tone quality is
to be preserved. Better results are obtained from Studio No. 1 with its volume of
75,000 ft?, even though it has received only rudimentary acoustic treatment. It
would not be true to say, however, that the small size is the only reason for the poor
compromise between tone quality and definition; Glasgow Studio No. 2, for example, is
smaller and gives better results. The table below compares the volumes and reverbera-
tion times of the smaller regional music studios:
Studios used for orchestras up to about 30 players
Studio
Approx. No,
of players
Volume
ft 3
Mean Reverberation Time
Empty
With Orchestra
Glasgow No. 2
24
24,000
0-99
0=82
Belfast No. 8
25
30,000
0=91
0°79
Manchester No. 1
20-25
33,000
1-10
0=99
Swansea No. 1
32
36,000
1°27
1=01
Charles St. , Cardiff
32
43,000
1=38
1=10
Bristol No. 1
30
58,000
97
0=87
It will be seen tliat Belfast Studio No, 8 has a lower mean reverberation
time than any other, including even Glasgow Studio No. 2 which has only four-fifths of
the volume of Belfast Studio No. 8. Reference to curves (c), (d) and (el of Pig. 7
shows that apart from two fluctuations the reverberation time falls as the frequency
rises, in contrast to the behaviour of many other studios. The additional absorption
by the orchestra will increase the rate of fall.
An analysis of other small-orchestra studios, which will not be given in
detail here, suggests that the mean reverberation time from 1,000 c/s to 4,000 c/s
with the orchestra present should be at least as great as that from 125 c/s to 500 c/s.
This conclusion may not apply to large studios, in which a relatively longer middle
frequency reverberation time is found to be acceptable, but it appears to be particu-
larly applicable to studios less than 60,000 ft 3 in volume.
In the case of Belfast Studio No. 8 the mean reverberation times in the lower
and upper bands are respectively 0°87 sec. and 0°71 sec, a fall of 2056. It appears
probable therefore that the definition and tone might be improved by increasing the
reverberation time above 700 c/s to about 0°95 sec. with the orchestra in the studio.
.It must be remembered, however, that the studio will not be used primarily for
orchestral programmes, and such a change would not improve the studio for other
purposes.
The cubicle has the same basic treatment as in 1948, but has been modified
to reduce the upper frequency reverberation (Pig. 5(b)). It is considered to be
satisfactory.
3.7. Continuity Studio (2,750 ft 3 }.
The Continuity Studio is of generous sizej it has an inner lining of 3 in.
breeze with a 2 in. cavity, standing on isolating layers of bituminous felt. The
floor, consisting of | in, boarding on 1-| in. battens, lies on a layer of quilted
glass wool. The Colterro lath ceiling is suspended in the same manner as that of
Studios Nos. 3 and 5. The acoustic treatment consisted initially of 1 in. and 2 in.
deep porous absorbers covered with slotted hardboard of 2056 open area, and suspended
frames of Tygan-eovered glasswool as in Studios Nos. 3 and 5. The reverberation
characteristic was found to be too high in the bass and too low in the 1,000 c/s
region. The anti-flutter ceiling absorbers were therefore replaced by membrane
absorbers and some of the £D% hardboard covers by unperforated hardboard or Tygan
fabric.
When this had been done, the shape of the reverberation characteristic was
satisfactory, but the mean reverberation time of 0°51 sec. was found to be too great.
Further modifications reduced the mean time to 0°40 sec, though the characteristic
(Fig. 3(d)) falls steadily with increase of frequency and the studio remains rather
boomy and coloured in the bass.
There is no control cubicle, the studio being monitored from a desk in the
adjacent control room. The insulation between the studio and the control room is
satisfactory and the monitoring desk area is partly enclosed for protection against
control room sounds by a screen and roof of Tentest "Rabbit-Warren" board.
3.8. Echo Rooms.
There are two echo rooms. Echo Room A is on the 3rd floor, lying adjacent
to the cubicle of Studio No. 5. Echo Room B is on the fifth floor and is normally
associated with Studio No. 8. Though identical in size with Echo Room A, it differs
in having a dividing wall along the centre line extending from one end of the room to
within 5 ft of the other end. The LSU 10 loudspeaker and the microphone are placed
on different sides of the dividing wall, well away from the communicating end.
The reverberation characteristics are shown in Figs. 8(a) and (b ) . That of
Echo Room A, which has no deliberate acoustic treatment, falls from 2^ sec in the bass
to 0°8 sec. at 8,000 c/s. Echo Room B has been acoustically treated with membrane
absorbers, and the reverberation time reaches its highest value of 2° 2 sec. between
700 and 2,000 c/s. It is found to be very suitable for use in conjunction with an
10
orchestra in Studio No. 1, for which purpose, a filter with a rising frequency-
characteristic is sometimes inserted into the loudspeaker circuit.
3.9. 4th Floor Listening Room.
A quality checking room is situated on the 4th floor next to the Continuity
Studio. The acoustic treatment consists of shallow porous absorbers, the only bass
absorption being provided by the plaster and Colterro lath ceiling. The reverberation
characteristic is shown in Fig. 8(c).
4. CONCLUSIONS.
Some conclusions of a general nature were .formed as a result of the
experimental work described above. They are stated briefly here as a guide for
future designs.
4. 1. Sound Insulation.
It is not practicable to put a music studio adjacent to any other studio
without providing complete isolation, e.g. by an open corridor or some equally large
cavity. Impact sound insulation is satisfactorily dealt with by the floating floor
constructions used in Belfast.
The double windows between studios and their cubicles should have glass of
thickness not less than - in. and - in. to avoid the possibility of "howl-round".
8 2 * *
The dimensions of cubicles and studios should not be identical, nor in simple ratios,
especially perpendicular to the common window.
4.2. Acoustics of Talks and General— Purpose Studios.
The three Belfast studios intended for speech are only fairly good. The
reverberation times all decrease from about 0°45 sec. at 100 c/s to 0*3 sec. at
8,000 c/s, but they are acceptable with suitable microphone corrections. Experience
with these studios in their intermediate and final stages confirms other experience in
showing that the reverberation time should not start to rise with decreasing frequency
until a frequency lower than 100 c/s is reached. Excessive reverberation between
100 c/s and 200 c/s gives rise to very serious colourations.
Breaking up of the ceiling areas by the use of suspended absorbing frames
was successful in eliminating flutters and rings, but absorbers of a similar area
forming part of the ceiling would probably have been equally effective.
Slotted hardboard of 20$ open area cannot be regarded as an effectively
transparent cover for porous absorbers. Alternative types of cover, e.g. Tygan
fabric, have been found necessary to provide adequate absorption above 2,000 c/s.
4.3. Music Studios.
In the design or re-treatment of small music studios it is necessary to
ensure that with the orchestra in the studio there is no general decrease of reverbera—
11
tion time with increasing frequency. The mean reverberation time between 1,000 c/s
and 4,000 c/s under these conditions should be at least as great as that between
125 c/s and 500 c/s. To achieve this object, the reverberation characteristic of the
empty studio should be a maximum in the former frequency range.
REFERENCES.
1. Research Department Report B.038, "Acoustic Measurements in Belfast Studios",
January 1349.
2. Mintz and Tyzzer, "A Loudness Chart for Octave Band Data on Complex Sounds",
J. A. S.A. 24 (January 1952), pp. 80-83.
3. Research Department Technical Memorandum B.1004, "The Acoustics of the
Studios at 1 and 1A Portland Place", November 1952.
12
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(a) Studio No.1 (75,OOOcu.ft.) (b") Studio No. 2 (4,000 cu.ft.)
(c) Studio No.3 (6,30Ocu.ft.) (d) Studio No.5 (6,700 cu.tt.)
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Fig. 2 — Reverberation characteristics ot studios.
(a) Studio No.6 (1,550 cu.tt.) (b) Studio No.8 (30,000 cu.ft) itloor carpeted.J_
(c) Studio No.8 (30,OOOcu.ft. no carpet.(d)Continuity (2,750 cu.ft.)
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Frequency in cycles per second
Fig.S-Reverberation characteristics af studios.
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(a) Cubicle to studio No.l (b) Cubicle to studio No. 2
(c)
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Fig.4-Reverberotion characteristics of listening cubicles.
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Frequency in cycles per second
Fig.5~Reverberation characteristics of listening cubicles
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14
Fig. 6 - Photograph of Studio No. 5 showing absorbers
suspended from ceiling to prevent flutters.
15
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(a)Temporary treatment November 1948. (b")After laying of wood strip floor (estimated),
-(c)Permanent treatment April 1952 (no carpet). (d)First modification March 1953.
I I | I | I I (e) Second modification (estimated). I
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Fig.7 Reverberation characteristics of studio No.8
2-4
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Fig.8-Reverberation characteristics.
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16
APPENDIX
SUMMARY OP MEASUREMENTS
Table 1
Dimensions of Studios etc.
Floor
Dimensions
Volume
ft 3
Notes
Studio No. 1
Ground
60 x 43!
x 29
75,000
(Temporary
Cubicle No. 1
Ground
11 x 10
x 10
1,100
J treatment
Studio No. 2
1st
22 x 17|
x 10
4,000
Cubicle No. 2
1st
18 x 16
x 11
3,100
Studio No. 3
■ 2nd
28 x 20|
x 11
6,300
Cubicle No. 3
2nd
15 x 14
x 10
2,100
Studio No. 4
2nd
22 x 14
x 14
4,100
( shell only )
Studio No. 5
2nd
29 x21|
x 11
6,700
Cubicle No. 5
2nd
16 x 13|
x 10
2,130
Studio No. 6
4th
15 x 12
x 84
1,550
Non- parallel walls
Cubicle No. 6
4th
17 x 84
x 104
1, 360
Studio No. 8
5th
49| x 29|
x 23
30,000
Cubicle No. 8
5th
20§x 13!
x 10
2,930
Continuity
4th
20 x 12!
x 11
2,750
Listening Room
4th
14! x 13
x 11
?;,070
Echo Room A
3rd
16 xl4
x 11
2,620
Echo Room B
5th
15! x 15
x 11
2,610
With dividing wall
Table 2
Measurements of Airborne Sound Insulation between Studios
Studio
Nos.
Prom
1
1
3 3
C.R.
8
To
3
5
1 5
3
Continuity
c/s
Insulation
*
dB
44
-
—
39 24
—
-
50
-
-
48 36
62
20
62
36
39
41 31
40
38
75
-
34 37
48
50
88
41
40
38 30
45
45
105
-
32 32
50
46
125
48
45
44 35
46
41
150
-
-
36
50
-
175
50
51
45 41
54
47
2lO
_
-
50
57
-
250
59
64
49 50
52
48
350
69
69
51 50
60
51
500
57 55
56
63
700
62 61
65
1,000
64 65
1,400
64 71
2,000
Before construction of the ceiling in Studio Ho, 1 B
17
Table 3
Sound Insulation between Studios and Cubicles
Studios Nos.
2
3
5 6
8
Continuity
Frequency
Insulation
dB
c/s
44
44
43
41
-
22
50
42
33
39
•43
32
62
40
35
52 32
41
34
75
40
53
45
39
29
88
53
50
53 21
43
38
105
43
49
43
42
31
125
40
45
46 33
52
32
150
45
54
-
43
36
175
42
41
50 32
49
39
210
-
46
—
-
38
250
47
49
47 35
49
42
350
50
50
50 37
48
46
500
53
51
56 48
47
49
700
58
55
64 47
58
53
1,000
64
54
64 54
56
51
1,400
64
51
62 51
56
-
2,000
60
51
54 49
55
-
2,800
59
-
58 46
58
-
4,000
66
-
62 47
59
~
Table 4
Howl— round between Studios and Cubicles (Pinal Condition)
The figures in this table represent the maximum sound— level gain in dB
between a microphone in the studio and the normal listening position in
the control cubicle, above which howl—round occurs . The experimental
chain comprises the microphone and monitoring loud— speaker normally used
with the studio.
Studio
Howl-round gain (dB )
Best mic. position
Worst mic. position
No. 2
22
18
No. 3
32
32
No. 5
32
32
No. 6
30
24
No. 8
32
32
Continuity
30
26
