adjustable acoustics
TRANSCRIPT
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If an audio space is used for only one purpose, it can be treated withsome precision. Even though a multipurpose audio space carrieswith it some compromises, economics may dictate that a given space
must serve more than one purpose. It is necessary to weigh the acousti-
cal compromises against the ultimate sound quality of the product.
For this chapter it is desirable to cast aside the impression that
acoustical treatment is an inflexible exercise and to consider means ofintroducing the element of adjustability.
Draperies
As radio broadcasting developed in the 1920s, draperies on the wall and
carpets on the floor were almost universally used to deaden studios.
During this time there was remarkable progress in the science of
acoustics. It became more and more apparent that the old radio studio
treatment was quite unbalanced, absorbing middle- and high-frequency
energy but providing little absorption at the lower frequencies. As
proprietary acoustical materials became available, hard floors became
common and drapes all but disappeared from studio walls.
A decade or two later the acoustical engineers, interested in adjust-
ing the acoustical environment of the studio to the job to be done,
turned with renewed interest to draperies. A good example of this
Ad jus t ab l e Acous t i c s24
4 7 3
C H A P T E R
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Source: THE MASTER HANDBOOK OF ACOUSTICS
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4 7 4 CHAPTER TWENTY-FOUR
early return to draperies was illustrated in the rebuilding of the old
Studio 3A of the National Broadcasting Company of New York City in
1946. This studio was redesigned for optimum conditions for making
records for home use and transcriptions for broadcast purposes. The
acoustical criteria for these two jobs differ largely as to the reverbera-tion-frequency characteristic. By the use of drapes and hinged panels
(considered later), the reverberation time was made adjustable over
more than a two-to-one range. The heavy drapes were lined and inter-
lined and were hung some distance from the wall to make them more
absorbent at the lower frequencies (see Figs. 9-13 through 9-16). When
the drapes were withdrawn, polycylindrical elements having a plaster
surface were exposed. (Plywood was in critical supply in 194546.)
If due regard is given to the absorption characteristics of draperies,
there is no reason, other than cost, why they should not be used. Theeffect of the fullness of the drape must be considered. The acoustical
effect of an adjustable element using drapes can thus be varied from
that of the drape itself when closed (Fig. 24-1) to that of the material
behind when the drapes are withdrawn into the slot provided. The
wall treatment behind the drape could be anything from hard plaster
for minimum sound absorption to resonant structures having maxi-
mum absorption in the low-frequency region, more or less comple-
menting the effect of the drape itself. Acoustically, there would be
little point to retracting a drape to reveal material having similaracoustical properties.
Adjustable Panels: Absorption
Portable absorbent panels offer a certain amount of flexibility in
adjusting listening room or studio acoustics. The simplicity of such
an arrangement is illustrated in Fig. 24-2A and B. In this example a
F I G U R E 2 4 - 1
The ambience of a room may be varied by pulling absorptive drapes in front of reflec-tive areas.
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ADJUSTABLE ACOUSTICS 4 7 5
F I G U R E 2 4 - 2 A
(A). The simplest and cheapest way to adjust the reverberation characteristics of a roomis to use removable panels. These photographs were taken in the studios of the Far EastBroadcasting Company, Hong Kong.
F I G U R E 2 4 - 2 B
(B). Close-up of hanging detail.
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4 7 6 CHAPTER TWENTY-FOUR
perforated hardboard facing, a mineral fiber layer, and an air cavity
constitute a low-frequency resonator. Hanging such units on the wall
adds low-frequency absorption, and contributes somewhat to sound
diffusion. There is some compromising of the effectiveness of the pan-
els as low-frequency resonators in that the units hang loosely from themounting strip. Leakage coupling between the cavity and the room
would tend to slightly destroy the resonant effect. Panels may be
removed to obtain a live effect for instrumental music recording, for
example, or introduced for voice recording.
Free standing acoustical flats are useful studio accessories. A typi-
cal flat consists of a frame of 1 4 lumber with plywood back filled
with a low density (e.g., 3 lb per cu ft) glass fiber board faced with a
fabric such as muslin or glass fiber cloth to protect the soft surface.
Arranging a few such flats strategically can give a certain amount oflocal control of acoustics.
Adjustable Panels: The Abffusor
Combining broadband absorption in the far field with horizontal or
vertical diffusion in the near field down to 100 Hz for all angles of inci-
dence is the accomplishment of RPG Diffusor Systems3 in their Abffu-
sor. The Abffusor panel works on the absorption phase grating
principle using an array of wells of equal width separated by thindividers. The depth of the wells is determined by a quadratic residue
sequence of numbers to diffuse what sound is not absorbed.
The Abffusor panels are approximately 2 4 and 2 2 ft. They
can be mounted in ceiling grid hardware or as independent elements.
Figure 24-3 describes one method of mounting the panels on the wall
with beveled cleats. The panels can be easily removed by lifting off the
cleats. The sectional drawing in Fig. 24-3 reveals the construction of
the unit.
The absorption characteristics of the Abffusor are shown in the
graphs of Fig. 24-4 for two mountings. Mounted directly on a wall an
absorption coefficient at 100 Hz of about 0.42 is obtained. With 400
mm of air space between the Abffusor and the surface, the coeffi-
cient is doubled. The latter is approximately the performance with
the Abffusor mounted in a suspended ceiling grid. Near perfect
absorbance is obtained above 250 Hz. The idea of obtaining such
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ADJUSTABLE ACOUSTICS 4 7 7
Cleat
1'11 5/8"
Rear view
Attach cleatto abffusor
0'4"
Internalwood bracing
Side view
Section
Abffusorcleat
Wall cleat
3'11
1/
4"
F I G U R E 2 3 - 3
Abffusor wall mounting detail.
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4 7 8 CHAPTER TWENTY-FOUR
wideband sound absorption plus sound diffusion is very appealing to
the designer.
Hinged Panels
One of the least expensive and most effective methods of adjusting studio
acoustics is the hinged panel arrangement of Figs. 24-5A and B. When
closed, all surfaces are hard (plaster, plasterboard, or plywood). When
100 1,000 10,000
Frequency - Hz
0
0.2
0.6
0.8
1
1.2
1.4
0.4
Soundabsorptioncoefficient
A Mounting
E400 mounting
F I G U R E 2 4 - 4
Abffusor absorption coefficients.
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ADJUSTABLE ACOUSTICS 4 7 9
opened, the exposed surfaces are soft. The soft surfaces can be covered
with 3 lb/cu ft density glass fiber boards 2 to 4 inches thick. These boardscould be covered with cloth for the sake of appearance. Spacing the glass
fiber from the wall would improve absorption at low frequencies.
Louvered Panels
The louvered panels of an entire section can be adjusted by the action
of a single lever in the frames commonly available for home construc-
tion, Fig. 24-6A. Behind the louvers is a low-density glass fiber board
or batt. The width of the panels determines whether they form a series
of slits, Fig. 24-6B, or seal tightly together, Fig. 24-6C. In fact, opening
the louvers of Fig. 24-6C slightly would approach the slit arrangement
of Fig. 24-6B acoustically, but it might be mechanically difficult to
arrange for a precise slit width.
The louvered panel arrangement is basically very flexible. The
glass fiber can be of varying thickness and density and fastened
A
B
F I G U R E 2 4 - 5
An inexpensive and effective method of incorporating variability in room acoustics isthrough the use of hinged panels, hard on one side and absorbent on the other.
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4 8 0 CHAPTER TWENTY-FOUR
directly to the wall or spaced out different amounts. The louvered
panels can be of hard material (glass, hardboard) or of softer mater-
ial such as wood and they can be solid, perforated, or arranged for
slit-resonator operation. In other words, almost any absorption-
frequency characteristic shown in the graphs of earlier chapters canbe matched with the louvered structure with the added feature of
adjustability.
Variable Resonant Devices
Resonant structures for use as sound absorbing elements have been
used extensively in the Danish Broadcasting House in Copenhagen1.
One studio used for light music and choirs employs pneumatically
operated hinged perforated panels as shown in Fig. 24-7A. The effect
is basically to shift the resonant peak of absorption as shown in
Fig. 24-7B. The approximate dimensions applicable in Fig. 24-7A
are: width of panel 2 ft, thickness 3/8 in., holes 3/8 in. diameter
spaced 1-38 inch on centers. A most important element of the
absorber is a porous cloth having the proper flow resistance covering
either the inside or outside surface of the perforated panel.
A B
F I G U R E 2 4 - 6
Louvered panels may be opened to reveal absorbent material within, or closed to pre-sent a reflective surface. Short louvers can change from a slat resonator (closed) toreveal absorbent material within (open).
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ADJUSTABLE ACOUSTICS 4 8 1
When the panel is in the open position the mass of the air in the holes
and the springiness (compliance) of the air in the cavity behind act as
a resonant system. The cloth offers a resistance to the vibrating air mole-
cules, thereby absorbing energy. When the panel is closed the cavity vir-
tually disappears and the resonant peak is shifted from about 300 to
about 1,700 Hz (Fig. 24-7B). In the open condition the absorption for
frequencies higher than the peak remains remarkably constant out to
5,000 Hz.
A studio designed by the late William B. Snow for the sound mix-
ing-looping stage at Columbia Pictures Corporation studios in Hol-
lywood used another interesting resonant device.2 Sound mixing
2
1
2
1
A
B
100 200 500 1,000 2,000 5,000
0
0.2
0.4
0.6
0.8
Frequency, HZ
Absorp
tioncoefficient
F I G U R E 2 4 - 7
(A) Pneumatically operated hinged, perforated panels used to vary reverberation in theDanish Broadcasting House in Copenhagen. An important element not shown is apourous cloth of the proper flow resistance covering one side of the perforated panel.(B) Changes in absorption realized by shifting the panel of (A) from one extreme toanother.
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ADJUSTABLE ACOUSTICS 4 8 3
The Snow design illustrates the extreme flexibility offered in com-
bining many types of absorbers in an effective yet inexpensive overall
arrangement.
Rotating Elements
Rotating elements of the type shown in Fig. 24-9 have been used in
radio station KSL in Salt Lake City, Utah. In this particular configura-
tion the flat side is relatively absorbent and the cylindrical diffusing
element is relatively reflective. A disadvantage of this system is the
cost of the space lost, which is required for rotation. The edges of the
rotating element should fit tightly to minimize coupling between the
studio and the space behind the elements.
At the University of Washington a music room was designed with aseries of rotating cylinders partially protruding through the ceiling.
The cylinder shafts were ganged and rotated with a rack-and-pinion
drive in such a way that sectionalized areas of the cylinder exposed
gave moderate low-frequency absorption increasing in the highs, good
low-frequency absorption decreasing in the highs, and high reflection
absorbing little energy in lows or highs. Such arrangements, while
interesting, are too expensive and mechan-
ically complex to be seriously considered
for most studios.A truly elegant solution to the rotating
type of adjustable acoustics element is the
Triffusor, another product of RPG Diffu-
sor Systems, Inc.3 shown in Fig. 24-10.
The Triffusor is a rotatable equilateral-
triangular prism with absorptive, reflec-
tive, and diffusive sides. A nonrotating
form of Triffusor is available with two
absorptive sides and one diffusive side,
especially adapted for use in corners. The
nominal dimensions of the Triffusor are:
height 4 ft, faces 2 ft across. In a normal
mounting the edges would be butted and
each unit supplied with bearings for rota-
tion. In this way an array of these units
Soft
Hard
F I G U R E 2 4 - 9
Rotating elements can vary the reverberation charac-teristics of a room. They have the disadvantage ofrequiring considerable space to accommodate therotating elements.
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4 8 4 CHAPTER TWENTY-FOUR
could provide all absorptive, all diffusive,
all reflective, or any desired combination
of the three surfaces.
Portable Units: The Tube Trap
A proprietary, modular, low-frequency
absorber with a number of interesting fea-
tures has been introduced by Acoustic Sci-
ences Corporation.4 Known as the Tube
Trap, it is a cylindrical unit available in
9-inch unit stacked on top of it is shown inthe corner of a listening room in Fig. 24-11.
A quarter-round adaptation of the same idea
is shown in Fig. 24-12. The construction of
the trap is shown in Fig. 24-13. It is basically
a simple cylinder of 1-inch glass fiber given
structural strength by an exoskeleton of
wire mesh. A plastic sheet designated as a
limp mass covers half of the cylindrical
surface. For protection and appearance, afabric cover is added.
Absorption coefficients are used in the
familiar equation:
(area) (coefficient) = sabins absorption
With the Tube Traps, it makes more sense to list directly the sabins
of absorption contributed to a room by each tube. The absorption char-
acteristics of the three-foot long Tube Traps and of the 9-, 11-, and
16-inch-diameter models are shown in Fig. 24-14. Appreciable absorp-
tion, especially with the 16-inch trap, is achieved below 125 Hz.
There is another benefit to be derived from stacking a couple of
Tube Traps in each corner behind the loudspeakers. The limp mass,
which covers only half of the area of the cylinder, provides reflection
for midrange and higher frequencies. This limp mass, however, does
not prevent low-frequency energy from passing through and being
F I G U R E 2 4 - 1 0
The Triffusor may be used in groups to provide vari-able acoustics in a space. Rotation of the individualunits can bring diffusing, absorbing, or reflectingsurfaces into play.
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ADJUSTABLE ACOUSTICS 4 8 5
absorbed. By reflection of the mid-to-high
frequencies, it is possible to control the
brightness of the sound at the listening
position. Figure 24-15 shows the two posi-
tions of the tubes. If the reflector faces theroom (Fig. 24-15B), the tube fully absorbs
the lower frequency energy while the lis-
tener receives the brighter sound. It is
reported that most users prefer the brighter
sound. The mid- and high-frequency
sound is diffused by its cylindrical shape.
If less bright sound is preferred, the reflec-
tive side is placed to face the wall. This
might possibly introduce colorationsresulting from the cavity formed by the
intersecting wall surfaces and the cylindri-
cal reflective panel. By placing absorptive
panels on the wall surfaces, as indicated in
Fig. 24-15A, this coloration can be con-
trolled.
Tube Traps can be placed in the rear
two corners of the room if experimentation indicates it is desirable.
Two Tube Traps may be stacked in a corner, the lower, larger oneabsorbing lower frequencies and the upper, smaller one absorbing
moderate lows and midrange energy. Half-round units are also avail-
able that can be used to control sidewall reflections or provide gen-
eral absorption elsewhere. Whether the Tube Traps plus carpet,
furnishings, structural (wall, floor, ceiling) absorption, etc., combine
to provide the proper overall decay rate (liveness, deadness) or not
must be determined either by listening, calculation, or measure-
ment. If flutter echoes are detected, steps must be taken to eliminate
them. There is nothing as effective as an experienced ear in fine tun-
ing the listening room.
Portable Units: The Korner Killer
The special form of RPG Diffusor Systems Triffusor with two
absorptive sides and one diffusive side is called the Korner Killer.
F I G U R E 2 4 - 1 1
An 11-inch-diameter Tube with a 9-inch one on topof it. When placed in room corners, they can providesignificant control of room modes.Acoustic SciencesCorporation.
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4 8 6 CHAPTER TWENTY-FOUR
Placed with the absorptive faces into the corner it directs the diffusive
face toward the room. This not only helps control normal modes, it
adds important diffusion to the room. The diffused reflections are
reduced 8 to 10 dB in the diffusion process, which would keep them
from contributing to perceptual confusion of the stereo image. This is
F I G U R E 2 4 - 1 2
A quarter-round adaptation of the basic Tube-Trap.Acoustic Sciences Corporation.
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ADJUSTABLE ACOUSTICS 4 8 7
Gilford701 fabric
Wire mesh skeleton
400-Hz limp mass crossover
Threadedinsert
3/4" solidend cap
Fibrouscylinder
Dacronfloat
F I G U R E 2 4 - 1 3
The construction of the Tube Trap. It is basically a cylinder of 1-inch glass fiber withstructural support. A plastic limp mass covers half the cylindrical surface, whichreflects and diffuses sound energy above 400 Hz.Acoustic Sciences Corporation.
0
5
10
15
20
Absorption/tubesabins
20 50 100 200 500 1 kHz 2 kHz 5 kHz 10 kHz
Frequency - Hz
3 ft 16"
3 ft 11"
3 ft 9"
F I G U R E 2 4 - 1 4
Absorption characteristics of three sizes of Tube Traps. The 16-inch unit provides good absorption down toabout 50 Hz.
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4 8 8 CHAPTER TWENTY-FOUR
in contrast to the higher level limp mass reflections of the Tube Trap
in Fig. 24-15B, which as early reflections, would tend to confuse the
stereo image.
Endnotes1Brel, Per V., Sound Insulation and Room Acoustics, London, Chapman and Hall (1951), p.242.
2Snow, William B., Recent Application of Acoustical Engineering Principles in Studios andReview Rooms, J. Soc. Motion Picture and Television Eng., 70 (Jan 1961) 33-38.
3RPG Diffusor Systems, Inc., 651-C Commerce Drive, Upper Marlboro, MD 20774, Telephone:301-249-0044, FAX: 301-249-3912.
4Acoustic Sciences Corporation, 4275 West Fifth Ave., Eugene, OR 97402, Telephone: 1-800-ASC-TUBE or 503-343-0727.
A B
F I G U R E 2 4 - 1 5
The positioning of the limp mass reflector gives some control over the brightnessof the sound in the room. (A) With the limp mass facing the corner, the absorbent side ofthe cylinder absorbs sound over a wide range. (B) If the limp mass faces the room, highsabove 400 Hz are reflected.
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