adjustable acoustics

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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the drapes were withdrawn, polycylindrical elements having a plaster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.



Adjustable Acoustics


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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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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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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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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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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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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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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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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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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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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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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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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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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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adjustable acoustics

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