Handbook for Sound Engineers

108 Chapter 5

Draperies are also porous absorbers of sound.
Included in the drapery category are drapes, curtains,
tapestries, and other fabric wall-hangings, decorative or
otherwise. Besides the type and thickness of material,
the percent fullness has an effect on how well draperies
absorb sound. (The percent fullness is a representation
of the amount of extra material in the drapery. For
example, 100% fullness would mean that a 3.0 m wide
curtain actually consists of a 6.0 m wide piece of mate-
rial. Similarly, 150% fullness would indicate that a 3.0 m
wide drape consists of a 7.5 m wide piece of material; a
6.0 m wide piece of material being used for an 2.4 m

wide drape, etc.) Fig. 5-18 shows the absorption coeffi-

cients for draperies with different percent fullness.^22

While spacing draperies from the wall does increase

absorption slightly, it would not appear to be as signifi-

cant as percent fullness, as indicated by Fig. 5-19.^22

5.2.3 Discrete Absorbers

Discrete absorbers can literally be anything. Even an

acoustical tile or foam panel is a discrete absorber. The

absorption per unit of a tile, panel, board, person, book-

shelf, equipment rack, etc., can always be determined. In

the context of acoustical treatments, there are two main

classes of discrete absorber that should never be ignored:

people and furnishings.

5.2.3.1 People and Seats

In many large spaces, people and the seats they sit in will

be the single largest acoustical treatment in the room.

Any acoustical analyses of sufficiently large spaces

should include people in the calculations. How the seats

behave acoustically when they are empty is another

important consideration. Empty wood chairs will not

absorb as much as the people sitting in them. A heavily

padded seat may absorb just as much sound as a seated

individual. A chair that folds up when not in use may

have a hard, plastic cover on the underside of the seat

that will reflect sound. Perforating the cover to allow

sound to pass in through the bottom of the chair when it

is folded up may be a worthwhile consideration for some

applications. For more information on the absorption of

people, seats, and audience areas in general, refer to

Section 7.3.4.4.4.

5.2.3.2 Acoustical Baffles and Banners

In very large rooms, such as domed stadiums, arenas,

gymnasiums, factories, and even some houses of

worship, absorbers need to be placed high on the ceiling

to reduce reverberant sound. Installing spray applied

acoustical treatments in such spaces is often uneconom-

ical because it would be too labor-intensive. To solve

this problem, prefabricated acoustical treatments that

hang from the ceiling are often used. Acoustical baffles

are typically 61 cm × 122 cm (24 in × 48 in)—or some

other relatively manageable size—and are often approx-

imately 3.8 cm (1.5 in) thick. The core material is often

a rigid or semi-rigid mineral fiber, such as glass fiber,

with a protective covering of polyester fabric, rip-stop

nylon, or PVC. Acoustical foam panels and other porous

Figure 5-18. The effect of fullness on the absorption of cot-
ton cloth curtain material, 500 g/m^2 (14.5 oz/yd^2 ).^22

Figure 5-19. The effect of mounting over an air space on
the absorption of velvet cloth curtain material, 650 g/m^2
(19 oz/yd^2 ).^22

125 250 500 1k 2k 4k

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

Octave band center frequency—Hz

ASAB

14% fullness

33% fullness

100% fullness

125 250 500 1k 2k 4k

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

Octave band center frequency—Hz

ASAB

Air space = 10 cm (3.9 in)

Air space = 0 cm (0 in)

Air space = 20 cm (7.9 in)