Handbook for Sound Engineers

Acoustical Treatment for Indoor Areas 113

fill). The air mass in the slots between the slats reacts
with the springiness of the air in the cavity to form a
resonant system, much like that of the perforated panel
type. In fact, Eq. 5-5 is again used to calculate the fR for a
slat absorber, but with the following equations for H, G,
and t':

(5-6)

(5-7)

(5-8)

where,
r is the slot width,
w is the slat width.

While G is often approximated to a value near 0.6, it
is not difficult to calculate. As with perforated absorbers,
the above will yield approximate results for the fR of a
slat absorber, which will be fine for most design
applications.
In a practical sense, the absorption curve can be
broadened by using a variable depth for the air space.
Another approach is using slots of different widths. In
the structure of Fig. 5-28, both variable air space depth
and variable slot width could be used. Porous absorptive
fill is shown at the back of the cavity, removed from the
slats. This gives a sharper absorption than if the absor-
bent is in contact with the slats. It should be noted that,

all other factors remaining the same, randomly placed

slats (yielding randomly sized slots) will lower the

overall absorption, while bandwidth is increased.^29

5.2.5 Bass Traps

Bass trap lore pervades the professional audio industry,

particular in the recording industry. Literature on the

subject, however, is scarce. The term has become a

catchphrase often used by acoustical product manufac-

turers to include a large variety of acoustical products,

often including what are simply broadband absorbers.

Very few bass traps are actually effective at absorbing

bass. It is simply quite difficult to absorb sounds with

wavelengths at or approaching 56 ft (17 m). To most

effectively absorb a given frequency at any angle of inci-

dence, including normal incidence the absorbing material

should be at least and, ideally, ¼ of the wavelength of

the lowest frequency of interest. For 20 Hz, this means a

depth between 1.7 m (minimum) and 4.3 m (ideal)! It is

relatively rare to find someone who is willing to build a

device that large to trap bass. This may be necessary in

the design of very large rooms, like concert halls, but it is

probably more interesting to ask what crime has the

20 Hz committed that it needs to be trapped? As we shall

see in the next chapter, the low end performance of small

rooms can be reliably predicted from a study of the distri-

bution of room modes. If the modes are distributed prop-

erly, trapping may not be needed. On the other hand,

imagine that the modes are not distributed properly and

the goal is to fix a problem room. If there is enough space

to build a bass trap that is large enough to have an impact

Figure 5-27. The effect of depth of air space on the absorp-
tion of a 105 “m thick microperforated absorber, the RPG
ClearSorber Foil from RPG, Inc.^28

Air space = 30 mm (1.2 in)

Air space = 50 mm (2.0 in)

Air space = 100 mm (3.9 in)

(^12516020025031540050063080010001250160020002500315040005000)
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
(^1) / 3 Octave band center frequency—Hz
ASAB
H r
wr+
= ------------
G^1
S
---^1
2
---SH
©¹
–= lnsin§·
tc t+= 2 Gr
Figure 5-28. A slat absorber with varying depth to widen
the effective frequency range of absorption.
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