Handbook for Sound Engineers

1254 Chapter 34

. (34-25)

Some rooms, of course, do not lend themselves to
analysis by the given equations. One example is the
Superdome-sized room that has no true reverberant field
because the individual reflections are spaced far enough
apart in time that they are more accurately called
echoes, not reverberation. These rooms may have an
apparent dramatic increase in reverberation when
excited by a high sound pressure level sound system
(similar to those used for concert sound reinforcement).
Another example is the acoustically dry conference
room that has no significant reverberant field because of
an abundance of carpeting, draperies, padded seating,
and acoustical ceiling tile. One approach to design in
both of these spaces is to use the simplified (outdoor
system) equations where needed, since they deal with
the direct sound. In the large space, echoes must be
considered (they are not included in any of the equa-
tions); in the small space, table top and other nearby
reflections must be considered.

34.2.3.6 A Modification for Low RT 60 Rooms

The mathematical model presented in this section
assumes a well-developed, statistically random rever-
berant field. Such a reverberant field is unlikely to exist
in a corporate conference room or a home living room
because of the abundance of sound absorption materials.
In many cases, in these small, acoustically dead rooms,
the outdoor sound system equations (Eqs. 34-1 through
34-8) can be applied successfully to describe the behav-
ior of sound in the room, Fig. 34-8.

Nearby echoes, however, must be considered. These
echoes may add to the useful sound at the listeners’ ears
or they may cause harmful comb filtering depending on

the arrival time of these echoes at the listeners’ ears

compared to the arrival time of the direct sound (see

Section 34.2.3.1.2).

When these nearby echoes are useful, they add to the

sound level in a way that can be predicted by a modifi-

cation of Eq. 34-13. Qualitatively, what happens in such

a room is that the attenuation of sound with increasing

distance from the source is greater than would be

predicted from Eq. 34-13. According to Eq. 34-13, the

LP from a source should not attenuate more than 3 dB at

any distance past Dc. In these rooms, however, the

actual attenuation of sound for distances past Dc is

somewhere between the value predicted by Eq. 34-13

and the value that would be predicted by the

inverse-square law (Eq. 34-1) as shown in Fig. 34-8.

V. M. A. Peutz, one of the originators of the Alcons

concept, has investigated this phenomenon, and the

following equation for attenuation in an acoustically dry

room is derived from his work.

(34-26)

where,

H is the room height,

V is the room volume.

Example:

Let

V = 4275 ft³,

H = 9.5 ft,

LP = 90 dB,

RT 60 = 0.4 s,

D = 10 ft,

Dc = 20 ft.

Note: D and Dc must be greater than calculated Dc

for Eq. 34-26 to work. Dc calculated for this room =

7.22 ft, so that Eq. 34-26 may be used with D=10 and

Dc= 20.

Then

Note the similarity between Eq. 34-26 and the

inverse-square law Eq. 34-1. The answer, 86.2 dB, is

between the 84 dB predicted by Eq. 34-1 and the

88.7 dB predicted by Eq. 34-13. Dimensions are

assumed to be in feet. Also, note that Eq. 34-26 should

be used only in rooms with very low calculated rever-

Figure 34-8. Attenuation with distance in a relatively dead
room. Courtesy JBL Professional.

RT 60 T RT 60 A

3

RT 60 B

3

=^3 +

Calculated response

Observed response

DC 2DC 4DC

Relative response - dB

Lpc LP 0.734

V

HRT (^60)
--------------------- Dc
D
–= log------
Lpc 90 0.734^4275
9.5 0.4
--------------------^20
10
–= log----- -
=86.2 dB