Handbook for Sound Engineers

Sound System Design 1267

berant field. Given the direct and reverberant field level
at a listener’s position, the answers to Questions 2 and 3
are simple to calculate. Because the software tools do
this so well, few designers attempt to design a complex
cluster without using them.

34.3.2.10.3 The Four Questions for the Complex Cluster

34.3.2.10.4 Question 1: Is the System Loud Enough?

For the complex cluster, a new approach is indicated.
That approach is to find the total direct sound level at
the listener’s position, then to find, independently, the

total reverberant sound level at the listener’s position,

and to add them to get the overall total sound level at

that position. Comparing and adjusting this overall level

with the desired LP (i.e., is it at least 15 dB above ambi-

ent noise?) will provide the required EPR. It is also pos-

sible to use the original indoor EPR equation (Eq.

34-16) by calculating a value for the Dc modifier N and

using this value of N in the critical distance portion of

the EPR calculations.

Many designers believe that, although the rever-

berant energy in a room can aid the perception of useful

loudness, it is the direct sound level that is most impor-

tant. The LPd can, of course, always be calculated via

the inverse-square law, Eq. 34-1. If a listener is in the

direct field of more than one loudspeaker, the direct

sound from all such loudspeakers can be added to

obtain the total LPd at the listener. If the LPd level at the

listener is high enough, the LPt will also be high enough,

thus, answering Question 1 satisfactorily.

34.3.2.10.5 Question 2: Can Everybody Hear?

For a complex cluster, the answer to Question 2 can be

obtained manually. However, given accurate data, soft-

ware tools like EASE and Modeler provide a detailed

and accurate answer that makes these tools the best way

to answer Question 2.

34.3.2.10.6 Question 3: Can Everybody Understand?

The Alcons equation, like the EPR equation, may be

used for a complex cluster by calculating a value of N as

discussed in Section 34.3.2.10.

Another method is to calculate the direct field and

the reverberant field at each listener’s position of

interest and perform a direct/reverberant comparison.

Since the Alcons concept is based on the direct/rever-

berant ratio, a knowledge of the actual numeric

direct/reverberant ratio is equivalent to a knowledge of

the numeric value of Alcons. However, the Alcons equa-

tion assumes a 25 dB SNR. In rooms with a higher level

of ambient noise, use Eq. 34-20. EASE and Modeler

also provide good estimates of intelligibility at each

listener’s position.

34.3.2.10.7 Question 4: Will It Feed Back?

This question can be answered by calculating the total

sound level (direct plus reverberant) reaching the micro-

phone from the cluster. If this level is equal to or greater

than the level expected from the talker at the micro-

Figure 34-23. A cruciform church showing distributed loud-
speaker coverage. Courtesy Bosch/Electro-Voice.

4.0'

A. Cruciform church, loudspeaker coverage

required over seating area only.

B. Scaled drawing of the seating area for the cruciform

church using 4-inch loudspeakers at 4000 Hz.

36.0'

36.0'

24.0'

48.0'

96.0'

24.0'

96.0'