Handbook for Sound Engineers

Sound System Design 1251

most accurate in rooms with reverberation times of 1.6 s

or longer and is not particularly accurate or useful in

acoustically dead rooms. In addition, this form of the

Alcons equation assumes at least a 25 dB SNR. See

Chapter 36 for a thorough discussion of speech intelligi-

bility including additional speech intelligibility

measurement standards.

(34-18)

where,

D 2 is the distance between the loudspeaker and the

farthest listener,

RT 60 is the room reverberation time,

N is a number that attempts to compensate for the fact

that there are most likely several loudspeakers in the

cluster and only one will be pointed at the farthest

listener (but all add to the reverberant field). Note that

some texts use N + 1 in place of N,

Q is the Q of the loudspeaker,

V is the volume of the room,

For metric distances, replace the constant 656 with the

constant 200,

D^2 is in m^2 for the SI system.

Example:
Let,
D 2 = 125 ft,
RT 60 = 2.5 s,
V = 500,000 ft³,
Q = 10,
N = 1.

One way to interpret N is the total number of loud-

speakers producing the same acoustic power as the

loudspeaker pointed at the farthest listener. For a more

detailed discussion of the concept of N, see Section

34.3.2.10.

34.2.3.4.3 Alcons Modified for Audience Absorption

and Talker/Listener Factors

The following equation for Alcons includes modifiers

for the absorption of an audience and for a talker with

poor articulation or listeners with less than normal hear-

ing acuity. Use this form of the equation when listener

or talker difficulties are expected or when the audience

area will be significantly more absorptive than the rest

of the room such as a large cathedral with marble walls

and ceiling but with pew cushions in the seating area

and carpeting on the floor.

(34-19)

where,

,

a is the average absorption coefficient for the room,

ac is the absorption coefficient in the area covered by

the loudspeaker (the audience area),

k is a correction factor for a talker with poor articulation

or listeners with less than normal hearing acuity and is

typically in the range of 1– 3%,

other factors are as before.

For metric distances, replace the constant 656 with the

constant 200,

D^2 is in m^2 and V is in m^3 in the metric system.

34.2.3.4.4 Alcons in Terms of RT 60 and Direct,

Reverberant, and Noise Levels

The following equation presents Alcons in terms of the

four factors most critical to intelligibility, which are

direct sound level, reverberant sound level, reverbera-

tion time, and noise level. Use this form of the equation

if the four quantities are known or can be reliably

estimated.

(34-20)

where,

,

,

Alcons

656 D 22 RT 602 N

QV

=-------------------------------------

Alcons 656 125

2 2.5^21

10 500 000 

=-------------------------------------------------

=12.8% (less than exceptable)

Alcons

656 D 22 RT 602 N

QV

=-------------------------------------+k

m^1 – a

1 – ac

---------------=

Alcons=100 10 2–>@ABCABC–+ 0.015+

A 0.32

ER+EN

10 ED++ER EN

---------------------------------------

©¹

–= log§·,

B 0.32

EN

10 ER+EN

–= log©¹§·--------------------------,

C 0.5

T 60

12

–= log©¹§·------- ,

ER 10

LR

=^10 ------

ED 10

LD

= ------ -^10