Handbook for Sound Engineers

Fundamentals and Units of Measurement 1659

(48-46)

M. The critical distance modifier (M) corrects for the

effect of a different absorption coefficient within the

path of the loudspeaker’s coverage pattern.

(48-47)

N. The critical distance modifier (N) corrects for
multiple sound sources. N is the number describing the
ratio of acoustic power going to the reverberant sound
field without supplying direct sound versus the acoustic
power going from the loudspeakers providing direct
sound to a given listener position.

(48-48)

%Alcons. The English language is made up of conso-
nants and vowels. The consonants are the harsh letters
that determine words. If the consonants of words are
understood, the sentences or phrases will be understood.
V. M. A. Peutz and W. Klein of Holland developed and
published equations for the % articulation loss of conso-
nants (%Alcons). The equation is

(48-49)

where,

Q is the directivity of the sound source,

V is the volume of the enclosure,

M is the critical distance modifier for absorption,

N is the critical distance modifier for multiple sources,

D 2 is the distance between the loudspeaker and the

farthest listener.

When , then %ALcons = 9RT 60.

FSM. The feedback stability margin (FSM) is required

to insure that a sound reinforcement system will not

ring. A room and sound system, when approaching

feedback, gives the effect of an long reverberation time.

A room, for instance, with an RT 60 of 3 s could easily

have an apparent RT 60 of 6–12 s when the sound system

approaches feedback. To insure that this long reverbera-

tion time does not happen, a feedback stability margin

of 6 dB is added into the needed acoustic gain equation.

NOM. The number of open microphones (NOM) affects

the gain of a sound reinforcement system. The system

gain will be reduced by the following equation:

(48-50)

Every time the number of microphones doubles, the

gain from the previous microphones is halved as the

total gain is the gain of all the microphones added

together.

NAG. The needed acoustic gain (NAG) is required to

produce the same level at the farthest listener as at the

EAD. NAG in its simplest form is

(48-51)

NAG, however, is also affected by the number of

open microphones (NOM) in the system. Each time the

NOM doubles, the NAG increases 3 dB. Finally, a 6 dB

feedback stability margin (FSM) is added into the NAG

formula to ensure that the system never approaches

feedback. The final equation for NAG is

(48-52)

where,

'Do and 'EAD are the level change per the

Hopkins-Stryker equation.

PA G. The potential acoustic gain (PA G) of a sound

system is

(48-53)

where,

Figure 48-7. Definitions of sound system dimensions.

Loudspeaker

Talker

DS

D 2

D 0

D 1

Listener

Microphone

Dc 0.3121** Qv

RT 60

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

** 0.057 for SI units

M

1 – atotal room

1 – aloudspeaker coverage area

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

N Total number of loudspeakers

Number providing direct sound

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

%Alcons 656**

RT 602 D 22 N

VQM

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

** 200 for SI units

DctDL

Gain reductiondB= 10 logNOM

NAG= 20 logDo– 20 logEAD

NAG 'Do–= 'EAD++ 10 logNOM 6 dB FSM

PAG 'Do+= 'D 1 – 'Ds–'D 2