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7-2

 

This list provides brief descriptions of various terms relating to this publication: 

ABSORPTION COEFFICIENT – The ratio of absorbed-to-reflected sound.  The absorption 
coefficient has a range of 0 to 1. 

“A” WEIGHTED DECIBEL – The ear is less sensitive to low frequency pitch at low volume 
levels, the “A” weighted correction curve is applied to SPL measurements to equalize the loudness 
of sounds over the hearing range.  Meters configured with the “A” weighted filter read out in units 
of dBA, short for “A” Weighted Decibel. 

AHJ – The “Authority Having Jurisdiction” is the organization or person responsible for 
approving fire alarm installations for occupancy. 

AUDIBILITY – A measure of loudness of a sound.  When used with respect to fire alarm 
systems, audibility is regarded as the evacuation signal level above background noise. 

CIS – The “Common Intelligibility Scale” is a standardized scale that correlates a variety of 
intelligibility measurement methods. 

CLIPPING – When part of the electrical signal path exceeds the capacity of the component audio 
“Clipping” can occur, causing a reduction in intelligibility. 

COMBINATION SYSTEM – A fire alarm system that is also used for non-fire alarm functions.  
Combination Systems contain not only typical configurations of fire alarm and emergency 
voice/alarm communications systems, but also sound systems for background music and paging 
for non-emergency messaging. 

COMMAND CENTER – The area of a building (usually near the entrance) that acts as the  
communications center for emergency personnel.  The command center is used to display the fire 
alarm system status and control the annunciation system.  This area typically includes the 
equipment required to generate and distribute messages throughout the building’s fire alarm audio 
network. 

CONSTANT VOLTAGE – Speakers used for a distributed emergency evacuation system are 
wired as a “Constant Voltage” system, where the voltage at each speaker is the same (typically 
25V or 70.7V) at the maximum power output of the amplifier. 

COVERAGE ANGLE – The angle where the speaker Sound Pressure Level (SPL) drops 6 dB 
from the on-axis SPL. 

COVERAGE AREA – The plane where the Sound Pressure Level (SPL) at the edge of the plane 
drops 6 dB below the on-axis SPL.  

CRITICAL POLAR ANGLE – The angle where the sum of the distance loss and the polar loss 
is 6 dB less than the on-axis Sound Pressure Level (SPL). 

DECIBEL – A unit used to express relative difference in power, usually between acoustic or 
electric signals, equal to ten times the common logarithm of the ratio of two power levels or  
20 times the common logarithm of the ratio of two voltage levels. 

DIGITAL RISER – A digital system that transmits multiple channels of digital audio throughout 
a facility.  A single pair of wire is used to transmit up to eight channels of digitally encoded audio 
signals.  The digital riser is considered a signaling line circuit, and is wired in a Style 4 or Style 7 
configuration. 

DIRECTIVITY FACTOR “Q” – A common representation of speaker directivity for speakers 
having a conical coverage pattern (typical of single driver speakers used in fire alarm 
applications).  Q is determined by an equation covered in Chapter 2 of this manual.