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3-2
The figure below lists the relative contributions of each frequency band:
Octave Band Contribution to Intelligibility
0%
5%
10%
15%
20%
25%
30%
35%
125 250 500 1000 2000 4000 8000
Frequency component of speech
Relative Contribution to intelligibility
Figure 3-1. Frequency of Speech Contribution to Intelligibility
Audibility is relatively straight-forward and deficiencies are relatively easy to correct. Intelligibility
is a more complex characteristic of an installed audio system, involving room acoustics, speaker
placement, and psycho-acoustic effects. There are many factors that affect the intelligibility of
messages presented over public address systems in public and private spaces. Some major
intelligibility factors include:
• Background noise.
• The configuration of the space being addressed.
• The acoustical properties of the materials on the walls, floors, and ceilings.
• The distortion and bandwidth of the sound equipment.
• The characteristics of the person speaking (male/female, accent, microphone technique, etc.)
Some of these factors are under the control of the system designer however many are not.
The challenge of good audio system design is to compensate for the factors which cannot be
controlled. This ensures that the system installed can provide intelligible messages in emergency
situations.
Continued on next page
Influences on Intelligibility
Introduction