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Distortion of the speech waveform can come from many sources, however it is usually exhibited
by an overdriven signal, causing the peaks of the waveform to be clipped. “Clipping” is caused by
some part of the electrical signal path within the fire alarm system exceeding the capacity of the
components. The most common cause of clipping is improper use of the microphone, where the
operator is shouting into the microphone, overdriving the system. Because clipping itself does not
reduce modulations, the intelligibility effects of clipping are generally not as severe as noise and
reverberation effects. A properly installed, operated, and maintained Simplex Fire Alarm Audio
System has minimal distortion and reproduces speech with excellent clarity.
Proper microphone technique can be a major speech intelligibility factor in emergency voice/alarm
communications systems. Unfortunately, this is one area over which the system designer has no
control. Although this is not normally considered during the measurement of speech intelligibility
or during system acceptance testing, it should be considered in the training and use of the system.
For this reason, when possible, use of pre-recorded messages or digitally compiled phrases may be
preferred. These messages provide a consistent sound level output and a controlled speech pattern
and pace that may be more intelligible than an unpracticed announcement via microphone.
Influences on Intelligibility, Continued
Distortion
Microphone
Technique