NTi Audio Acoustilyzer AL1 User Manual
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STI-PA - Speech Transmission Index for Public Address Systems
A rising awareness of security issues from unintelligible speech, new
technological means and the shortcomings of RASTI triggered the speaker
manufacturer Bose and the research institute TNO to develop a new method
for speech intelligibility measurements of PA installations. The result of
these efforts is STI-PA, which allows quick and accurate tests with portable
instruments.
Like RASTI, STI-PA applies a simplified procedure to calculate the MTF.
But STI-PA determines the MTF by analyzing all seven frequency bands,
whereby each band is modulated with two frequencies.
Supposing that no severe impulsive background noise is present and
that no massive non-linear distortions occur, STI-PA provides results as
accurate as STI. If however impulsive background noise is present during
the normal system operation hours, it is usually possible to mitigate the
effects by also acquiring a measurement at a more favorable time e.g.
under slightly different conditions in the area, or during the night time - and
to calculate an unbiased overall measurement by using the results of both
test cycles.
NTi Audio STI-PA measurement table:
Portable STI-PA analyzers, e.g. NTi Audio’s Acoustilyzer, are able to
evaluate speech intelligibility within 15 seconds per position and are thus
well suited for wide-area measurements and high productivity.