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Model DSP2B & Model DSP2BP
User’s Manual
OPTion STICis
®
- Intelligibility & Privacy Meter
U.S. Patent 6,792,404
Now Intelligibility and Privacy Can Be Measured
Meet or Exceed International Standards
Test CD Developed by TNO Human Factors Research Laboratory in Holland -
Developer of STI in Europe
Speech intelligibility is not a physical quantity like Amperes, Volts or Btu’s. It is the degree to
which we understand spoken language. Speech is not necessarily intelligible simply because it is
audible. Everyone has experienced speech signals that are loud enough but overly reverberant,
suffer from echoes or distortion and therefore are not understandable. Announcements in
airports, train stations and places of worship often suffer from this problem.
Intelligibility
Intelligibility is measured by sending a test signal through the sound system and into the
listening environment. The test signal is an artificial reproduction of a range of modulated sound
frequencies important to human speech. The meter measures the received signal, compares it to
the signal specification and computes the Modulation Transfer Function (MTF). The MTF is a
measure of the change in or distortion of the signal as it passes through the sound system and as
it is affected by the listening environment. The MTF is used to compute and display the Speech
Transmission Index (STI) on the meter. The meter can convert the STI to a generic Common
Intelligibility Scale (CIS), which permits direct comparison to other test methods.
It should be noted that any “measurements’ are actually predictions of voice intelligibility.
Subject based test methods can gauge how much of the spoken information is correctly
understood by a person or group of persons for that particular test. When properly done, that
resulting value is a prediction of how much of the spoken word will be correctly understood by
others at some other time. Intelligibility meters have been developed and use a test method that
has been correlated with accepted subject based test methods. Therefore, the numbers presented
by a meter are also predictions. Still, it is common to refer the results as measurements. This is
exactly the same as audibility measurements. The definition of a decibel for sound pressure
levels is the ratio of the measured sound pressure level to the threshold of human hearing. That
is, zero decibels is the threshold of audibility. However, that threshold is a subjective number
determined many years ago by testing a large group of people with “normal” hearing.
The Speech Transmission Index (STI) is a quantitative method of measurement covered in the
International Electrotechnical Commission (IEC) standard 60268-16.
These Gold Line meters require the use of a STI-PA test signal which was developed by TNO
Human Factors Research Laboratories in the Netherlands, in conjunction with Bose and Gold
Line. This test signal and the measurement methods used by meter has been verified to correlate
to established subject based test methods. The STI-PA test signal is provided on an audio CD
and is input to the system under test. The signal can be input by a direct connection or by
playing the signal over a calibrated loudspeaker and inputting through the system microphone.
