July/August - 38

COMMUNICATION TECHNOLOGY
measurement microphone. Distortion in the audio system,
background noise and reverberation affect the transmissions,
which the measurement mic will detect. Comparing
the original signal with the received signal determines
the STI rating. A rule of thumb for the threshold of 100%
intelligibility is usually a signal-to-noise ratio (SNR) of 12
decibels (dB), meaning the signal should be roughly four
times louder than the noise floor.
The Speech Transmission Index for Public Address
Systems, or STIPA, is a modified form of the STI scale
regarding intelligibility ratings. It's considered a fast and
accurate way to measure the STI of a given area, taking into
account the quality of the loudspeakers, signal interference,
room acoustics and more.
An important point to remember is that effective coverAn
important point to remember is
that effective coverage in a space
depends more upon loudspeaker
placement, the type of loudspeaker
and spacing between loudspeakers
than on the loudspeaker's volume
PHOTOS BY JEFF BARRET
For causeways, concourses and
gates, beam-steering technology
allows loudspeakers to tightly control
the sound and place it where needed
while keeping it away from surfaces
that may cause echoes, reflections
and reverberation.
and all they hear is muffled speech, they'll quickly give
up trying to understand what's being communicated. The
higher the speech intelligibility, the easier it is for listeners
to retain information and understand what actions, if any,
they need to take.
Speech Intelligibility and STI/STIPA
Simply put, speech intelligibility is the percentage of speech
a listener can understand. Intelligibility is affected by the
volume level and quality of the speech signal, the type
and level of background noise, room reverberation and
more. It's an essential consideration for AV systems in
transportation hubs.
The Speech Transmission Index (STI) is a numeric representation
measure of communication channel characteristics
whose value varies from 0 (bad) to 1 (excellent). STI
predicts the likelihood of syllables, words and sentences
being comprehended. The only way to provide an accurate
speech intelligibility rating is to measure it in the space or
room under typical ambient conditions (which is highly
recommended). To measure STI, a test signal is played
through the audio system, which is then measured with a
38 \ AIRPORT BUSINESS / JULY/AUGUST 2023
age in a space depends more upon loudspeaker placement,
the type of loudspeaker and spacing between loudspeakers
than on the loudspeaker's volume. Achieving sufficient
intelligibility is more than just a matter of turning up the
volume or increasing the wattage of the loudspeaker. Many
poor communications result from trying to compensate for
an insufficient number or poor placement of loudspeakers
with too much volume.
For transportation causeways, concourses and gates,
beam-steering technology allows loudspeakers to tightly
control the sound and place it where needed - on the
passengers - while keeping it away from other surfaces
that may cause echoes, reflections and reverberation. Beam
steering allows system designers and integrators to control
sound dispersion in the vertical axis on a driver-by-driver
basis (most loudspeakers have more than one driver), focusing
the sound where they want rather than broadcasting it
uselessly towards the ceiling or floor. Beam-steering loudspeakers
are typically much taller than wide and mounted
flat against the wall since the beams are digitally steered. For
that reason, they're much more cost-effective than flying
a line array and much easier to blend into the building's
architecture. Properly installed and tuned beam-steered
arrays can result in as little as a 2 dB drop in volume front
to back (closer to the loudspeaker vs. further away).
Audibility and Intelligibility in
Emergency Situations
Most transportation hubs have strict requirements for audibility
and intelligibility per local or national fire codes such
as NFPA 72. Think of the adage " loud and clear " - " loud "
refers to audibility, while " clear " refers to intelligibility. Both
are critical to providing proper instructions in an emergency
to building occupants, but the terms are not the same.
Audibility is the state or quality of being perceptible by
the human ear and is measured in A-weighted decibels
(dBA). A-weighting gives more value to frequencies in the
middle of human hearing and less importance to frequencies
July/August

Table of Contents for the Digital Edition of July/August
