A brief review of psychoacoustics and intelligibility will make it easier to understand the unique and effective approach to ambient noise compensation in
Psychoacoustics is the study of the physiology and psychology of the human ear-brain system and its interaction with the acoustics of the listening environment.
Intelligibility is a quantitative measure of the human ability to comprehend speech signals in a given environment. It is expressed as the percentage of the number of words understood correctly to the total number of words.
Intelligibilty Test Results
intelligibility testing of SmartSpeaker was carried out independently
by Mr. Claude Fortier at State of the Art Electroniks, Ottawa. The
tests included measurements of Speech Transmission Index (STI) and
Percent Articulation Loss of Constants (%ALcons) with and without
SmartSpeaker in noisy and quiet environments. A stereophonic
recording of real ambient noise was used to create the noise for the
field test. The Larson-Davis MLSSA system was used to measure
One 4-inch, 2 Watt
loudspeaker producing 65dBA signal SPL at the MLSSA system's
test microphone with SmartSpeaker turned
placed 0.5m from loudspeaker (on axis). Test Signal:
Weighted (spectrally shaped) pseudorandom noise generated by the
MLSSA system. Ambient Noise: A 15 second segment of a
stereophonic recording of subway noise played-back in far-field
with stereo reproduction equipment.
composed primarily of
human speech (multi-talker babble) with some low-level rumble
and hiss from the rolling stock. The background ambient noise
level of the test environment was 42dBA, primarily wide-band
SmartSpeaker: Ambient noise
pickup microphone was placed 0.2m from the loudspeaker and 0.54m
from the test microphone (180o
Power amplifier gain was adjusted for high signal-noise ratio
and intelligibility at the test microphone in the absence of
Where the comprehension of delivered speech is deemed important, intelligibility - not audibility - must define the efficacy of a PA system. Overall, less than 85% intelligibility is considered to be inadequate for most PA applications.
In public environments, the most important cause of intelligibility loss is the interference caused by ambient noise. The impact of ambient noise on speech intelligibility depends in a complex way on several statistical properties of the noise, such as average amplitude, amplitude fluctuation and frequency spectrum etc.
Unlike audibility, intelligibility degradation occurs rapidly when the average noise intensity begins to approach the average PA signal intensity.
The graph shows a typical "inverted S" intelligibility graph. The vertical axis represents intelligibility from 0 to 100%. The horizontal axis represents the ratio of the environmental noise level to the PA signal level on an open-ended logarithmic scale.
The centre line on this scale indicates when the PA signal level equals the environmental noise level.
On the left of this line, the PA signal level exceeds the environmental noise level, intelligibility increases rapidly and levels off in Section 1.
Further to left, the intelligibility remains high but now the system begins to sound intrusively loud in Section 2 as the PA signal level continues to rise in relation to the level of ambient noise.
Optimum listening conditions lie in Section 1, where the signal is highly intelligible without being intrusively loud.
On the right of the centre line, Section 3 shows a rapid decline in intelligibility which fades into complete inaudibility in Section 4 as the environmental noise level increases in relation to the PA signal level.
Audiostat optimises the PA signal to keep intelligibility in Section 1 - automatically, seamlessly and in real time.