NTi Audio Acoustilyzer AL1 - Manual (Page 90)

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individually write down what they believe to have understood. The
statistical analysis of these notes results in a value representing the
percentage of the listeners who have understood the messages correctly.
Important procedures according to this method are PB-words, CVC or SRT
(Speech Reception Threshold). However, conducting such tests is rather
costly, and in some hazardous locations, even impossible. Therefore,
these methods are mainly used to verify technical principles during
development.
Technical Methods:
Already back in 1940, Bell Laboratories started to develop measurement
technologies to determine the speech intelligibility. Nowadays, highly
developed algorithms such as SII (Speech Intelligibility Index) and various
forms of the STI (Speech Transmission Index) measure speech intelligibility.
These techniques take care of many parameters which are important for
intelligibility such as:
• Speech level
• Background noise level
• Reflections
• Reverberation
• Psycho acoustic effects (masking effects)
Measurement techniques use synthesized test signals that don’t sound like
speech, but which have many common characteristics to real speech signals.
The averaged frequency response, as well as low frequency modulation,
is based on human speech. Since the speech of men’s and women’s
voices sound different, some measurement techniques use separate male
and female test signals: Speech intelligibility measurements acquire and
evaluate the signals in a manner characteristic of the way human speech is
altered before reaching the ears of a listener. Extensive investigations have
revealed the relationship between the alteration of speech characteristics
and the resulting speech intelligibility. These findings are incorporated into
the speech intelligibility meter that is able to display the intelligibility result
as a single number between 0 (unintelligible) and 1 (excellent intelligibility).