Intelligibility – The Working Edge For Sound Contractors


Intelligibility – The Working Edge For Sound Contractors – PDF

Acoustic Sciences Corporation
Eugene, Oregon USA

Presented by Art Noxon at the 1993 NSCA Conference & Expo in Orlando Florida – Acoustic Alignment For Intelligibility.
1993 NSCA Conference & Expo in Orlando, Florida

Intelligibility - The Working Edge For Sound Contractors

Speech is our primary method of communication. It is therefore important that uttered speech is received intelligibly. The intelligibility of speech depends (in part) on the acoustical properties of the enclosure in which the speech is transmitted from speaker to listener. Another important factor determining the speech intelligibility is the background noise level.

Although there have been many attempts to objectively quantify the speech intelligiblity, the most widely used parameter is no doubt the Speech Transmission Index (STI) and its derivatives. The STI is based on the relation between perceived speech intelligibility and the intensity modulations in the talker’s voice, as described by Houtgast, Steeneken and Plomp. The STI method is described in the IEC 60268-16 standard.

When a sound source in a room is producing noise that is intensity modulated by a low frequency sinusoidal modulation of 100% depth, the modulation at the receiver position will be reduced due to room reflections and background noise. The Modulation Transfer Function (MTF) describes to what extent the modulation m is transferred from source to receiver, as a function of the modulation frequency F, which ranges from 0.63 to 12.5 Hz. Hence, the MTF depends on the system properties and the background noise.

With the introduction of Dirac 6 and the Echo Speech Source, speech intelligibility measurements can be performed very quickly and easily. The Echo delivers a calibrated signal that is used by Dirac to calculate different speech intelligibility parameters. The Echo / Dirac combination also performs well with high levels of background noise. It is therefore no longer necessary to work with external equalizers or to set the output level. Quite often, the difficulty of emitting a signal with the correct speech spectrum and level, meant that shortcuts were taken which led to questionable results.


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