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Speech Intelligibility: Gathering Additional Pieces Of The Audio Puzzle
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Figure 1 - For speech to be intelligible, the direct sound field level must exceed the level of the reverberation and/or noise at the listener location.

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  Acoustic Measurement, Pat Brown

When most people think of sound reinforcement systems, or audio systems in general, they rarely do so with regard to speech intelligibility (SI).

It’s usually about the music, with the assumption that if a system sounds good for music, it should work for speech.

While this can be true in small, well-behaved rooms, it is seldom true in large spaces.

My first sound system disaster was the design and installation of a large, expensive sound system that failed to produce acceptable SI. Its excessive excitation of the room was lovely for the choir and symphonic music, but a talker at the lectern was unintelligible at all but the closest seats.

I quickly exhausted the “quick fix” solutions to no avail. These included equalization, re-aiming the loudspeakers and trying a different microphone.

It became painfully apparent that I had placed the wrong loudspeakers in the wrong place, and no amount of signal processing could change that. I found a copy of Sound System Engineering – 1st Edition by Don and Carolyn Davis at the local electronics store. As I methodically ingested it I found that I had broken most of the rules for maximizing SI. I fixed the system at my own expense, and was determined to never again design an unintelligible sound system.

In short, as a contractor I had some of the pieces of the puzzle, including basic audio knowledge, lots of tools and some equipment franchises. I knew the makes and models of all the popular mixers, amplifiers and loudspeakers. Until that project, my musician background had always enabled me to tweak my systems to make the musicians happy. I considered myself a pro.

What I didn’t have was an understanding of the science behind the sound. When it comes to SI, you leave the touchy/feely world of music “quality” and are left with the cold, hard variables of communication theory and principles.

Clearly, the way to avoid repeating the same mistake was to:

1. Learn the principles that govern SI
2. Develop a system design workflow that places SI at the forefront, rather than making it an afterthought.


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