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Speech Intelligibility In Design

If music reproduction drives the sound system design process, the cart is before the horse.

SI is becoming law. It already is in Europe. Sound reinforcement systems in public spaces must meet a minimum Speech Transmission Index (STI) score for public safety reasons. The rating method isn't perfect, but it's vastly better than nothing at all. The legal aspects of SI mean that architects must consider room acoustics as part of a building design. Buildings must not only be aesthetically pleasing, but also be acoustically tolerable.

The requirements for good SI are basic and straight-forward. It's all about ratios. The system must have adequate bandwidth, adequate signal-to-noise ratio, adequate direct-to-reverberant ratio, and be devoid of interfering reflections. Each of these conditions can be quantified by measurement. The relationships between them are well understood, which means that improvements in SI can be predicted in advance and verified once they're implemented. SI is subject to the laws of physics. There's a cause and effect relationship between the relevant parameters and their effect on the end results.

Music generally makes a poor choice for judging the accuracy of a sound system. This is because the music itself is a form of art with no objective right or wrong attributes. This is why I always initially evaluate the performance of a sound system with speech as a program source. Humans are calibrated regarding speech. If something is wrong, the brain will know it. Music is subjective, while SI is objective. It can be measured and graded, requiring expertise and special equipment on the part of the investigator.

If you're a contractor or consultant who has invested significant resources into learning audio and acoustics, you should go one step further and learn to market speech intelligibility as a product. SI means job security. There are precious few experts in this arena. You can't import it from China or buy it on eBay or at a warehouse store. It doesn't go to the low bidder, and the electrical or IT contractor doesn't want to be bothered with it. There aren't any magic processors that fix bad rooms and poor loudspeaker choices. Methods that improve SI in one venue may be ineffective in another. SI is like a custom suit that must be tailored to fit the wearer. And there will always be a need for good tailors.

The professional audio and AV industries would gain credibility as an engineering field if SI were pushed to the front of the sound system design process. Sound systems that are highly intelligible provide a strong foundation for high-clarity music reproduction. We only need to add channels and extend the bandwidth to render a good speech system capable of excellent music reproduction. Order to disorder — it's a one-way street.

Pat Brown is president of Synergetic Audio Concepts (Syn-Aud-Con) Inc. and Electro-Acoustic Testing Company (ETC) Inc. Syn-Aud-Con conducts training seminars in audio and acoustics worldwide for those who operate, install, and design sound reinforcement systems. ETC Inc. performs precision loudspeaker testing for the audio industry. He can be reached at

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