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Intelligibility

Intelligibility has a specific meaning and can be measured, unlike fidelity is not a subjective thing. Originally measured by reciting a hundred or thousands of random words and then listeners record what words they heard. It was found MANY things were involved or govern intelligibility.

In commercial sound where all the acoustic problems are worse and the required sound level higher, there was an early formula called the Hopkins Stryker equation which had some predictive ability.

That can be seen here in the original H.S. equation, note that all things being equal, the more sources you have the worse intelligibility is, the less directivity the worse it is, the less absorption, the worse it is and if the speakers are not aimed at the listeners, the worse it is.

http://acousticworx.com/sound%20system%20design%20hopkins%20stryker%20formula.html

This was superseded by the modified equation and then the ALCON and STI measurements.
Now the current ‘best” language independent predictor of intelligibility which has legal standing is the STIpa measurement.

This measurement is based on the modulation transfer functions at a number of different frequencies weighted to a voice spectrum.
The MTF measurement in acoustics is one which quantifies how much of the information in the signal reaches the microphone. Here is an example of an MTF as used in optics.

http://www.normankoren.com/Tutorials/MTF.html

To be clear while intelligibility can go hand in hand with pinpoint stereo imaging, preserving information as indicated in MTF’s is not a requirement to enjoy the sound.
For example, a choir is a large room may sound dreamy and a recording of a choir played in the same room may also sound dreamy even though there is zero intelligibility and no sense of stereo image.

While STIpa can be easily measured;

http://www.studiosixdigital.com/audiotools/sti-pa/

I am not sure it is useful in a living room, on the other hand, if one were comparing different loudspeakers in the exact same locations or listening position in a room or anechoically, then the MTF measurements at the heart of the STIpa are I think useful as a way to compare “how much of the recording gets to the listening position” with one speaker vs another. Arta has a MTF routine.

What shows up too is that the more it preserves the signal here, the more generations one can record and play back music using the loudspeaker and measurement mic in the loop, before subjectively falling apart.

Here, most loudspeakers sound surprisingly bad by generation one or two,few reach three, by far they are the weakest link if being faithful to the signal is the yardstick.
Best,
Tom Danley


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