Subjective quality:

Clarity refers to how clear the sound quality is. Can you hear every separate note of a fast-tempo soloist’s coda distinctly, or do the notes tend to blur into one another? Some blending is often desired for music, but for speech and opera, greater clarity leads to better speech intelligibility.

Objective measure:

Clarity is produced when a room has a high ratio of early sound energy to later reverberant energy. Early sound energy is that which arrives at the listener within 80 milliseconds of the direct sound from the source to the listener. A popular objective measure for clarity is the Clarity Index, c80. This is defined as the logarithmic ratio of early sound energy, arriving in the first 80 ms, to late sound energy, arriving after 80 ms:

c80 = 10 log (early sound energy/late sound energy)

The units of c80 are decibels (dB).

Like many of the acoustic qualities discussed on this website, c80 is dependent upon frequency. Therefore, c80(3) has been developed to give an overall idea of what a room’s clarity is. c80(3) has been defined as the average of c80 values at frequency octave bands centered at 500 Hz, 1000 Hz, and 2000 Hz.

Listen to spaces with different c80‘s.

Optimum values:

In general, acceptable values for c80(3) for concert halls are between +1 dB and -4 dB.

Click for c80(3)‘s of famous concert halls.

How to design:

To increase clarity, one should increase the amount of early sound energy relative to late sound energy. This could be accomplished by adding absorption in areas farther from the sound source.

Back to Reverberation

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