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The Speech Intelligibility Calculator

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This page will calculate reverb time, critical distance, and expected %ALCONS for the room you input here. The results show the inter-relationship between reverberation time, room finishes, speaker selection and speech intelligibility. This calculator is provided as an educational tool to demonstrate this relationship, and any use made of the information here is done at the sole risk of the user. (Let's face it, if it were really this easy would we be giving the information away for free?) Work your way down the page, inputting data where indicated, then push the button to calculate the results. Caveat

Input Room Data

Room Details

Input the room dimensions and the additional acoustical finishes in this section. This calculator is based on the Sabine reverberation equation, which makes certain assumptions about the distribution of acoustical treatment or absorptive surfaces. This has been done to keep this a fairly simple calculator. Difficult acoustic spaces will require more detailed reverberation calculations to ensure an accurate prediction of acoustical and sound system performance. The absorption values for the materials shown here are also based on a specific example of each type, and for a specific mounting method, they cannot be generalized across all materials of this type. Room shape will also affect the speech intelligibility, as focusing surfaces and late reflections will dramatically degrade speech intelligibility, as will exterior or interior noise sources. It would be a very bad idea to stretch these assumptions to cover a specific application of your own, as large errors would be possible. Problems with many variables, such as predicted acoustical and sound system performance, cannot be generalized effectively. (Ok class, first of all, let's assume spherical cows...")

Room length
in Metres

Room width
in Metres

Room height
in Metres

Carpet w/underpad
in square Metres

Ceiling tile
in square Metres

25mm fibreglass
in square Metres

Number of
Seated Audience
Input Speaker System Data | Jump to Recalculate Button

Speaker Details

Input loudspeaker coverage and quantity
Horizontal coverage
Default value
Enter new value
in degrees
Vertical coverage
Default value
Enter new value
in degrees
Input number of loudspeakers
active in the room

Calculated Loudspeaker Q and Directivity Index
No Input Required in These Two Boxes

Loudspeaker Q
calculated here
Directivity Index
calculated here
in Decibels

Input Listener Data | Jump to Recalculate Button

Note that the theoretical Q or Directivity index of a loudspeaker is very theoretical. Real live loudspeakers tend to vary significantly from theory, and one manufacturer's 90x40 loudspeaker is not often the same as another's. Loudspeakers also vary widely in directivity with frequency. Where a speaker may actually have a Q of 12.8 at 2kHz, it is likely that that the directivity value would drop to a Q of 4 at 500Hz, and as low as a Q of 1 at 125Hz. This is why we have only evaluated the 2kHz value for critical distance and the related %ALCONS. A thorough acoustical and sound system design would include the prediction of expected sound system behaviour across the entire audio bandwidth, taking into consideration the variations in loudspeaker coverage and variations in reverberation time. This is where computer modelling is very handy.

Also note that with loudspeakers, more is not necessarily better. In very reverberant spaces, adding more loudspeakers can dramatically reduce intelligibility. It is very easy to get into a no-win situation, and acoustical treatment is the only course of action to deliver speech intelligibility.

Listener Details

Distance from talker or loudspeaker to listener
in Metres

You can change the values and re-calculate.
Reverb time results | Critical Distance results | %ALCONS results

Reverb Time

This is the reverb time you would expect to see in an unfurnished room with gypsum wallboard walls and ceilings, a concrete floor plus the carpet, ceiling tile, acoustical treatment and audience added. If the basic room materials are different than described, the resulting acoustical characteristics will be very different than predicted here.

125Hz RT60
in Seconds

250Hz RT60
in Seconds

500Hz RT60
in Seconds

1,000Hz RT60
in Seconds

2,000Hz RT60
in Seconds

4,000Hz RT60
in Seconds

Change Room Parameters | Critical Distance Results | %ALCONS results

These would be the criterion values for reverberation time

Application 125Hz 500Hz 2000Hz
Classroom 0.9 sec 0.6sec 0.6sec
Church or theatre for speech or amplified music 1.3 sec 1.0 sec 1.0 sec
Church or theatre for music 1.8-2.0 sec 1.5 - 1.8 sec 1.5 - 1.8 sec
Convention facility 1.8 sec 1.5 sec 1.5 sec
Gymnasium for teaching 1.8-2.0 sec 1.5 - 1.8 sec 1.5 - 1.8 sec
Small arena (500 -2000 seats) 2.75 sec 2.0 sec 2.0 sec
Large arena (2000+ seats) 3.25 sec 2.75 sec 2.75 sec

Why does excess reverberation affect speech intelligibility?

Critical Distance (Dc )

Critical distance is defined as the distance from a sound source located in a reverberant field, where the direct sound is equal in level to the reverberant field level. Beyond this distance from the source, there will be no further drop in sound level, as the room's reverberant field is defined as having a uniform sound level (in fact that's how to tell when you're in the reverberant field). Critical Distance is affected by the directivity of the source. With an optimum signal-to-noise ratio (a complete lack of background noise) and short reverb times, you could still understand speech beyond critical distance, but the optimum signal-to-noise ratio is rarely encountered. With long reverb times Critical Distance becomes the limiting distance for speech intelligibility.

This is the Critical Distance for an unaided talker

This is the Critical Distance for
the loudspeaker coverage selected

m m

Change Room Parameters | Change Speaker Data | Change Listener Distance


This is the expected %ALCONS (or Percentage Articulation Loss of Consonants) for an unaided talker and the loudspeaker selected above. This refers to the percentage of consonants that will be lost or misunderstood in speech, and it is in these consonants that intelligibility lies. The maximum acceptable %ALCONS for speech communication would be 15%, as long as there is a minimum of 25dB of signal to noise ratio, and the spoken information is being delivered at a pace suitable for the acoustic environment. A more suitable maximum %ALCONS for familiar speech would be 10% (this would also apply to life safety systems, voice warning or fire evacuation page systems). For school and education environments or in public spaces where people may not speak english as a first language, a suitable maximum %ALCONS would be 5%. As you try different values, note the effect of reverberation time, speaker selection, number of speakers, and listening distance on %ALCONS. If you are seeing values in excess of 100% the listener distance is beyond 3.16 times critical distance. More detailed information on %ALcons can be found in Appendix X on page 639 in Sound System Engineering by Don and Carolyn Davis

This is the %ALCONS of an unaided talker

This is the %ALCONS of
the loudspeaker of the
coverage selected

% %

Change Room Parameters | Change Speaker Data | Change Listener Distance
©1997 Mc2Systems Design Group

It is possible to input data into this calculator that will result in invalid or erroneous results. This information is provided for educational puposes only, with no warranty of its accuracy, or applicability, and any use made of this information is done so at the sole risk of the user. Rather than take that risk, you should be hiring an acoustical consultant.
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Speech Intelligibility Page | Directivity Page | Reverb Demo Page
Mc Squared System Design Group, Inc Mc Squared System Design Group, Inc
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