Home Cinema Design and Dialogue Intelligibility - October 13th, 2008
Home cinema designers need to be able to predict speech intelligibility, measure it accurately, and understand the anomalies present in real home cinema environments that are not currently easily detected in advance of measurements. Key parameters effecting dialogue intelligibility are:
1. Signal-to-noise (S/N) ratio
2. Reverberation time (RT60)
3. Distance from the source
4. Source misalignments
5. Reflections under 1 ft of path length difference
6. Reflections that are late in time (100+ msec) and higher in level than energy near them.
The first three parameters are predictable within reasonable tolerances at the drawing stage of the designated home cinema room. The last three are classified as anomalies that occur through oversight or error.
In 1971 V. M. A. Peutz published his equation for the percent of articulation loss of consonants (%ALcons) in speech. Presently, only the Peutz equations allow a workable estimate of %ALcons at the design stage, for a home cinema room. They can be useful in matching the cinema room to the sound system installed.
The %ALcons at a listener is dependent upon a too early or too late return of reflected energy. The distance travelled or the signal delay is not the only factor. The amount of interference detrimental to %ALcons is also very level-dependent. These too early and too late reflections are corrected by careful placement of devices, precision signal delays, or selective acoustic wall treatments, like absorptive acoustic panels and diffusers.
Speaker misalignment is probably the most common cause of reduced intelligibility in home cinema systems. Misalignment causes spurious lobes to be radiated by the cinema speakers, which can excite wall surfaces. This causes an increase in the level of the reverberant sound field.
When all else fails, reducing the distance from the speaker to the listener will solve the intelligibility problem. We have never been in an environment where it’s legal to work that you could not communicate face to face.