Theoretical Considerations of Acoustics in Rooms – Speech Intelligibility and Directivity Index

As mentioned earlier in the acoustic post on home theater, to create a home theater regardless of the shape and size of the space, we need to consider the best equipment with the best layout in space. To implement the above, we will explain the theoretical issues. We have already gone to the Acoustic Parameters and Time Criteria section and the Energy Criteria and Spaciousness section, and now we are going to discuss the topics of speech intelligibility and conductivity index.

Speech Intelligibility

Speech intelligibility is a useful parameter to determine the audibility of speech e.g. reproduced by a center channel in home theaters, in a room. A common way to assess speech intelligibility is by measuring the speech transmission index STI, which determines the modulation transfer as the ratio between the transmitted and the received degree of modulation.

As speech can be regarded as a signal which is modulated in its amplitude, the speech information is therefore the degree of the associated modulation. The degree of modulation in the signal and thus the audibility of speech will be reduced by a low signal-to-noise ratio (SNR), individual reflections or adverse reverberation in a room. [5] To achieve a good intelligibility, the SNR should stay within 5-20 dB. A practical example in case of home theaters would be the ratio between speech and everything else (background music, ambient sounds, etc.) interfering the audibility of speech in a soundtrack.

Another useful method for acoustical practitioners is suggested by professionals. To find out about influence of a room on speech intelligibility a simple test can be done by assessing the audibility of having a conversation between one person in the location of a loudspeaker (preferably a center loudspeaker, where commonly speech in home theaters comes from) and another seated in the audience area.

Nevertheless, reflections also have the ability to act very supportive on improving speech intelligibility especially if they are occurring within the first 50 ms. In this case early reflections have the same effect on speech intelligibility as increasing the level of the direct sound.

Directivity Index

The Directivity Index is an indicator of the ability of the sound source to confine the sound energy into a narrower radian:

Directivity Index

where Directivity Factor Q is the ratio of the squared sound pressure level at one measured point with a certain distance (on-axis) to the mean squared SPL over all directions at the same distance.

Sound energy from an omnidirectional sound source would represent a Directivity of Q = 1, resulting in a DI = 0 dB

What has been said so far and what we will discuss in detail in the future, will pave the way for creating a home theater of the highest quality. The audio products offered in Saba Smart systems, which include Dali speakers, Yamaha amplifiers and inakustik cables with their great variety and high quality, make it possible to implement any type of equipment according to the relevant needs.

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