Published in July 2007

Assisting the Hearing Impaired, Part 2
By David Lee Jr., PhD

'A little thought, some basic equipment and, voila!, communication happens!'

Editor’s Note: Part 1 of this discussion appeared last month.

    Critical Distance (Dc) is a concept used in sound system design as it relates to music clarity, speech intelligibility and even avoidance of feedback. Its applicability or relevance increases with increasingly more reverberant spaces. Let’s start with a definition of Dc and a subjective example of how to experience it.
    Dc is the distance from a sound source in a reverberant room where the direct sound pressure level (SPL) and the reverberant level are equal. Imagine yourself and a friend in a quiet, but highly reverberant, space such as a large cathedral with a high vaulted plaster ceiling, stone walls, a slate floor and wood pews without cushions. You and your friend stand facing each other about four feet apart; you’re facing toward the chancel and he’s facing the nave.
    He speaks to you in a raised voice. You mostly hear the direct sound of his voice, but also a “field” of reverberant sound at a low level in the background. Now you begin walking backward slowly and concentrating on the level of these two sounds. The further you go, the lower the level of the direct sound and the more you notice the reverberant level. At some distance, you determine that the direct and reverberant sound levels are equal. This is the critical distance.
    Why does it matter? How do you use this?
    Let’s first consider its application to achieving adequate speech intelligibility in sound system design. Speech intelligibility is proportional to the direct-to-reverberant sound ratio (Ld/Lr). At Dc, Ld/Lr = 1. The intelligibility might be fair at that point, but it will diminish at greater distances. Ideally, we want to have a positive Ld/Lr or, stated another way, we want all seats to be at a closer distance than Dc. To do this, we somehow have to increase Dc. One option is to reduce the reverberation time (RT60), but this may be impractical. Another option is to move the sound source(s) closer to the listeners. A third option is to increase the directivity factor (Q) of the sound source(s).
    Going back into your imagination, remember where you were in the cathedral at some distance from your talking friend where you perceived the direct sound of his voice and the reverberant sound field to be at the same level. Now he cups his hands around his mouth and you can once again hear the direct more than the reverberant sound. You begin walking slowly backward until, once again, the two sound levels are equal. You’ve established a new Dc.
    Now he picks up a cheerleader’s bullhorn and speaks through it, aimed in your direction. The direct sound is now much louder than the reverberant sound and he is perfectly intelligible! You start walking slowly backward again, but before you reach Dc, your back hits the rear wall. The horn through which your friend is speaking has sufficient directivity to ensure a positive Ld/Lr all the way to the back of the room.
    Now you walk along the rear wall off to one side. As you go, you notice the direct sound level is diminishing quickly and, along with it, speech intelligibility. What has happened? You walked out of the horn pattern to where this high Q device no longer provides a high enough Q. It would take another horn to cover this listening area. This is the beginning of a horn cluster design.
    Considering that the reverberant sound field remains at a virtually constant level in highly reverberant rooms, there is another way to apply our knowledge of Dc. If a sound system is designed such that its potential acoustic gain (including a feedback stability margin of about 6dB) is equal to or greater than the required acoustic gain, it will not in the “normal” hearing and hearing-impaired worlds.
    For additional information, download “Breaking the Sound Barrier in Your Church” at
    I believe that helping worship leaders find ways to enhance the worship experience for every person is vital to those of us who consult in the house of worship market. Perhaps some of you have solutions to address this crucial communication need. I would like to know what you think about this need, and to hear your solutions.

David Lee Jr., PhD, CEO of Lee Communication Inc., Orlando FL, is a licensed minister and has more than 25 years of experience as a systems integrator. He is a member of Sound & Communications’ Technical Council. Send comments to

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