Late Reflections and Echoes
Spoken language is a fragile thing, it doesn't take a great deal of background noise, or distortion of the signal, to degrade speech intelligibility. The ear and brain are a remarkable speech processor combination, but when working with distorted speech signals, even they have their limits. One of the most damaging distortions to a speech signal is a late reflection, or series of reflections, that arrive after the direct speech signal. We can detect these late reflections as separate signals when they are delayed more than 80 milliseconds (0.080 seconds). These late reflections, when loud enough, mask subsequent speech signals. For listeners, these delayed signals can render speech unintelligible. You may have experienced this in outdoor sports events, where signals from several loudspeakers may arrive at a single listening position.
Multiple arrivals delayed from 80 to 300 milliseconds:
As difficult as this is for listeners, when the announcer, or person making the speech hears these echoes, it can be impossible to speak over without a great deal of concentration. You may have experienced this when making long distance phone calls, you occasionally here your own voice through the ear piece with a noticeable delay. If this line echo is loud enough, you can find yourself repeating the same word or having to make radical changes in your speech cadence. In the world of architecture, this problem can be generated in rooms that are designed as a semicircle, with the talker at the centre of the radius. This is a common design for churches, and the pulpit is often near the focal point. The Pastor or Minister will often have to work through this late reflection (anywhere from 80 to 150 milliseconds in rooms that are 40 - 75 feet deep) that only he will be able to hear. The audience will be unaware of the struggle that he is having in talking over this echo, but they will be aware of odd pauses or strange cadence in his speech.
150 Millisecond slap echo:
Return to Common Acoustical Problems
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