A microphone is a component of any voice recording or playback system. Its function is to convert acoustic sound waves into corresponding electronic signals. This signal can then be recorded, sent out, amplified or corrected. However, a microphone cannot effectively distinguish which sounds are needed in an environment with strong echoes. The placement of a microphone in such an environment is not likely to contribute to the improvement of the acoustic environment. This article starts with the "critical distance" to talk about the location selection of the microphone.
1. What is the critical distance?
In every room, there is such a distance (measured from the position of the speaker) that the intensity of the direct speech is exactly equal to the intensity of the speech that is reflected back. Acoustically, this distance is named the critical distance, which is abbreviated as Dc.
2. Why is Dc important for determining where the microphone is placed?
If a microphone is placed at a distance from the speaker's Dc or further, the sound quality of the speech will be poor. This poor sound quality is often described as "reverberation," and "the sound of the bottom of the barrel." The speaker's words will be difficult to distinguish, because the reflected sound overlaps with the speech sound and the speech sound becomes blurred.
3. How to measure Dc?
Tools needed: 25-foot tape measure, sound meter (Radio Shack part 32-2050 or equivalent), portable speaker with FM radio.
1) Place the subwoofer at one end of the room - the position of the speaker, and use the FM radio to continuously move around the stations. This continuous "white" noise will be used instead of the speaker's voice.
2) Use a tape measure to measure the distance from the sound to the other end of the room and fix the tape. This is a reference data for the critical distance.
3) Adjust the measurement item of the sound meter to the position of “A”, adjust the response to “slow”, and adjust the range to “90” dB. Use the distance measured by the tape measure as a guide and place the sound meter one foot away from the speaker.
4) Constantly increase the volume of the speaker until the pointer of the sound meter points to “0”, that is, the sound pressure level (SPL) at this time is 90 dB.
5) Move the sound meter to a position two feet from the sound, and the reading of the watch should be reduced by 4-6dB.
6) Re-adjust the range of the sound meter to “80” dB and move the sound meter to four feet from the speaker. At this point the reading of the watch should drop 4-6dB again.
7) Continue to move the sound meter by multiple times according to this ratio. After each distance is doubled, the reading of the meter should drop 4-6dB each time when the critical distance is not reached.
8) When the watch continues to move, if the reading of the watch is not reduced by 4-6dB but remains in a relatively stable state within a few feet, record this distance, which is Dc, the critical distance.
4. How to use Dc to guide the selection of the microphone?
In general, an omnidirectional microphone should be placed at a distance of thirty percent from the speaker's critical distance. For example, if the critical distance is 10 feet, then an omnidirectional microphone should be placed 3 feet from the speaker. A one-way microphone (heart, super heart, or gun microphone) should be placed at a distance of fifty percent from the speaker's critical distance. For example, if the critical distance is 10 feet, then a unidirectional microphone should be placed 5 feet from the speaker.
5. What if the microphone must be placed at a 50% supercritical distance?
a. Reduce the reflection of the room by acoustic methods. This will increase the critical distance.
b. Accept this unacceptable acoustic effect due to a >50% of the critical distance.
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