CHEAT SHEET
Positioning Microphones
Correct microphone placement and positioning can have a dramatic effect on the quality of sound.
MOST COMMON LOCATION PROBLEMS
Too far: The mic picks up too much unwanted sound. Too close: Depending on type of mic, this can overload the mic or preamp circuitry, causing distortion. In dynamic mics, this causes the “proximity effect,” which results in an emphasis of low frequency energy. Off-axis: With directional mics, there may be coloration of the sound or dramatic attenuation of overall level due to being outside the coverage angle. Adjacent sound sources: Leakage of unwanted sounds can make it difficult to control the level of the mic’s intended source. Feedback: Generally, microphones should be never be located in front of loudspeakers.
COVERAGE AREA
The coverage angle describes the useful “working angle” of a polar pattern. The left and right borders of this angle are the points at which the sound source is 6 dB lower in level than a source directly on-axis. Using the following guidelines, with a known coverage angle, you can estimate how wide an area a directional mic will cover. s For a cardioid, the “stage width” is 4.4 times the distance of the mic from the stage. For example, if the mic is 10 feet from the stage, it’s picking up about a 40-foot wide area. s For a supercardioid, it is 3.1 times the distance. s For a hypercardioid, it is 2.6 times the distance. s For a bidirectional, it is 2.0 times the distance.
DIRECTIONAL CHARACTERISTICS WHY AIM?
Aiming a microphone is important whenever the potential for feedback exists or when there is significant ambient noise present. If either of these are coming from a specific direction (such as a monitor speaker or a nearby drum kit for example), it may be more important to aim the mic away from the undesired sound source even though that puts the desired source slightly off-axis. Even when feedback and noise are not a factor, it is still desirable to aim any directional microphone at the sound source because the off-axis sound quality of the microphone can be quite different than its on-axis sound.
Polar Response Pattern Coverage angle Angle of maximum rejection (null angle) Rear rejection (relative to front) Ambient sound sensitivity (relative to omni) Distance factor (relative to omni)
Omnidirectional 360° — 0 dB 100% 1
Cardioid 131° 180° 25 dB 33% 1.7
Supercardioid 115° 126° 12 dB 27% 1.9
Hypercardioid 105° 110° 6 dB 25% 2
Bidirectional 90° 90° 0 dB 33% 1.7
CRITICAL DISTANCE AND MICROPHONE PLACEMENT
Critical Distance (Dc) is the distance (measured from the talker) where the direct speech and the reflected (or reverberant) speech are equal in intensity. If a microphone is placed at Dc or farther from a talker, the speech quality picked up will be very poor. An omnidirectional microphone should be placed no farther from the talker than 30 percent of Dc. For example, if Dc is 10 feet, an omnidirectional may be placed up to 3 feet from the talker. A directional mic (cardioid, supercardioid, or shotgun) should be positioned no farther than 50 percent of Dc. For example, if Dc is 10 feet, a directional may be placed up to 5 feet from the talker. How to estimate Dc: s Place a “white noise” sound source (for example, a boom box tuned to an unused FM frequency) in one end of the room in place of a talker. s Extend a tape measure from the source to the far side of the room. Lock the tape measure in place. It is the reference for distances. s Using a sound level meter set to “A” weighting, “slow” response, “90” dB range, place it 1 foot from the boom box. s Increase the boom box volume until the sound level meter needle points to “0,” which is 90 dB of sound pressure level (SPL). s Move the sound level meter back to the 2 foot mark. The meter reading will drop 4 dB to 6 dB. s Reset the meter to the “80” dB range. Move the meter to the 4 foot mark. The meter reading should again drop 4 dB to 6 dB. s Continue to double the distance each time the meter is moved. When the distance is doubled, the meter should drop 4 dB to 6 dB if Dc has not been reached. s During one of these meter moves, the meter reading will not drop the predicted 4 dB to 6dB, but will remain relatively constant in level over several feet. Note the distance where the meter reading first remains steady. This is the Critical Distance.
Polar response pattern: This is a graphical representation of the microphone’s output level when the sound source is at different positions. Where the line is farthest from the center point, the mic is most sensitive; where it approaches the center point, there is little or no sensitivity. Coverage angle: The angle between the points where sounds will be 6 dB lower than on-axis sounds. This is the maximum useful pickup angle for the mic. Angle of maximum rejection (null angle): This is the angle at which the mic picks up the least sound. Ideally, this part of the mic should be aimed at monitor speakers. Rear rejection (relative to front): This is how much lower in level sounds coming from behind the mic are compared to sounds com-
ing from in front. Note that with a hypercardioid mic, sounds from behind the mic are only 6 dB lower, so you’re picking up almost as much in back as in front. Ambient sound sensitivity (relative to omni): This is how much ambient noise the mic picks up overall, compared to an omnidirectional mic. In situations with lots of noise coming from nowhere in particular (in the middle of a crowd for instance) the hypercardioid picks up only 25 percent as much noise as an omni. Distance factor (relative to omni): This is the closest thing to a measurement of microphone “reach.” The Distance factor tells you how much farther you can put a mic from the source compared to an omni and still pick up the same amount of ambient noise. In other words, if an omni worked at 1 foot, a supercardioid would work equally well at 1.9 feet.
MICROPHONE PLACEMENT AND MONITORS LOUDSPEAKERS
Depending on the polar pattern of the mic, you can increase gain before feedback by positioning the monitors out of the rear lobe of the pick-up pattern
MICROPHONE ≠ EAR
Many users assume that microphones are a mechanical version of our ears. But our ears are attached to our brains, which filters, focuses, and processes all of the sounds collected by the tympanic membrane so that we hear what we want (or expect) to hear.
THE MYTH OF REACH
Myth: A microphone reaches out to capture sound. Fact: A microphone only responds to sound waves that travel to its location; it cannot “reach” out and capture the sound wave from a distance. A microphone measures local rapid variations in air pressure and provides an electrical output that mirrors these variations.
SOURCE: SHURE
98 FEBRUARY 2008 | proavmagazine.com