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Using Microphone Techniques To Attain A Better Result

Approaches for solving problems and improving quality

By Bruce Bartlett October 23, 2015

The type and placement of microphones can make a big difference in the overall sonic landscape.

But when you get close, the part of the instrument that the mic is near is emphasized.

The tone quality that is picked up very close may not reflect the tone quality of the entire instrument.

For example, the sound hole of an acoustic guitar resonates strongly around 80 Hz to 100 Hz.

A mic placed close to the sound hole hears and emphasizes this low-frequency resonance, producing a bassy, boomy timbre that does not exist at a greater micing distance.

The close-mic’d sound is harsh, too. To make the guitar sound more natural when mic’d close to the sound hole, roll off the excess bass on the console or use a mic with a bass roll-off in its frequency response. Also dip out some 3 kHz to reduce harshness.

A sax mic’d in the bell sounds like a kazoo. To mellow it out, cut around 3 kHz and boost around 300 Hz. And if it’s possible to attain adequate gain-before-feedback with mic positions that sound more natural, by all means do so.

This placement likely emphasizes low-end resonance. (click to enlarge)

Making Contact
Another approach is to use contact pickups in tandem with microphones.

A contact pickup can solve feedback problems because it is sensitive to mechanical vibrations, not sound waves.

A pickup for an acoustic guitar usually sounds good near or under the bridge.

Unfortunately, the guitar sounds electric with a pickup because it misses the acoustic string sounds.

Many sound operators have had success with a hybrid method that combines a pickup with a mini mic. A pickup mounted under the bridge picks up the lows and provides volume and punch.

A mini hypercardioid mic is mounted just inside the sound hole facing in. It provides the treble and the clean acoustic string sound.

The pickup and microphone are mixed in a small two-input mixer provided as part of the system. The combination of the pickup and microphone provides a loud, punchy, yet natural sound with all the crispness of a real acoustic guitar.

It often helps to send the pickup signal just to the stage wedges (where feedback is worst), and send the mic signal just to the house loudspeakers. Using as few mics as possible can also be helpful. The more mics in use, the more likely it is to produce feedback.

The gain-before-feedback ratio decreases 3 dB each time the number of open mics doubles. Two mics at equal levels have 3 dB less gain than one mic; four mics have 3 dB less gain than two mics, and so on.


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About Bruce

Bruce Bartlett
Bruce Bartlett

Recording Engineer
   
AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are Practical Recording Techniques and Recording Music On Location.
http://www.bartlettaudio.com

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