Suppose you want to record a musician who plays guitar and sings at the same time. The vocalist is self-accompanied on acoustic guitar.


This seems like an easy miking situation. Just put a mic on the singer, another on the guitar, then mix the two mics.


But how does it sound? 


No matter how good the mics you use, often the singer’s voice sounds thin, hollow or filtered. That strange sound can occur whenever you mix two mics that are picking up the same sound source. The vocal mic picks up the singer’s voice, and so does the guitar mic.


The vocal picked up by the guitar mic is delayed because the vocal sound travels a longer path to that mic. The two vocal signals in the mix — direct and delayed — interfere with each other and make a hollow sound.


How does that happen? When you combine a signal with itself delayed, at equal levels, certain frequencies cancel out. In the frequency response of the mixed signals, a row of notches appear at the frequencies that cancel. This response is called a comb filter effect because it looks like the teeth of an inverted comb. The frequencies that cancel depend on the delay. In general, if two mics pick up the same sound source at different distances, at nearly the same level, and their signals are fed to the same channel, that might cause phase cancellations. These are peaks and dips in the frequency response caused by some frequencies combining out of phase. The result is a colored, filtered tone quality that sounds like mild flanging.


There are several ways to solve the problem:First record the guitar, then overdub the vocal. This method sounds the best because the isolation is perfect, and you can use any mic techniques you want.


Some guitarists, however, are not comfortable with this method. They need to sing and play at the same time. That’s when you use one of the following techniques:


In your DAW, delay the vocal mic signal by about 1msec. Then the signals of the two mics will be more in-phase, preventing phase cancellations when they are mixed to the same channel. Mic the voice and guitar very close. Put a foam pop filter on the vocal mic and angle it upwards. The singer should have lips lightly touching the pop filter.


Try aiming the guitar mic 3″ to the right of the sound hole (toward the neck) and 3-4″ away. Roll off the excess bass with your mixer’s EQ.


Or, use a mini mic on the guitar’s surface — it won’t pick up much of the vocal. Or, use a pickup on the guitar instead of a mic.


Another suggestion: Place two bidirectional mics side-by-side so their grilles touch. This gets rid of any delay between their signals. Aim the “dead” side of the vocal mic at the guitar; aim the dead side of the guitar mic at the mouth.Use just one mic, or a stereo mic, midway between the mouth and guitar about 1 foot out front. Adjust the balance between voice and guitar by changing the mic’s height.


– Bruce Bartlett (, author of “Practical Recording Techniques 6th edition” and “Recording Music On Location.”


Originally posted 2010-08-19 11:54:52.