I have been through your book "Recording Music On Location" several times. I have a question in preparation for purchasing some microphones.
I am recording a brass band, and the setup in the group is not so much wide, but more deep. By using only two mics out front, I am fearful that the back row of players, which are situated in the middle, may be weak in comparison to the front.
Especially since no microphone is pointing directly in that direction, unlike instruments on the side.
To overcome this, I was thinking maybe a third mic (panned center) placed more over the group on center, or even over the back-row players to help reinforce their sound. Do you think this is wise? I have done some reading on the Web and have come across something called the 'comb filter effect' with using three mics. Is this an issue? Or should I just use three spaced omni mics up front?
Reply: Will you get a chance to experiment with different techniques? Here are two suggestions:
1. How does the balance of the brass band sound in the audience? The composer may have written the dynamics of the compositions so that the back-row instruments are well balanced with the front-row instruments, as heard by the audience or the conductor. If the audience and conductor hear a good balance between the front and back rows, the mics should too -- as long as they are not too close to the front row.
So you might try a spaced pair of cardioid mics (CM-700) out front (maybe with a center mic), raised up high, aiming straight ahead when viewed from above, and angled down at the back row when viewed from the side. Using cardioids rather than omnis lets you place the mics farther from the group for the same direct-to-reverb ratio. If you use omnis, they will need to be placed fairly close to the front row, which might upset the balance.
You could also try a pair of CM-700 cardioid mics in an ORTF arrangement (angled 110 degrees apart, spaced 7 inches horizontally) or NOS arrangement (angled 90 degrees apart, spaced 12 inches apart).
Raising the mics up high on booms places them farther from the front row, but not farther from the back row. This improves the front-to-back balance.
2. Using a close-up spot mic to reinforce a quiet instrument section is common practice. I'd place it fairly close to the back row of players, maybe 6 feet. Bring it up in the mix just enough to improve the balance.
With this technique, the sound source (the back-row players) are picked up by three mics at different distances: the close-up spot mic and the distant overall mics. This might cause comb filtering, which is a series of notches in the frequency response caused by various frequencies being in or out of phase between mics that are at different distances from a sound source. It occurs when the signals from the close mic and distant mic are similar in level (within about 9 dB) and similar in spectrum. Spectrum is the level vs. frequency of a sound source -- the tonal balance, or the ratio of fundamentals to harmonics.
Comb filtering is more of a problem in theory than in practice. The close-up spot mic will pick up a different spectrum than the distant mic, so comb filtering might not be audible.
Also, you can delay the spot mic so its signal coincides with that of the distant mics. This prevents comb filtering because it puts the spot-mic and distant-mic signals in phase. The formula for the needed delay is T = D/C, where T = delay in seconds, D = distance between spot mic and distant mic in feet, and C = speed of sound, 1130 feet per second. Some engineers recommend adding about 10-20 msec of additional delay to the spot mics so that they don't "stick out" so much in the mix.
This delay can be done by a digital delay unit, or by a computer audio editing program. Slide the spot-mic track to the right (later in time) by the correct number of milliseconds.