Some time ago I decided I wanted to find out how the location of a microphone near a banjo affects the tone quality you hear.
To make these tests scientific as well as subjective, I measured the spectrum of the banjo in several different mic locations. The spectrum of a musical instrument is its output vs. frequency.
It affects the tonal balance or timbre. It is the relative levels of the fundamental frequencies and harmonics, and can be measured with a real-time analyzer.
As an experiment, I measured the spectrum of a Chuck Lee open-back banjo in several different mic locations to find out what the differences were. I also compared the sound by recording the mic signals and listening to the playbacks at equal listening volumes.
For example, this screen capture shows the frequency spectrum of a banjo picked up at 1 foot (green) and at 3 inches (yellow).
The 3-inch placement sounds bassy (more low frequencies than the reference) and dark (less high frequencies than the reference) I placed a flat-response omni measurement mic 1 foot from the center of the banjo head as a reference. Compared to the reference, the spectra of other mic positions exhibited the following differences:
Six inches from the center
Warmer or thumpier than the reference. +8 dB from 100-180 Hz.
Comments: The closer the mic is to the banjo head, the more warmth or bass you get. That’s not due to microphone proximity effect in this case, because I used only an omnidirectional mic, which does not have proximity effect.
Instead, the banjo’s bass is weak at a distance because the low frequencies from the front of the head are partially out of phase with the low frequencies from the back of the head, so they partly cancel out at a distance.
The same thing also happens with an open-back guitar cabinet.
Three inches from the center
Bassy and dark. The hand gets between the strings and the mic, so the highs are diminished about -4 dB from 2kHz to 10 kHz.
Lows are boosted +14 dB from 100-200 Hz compared to the reference mic position.
Six inches away, halfway between the bridge and lower rim:
Good match with the reference, but a little warmer due to a +6 to +9 dB rise from 100-180 Hz. A nice, natural sounding mic location (in my opinion).
Six inches from the lower rim
Thin, due to an 8 dB loss from 160-360 Hz compared to the reference mic location.
Six inches from where the neck meets the pot
Similar to the reference, but a little warmer due to a +7 dB boost from 100-200 Hz. This is another natural-sounding location, but is not as loud as halfway between the bridge and lower rim.
Three inches away, halfway between the bridge and lower rim
With this measurement we saw +10 dB at 400 Hz, -5 dB above 3 kHz, +10 to +12 dB from 100 to 180 Hz. In musical terms, bassy and dark.
Three inches from the lower rim
Midrangey—thin and dark. -4 dB from 180 to 250 Hz, -4 dB above 4 kHz.
Three inches from where the neck meets the pot
Similar to the reference but +6 dB from 100-140 Hz.
Obviously, the purpose of these measurements is not to tell you how to mike your banjo. That depends on your instrument and your personal taste in tonal balance. Each instrument is different, too.
However, hopefully these tests may shed some light on how mic placement affects the reproduced timbre of an open-back banjo.
So, here’s a quick summary of what I found.
• Mic 12” from the center of the head: A little thin or weak in the bass. (But a cardioid mic with proximity effect might not sound thin here).
• A “natural” sound is available halfway between the bridge and lower rim, or where the neck meets the pot.
• Miking near the rim sounds thin.
• Close miking sounds bassier than distant miking but provides more gain before feedback and more isolation from floor monitors and other instruments.
In the end there are many more mic placements to try, but hopefully this give you a starting point if you’re looking for a new direction.
AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are “Practical Recording Techniques 5th Ed.” and “Recording Music On Location.”
More articles by Bruce Bartlett on PSW:
Recording Microphone Techniques To Produce Warm, Spacious Stereo
Remastering Jazz Classics: The Dave Brubeck Quartet, Art Pepper, and Sonny Rollins
Deconstructing Hip-Hop To Hear How The Mix Comes Together