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Part 4: A Scientific Approach / Other Methods
Scientific Starting Point
Figure 7 shows a scientific approach to mic placement on a Beatles AC30 VOX amp. The tech in the picture is using test instruments rather than his ears to find the "sweet spot." Sound waves coming out of a guitar amp emanate in all directions, setting up patterns much like a stone produces when thrown into a very still pond. Like in the water, sound waves interact, producing multiple reflections and standing waves where sonic energy coalesces. The idea is to place the mic where sound waves add together.
Fig. 7: Scientific Approach
To find this position, you must measure the output level of a microphone while you move it around in front of the amp. First, set up for a guitar overdub with your microphone and fix the headphones so the musician can hear himself. Next, unplug all headphones and send a steady 700Hz oscillator tone into the guitar amp's input jack. Place a large VU meter so that you can see it from wherever the guitar speaker cabinet or combo amp is located. (If you can't get a sight line to the VU, plug a volt meter into the cue system and read the level there). Set the meter to read mid-scale with the cue system level control. And wear ear protectors! Let's make this science project as painless as possible.
Move the microphone around in front of the amp. In doing this, you'll find many peaks and dips in level. I try to stand behind the speaker so that my body doesn't affect this measurement process. Obviously, the level goes up as the mic gets closer to the speaker, but if you keep a fixed distance and then move the mic left, right, up or down, you'll find a peak level. This is where you should set the mic. Make sure you like the sound you get using the scientific starting point -- you may hate it!
Recording the Electric Acoustically
Fig. 8: Recording the Electric Acoustically
But wait, there's more
Images courtesy of the Oliver Leiber Collection.
Barry Rudolph is an L.A.-based recording engineer. Visit his website at www.barryrudolph.com.