#3: Miking an amplifier

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Last weekend, I was helping a friend record a few songs at the lab/studio, and we decided to revisit a microphone experiment we had tried last year. This time around we compared the frequency responses of two microphones and two miking positions. Here's what we found.

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EXPERIMENT
We recorded an amplified Rhodes piano using two Shure SM57 and two Rode NT5 microphones. The first SM57/NT5 pair (placed side-by-side) was aimed directly at the centre of the amplifier's speaker cone, a few centimetres from the amp face. The second pair was placed roughly 15 cm below the speaker centre, aiming upward at about 45 degrees.

The performance was recorded with Reaper (Cockos) using all four microphones simultaneously. Each track was then fed into MATLAB to calculate the power spectrum (i.e., the power levels across different frequencies), which was then normalized (for a more straight-forward comparison) and plotted in Excel.

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RESULTS
Here are the results! Each of the four spectra is plotted twice; this way, we'll be able to see the different mic and position comparisons more easily. Now, one thing that's immediately clear is that the recorded song was in the key of A! How do we know that? Well, there are nice big peaks around 110, 220, 440, and, to a lesser extent, 880 Hz. Each of these frequencies corresponds to a different octave of the note A. (The peaks also give us four clear points of comparison.)

Microphones. First, let's look at the microphone results. Regardless of position, we see a couple trends: the upper bass range (roughly 160–250 Hz) is stronger for the SM57, whereas the NT5 is more sensitive to the low-mids (roughly everything above 250 Hz on these plots). When the mic is positioned directly at the centre, the SM57 continues its sensitivity from the upper bass range all the way down, but when angled the NT5 is more sensitive to lower frequencies.

Positions. From the plots below, we can see that for both microphones angled placement emphases the upper bass range (about 160–250 Hz) and centre placement emphases the low-mids (250 Hz and up). On the low end of things, around the 110 Hz peak, the two microphones respond differently to positioning: the sensitivity of the SM57 to low frequencies drops dramatically with angled placement. The low-range response of the NT5 is much less sensitive to changes in angle.

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SUMMARY
So, what's the verdict?

• The SM57 has stronger low-frequency response (we can say it's warmer), and the NT5 is more sensitive to higher frequencies—it gives a brighter sound.
• Angled microphone placement contributes to a warmer sound, whereas centre (i.e., "straight-on") placement produces a brighter sound.
• For the NT5, the low-end response is similar for both placements, but the SM57 experiences a significant drop in low-end response for angled placement.