#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.

#1.3: The Life of Strings

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In the last post, we looked into the differences between soft and hard picking (besides the volume!): harder sounds were slightly less bright and softer sounds had greater sustain (more steady volume). Let's now return to our first experiment: we're onto the third week for a new set of strings, and we've got some new sustain and brightness results to show for it.

Let's compare the new information to the past two weeks' results, keeping in mind that the first week's samples were played using a pick, and a finger has been used to pluck the strings since then.

We don't yet see any general trends for sustain, but we unexpectedly see an increase in brightness for most strings. The total playing time is still only at 18 hours, so let's accumulate a few more recordings and then take another look...

#2: Attack of the Guitar Pick!

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Let's say you record the same guitar part two times. The first time you play the part very softly, gently plucking the strings with every note and chord. The second time you play as hard as you can, hitting all the notes accurately, but with all of your might. Our intuition says that both recordings would sound different, even if the volumes were matched. Louder sounds louder even if isn't... OK, but how? That's what we're going to look at today: how does "playing harder" affect guitar tone?

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THE PLAN
We'll pluck the same string over and over again; sometimes we'll pick harder, other times softer. Then we'll look for differences in the sound. First, we'll look at our classic sound characteristics—brightness and sustain—for the full duration of each pluck (see first blog). Then, we'll separate each pluck into two parts: the attack and the decay. For each part, we'll determine the brightness and sustain separately.

As usual, the guitar is a Fender Telecaster (American Standard) with D'Addario regular light strings (0.010–0.046). The 3rd string (G) was used for this experiment.

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RESULTS!
> The whole signal. Brightness goes down (a little) with harder picking, and sustain drops significantly. There may be a relationship between these two trends: higher harmonics ring longer than lower harmonics ("On the sensations of tone," Part I, Chapter IV, H. Helmholtz), and if louder sounds aren't as bright, as we see here, then it would make sense that they don't ring for as long. (It may be a good time to note that these sustain values are based on a drop of -20 dB from the signal maximum. If we were just looking for the point where the signal got silent, then louder sounds would certainly ring for longer, as our intuition would suggest.)

> Attack! The brightness trend here is similar to that for the entire signal: a slight decrease in brightness for louder sounds. Because the attack region is so short (one second), an alternate method for expressing sustain was used: the volume drop across the region. There may be greater sustain (i.e., less volume drop) over the attack region for louder sounds—opposite to the effect for the entire signal—but the data in the plot below are too messy to say for sure.

> Decay... The results here are similar to those for the whole signal: louder sounds are less bright and drop faster in volume.

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CONCLUSIONS
Playing harder or softer leads to differences in sound beyond just a change in volume. A softer sound has greater sustain and is a little brighter, whereas a harder sound has a stronger fundamental frequency and decreases in volume more quickly.