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Full Version: Naughty Experiments with Tape and Microphones
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Here is a truly insane experiment.

Have you ever wondered why small diaphragm mics have a ring of slits or holes about a centimeter or so back from the capsule?

The purpose of these holes is to allow a certain ratio of sound waves to enter the rear of the capsule but delayed and/or filtered. When a sound comes from directly behind the mic, the time it takes to reach the diaphragm from the front and back is equal and the sound is cancelled out. That is how single-diaphragm mics like these achieve directional patterns, a technique invented by Shure nearly a century ago, which they called "Unidyne", probably because it allowed a single diaphragm capsule to have directional behavior. A more technical term would be 'pressure-gradient'.

Before the Unidyne and of course the Dual-Diaphragm method pioneered by the Germans (which is used on almost all large diaphragm condensers, where a diaphragm-backplate-diaphragm sandwich picks up sound to the front and back, which is then combined electronically to achieve the desired polar pattern(s)), there were only two patterns available: Omnidirectional, or Pressure microphones, and Bidirectional/Figure-8 or Gradient microphones. 

While both 'pure' patterns are of course extremely useful, even today, there's a big problem in situations like sound reinforcement or critical recording where the noise (such as another instrument or a stage monitor) you're trying to null out may be directly opposite the performer/instrument you're trying to capture, and other situations where you want to pick up, say, a semi-circle of performers with a single mic, but the omni is too distant sounding while the bidirectional badly favors those standing in the middle of the semi-circle. The most rudimentary attempts to create intermediate patterns between the two involved essentially putting two capsules, one omnidirectional and the other bidirectional, in very close proximity and perfect alignment, then electrically combining their output. The problem with this is that the highest audible frequencies are only a few centimeters long, so comb filtering and other issues might occur unless the capsules are incredibly close. So, the Unidyne method was a really quite remarkable breakthrough.

If you look at an omnidirectional capsule for a small diaphragm condenser, it has NO visible holes (ok, maybe just a tiny vent somewhere to equalize pressure so it doesn't explode when a big pressure change occurs!). This is because it is designed to be, at least in theory, a pressure microphone: it doesn't matter what direction the sound is coming from, the wiggly membrane of the diaphragm in theory will respond equally (until you get into fun stuff like the body of the mic masking certain sounds, filter and mesh materials, resonator plates and cavities, but I digress).

Well... that's where things get naughty and definitely not within warranties...  Angel

What if you took a small diaphragm condenser and covered up the vent holes? In theory it would make an omnidirectional mic, right? But what happens to the frequency response?

In theory I assumed the mic would get incredibly bright because it's commonly known that pressure microphones (omnis) are more or less insusceptible to proximity effect, and at this distance (like 6 cm from my mouth), a lot of that low end is due to the proximity effect.

I tried this experiment with an old NT5 I don't use much and some masking tape:

(masking tape or painter's tape is a no-residue, relatively weak paper tape used for making sharp, clean lines when painting; it's commonly available in the US in hardware stores, but I assume it can be found under a similar name in Europe and elsewhere. I chose it purely because it is easy to remove and in theory should be quite absorbent of sound)

A few takeaways:
  1. The effect isn't that noticeable even if there is the smallest of holes left open. It's still audible for sure, just not as big as when the final gap is covered.
  2. The cavity seems to become resonant as the tape covers more and more of the vents, you can hear a few particular bands of resonance.
  3. The final result is... a super bassy sound? This went against my prediction that it should get brighter. My best guess is this has to do with dampening of the diaphragm being calibrated to compensate for the sound of the capsule with both sides active, OR that the tape is actually acting as a filter as it just can't absorb low frequency sound that well?
  4. The mic gets CONSIDERABLY hotter as the holes are covered. Again, totally unsuspected.
I then tried the same experiment with a ribbon mic. Almost all ribbon mics are quite consistent gradient (bidirectional) mics, in many regards, the most consistent bidirectional patterns possible technologically. So, if we block off the back, again, it should in theory become a pressure mic instead?

I had predicted that in this case the mic would become incredibly bright... and in this case I was right! Never in a million years would I suspect you could get that much highs out of a ribbon mic!

Again, it seems some noticeable resonances have appeared. I wonder if these are due to the cavity left between the tape and rear of the diaphragm/ribbon acting as a resonator.

To me the most useful situation for both of these is to make some old-timey sounding mic effect. They both seem to have something strong around 3kHz which helps that 'old radio' sound a lot. You could alternately use it for special recording purposes or effects, especially since off-axis it should have some TRULY INSANE coloration I can't even imagine.

So, if you ever have a SDC which is impossibly bright or a ribbon which is impossibly dark... a little masking tape might be all you need, supposing you don't mind EQ'ing out the horrific resonances?  Rolleyes