“Sub-Kick” Microphones

What is a sub-kick microphone?

It’s actually a speaker, operating in reverse (an input versus output device). Sounds weird, huh? But, it has an extremely useful purpose when used in tandem with the usual internal kick drum microphone.

The idea is to capture frequencies below 60Hz, which in turn will produce a much “punchier” sounding kick drum when blended with the internal microphone during the mix.

If you’re not a geek like me and have no interest in sub-kick build or operation, feel free to skip to the end of this article for a video which demonstrates the kick drum sound enhancement a sub-kick microphone provides.

During DAW mixing, applying various plugins for things like EQ and compression to the kick drum track is common practice, in order to obtain a nice blend of punch, low-end and “click” from the pedal beater striking the batter head. But, if recorded correctly, a sub-kick microphone track can usually be added to the mix “raw”, requiring no additional processing.

Hopefully I’ve held your attention so far, so let’s get into a high-level explanation of how this basically works:

  • A speaker is wired to an XLR cable, then placed in front of the resonant (front) bass drum head, pointed at a non-ported head surface.
  • Air pressure from the resonant drum head actuates the speaker cone with each kick, producing a very low frequency output signal.

Using a speaker as a microphone offers a very large diaphragm (the diameter of the speaker), which is perfect for capturing low frequencies. “Normal” dynamic microphones simply can’t respond to those frequencies, because of their small diaphragms.

There are various off-the-shelf sub-kick microphones available, which can easily run over $300, not including a stand. The good news is, you can easily build one yourself. In fact, if you have all required components, you can accomplish this for free! Even if you don’t have those parts on-hand, you’ll still spend far less than $300 buying them from cheap sources, such as eBay.

As a side note, a sub-kick microphone can also be used to mic a bass amp, with thunderous output as a result.


DISCLAIMER:
– If you decide to use this build procedure, you do so at your own risk.
– Be sure to read this entire process before beginning your project.
– Some research on the Internet will no doubt provide you with additional help.
– I will not be held responsible for you damaging your own equipment!


The first things you’ll need are an XLR cable you’re willing to sacrifice, and of course a suitable speaker.

My recommendation is 8-10″ in diameter, with an impedance of 6-8 ohms. If the speaker diameter is too large, it will result in a less responsive microphone, since it will require more air pressure to actuate the cone. Too small, and you’ll wind up with excessive attack, adding frequencies that are undesirable in sub-kick sound. Leave those frequencies to your internal kick microphone to handle.

Remember, the entire purpose of a sub-kick microphone is to capture very low frequencies, adding much more punch when miking your kick drum.

CAUTION:
Due to speakers having a very low impedance (usually 6-8 ohms), wiring them directly to an XLR preamp input will result in a VERY HOT signal which can easily fry the amplifier! In my case, the signal was so high that it was totally unusable, even with the mixer input trim at zero and level fader all the way down to -50dB.

To get into just a bit more technical example of this:

Say I’m using a 6 ohm “microphone” (the speaker itself) and connecting it directly to a preamp. Because 6 ohms is such a tiny amount of resistance compared to a typical low impedance microphone (most of mine run from 150 to 600 ohms), you could possibly send the preamp to an early grave, especially those with no protection circuitry. It’s almost like just shorting the positive and negative XLR pins together.

No pad - Hot signal - Peak LED on
Speaker connected directly to mixer. Even tapping the speaker cone extremely gently causes the “Peak” LED to illuminate with every tap, indicating pre-amp overload. Don’t try this at home kids!

To account for this overload, an attenuator (AKA “pad”) circuit is absolutely required, typically for a reduction in signal of at least 20dB.

Don’t let attenuators scare you away, as they can actually be the easiest part of the whole project. How?

What I did is buy an in-line -20db attenuator from Amazon for $9.49, including free shipping.

Pad - In-line 20dB attenuator
An inexpensive in-line 20dB attenuator. Pre-amp overload problem solved.

This allowed me to bring up both the input trim and level fader for plenty of signal, yet absolutely no pre-amp overload.

There are also attenuators available that have adjustable signal level reduction, but they are more expensive and not usually required.

Alternately, if you have a decent knowledge of electronics, you can build one yourself using a few resistors. Here’s a web site that explains pad circuits in general, which is full of great information if you decide to go that route.

My school of thought:
Even though resistors are so cheap, running around to find them along with the actual construction of the circuit itself is just not worth my time and effort, considering you can buy in-line attenuators delivered to your door at such low prices, such as the one above. Things come out looking a lot cleaner too!


MY BUILD

Now, you won’t be able to follow this build procedure exactly, unless you happen to have the same parts I used on-hand (unlikely). But, it will still give you the general idea, and you can then make your own choice of materials from there.


My Scavenged Parts

I’ve owned a 500 watt Klipsch ProMedia 5.1 speaker system for a long while now, that used to be connected to my old Alienware gaming PC. It was a great sounding system, but their BASH amplifiers are notorious for various failures of all sorts of components. Two months after I bought the system it went totally dead, producing no sound at all. Klipsch repaired it under warranty and sent it back to me, after which it operated just fine for about 3 years.

Then, all six speakers started cracking whenever the control pod knobs were adjusted. They’re optical, so it wasn’t a matter of just spraying some DeoxIT into them for the usual purpose of removing dust and/or oxidation. Shortly after that, the control pod quit completely, and the system has been sitting boxed in a closet for some 5 years now.

When I got the idea of building a sub-kick microphone, I realized that I already owned the basic materials, which were inside the Klipsch system sub-woofer. The sub-woofer enclosure actually contains not one but two side-firing 8″, 6 ohm speakers, although only one will be used as the sub-kick microphone. It also has an external port leading inside, right between the two speakers.

Klipsch sub-woofer - Stock
The Klipsch ProMedia 5.1 sub-woofer/amp cabinet, in “stock” condition.


I’m only including the next two photos for the sake of completeness, as they obviously won’t apply to you unless you’re cannibalizing the same sub-woofer. Here’s a look at the BASH amplifier assembly removed from the rear of the cabinet, and the internals which are mounted to this input/output plate (right click and open in new tab to enlarge). I found a bunch of resistors, capacitors and FETs on several of the circuit boards cooked to a crackly crunch… An ugly situation that I wasn’t going to waste time trying to salvage anyway.

Klipsch ProMedia 5.1 rear plate and BASH amplifier assembly.


Here’s a look inside the sub-woofer cabinet with the amplifier assembly removed, which left a lot of room in there. You can see the port tube I mentioned earlier between the two speakers. To my delight, I found both speaker cones were in perfect condition.

Klipsch sub-woofer - Gutted
“Gutted” sub-woofer cabinet interior.


You don’t want a grill of any sort covering the speaker and blocking air flow, so be sure to remove it if present.

This speaker has a steel grill, press-fit onto the outside of the metal support struts. To remove it and avoid damage to the speaker cone and/or foam, I carefully threaded a paper clip through two holes at the center of the grill (being careful not to poke holes in the cone), twisted the ends together, and pulled that directly away from the speaker using a pair of pliers. Pop, out it came, like nobody’s business.

Speaker grill removed, using the mighty paper clip tool!


Here’s my Sennheiser e902 internal kick microphone, which will be used in tandem with the sub-kick microphone. It does record wonderfully on its own, with a nice blend of punch, low-end and beater click. However, as far a punch goes, I always seem to want more, more, more! Thus, this sub-kick microphone build idea was born.

Sennheiser e902 Kick Drum Microphone
Sennheiser e902 internal mic position
Sennheiser e902 internal kick drum microphone.


Rather than removing the speaker and mounting it on a stand, I thought to try sitting the sub-woofer cabinet on its open end. In review, the speaker in use has had its grill removed and is facing the non-ported area of the front resonant bass drum head. This raised the speaker to just about where I wanted it for my 22″ DW bass drum, although I felt a bit higher might be more optimal. It turned out to be a non-issue, as it functions perfectly right where it’s at. It really couldn’t have worked out any better.

I didn’t bother removing the other speaker facing front in this photo, since it wouldn’t have any significant effect on sub-kick sound. When using a speaker in a normal fashion as an output device, of course cabinet build is very important, but not so when using it “in reverse” as a microphone. In fact, a speaker used as a sub-kick microphone doesn’t require any sort of enclosure at all. A nice plus in my case is the cabinet is safely storing the unused speaker which is facing front, in the event the one in use at rear decides to go south on me.

Sub-kick mic position
Sub-kick microphone cabinet positioning.


Prepping the XLR cable

Cut off the female XLR connector (the one with no pins) and strip off the outside black insulation. Then, do the same for the two internal wires. These are routed through the cable to the positive and negative pins of the remaining male XLR connector (pins 2 and 3, respectively). Trim off any cotton padding and the ground wire, as it’s not required. You should wind up with something like this:

Sub-kick mic XLR cable prep
XLR cable preparation complete.

Since the color of the two internal XLR cable wires can vary, trace the XLR pins using an ohm meter or other continuity tester to be sure you’re connecting them to the positive (XLR pin 2) and negative (XLR pin 3) speaker terminals. Before making permanent connections, you can perform this little test to ensure everything is wired correctly:

Temporarily twist the two XLR cable wires onto the speaker terminals, and plug the cable into a mixer (with the pad circuit in place!). Then, gently tap the speaker cone. If there is no sound, you’ve got them reversed… Swap the connections, re-test, and all should be well.


A 1/4″ hole was drilled into the cabinet for routing the cable inside.

I used butt splices for finalizing the connections, since my speaker already had wire leads soldered on. If yours does not, a bit of soldering of the XLR cable wires to the speaker will be in order.

I’d also highly recommend you add some sort of stress relief for the cable. In my case, I used a small plastic wall bracket intended for mounting wire shelves, along with a few wraps of electric tape around the outside of the cable in order to ensure it wouldn’t slip through.

Sub-kick mic wired
Completed internal wiring with stress relief, tidied up with Velcro tape.


Mounting

As previously described, luckily in my case I didn’t have to worry about this. Looking around on the web at other DIY sub-kick microphone projects, most people don’t bother placing sub-kick microphones in any sort of enclosure at all. As I mentioned earlier, it’s not required for what is now an input device (see this article’s featured photo for an example).

I’ve also seen them mounted in old tom or snare shells, but that’s just for appearance. You might be playing live and worried about its on-stage looks, so in that case do whatever trips your trigger to pretty it up. I don’t think mine looks too bad as-is when socked-up near the resonant head. Hell, maybe I’ll stick a nice bouquet of flowers into the port hole.

Besides, the objective is improved kick drum sound, not appearance.

So, here’s where you’ll need to get creative if you’ve just got a loose speaker. You can modify old microphone, snare or cymbal stands, for example. One build I found while surfing was particularly creative, if not the prettiest thing in the world. This fellow fabricated a small wooden stand, screwed a metal plate onto the vertical piece, and simply let the speaker’s magnet hold it in place. Ingenious.


Okay, we’re done!
Now, check out this demo video for the results.

Headphones recommended, in order to clearly hear the differences in sound.


Summary

There you have it… Total cost for my DIY sub-kick microphone: $9.49 (for the in-line attenuator that I was too lazy to build myself).

If you’ve read this entire article, you now know just how easy constructing a sub-kick microphone is. When it comes to enhancing your kick drum sound, believe me, it’s well worth the small amount of effort required.

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