Building a Kick-Drum Mic out of a speaker…
By Gabe Herman
There is much written about this topic all over the web, and after studying a lot of pages, with a lot of great information, I thought I would give this project a shot and try to combine some of the things I’ve learned from my internet research with some of my own ideas and try to document my process as much as possible for others who might be interested in doing something similar.
For those who may not have come across this idea before, the concept is fairly straightforward. Mechanically speaking, there is not a whole lot of difference between a moving coil dynamic loudspeaker and a moving coil dynamic microphone. Both rely on the theory of electromagnetic transduction to convert voltage into sound, or sound into voltage. The theory states: “When a piece of conductive metal crosses an electromagnetic flux, then a corresponding voltage of a specific polarity and amplitude will be created within that metal”.
Moving-coil loudspeakers are a great example of how this theory is used in audio applications. Typically, they are used to transform an electronic representation of sound (Audio) into something which can be heard (Sound). To simplify its basic working mechanism; A audio signal is sent to a coil of wire that is suspended around a permanent magnet. As the audio signal (voltage) changes from positive to negative (alternating current), the coil reacts by moving back and forth towards the various poles of the magnet that propagate the attracting force (negative to positive and visa-versa). The other end of this coil is attached to a diaphragm that is sometimes referred to as a cone or driver. As the coil moves from one pole to the next, it causes the air in front of and around the cone to either increase in pressure as it moves forward, or create a vacuum of pressure when the cone retreats backward. These oscillations in air pressure creates what we perceive as audible sound.
A dynamic microphone is essentially the same working-mechanism. The only difference is that a microphone is supposed to capture acoustic energy and transduce it into electrical audio signal. In this case the construction is more or less the same except that the diaphragm is much smaller than the cone of a typical loudspeaker. Disturbances in air pressure cause the diaphragm to move back and forth. Since the diaphragm is connected to the coil of wire, this causes the coil to move across the magnet thus creating alternating current in the coil.
Since there is no basic difference between a speaker an a microphone, then it stands to reason that one could take the place of another. (If you ever wish to be daring, you can experiment with this idea by hooking up the output of a pair of headphones into the input of a microphone pre-amp and speaking into one of the ears…D.J.s have been doing this forever.)
While it’s possible to achieve transduction using a speaker as a mic, the fact is that on the whole, speakers really don’t sound that good in reverse. In fact, they sound down-right horrible as a vocal mic. But in certain circumstances, they do offer something that most vocal mics can’t. This is due to the fact that large cones are inherently better at responding to lower frequencies than smaller diaphragms such as the ones present in most vocal mics. This has a lot to do with the correlation of the wavelength of lower frequencies as is corresponds to the circumference of the cone. When recording instruments where low frequencies are critical such as a kick-drum or bass amp, speaker drivers can actually provide tons of glorious sub-frequencies that most dynamic microphones as well as large diaphragm condenser mics just can’t re-produce. Used on their own, sub-kick mics aren’t that clear a representation, but when added in moderation to standard kick-drum mics closer to the beater head of the kick-drum pedal, they add beefy, percussive and skull-shaking low end that can be heard on a lot of popular drum recordings.
Engineers have been building these things for decades, so it’s not a new idea. In fact, Yamaha has a sub-kick mic that is now being manufactured and sold for around $400.00. While I admit, it’s always easier to buy one, building one yourself is easy and shouldn’t cost you a whole lot. I built the one featured in this article for about $25, and it sounds every bit as awesome as the $400.00 model. (The difference, I think, must be in the materials for the shell and the fancy logo on the front.) Hey, desperate economic times call for creative solutions!
In this how-to article, I’ll be showing how I built my own, but I encourage anyone interested in building one for themselves to take some time to research and experiment and come up with their own design. As I have mentioned before, some of my ideas are not unique, so I’ll do my best to include links to resources where possible.
Also, I will list some parts as being available at radio shack. Yes. Radio Shack, “You’ve got questions, we’ve got blank, vacant expressions” I list this place as a resource because it is accessible to almost every american. I would strongly suggest that if you plan on going to radio shack to buy some or any of these parts, know what you are looking for before you show up. Otherwise, I can almost guarantee you that they will not know what you are looking for and are not likely to be able to help you find them. If you want another good resource, www.digikey.com is a great online dealer for just about any and everything electronics based under the sun.
So without further ado, here’s how to make your very own sub-sonic functioning work of art:
(1) 10″ subwoofer driver (8 ohm)
(1) 10 1/4″ Cardboard Sonitube concrete pillar forming tube
(2) XLR recessed male wall mount
(1) DPDT Submini Toggle Switch
(1) Mini Breadboard (radio shack)
(2) 1/4 Watt 470 ohm resistors (1-2%)
(1) 1/4 Watt 100 ohm resistor (1-2%)
24 gauge wire
Power drill and bits (3/4″ bit for XLR mounts, the rest standard sizes)
Tight-Bond Wood Glue or Gorilla Glue. (Contact cement might work well too)
Dark Lead Pencil and a Sharpie marker
Spray paint (as many colors as you like)
(1) Roll of white, adhesive contact paper
(1) Roll of Blue painting tape
(1) Exacto-Knife (Type A with #11 Blades)
About the materials:
Speaker: I decided to use a 10″ sub-woofer for my speaker element, mainly, because it’s what I had lying around at the time. From my research, I found that there are some things to consider when choosing a speaker. First off, there’s no need to buy anything expensive. The main things to consider are the size of the cone and the stiffness of the material that the cone is made out of. I’ve read that it’s not advisable to use anything smaller than a 6″ cone, as 4″ is really too small to capture the low frequency waves we’re looking for, and because as you get smaller, the speakers get lighter and will distort quicker. Anything larger than 10″ is really getting to be large enough to be in the way of other mics and will probably get a little heavy.
Another thing I’ve read is that stiffer cones don’t have as much reflex as softer rubber-gasketed cones and thus are not as responsive. I’ve not tested either of these theories yet, but I can certainly see the argument for them and I’d love to hear from anyone who has already gathered test results on this.
Electrical Components: Don’t go overboard on these either. It’s nice to use good resistors when you can, but I’ve found that it really wont make a noticeable difference if you go with the cheapo radio shack types.
Shell Enclosure: Some people prefer to make their speaker housing out of a wooden drum shell. This is actually what I wish I could’ve done, but I didn’t have one available and rather than spend $40 on one, I thought I would try my best to make do with what cheap materials I could find. The cardboard concrete form tube turned out to be perfect and only cost me $7.00 at the Home Depot.
How it comes together…