Making Tube & Contact Microphones

Overview

    A tube, or hose, microphone is a common electret microphone which is connected to a length of tubing.  The open end of the tubing can then be treated just like it was a regular microphone.  It can be routed or hid in small, confined spaces or even placed at the focal point of a parabolic reflector.  It can also be mounted near the corner on a glass window.  This will cause the windowpane to act like a boundary reflector and will slightly increase the gain of the microphone.

Tube microphones are ideal for through-the-wall monitoring applications inside hotels, apartments, $2600's corporate office, and even Mosques.  Refer to GBPPR 'Zine, Issue #18 for information on building a silent drill to help during those covert microphone installations.

Tube microphones can even be trimmed to resonant at a certain audio frequency, if so desired.  Just divide the speed-of-sound at sea level (1,100 feet per second), by the target frequency, in Hertz.  This will be the audio wavelength, in feet.  Trim the tube to this length.

Example

Resonant Wavelength for a 1,000 Hz Tone:

  1100 ft/s 
  --------- = 1.1 feet or 13.2 inches
  1000 Hz

    A contact microphone is a microphone which only responds to direct sonic or physical pressure.  An ideal contact microphone should not respond to any nearby "sounds."  This allows for the use of enormous voltage gains (90+ dB) during the pre-amplification stage without worrying about any feedback oscillations.  You'll often see contact microphones used on the National Geographic Channel when they record the "sounds" of ants and other insects walking or crawling along.  Of course, that is gay.  We'll be using them to monitor our neighborly terrorists and break into safes.

For those really quick-and-dirty operations, you can tape a standard speaker or phonograph cartridge to the wall or object you are trying to "hear" through.  Then run the output into a standard microphone pre-amplifier.  If you use a speaker, be sure to add a 8-to-600 ohm matching transformer.

The contact microphone microphone covered in this article isn't perfect, but it does work.  It'll give the enterprising spy a good starting point.  Oh...  If you don't want to make a contact microphone, you can buy them commercially as "guitar pick-ups."  But where is the fun in that?










Pictures

Overview of the 1/4-inch O.D. tubing used for testing.  This is the tubing which will carry audio from the target area to the electret microphone element.  On the upper left, is clear vinyl tubing.  Vinyl tubing is very flexible and easy to work with, but can be hard to route through "tough" areas because of its tendency to bunch up.  Below that is white polyethylene tubing.  This is just like the vinyl tubing, only much more rigid.  A drawback to polyethylene tubing is its tendency to stay in a "rolled-up" form when trying to route it.  To the left of that is regular copper tubing.  Copper tubing is probably the best for short runs or through-the-wall applications.  It is surprisingly flexible and easy to work with.  Copper tubing is also the only thing that will work in areas saturated with large amounts of electromagnetic interference.  Two drawbacks to copper tubing is needing a tubing cutter to properly trim it and the fact it will show up using a metal detector during a TSCM sweep.

Example of a tube microphone using a 1/4-inch electret microphone element from an old cellular phone (circled in red), a brass "Male Elbow - 1/4 IN. O.D. Tube X 1/4 IN. Male Iron Pipe" compression adapter, and a BNC jack.

Overview of how the compression fitting works.  It works the same if the tubing is copper, vinyl, or polyethylene.  The little brass compression rings can only be used once, so purchase alot of them.  The inside of the threaded pipe end of the adapter will be tapped using a 3/8-inch, 32 TPI tap.  This tap can be hard to find, but McMaster-Carr (www.mcmaster.com) carries them.

Completed tube microphone.  The electret microphone element is sandwiched between the polyethylene tube and the inside rim of the compression fitting.  The other end was tapped and the BNC jack screwed in.  The body of the electret microphone element is compressed against the inside of the compression fitting so it all shares a common ground and only one wire is needed to connect the center pin of the BNC jack to the microphone's + (positive) terminal.

Inside rear view of the compression fitting showing the electret microphone element.

Example of inserting the electret microphone into the compression fitting.

Tapping the threads on the inside of the brass compression fitting.  Brass is soft and easy to tap.  Once started, turn the tap handle about 1/4 of a turn, then back it out a bit to "break" the chips forming inside the hole.  Be sure to use lots of lube.

If you don't have a 3/8"-32 tap, or don't want to use a BNC jack, you can use a brass pipe cap instead.  Drill out the center of the pipe cap with a 5/32" drill bit and fit a common 3/32" (mini) mono jack.  Use a rubber O-ring to prevent anything from shorting out when you screw the cap to the compression fitting.  The threads on the pipe cap and compression fitting may not be the same, but for this application, it still works out O.K.

Connecting the center pin of the BNC to the electret microphone element.

Completed tube microphone adapter which is completely shielded and can be quickly connected or disconnected using normal coaxial cables.

Examples of completed tube microphones.  The top example has a short length of polyethylene tubing and a "straight" microphone holder.  The middle example has a short length of copper tubing and a "right-angle" microphone adapter.  The one below that has a 1/4-inch to 1/4-inch brass compression adapter to allow for the quick connection or disconnection of different tubes.  Use a short piece of copper tube between the two fittings to keep the electret microphone element in place.  This helps to protect the microphone element from any damage by pressing the audio tube in too far, or from overtightening the compression nut.

To connect two pieces of 1/4-inch vinyl or polyethylene tubing, use a short piece of 9/32-inch O.D. brass tubing.

For the contact microphone, you'll need to do things a little differently.  Now this is a total hack, but it appears to work quite well.  First, you'll need a Kobitone 1-inch diameter ceramic microphone (Mouser Part # 25LM037).  Two are shown in the lower left of the picture.  One of the microphones is opened, with the foil removed, exposing the ceramic element.

Next, you'll need some steel-reinforced epoxy putty (cold weld).  J-B Weld is the one and only.  This will be used to fill the vibration pick-up "cap" and for the connection to the ceramic element.

Next, you'll need a 3/4-inch copper pipe end cap (which should actually be 1-inch in diameter), some 1/4-inch copper tubing, and a good tubing cutter.

Use the tubing cutter to cut the copper pipe end cap so it is only 3/8-inch high (closed end).  Then cut a piece of copper tubing 1/2-inch long.  File or debur the edges of the pipe and the tubing so everything is smooth and level.  Mix the cold weld epoxy and fill the copper pipe end cap.  Smooth and level the epoxy so it all looks pretty.  Before it cures, place the short 1/4-inch tubing stand-off in the center of the end cap, and press the epoxy all around the sides.  Smooth and level the epoxy again.  See the photo for a better idea of what you're trying to accomplish.

Carefully take apart the ceramic microphone.  You need to pry the cover's edges with needle-nose pliers to remove it, then use a X-acto knife to cut away the aluminum foil from the ceramic element itself.  The aluminum foil is glued to the ceramic element in the center, so be careful not to pull on it, or the element will break.

What you are basically trying to do here is replace the microphone's foil "air pick-up" with a "vibration pick-up" sensor.

Place a bit of cold weld inside the 1/4-inch tubing stub and gently press the ceramic microphone body onto the stub, centering on the ceramic element itself.  Use little bits of art foam around the edges to keep the microphone's body from twisting and breaking the ceramic element.  Finish up by connecting a length of shielded wire to route the signals from the microphone.

Be very careful with this setup as it is extremely fragile.

Completed contact microphone.

To listen through walls, connect this to a standard high-impedance (10k ohms) microphone pre-amplifier.

To help in cracking safe combinations, connect it to a very-high gain, very-low noise op-amp and run the output to an oscilloscope or strip chart recorder.