Make a Contact Microphone

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Introduction: Make a Contact Microphone

A contact microphone is a type of microphone that picks up vibrations from contact with objects, as opposed the the more common type of microphone that picks up pressure waves in air. Here's a demo of a contact mic attached to an alarm clock, along with finger tapping on the table about a foot away from the clock:

In this Instructable, we show you how to construct a professional quality contact mic that is very rugged and sounds good. It has a 1/4" female jack so that you can connect it to an amplifier with a standard guitar cable.

This particular version of a contact microphone was designed for the premier of The Immortal Flux, a composition for drum kit and prepared drumhead orchestra written by Glenn Kotche, drummer for the band Wilco, at the Solid Sound Music Festival, sponsored by D'Addario.

To learn more about about the common ground between technology and percussion music, including tips on composing your own works from ensemble Third Coast Percussion, see this Instructable on the WAVES project.

Step 1: Parts and Tools

Parts

  • 27mm diameter piezo bender, also known as a "buzzer element", such as this part from Digi-Key
  • 8" length of 2-lead, shielded balanced microphone cable, such as Mogami W2582
  • 1/4" inline female mono jack (sometime called TS or "tip-shield"), such as Rean/Neutrik NYS2202P
  • cap from 20 oz pop bottle

Tools


  • soldering iron and solder
  • wire strippers
  • flush cut wire cutters
  • 1/4" and 5/16" heat shrink tubing
  • hot air gun or candle
  • hot glue gun
  • sandpaper

About Piezo Elements (Benders)

A piezo element is a metal disk with a circle of piezoelectric crystals in the middle. When the crystals are made to vibrate, they produce an oscillating voltage. Conversely, if you apply an oscillating voltage to a piezo element, it vibrates--that's why they also work as buzzer elements. A piezo element has 2 terminals: the inner circle which is the signal terminal and the metal disk which is ground. A good discussion of piezo elements and how they work can be found at Open Music Labs.

About Balanced Microphone Cables

Since a piezo has 2 terminals, we need a cable with 2 conductors to connect to it. You might think that simple speaker wire would do, but there's another consideration--cables can act as antennas that pick up stray electrical signals in the environment that can cause audible hum when plugged into an amp. To counteract this, you need to use shielded cables that have a sheath of wire surrounding the conductors. Shields are typically made of thin copper wire that is either wound in a spiral or braided around the conductors. (Foil shields are also used for cables that a part of permanent installations, but are not appropriate for this project.) Audio technicians typically agree that the highest quality (and most expensive) mic cables use braided shields, but spiral wound shields are perfectly fine for this project, and are somewhat easier to work with. In addition to the 2 conductors and the shield, mic cables may also have "filler" that helps the cable maintain its round shape. This filler may be plastic lines or bundles of thread, as pictured above. It doesn't matter which type you use for this project--if your cable has filler in it, you'll just need to snip it off before soldering your connections. You can buy bulk microphone cable from online suppliers such as Redco or Parts Express, or else you can just buy a mic cable from your local music store and just cut it. Do not use guitar cable for this project--it has a very different arrangement of a single conductor surrounded by a shield which is not appropriate for connecting to a contact mic in the manner we suggest.

Step 2: Strip Cable End for Jack

  1. Start with an 8" length of mic cable.
  2. Carefully strip around a half inch of the plastic or rubber cable housing from one end using a sharp hobby knife or an adjustable wire stripper. (Strip just enough to be able to make connections to the jack.) Be careful not to cut the wires or shield inside--just slowly score the housing until it splits and then pull it off.
  3. Separate the copper shield wires into a bundle. If there is filler, snip it off (be careful not to cut the wires!)
  4. In a typical mic cable, one of the insulated conductor wires is either blue or red and the other is clear. We'll use the clear wire as ground. Strip the ground wire down to as close to the edge of the cable housing as you can and twist the wire together with the shield.
  5. Strip off around 1/8" inch from the end of the signal wire and twist it into a bundle.
  6. Bend the stripped end of the signal wire 90 degrees, to create a little tab that we will insert into the signal terminal of the jack.

Step 3: Solder Leads to Jack

The jack has two terminals: a signal terminal that goes to the tip of a plug and a shield terminal that goes to the grounded plug housing and the cable shield. The signal terminal is usually a metal tab connected to the body of the jack with a small hole in it. Some jacks may have two signal tabs. If the jack is a mono jack, then you may connect the signal to either one of these. The shield terminal is usually a long strip of metal that may have a folded tab that forms a "pocket" for connecting the ground wires, or there may be a small hole in the bottom of the strip. The figure above is of the pocket variety.

If you plan to do a lot of soldering, it's worth investing in a good iron, as well as a good set of strippers and wire cutters. I use a temperature controlled iron set at 700 degrees with a chisel tip. This transfers a lot of heat quickly and melts the solder fast, which I prefer. There are a number of good Instructables on how to solder.

Before you connect the wire leads to the terminals of the jack, you should "tin" the wires by applying a small amount of solder to the wires. I use a cheap "helping hand" tool with alligator clips to hold my work while I solder.

After you've tinned the wires, put a small blob of solder in the "pocket" of the lead terminal. You should touch the soldering iron to the inside of the pocket to heat it up and then slowly feed the solder into the point where the tip of the iron and the pocket terminal meet.

Now we're ready to connect the wires. Fit the angled end of the signal wire through the hole in the signal terminal and solder it into place with a small amount of solder. Next, slide the ground wire up into the pocket and solder it into place. Use flush wire cutters to cut the wire ends off flush with the terminals. Using pliers, crimp the tabs on the end of the ground terminal to grab the cable housing. When you're done, it should look like this:

Note that there is little excess solder and there is clean separation between the signal and ground connections. It's worth taking your time to get a clean result.

Here's an example of a plug that has a small hole instead of a pocket for the ground wire. To connect the ground wire, simply bend it down to fit through the hole. Note that a large bundle of wire won't typically fit through the hole. To get a smaller bundle snip of half of the shield wire before you twist the shield to the ground lead wire.

Step 4: Heat Shrink Tubing Over Jack Connections

Heat shrink tubing over the jack connections serves 3 purposes:

  • It protects the signal connections from shorting to the grounded shield housing
  • It adds mechanical strength to the connection
  • It reinforces the place where the cable enters the jack, to keep it from breaking from too much flexing. (You know how annoying it is when the cord on your phone charger wears out at the joint!)

  1. Cut a 2" length of 5/16" diameter heat shrink tubing and slide it over the jack terminals as shown
  2. Turn the tubing slowly over a heat gun or candle until it shrinks
  3. If the jack came with a plastic jacket, slide that over the shrunk tubing.
  4. Screw on the housing.

Step 5: Prepare the Cable End for the Piezo

  1. Strip off a length of cable housing that's the same as the diameter of the piezo, a little more than an inch.
  2. Completely trim off the shield wires and any filler material
  3. Strip 1/8" inch of insulation off the ground wire and fold it over 90 degrees
  4. Trim the signal wire to a length near the middle of the inner circle crystal contact and strip off 1/8" of insculation
  5. Tin the ends of the wires with solder--be generous with the amount of solder
  6. Shrink on a short length of 1/4" diameter heat shrink tubing (around 3/8") over the cut end of the cable housing. This is to prevent any stray fringe wires ends from the copper shield from touching the the crystal contact when we solder the leads in place.

Step 6: Solder the Leads to the Piezo

  1. Melt a small blob of solder near the perimeter of the metal ground disk. To do so, first hold the iron to the disk and then feed a little bit of solder to the point where the iron and the disk meet. Be careful no to let the solder short to the inner crystal circle.
  2. Hold the end of the ground lead on the solder blob and touch the lead and the blob with the soldering iron until the solder melts and forms a joint.
  3. Solder the signal lead to the crystal terminal. If the wire lead doesn't sit nicely against the terminal, it may help to melt a little blob of solder onto the crystal terminal like you did for ground, and then solder the lead to the blob.

Step 7: Test the Microphone

Plug the mic into an amp using a guitar cable. Test it by tapping on it.

Step 8: Mount in Bottle Cap

Cut a notch into a standard 20 oz drink bottle cap that is big enough for the cable to fit loosely. I do this by first drilling a 1/4" hole on a drill press (hold the cap with pliers or vice grips while drilling), cutting away the excess with wire clippers, and then using a Dremel or file to enlarge. Sand off the nubs from where the bottle cap attached to the ring on the bottle neck.

Apply hot glue over the solder joints and along the joint between the cable and piezo

Apply a generous ring of hot glue around the inside edge of the bottle cap. The glue should come right up to the rim of the cap. Watch your fingers--hot glue is hot!

Set the piezo and cable assembly into the bottle cap so that it is uniformly flush with the rim and hold in place until the glue is firm. It's important that you get it as close to flush as possible, so that the piezo will make good contact with a surface. Apply a ring of hot glue around the joint between the cap and the cable.

Dip the bottle cap and piezo into Plast-Dip just past the joint. Be sure to stir the Plasti-Dip well before you do the dipping! When you dip, lower the mic slowly (take a few seconds) and then remove slowly. Hang the mic by the jack, remove drips with a paper towel, and let dry for a few hours.

You're done!

Step 9: Compose and Play Some Music!

In this Instructable, Peter Martin of Third Coast Percussion provides tips on composing and playing percussion music.

Step 10: Using the Contact Mic As a Guitar Pickup

You can use this contact mic as a pickup for an acoustic instrument such as a guitar. If you do, you'll probably want to use a longer cable than 8", so that the jack clears the top. To attach the contact mic to the guitar, use a little bit of poster mounting putty. You can further secure the mic and cable using gaffer tape, which holds well but doesn't leave a residue when you remove it. You can experiment on where to place the mic to get the best sound, but just below the bridge on the side of the high strings works pretty well.

One issue with using a piezo contact mic as a guitar pickup, is that if you connect it directly to a typical guitar amplifier, some of the bass gets filtered out. Using a preamplifier with a high input impedance can correct this problem. I've written this Instructable on making a Guitar Contact Microphone Preamplifier as a solution to this problem. It's a simple design that fits inside of a mint tin, and also includes some basic explanation on how the preamplifier works.

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    32 Comments

    Hi, so I had a go at making this for a uni project. First of all thanks for the instructable, it really did help!

    I am however, facing an issue. It seems my mics are picking up room sound. Everything I can find on these mics suggests that shouldn't be the case, that you could talk at them and not get anything. However, when i put the mic on the floor near my laptop and play music out of it, I can hear that music from the mic signal. Not sure if this is actually how they work, or whether I've done something wrong?

    Thanks again for your time!

    Have you actually tried this with a simple (unbalanced) cable like a guitar cable? I would have thought it would be as good if not better. The way you have it set up, it's not actually doing balanced audio - for that you need the shield grounded, and the two inner conductors carrying mirror-image signals of each other which get subtracted from each other at the receiving end, thereby cancelling out interference. None of the conductors should be connected to each other, and you would need considerably more circuitry to achieve it. Random google hit with more info http://www.aviom.com/blog/balanced-vs-unbalanced/

    Would it still work if I used a 3.5mm audio jack with shielded cable plugged into the aux. input on something like a computer speaker (the computer speaker has a built in amp) with the other end soldered to the piezo? Also, would hot glue work in place of solder?

    Hot glue is an electrical insulator, it wouldn't work well in place of solder. A soldering iron and solder cost under £10 or approx $10, not a big expense and you'll probably find yourself mending good earphones that broke at the jack from metal fatigue, a soldering iron saved an expensive computer headset multiple times as the wire kept getting pulled when I stood up if it was caught on something; you'll find that you end up mending items that you'd usually throw out and re-purchase, usually at a much higher price than the soldering iron and solder. Some flux; even though most soldering wire is coated in a bit of flux using soldering flux means you get better connections on some metals which seem to repel solder otherwise.

    Would these be a solution for amplifying a tap dancer? Thinking of building and attaching a few of these to a stage surface or making up a 4 by 8 sheet of plywood with contact mics embedded in the surface. Would appreciate others thoughts on this. What about the need for pre-amps? Optimally, would like to hear the metallic click of the taps rather than thudding of the footfalls.

    Probably not since the sound is made by the tap dancer's shoes clicking off the surface, standard microphones (directional microphones with a cat's tail over it) should work better think of the surface of the floor as a percussion instrument and the shoes as drumsticks attaching one to a drum-skin wouldn't work well, but attaching it to the wooden body would. If you attached contact mics to the wall just above the floor they might work but if you want the sound re-created perfectly you're better off with conventional mics, probably two front and two rear for even sound distribution.

    I am trying to build a throat microphone. I have been reading about the types of cable to use. I would have about 18 inches of cable going from the piezo (on a collar on the neck) and then that would go into a longer cable with a female 1/4 inch to an effects box. I saw one site where they joined the ground wire with the shielding and connected that to the outer ring of the piezo. But here it says I should trim the shielding. I have been having ground noise issues on another piezo I had lying around that already had a cable on it. There I was using a small diameter shielded cable with the shield connected to the outer ring and inner wire connected to the inside of the piezo. When I connected it to a 12 foot guitar cable I got a lot of buzzing. Now I'm trying to do it the correct way. I'm also uncertain how the long cable to the amp should be set up. Shield and ground together or trim shield on both ends? Also I wonder if there is a substance to go over the bottom of the piezo that will allow maximum response against the skin or if I should leave the bottom metal part as it is? Thanks

    wow, how many did you make? i count 30 ish on that rack :D

    Nice job. Going to make a few of these, if only for the fun of it :)

    We made 55 of them (50 + 5 spares) in 2 batches for Glenn Kotche's "Prepared Drumhead Orchestra". Got a small production line going to strip cable, solder, and prepare bottle caps. Bought the bottle caps online from someone on Etsy--last thing I needed was to drink 55 bottles of soda! Kinda fun, once you get a rhythm going. Many online suppliers (DigiKey, Mouser, etc.) have price breaks at 10 pieces, and quality mic cable can be cheap in bulk, so it's worth making a batch of them and giving them as gifts if you don't need them all.

    Great 'ible. Also brings back many memories of when I was an engineer at FRAP (Flat Response Audio Pickups), back in the early eighties. One question arises: are you getting enough of a signal out without requiring a preamp? The pickups we designed were 3D point-source pickups (sorry I can't say more, but an NDA was part of the gig!). Those were small enough that they required special handling in the form of a very high input impedance preamplifier (above 20 MegOhms, if memory serves) as well as extremely low impedance microphone cable (on the order of 9pF/ft) in order to keep the high frequencies from rolling off as well as keeping handling noise to a minimum. The small size allowed for precision mounting in the hot zone, a point on a vibrating surface that was the freest of anomalies caused by resonant nulls and provided the purest analogue of the sound being reproduced. While there at FRAP, I installed hundreds of units in acoustic guitars as well as many other stringed instruments and other vibrating surfaces. The President and founder of the company had a recording he made one very late night, of the suspension cables on the Golden Gate Bridge, being struck with a rubber mallet! Talk about low frequencies sounds!

    In order to ensure the hottest point on any stringed instrument that was being sampled, I developed an empirically derived formula which stood the rigours of many tests.

    Taking the string length of the longest string (using an acoustic guitar as an example) find the distance from the nut to the bridge saddle of the 6th string; multiply by 1.01, divide the product by 7 and rotate the result by a -90 degree right-hand vector, which on the guitar, places the hottest node just behind the saddle on the lower bout (the Treble Side, as it happens). The transducer was placed either on the surface by the use of a non-degrading non-solvent based wax developed to allow placement on valuable old concert violins, violas, cellos and contra-basses from the masters, that Stradivari, Amati and Guanari made, and designed to keep the owners of these rare instruments happy that we were not adversely affecting the finish on these irreplaceable instruments. In fact it left no mark whatever on any instrument in the many installations done over the entire history of the company or even afterwards. For more permanent installations, Silicone (GE Autoseal was the preferred type) replaced the proprietary wax, and the tranducer was placed on the underside of the guitar top (called the table) at the same node and usually reinforced by being placed against an internal strut at that point. Internal installations then exited through a machined jack that acted as the bottom strap end pin as well as an 1/8" jack (to keep capacitance down, like the choice of cable as previously mentioned.

    I wonder that accelero-metric transducer as large as the buzzer piezo element you are using would be able to accurately reproduce the sound from a vibrating node that large. It would seem to me that the smaller the pickup area, the better the chance of picking up only the hottest reinforced vibration node on the affected surface.

    Have you done any experiments using a smaller element and a preamplifier (like a FET type circuit to supply the necessary signal level as well as the required high impedance supplied to the source)?

    Well, this is certainly longer than the brief comment I intended to make, but I thought it might be of interest to other experiments in piezo technology to know some of the history, including that of the first production units to use that technology for musical instruments.

    Happy experimentation to all tinkers out there,

    keep up the good work!

    labernache