One way to pick up electromagnetic signals is to use a simple coil of wire and an amplifier. A pickup consists of yards of thin copper wire wrapped around an iron slug. We are going to use a bobbin with magnet wire. In the center of the bobbin we will place a magnet. Solder the two ends of the magnet wire to tip and sleeve of a plug.
Plugged into an amp this coil acts like a radio antenna for low frequencies.
Plug the tap coil into the portable amp and move the coil over appliances like a stethoscope. Sometimes you will hear different types of sounds from the same
appliance. Pass the coil slowly over a laptop, and note the change in sound as you move from the CPU area to the RAM to the disk drive to the CDROM. Listen to small motors in fan, vibrators and toys; notice the change in pitch as you change the motor speed. Take a ride on the subway and listen to the motors and doors as you come in and out of stations. What about a neon sign.
Jérôme Noetinger, Andy Keep, Nathan Davis and others have made beautiful use of this secret magnetic music.
The stethoscope-like accuracy of the coil moving over a circuit board makes it a useful, non-destructive device for pinpointing the location of interesting sounds.
If you move the coil near the speaker of your amplifier it will begin to feed back
with the coil that moves the speaker cone. As with feedback between a microphone and speaker, the pitch is affected by the distance
separating the two parts, but here the pitch changes smoothly and linearly, without the odd jumps caused by the vagaries of acoustics, giving you a Theremin-like instrument. Try this with a full-size guitar amplifier for greater range.
Homework:
Look up Jérôme Noetinger, Andy Keep, Nathan Davis and describe how they have used magnetic music. Describe your own idea for using a coil to listen to electromagnetic waves.
The rougher the metal surface the better. Tap the nail or clip on the plate. When it touches the metal the nail completes the circuit and sends 9 volts to the speaker coil, making the cone jump. By scraping the nail across the metal you can produce scratchy, percussive sounds. Put gravel, loose change or dried lentils inside the speaker cone for additional rhythmic accents.
Avoid holding the nail on the metal for an extended period of time, as your
speaker will get hot when presented with a steady DC voltage. Shorter pulses of current is better .
Plug your headphones into the input jack on
your amp and speak into it and listen.
Any dynamic (i.e., coil & magnet) microphone can be used as a very
quiet speaker or headphone. Generally microphones have delicate coil windings
and can be easily blown out, so BE CAREFUL.
IF THE MIKE USES
A BATTERY OR PHANTOM POWER OR IS REALLY, REALLY EXPENSIVE,
DON’T USE IT BACKWARDS.
According to Nicolas Collin's Hardware Hacking rules:
Rule #8: In electronics some things are reversible with interesting results, but
some things are reversible only with irreversible results.
Contact Mike
Contact mikes are great for amplifying hidden sounds in everyday
objects. The trick is making positive physical contact with the vibrating object.
Try: guitars, violins, drums, pots & pans, wrists & knees, foreheads,
pinball machines.
Another common principle of reversible sound translation is the Piezo-electric
effect, which depends on the electrical properties of crystals, rather than
electromagnetism: bang a crystal with a hammer and it will generate a pretty
sizeable electrical signal (enough to illuminate an LED); send an electrical
current into a crystal and it will twitch. Piezo-electric disks, made by bonding a
thin layer of crystal to thin, flexible sheet of brass, are everywhere today, inside
almost everything that beeps: appliances, phones, toys, computers, etc. Because
they are manufactured in huge quantities they are cheap, and they
happen to make even better contact microphones than they make speakers.
Drum trigger and commercial contact mikes are often made from Piezo disks.
Depending on your piezo, there will be 2 or 3 wires connected to the disk. One will always be
connected to the metal portion, somewhere near the edge; this is the GND connection. One will connect to the main part of the inner circle; this
is the “hot” connection. In some cases there will be a narrow, tonguelike
shape differentiated within the crystal, to which the third wire connects; this is an extra connection.
Cut the connecting wires so that they protrude about 2” from the disc. Strip
about 1/2" of insulation from the ends of the ground and hot wires. Leave the extra wire, if it exists, unstripped.
Shielded cable consists of stranded wire inside insulation, which is then
wrapped inside a layer of braided or twisted wire, which is in turn covered by
another layer of insulation.
Shielded wire is used to protect audio signal from hum and other
electromagnetic interference. Shielded cable comes with any number of internal
conductor wires, but for audio purposes most cable has one or two internal
conductors plus the shield. Use cable with a
single internal conductor plus shield.
Rule #9: Use shielded cable to make all audio connections longer than 8",
unless they go between an amplifier and a speaker.
Cut a 5" section of shielded cable. Strip
back 1" of the outside insulation. Unbraid the shielding and twist into a single
thick strand. Now strip back 1/2" of the inner insulation, and twist the center
conductor into a neat strand. Keep the two strands separate. Tin both strands,
being careful not to melt back the insulation.
Connect the hot wire from the Piezo Disk to the inner wire from the
shielded cable. Solder them together.
Connect the ground wire from the
Piezo disk to the outer wire from the shielded cable. Solder them together. Wrap
both joints separately with a bit of electrical tape so that they cannot short against
each other.
Strip the free end of the shielded cable 1” outer insulation, twist
shield, 1/2” inner insulation, twist conductor, tin the wires.
Unscrew the plug you are using to connect the contact mike to your
amplifier/recorder. Slide the barrel onto the wire before you solder. Connect the shorter lead that connects to
the tip of the plug to the inner conductor of the shielded wire and is also connected to the hot wire of the piezo.
Connect the sleeve to the outer conductor of the shielded wire, which is also connected to the GND wire of the piezo.
The signal comes from the white part of the Piezo
disk, while the ground is the brassy bit; on the audio plug the tip carries the signal and
the sleeve is the ground.
Plug into your amp and check that your new contact mike works—tapping
the mike should make a solid thunking sound. If there is no sound check the
joints at both ends of the cable—make sure they are good and there are no
shorts. If there is hum you may have connected the hot and ground wires to the
wrong conductor of the cable—desolder and reverse them. If it works, screw
the barrel down onto the plug and test again.
When you are sure you have an electrically functional contact mike, cover the
ceramic side with a piece of electrical tape—you can trim it around the
circumference with scissors or a knife, or you can wrap the edges over to the
other side of the disk.
Amplify the contact mike into a raw speaker: Place a speaker on its back, like a
candy dish. Rest the contact mike inside the cone and turn up the gain. The contact mike should jump up and down as it feeds back with the speaker.
