Hardware Hacking 15Chapter 4: In/Out (the Eighth Rule of Hacking)
You will need:
- A battery-powered mini amplifier.
- A pair of headphones or a small speaker.
- A pair of jumper leads and a jack to fit the input jack of your amplifier.
Electromagnetism
There is a beautiful symmetry to the principles of electricity that are most
commonly used to translate acoustic sound into an electrical signal and then
back into sound again. Inside every dynamic microphone (such as a typical PA
mike) is a lightweight plastic membrane affixed to a coil of fine wire encircling a
cylindrical magnet. Madonna sings, and her sound waves jiggle the membrane
which moves the coil in the field of the magnet, generating a very small electrical
current. This current is amplified, equalized, flanged, reverberated, compressed
and finally amplified even more before being sent back out to a bigger coil
wrapped around an even bigger magnet. Now this shimmering electromagnetic
field pushes and pulls against the big magnet (think of the two magnetic Scotty
dogs, forever trying to align themselves nose to tail), moving a paper cone back
and forth, producing sound waves of....a louder, possibly improved, Madonna.
A record player cartridge is just a microphone with a needle where the
diaphragm should be; and record cutting heads are just beefy backwards
phonograph cartridges. Headphones are just tiny speakers. The telephone tap
coils we used earlier are just electromagnets with no moving parts, receiving and
emitting electromagnetic waves rather than sound waves.
Not only is the same electromagnetic force used for both input and output
devices (microphones and speakers), but sometimes the gizmos themselves are
interchangeable. Try plugging a Walkman headphone into the input jack on
your amp or cassette recorder; speak into it and listen -- more than one band’s
demo tape was recorded this way. Plug any small speaker into the input of the
amp. According to legend, Motown engineers recorded kick drum with a large
speaker placed in front of the drumhead -- a sort of a sub-woofer mike. These
alternative microphones don’t sound as generically “good” as a $5000- Neumann
tube mike, but (as Motown’s sales have shown) for special applications they can
be very useful.
Likewise any dynamic (i.e., coil & magnet) microphone can be used as a very
quiet speaker or headphone, but microphones have very delicate coil windings
and can be easily blown out, so BE CAREFUL. Also, condenser mikes (like the
“plug-n-power” mikes for cassette recorders, or expensive studio mikes) use a
different, not-so-easily reversible principle of translation, so: IF THE MIKE USES
A BATTERY OR PHANTOM POWER OR IS REALLY, REALLY EXPENSIVE,
DON’T USE IT BACKWARDS.