The
Contact microphone comes in a variety of guises and is also
known as a transducer, a pickup or a piezo, and is a form of
microphone specifically designed to pick up audio vibrations
in solid objects. Normal studio mics are all built around the
principle of detecting vibrations in the air but Contact mics
are different in that they are designed to pickup surface vibrations.
This means that if you taped one to a wooden board and then
stood infront of it and sang. the chances are it wouldn’t
pick anything up. But tap, scrape, or in some way touch the
board and even the softest vibrations will be picked up by the
contact mic. This means that as a vocal mic its a pretty naff
idea and to get anything at all you would have to tape it to
your neck. Even so the principle simply doesn’t lend itself
to recording vocals and even the cheapest dynamic mic will make
a much better job. However where they do come into their own
is as instrument mics particularly in live situations and there
are a wide range of contact mics manufactured specifically for
bridged instruments such as violins, guitars, cello and bass.
With these the contact microphone can be stuck to the body of
the instrument or placed under the bridge and their unique selling
point is that in a live situation because they pick
up the vibrations directly from the instrument they’re
virtually free from acoustic feedback which is airborne.
You can undoubtedly get a better sound from a large diaphragm
studio condenser mic but for ease of use and price the contact
mic is hard to beat in the live situation and brands like Accusound
in sunny Sheffield make some great sounding contact microphones
for a wide range of instruments.
Design wise, contact microphones are passive and high-impedance
and this can cause them to sound 'tinny' so it’s really
important to use a preamp specifically designed for use with
a contact microphone to get the full frequency spectrum capable
of the device. Again check out Accusound.
But
there’s another studio use for contact mics that doesn’t
matter particularly how good they sound and that’s as
drum triggers They can be plugged into a trigger to MIDI converter
(Roland and Alesis make inexpensive units) or hooked up to a
sampler or drum machine with a trigger input. And it's studio
practice today to have a range of good drum samples to replace
or enhance the sound of the recorded kit.
The most commonly available contact microphone element is made
of a thin piezoelectric ceramic round glued to a thin brass
or alloy metal disc.The physics is very interesting and based
on a principle that certain crystals and ceramics can generate
a small electric current when pressure is applied to them. It
was first discovered in the 1880s but today it has a multitude
of industrial uses. More recently, flexible PVDF fluoropolymer
piezo film has been developed and is available in tabs, strips,
cables and even large sheets and is often used buried under
roads as traffic sensors.
Depending on the type of contact microphone you are using, it
will either be wrapped around the object which will be vibrating
or it will be stuck onto the object with some sort of adhesive
tape or attached via clips allowing the vibrations of the object
to be picked up by the microphone and recorded. Some contact
mics are also sealed so that they are waterproof and you can
get some amazing results with contact mics in water.
I've
used a variety of contact mics over the years but my personal
favourite is the C-ducer
contact strip mics specifically for the piano. We have a set
in the studio and while you can achieve better results from
more expensive condenser mics the C-ducers are brilliant in
a session where they’re are a group of musicians playing
live and you need some separation on the piano. A few years
ago I did a lot of mobile recording in local jazz clubs with
an 8 track Tascam tape machine and a small Studiomaster desk.
I really enjoyed these sessions as some of the players were
great musicians and you never quite knew what you would get
on the night. The track featured here is a typical live jam
with the piano recorded with 2 C-ducer contact mics taped to
the sound board under the piano. By using two strips, one on
the bottom end and one for the right hand and panning them,
you get a really neat stereo piano sound. I think they really
works in situations like this and it's great to have the piano
without any crosstalk.
