Below are audio recordings of my piezoelectric transducer on various instruments, with and without the preamplifier.
Unless otherwise indicated, the piezoelectric disc itself was placed on the top face of each instrument, as close as possible to the center of the bridge.
More instruments will be added as I get the chance to record them.
Technical note: The sound files are unedited. In a recording or performance scenario, EQ would be used to cut frequencies between 2kHz-10kHz for a more natural sound.
Use headphones / good speakers to hear the difference clearly!
Instruments I have tested on, but not recorded:
- cello
- double bass
- santoor**
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** hammered dulcimer from India
The major complaint about piezoelectric contact microphones is the 'tinny' or 'shrill' sound that is often associated with them (you can hear this in the recordings above on the left). Common commercial audio devices are not designed with piezos in mind - so what happens is an impedance mismatch** between the piezoelectric element and the audio input. As a result, a lot of the natural low frequencies of an instrument are irrevocably lost in the wire, changing the sound in a way that cannot be recovered.
For this reason, instruments that use piezoelectric pickups nearly always incorporate some form of circuitry (the preamplifier) that stands between the piezo element and audio input, so that they never 'see' each other, but passes the audio signal along. This usually also includes volume and tone controls to amplify and 'shape' the sound.
** The piezoelectric disc being a very high-impedance output and most audio inputs being much lower impedance, this mismatch creates a "high pass filter" which reduces the low frequencies that are detected.
Watch this video from Bell Telephone Labs (now AT&T) for a really interesting demo of waves, and the importance and implementation of impedance matching.
Notes about this pickup/preamp:
The preamplifier will not work with a standard mono guitar cable. It comes with a custom TRS->XLR cable; after which a standard XLR microphone cable can be used to connect it to an input.
The piezoelectric transducer will work with any other commercially available preamplifier; however, this preamplifier cannot be used with other transducers as it has been engineered to work specifically with the one presented here.
Phantom power is required for the preamplifier to operate.
Advantages
Disadvantages
The contact microphone can be attached to nearly any instrument or surface to pick up sound. It could even be used to record sounds underwater, with suitable waterproofing.
No batteries needed for the preamplifier, as it is powered by phantom power (available on most common audio interfaces). If phantom power is not available, the piezoelectric mic can still be used without the preamp, and it will sound like a generic piezo microphone (with the usual disadvantages).
Because the microphone outputs a balanced signal, it is highly resistant to electromagnetic interference such as the 'hum' from mains electricity (provided that the equipment it is connected to is properly set up).
By design, there is no gain/volume/EQ control on the microphone or preamp; all adjustment (if any) must be done at the mixer.
Since the microphone is in direct contact with the body, a small change in location could potentially have a noticeable effect on the timbre; and it will inevitably pick up more handling noise than a conventional microphone. With that in mind, the microphone will pick up sound from anything that it is attached to, but may pick up an unwanted amount of mechanical noise from heavily percussive instruments, and will work poorly on most wind instruments.
