Some of the most interesting work I have found in awhile is the research by David Rountree who is an audio engineer.
See this page
The relevant bits that we are interested in deal with different microphones.
He also had a very interesting section on working with a bell jar and obtaining evp`s.Another quote from his work was
"...using a B&K bench oscilloscope and a very large detection coil.Clearly this demonstrates a correlation between EVPs and EMF."
Microphone Selection
Capacitor or Condenser – I have never captured an EVP using a capacitor microphone. Capacitor microphones respond to changes in capacitance. I would like to continue this research if I ever capture an EVP, with capacitance field readings during the event. So far I have not been able to develop a device to measure any significant capacitance fluctuations during a paranormal event.
Electret – Many EVPs have been captured by both the community at large as well as verified by my own research. Extensive testing has shown that Electret microphones are deaf to EMF. Due to the supply voltage, the interconnecting wiring will pick up EMF only if it is higher in amplitude than the supply voltage. This means that EVP captured with electrets microphones are either Audio Voice Phenomena, or intentionally recorded data (Hoax), or simply pareidolia.
Dynamic – By far the most common method historically of capturing EVPs. These microphones respond to vibration as well as magnetic fields.
Ribbon – I have captured several EVPs while using ribbon microphones. However, due to their delicate nature, I do not recommend them for field use. Ribbon microphones respond to sound pressure variations as well as magnetic field fluctuations.
Carbon – I have never used a carbon microphone in field study, but there are reports from the past of ?ghost voices? being heard over the telephone. I suspect this is due to induction effects in the wiring, and not the device itself.
Piezo – I have never been able to capture an EVP with a piezo microphone. They respond to vibrations, but because of their crystal nature, it may be possible for them to respond to magnetic fluctuation, if they are extraordinarily strong.
Laser – I have not been able to test a laser microphone in this application.