Acoustical engineer Peter Riccardi has put together a MEMS instrumentation microphone that can trade blows with lab-grade commercial instruments — yet which costs under $30 in parts.
"Acousticians and acoustical engineers often rely on laboratory grade measurement microphones to acquire physical signals during experiments," Riccardi explains. "Typical condenser microphones from companies such as PCB PIEZOTRONICS could run a bill of over $1000 for a free-field microphone. The quality of such microphones is superb, but there are alternatives available to the hobbyist and enthusiast which will get you comparable performance for a fraction of the cost."
That's exactly what Riccardi has designed: a do-it-yourself equivalent to expensive lab-grade measurement microphones, but using low-cost parts a 3D-printed housing to keep the cost as low as possible — though still delivering on the goal of the same characteristics as its commercial competition for the audio band between 20Hz and 20kHz.
"The electronics is split into two boards for the prototype," Riccardi explains. "The first board is the MEMs carrier board. Four MEMs elements are mounted on a circular PCB with small through-hole pads scattered around for the necessary power and signal connections. The second board is the Motherboard or the pre-amplifier. This contains the inputs and signal conditioning for the MEMs outputs, as well as the IEPE conversion and output modulation — all done with adjustable Zener diodes."
There's a reason for packing four elements into a single microphone: Noise reduction. "By tying the outputs of multiple noise sources together in parallel (via small valued resistors) the output noise floor has an overall reduction which therefore increases the signal to noise ratio," Riccardi writes.
"Condenser microphones have excellent signal to noise ratios with typical sensitivities being on the order of 50mV/Pa," Riccardi continues. "By using four MEMs elements in parallel, the overall signal to noise ratio is improved at minimal effort and cost."
The finished prototype is housed in two-part 3D-printed cylindrical chassis, designed to mimic the size of commercial microphones. In testing, the assembled prototype offered a similarly-flat frequency response coupled with high enough sensitivity for real-world use — "below 200Hz," Riccardi notes, "its performance might even be better than the [commercial] microphone."
Riccardi's full write-up is available on his Hackaday.io project page, along with SolidWorks and Altium design files for the housing and PCB respectively.