Reference mics are vital tools for audio work. They’re prized for their flat frequency response, and are often used for characterizing the audio response of a room or space. OpenRefMic aims to be an open source design for producing reference mics without paying exorbitant retail prices.
The heart of the build is a preamplifier that runs off standard 48 V phantom power, and is responsible for both biasing the electret microphone element and acting as a buffer for the mic signal. It’s designed specifically to work with the PUI AOM-5024L-HD-F-R mic capsule, chosen for its good performance and low noise characteristics. However, other electric mics should work, too. The hardware is wrapped up in a 3D printed case which can readily be made on most basic printers. It’s complete with a press-fit grille that holds the mic capsule in place.
The prime goal of the project is low noise; the project creator, [loudifier], notes that most commercial reference mics focus first on flat frequency response and then reducing noise. OpenRefMic performs well in this area, and its lack of a perfectly flat frequency response is countered with calibrated equalization. It also works with regular pro-grade XLR cables and phantom power, rather than needing fancy laboratory-spec cables and interfaces.
The final result is a credit to [loudifier], who demonstrates a strong understanding of the principles of reference mic design. We’ve seen some other great low-cost reference mics recently, too!
Exorbitant prices?
https://www.thomann.de/de/messmikrofone.html?oa=pra&gk=MIME&filter=true
I was thinking the same thing while looking at my miniDSP UMIK-1 and UMIK-2. There’s plenty of reference measurement mics out there that aren’t obscenely expensive.
Seems it goes from reasonable through monster cable buyer to “even his radio alarm is Bang and Olufsen”. What is this comparable to, base level electret, or fancier lower noise midrange?
Hey, this is my project. Yes, there are plenty of cheap mics with flat response, but there isn’t anything in the budget range with a noise floor lower than about 30dBA. There is a video on the GitHub comparing this mic’s 18dBA noise floor to a Dayton EMM-6 at ~34dBA and the difference is pretty dramatic.
I think [Loudifier] is a cool name,
I wish I had thought of it!
How much do you estimate the BoM to be for this build?
$40, not including the 3D printed parts. There is a complete BOM on the GitHub, including the connectors and PCB price from OSHPark.
I don’t know enough about microphone noise to say if the microphone element or the amplifier stage is the dominant noise source, but if I’ve done the calculations correctly the amplifier stage noise can be reduced by a couple of dB by halving the feedback resistors.
The self-noise of the mic element will usually be the dominant noise source in the system. Depends on the dynamic range of the input and the sensitivity, SNR, and AOP of the mic. You can adjust the supply voltage and feed resistor for an electret to tune the sensitivity and AOP to a certain extent, and you can also add some gain if your preamp is low noise.
I haven’t had a chance to measure the preamp by itself, but I’m pretty sure the preamp noise is significantly lower than the electret, but the feedback resistance could definitely be lowered if it is degrading the total system noise.
Never mind how flat; what does it sound like? ;-)
I have little use for an imstrumentation mic, I’m more interested in accurate low-noise mics for field recording. So, how does the capsule (PUI AOM-5024L-HD-F-R) sound? Has anyone out there used it in any DIY recording mic builds with good results? How does it compare to the Primo EM272? Thx.
Sounds great. The closest thing spec-wise I have found is the UMIK-2. Everything else is either very expensive or has an unknown frequency response or directivity. Check out the field recording comparison on the GitHub. https://github.com/loudifier/OpenRefMic
I didn’t know about the EM272, will add that to the list of mics that I will modify the design for. Might be tough to fit it into the 1/2″ form factor, since it is slightly larger.
The PUI capsule is very similar. The noise floor is about the same, but it looks like it needs a little more EQ than the EM272. The PUI mic is also available from Digikey, so it is much easier (and cheaper) to find.