PreSonus PRM1 Measurement Microphone by Sound-Guy
PreSonus PRM1 Precision Reference Microphone
While making some changes to the acoustic treatment of my project studio, I decided I should get a new measurement mic since my old standard mic was several years old. I could send it to a calibration lab, but that generally costs more than a new mic! I looked at several possibilities and the PreSonus PRM1 looked like a good choice and was recommended by my friendly audio equipment supplier.
What is it?
A measurement mic is a (usually) small diaphragm condenser omnidirectional microphone with a very flat frequency response over the audio range, often supplied with a calibration curve or calibration file. Measurement mics are not intended for recording instruments or vocals since they generally have a relatively high self-noise level and a limited dynamic range. They are intended to be used for measuring test tones and noise bursts at sound levels of 70-90 dB SPL for to help tune a room.
The PRM1 has a ¼” pre-polarized electret-condenser capsule, is about 8 inches (200 mm) long and comes in a nice plastic hard case, with a mic clip (called a shock mount in some literature, which it is not) and a foam windscreen. It requires 9-52 Volt phantom power, but has a reasonably high output (rated at 14 mV/Pa, or -37 dB/Pa into an open circuit). It has an output impedance rated at 200Ω, an A-weighted noise figure of 26 dB, and a maximum sound level (SPL) of 132 dB SPL for 1% distortion. Although not intended for recording music and voice, it could be used for such material as long as the self-noise level and maximum SPL are not limiting its application.
PreSonus of course make both live sound and studio gear and the PRM1 was designed for use with the Smaart System Check wizards that are integrated into PreSonus UC Surface control software for several of their StudioLive mixers. But it is also useful for any audio frequency measurement task.
Why a Mic?
In changing room treatment I wanted more than just qualitative listening tests. I wanted to evaluate frequency response and other measures such as harmonic distortion and signal decay times using Room EQ Wizard (REW), a wonderful donation-ware program ( REW - Room EQ Wizard Room Acoustics Software ) that enables measuring many audio parameters, both in a room (speakers, amps, mics) and in a DAW environment (such as distortion measurements of a console plug-in).
In measuring room acoustics an omni mic is needed since it “hears” audio from all directions, the direct sound from speakers as well as reflections from walls and ceiling. REW includes the ability to automatically apply a correction curve if you have one for your mic, a great feature. The PRM1 does not include a calibration file, but while testing it I was given the chance to use a measurement mic that had a calibration file accurate to within better than 1 dB from 20 Hz to 20 kHz, so I figured I could run both mics and compare the results.
Easier Said than Done
Comparing two mics is relatively easy if you have an anechoic chamber, but in a room, even an acoustically treated one, it’s not a simple task. I tried putting the mics active ends together, and this worked OK at low frequencies, but above a kilohertz or so there was acoustical interference between them since each was blocking some room sound from the other. And at high frequencies, where wavelength is very short, any reflected audio can cause partial cancellation or reinforcement of the sound that varies results a Db or more with only a fraction of an inch or cm variation in location.
So I tried recording a frequency sweep with the calibrated mic using its correction file, and then the PRM1 placed, as best I could figure, at the same location (within about half an inch, or 1 cm) at a distance of about one foot (30 cm) from my PreSonus Sceptre 6 speaker cones. This worked very well up to a few kHz, but above 10 kHz the tiniest movement of one or the other mic created variations of a dB or more.
Since the mid-range is the most important part of the audio spectrum I wasn’t concerned about the top octave so much, and ran several tests up to 10 kHz that resulted in fairly consistent measurements.
The resulting plot is the “error” of the PRM1 versus the corrected calibration mic with a vertical scale of 1 dB per line. As you can see the PRM1 appears to be very flat, less than half a dB variation, up to a couple kHz, and has only a small 1.2 dB peak at about 7 kHz. This is admirable performance - the reference mic without using its calibration curve had a peak larger than 3 dB in this frequency range!
Although I tested only one PRM1 and can’t claim they are all identical, this was impressive performance for a $100 measurement mic. For its intended use of tuning a sound system to perform accurately, it appears to be an excellent choice.
Product Info: PRM1 | PreSonus