This month’s FRONT of HOUSE features a buyers guide on measurement microphones (on page 15). With that in mind, let’s take a look at measurement mics, how they differ from “normal” microphones, and how you can achieve accurate measurement results.
Do You Really Need Another Mic?
Yes, you do. Many engineers use SPL or spectrum analysis apps on their phones or iPads, but do you want to trust the accuracy of the microphones built into those devices? You have no way of knowing if the frequency response is flat, or if there’s any sort of calibration to ensure accurate SPL readings (That’s one of the reasons you’re better off using something like the iTestMic from Studio Six Digital with your phone). It’s also a bad idea to use a “normal” instrument microphone for measurement purposes. Most of the mics we use for instruments are directional. An omnidirectional mic is a better choice when taking RTA and SPL readings because an omni mic (a) behaves more like your ears and (b) takes into consideration direct and diffuse sound — which influences measurement results. Another issue with most of the mics we use on stage is that (because they are directional) they exhibit proximity effect, which can skew the results of RTA.
The good news is that you can get a decent measurement microphone without breaking the bank. It’s not that difficult these days for manufacturers to produce an omnidirectional condenser mic with a fairly flat response from 100 to 10 kHz, and a dynamic range that can handle SPLs ranging from 40 to 130 dB. Those characteristics can accommodate most of the room measurements you’ll need to make. As you can see from our Buyers Guide, prices range from under a hundred bucks to over a thousand, with plenty of choices in the $200 range. You might gripe about the investment, but the truth is that a good measurement mic can last your entire career, as it won’t be getting whacked with a drumstick several times a night.
What do you get when you climb the price ladder? Flat response across a more extended frequency range, precise matching in a stereo pair, consistency between samples of the same mic, long-term stability, lower self-noise, a calibration chart specific to the particular microphone you have purchased, and an electronic calibration file (more on this in a minute).
When you’re reading through the specs of some of these microphones, you might see them described as “Class 1” or “Class 2.” These labels refer to an IEC standard (61672) for sound level meters, with Class 1 being a device suitable for lab and critical applications, and Class 2 being a device suitable for use as a general field instrument. Class 2 is sufficient for our type of work, but if you think you might need to do an SPL measurement of penguins squawking in their native habitat, Class 1 devices are certified to maintain their tolerance down to a temperature of around 15 degrees Fahrenheit.
Some measurement mics are supplied with an Electronic Calibration File (ECF), a file containing the factory-measured frequency response of your individual microphone. ECFs are usually obtained free of charge by emailing the manufacturer with the serial number of your microphone. Suppose that your XYZ-1 measurement mic has a deviation in the frequency response of 1 dB at 4,136 Hz. An RTA reading made using that mic will show a boost in the response at that point, but how do you know if the boost is real, or if it’s a byproduct of the frequency response of your particular mic? That’s the point of the ECF. Software that can import an ECF will “know” about such a deviation and will compensate, resulting in more accurate measurements.
Calibrating your software for SPL measurement can be a bit trickier, and may require a microphone calibrator. Rational Acoustics has an excellent video on the subject at www.plsn.me/FOH-RA.
It may sound silly, but it’s important to know which direction to point a measurement microphone. You’re thinking, “If the mic is omnidirectional, it doesn’t matter where you point it.” Incorrect. Almost all omnidirectional microphones become somewhat directional at very high frequencies. [A quick refresher from Audio 101: Low frequencies have long wavelengths that easily bend around objects — including microphone housings.] When frequency increases to the point where the wavelength is smaller than the microphone’s diaphragm, soundwaves can no longer bend around the mic housing to reach the front of the capsule, thus making the mic slightly directional. This is sometimes referred to as a microphone’s acoustic shadow, and is why the majority of measurement mics have the same tapered shape.
Out Here in the Fields
As a result, measurement mics are optimized for either free-field, diffuse-field or pressure-field use. Free-field refers to an area where sound is not reflected from any objects (an anechoic chamber would be a pretty good example of a free field) and free-field mics can be used to make direct sound measurements of loudspeakers. A diffuse-field is one in which sound arrives simultaneously from different directions, i.e., most of the rooms we work. A pressure-field is usually a very small area in which sound has the same amplitude and phase anywhere within the field (think PZM).
Why do you care? Take a look at Fig. 1, which shows the frequency response of the beyerdynamic MM1 measurement mic.
Starting at around 8k Hz, the frequency response of the MM1 rises to a 5 dB peak between 15 and 20 kHz at 0 degrees, but the curve is pretty much flat up to that point at 90 degrees off-axis. That’s because the MM 1 was designed to have a flat frequency response in the diffuse field and should be pointed 90 degrees from the source, i.e., toward the ceiling. Pointing it straight at the P.A. system will probably result in RTA readings with a skewed high-frequency response. Don’t complain to me if you’re trying to use a pair of pliers to hammer a nail. If your software can import an ECF, make sure you’re using the correct file, because some manufacturers furnish different files for 0 and 90 degrees.
And you thought pointing a microphone at the ceiling was a silly idea…
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
