IEM Measurement Tools - Audix TM2 and SLS-Audio MiEMi
I spent some time today playing with both of these tools designed to allow you to measure In Ear Monitors with an audio analyzer. In this post I'll be discussing the Audix TM2 and the SLS-Audio MiEMi.
Here are my comments on both - please note, this is not intended to be an exhaustive or official review, just my initial impressions.
MiEMi
This unit is an aluminum coupler designed specifically for use with the iSemCon EMX-7150 measurement microphone (which works out well because the EMX-7150 is my go-to for measurement work). There is also a less expensive, 3D-printed version of the coupler that is sold to fit a variety of other mic sizes and is, according to the website, less accurate. The rubber coupler is easily replaceable. You fold the coupler in on itself and then press the IEM in.
Audix TM2
This unit is a standalone measurement coupler, which means its measurement mic is built into the unit itself. You connect an XLR cable to the base and engage phantom power on your interface.
It ships with a few different adapters - "small" and "large" for measuring the two standard sizes of IEMs without the eartips attached, a "custom" adapter for measuring with the eartips attached, and a fourth that fits to the standard half-inch diameter acoustic calibrator for calibrating the TM2 for SPL. These come in a zip case that also holds the TM2.
The TM2 also ships with a printout of the unit's frequency response deviation from the benchmark unit, done with an Audio Precision analyzer, and a USB keydrive containing the corresponding correction file. The deviations were all less than +/- 1 dB below 8 kHz (above 8 kHz, IEM measurements get really tricky due to coupler resonances and insertion variance, so I'm not usually paying a ton of attention to the top octave.). Here's the correction curve from the particular unit I tested:
Testing...Testing...heyheyYahTssTss
Okay, here's what you all came to see, I'm sure. With both of these units, you are setting the coupler up as your Measurement input to the analyzer and using a loopback for the reference signal, just as you would with a typical dual channel measurement setup. The only difference is that we also need to drive the IEMs themselves via the IO's headphone out. Honestly, it's probably not a terrible idea to do an idiot-check validation transfer function between the Line level outs and the Headphone outs on your interface and make sure that comes up flat before you do more testing.
My two concerns with devices like this are accuracy (how close is this result to the actual response?) and precision (do I get the same answer every time?). This is where some of the guerilla DIY IEM test rigs fall flat. I will answer both questions at once. The image below shows the left side of my Mackie MP-240 IEMs (which are fantastic for the price!) measured six times: three times each with each rig. We see that not only do we get the same answer each time, but both systems seem to agree. (1/12 oct smoothing)
The trick with these is that even small variances in insertion can create significant differences at the frequency extremes (below 100 Hz and above 8 kHz or so). The MiEMi is definitely more fussy in this regard, and it took a little practice to be able to get repeated measurements. The Audix unit is a lot more stable and seems to have a more robust grip on the IEM. It can help to provide some strain relief on the cable, or to rest a thumb on the IEM to keep it in place during the measurement, but you have to be really careful not to make any motions that would bump the mic. The TM2 is a lot more forgiving in this aspect.
That being said, I'm really interested in the broad strokes here: does L match R? Do your IEMs measure the same tonight as they did last week? Much as with measuring a PA system, this is about the forest, not the trees.
As an interesting aside, it seems that a lot of IEMs have a HF problem. Here we see my Mackie MP-240 in blue and Shure SE-315 in pink, normalized to the mid-band (500 Hz). It will be interesting to see, as more data becomes available, whether this is simply a limitation of a non-laboratory grade coupler.
The Shure HF takes a bump up above 1 kHz and then falls off a cliff completely above 4 kHz, which seems to be a common trend. The SLS website has measurements of some other common IEM units and a lot of them exhibit the same trait.
(Heads up - their screenshots are magnitude on top, phase on the bottom, the opposite of what appears here. Many of them also use heavy smoothing and a greatly compressed vertical scale of +/- 35 dB or even more in some cases, which can make the responses seem flatter than they really are. Just some things to be aware of when browsing the data on your own.)
My subjective perception of the SE-315's is that they had a little too much going on in the hi-mid band and it's interesting to see that borne out in the data.
SPL Calibration
Another interesting (and dare I say very important) aspect of this is the ability to calibrate the TM2 for SPL and have an idea of what sort of sound exposure our artists are enduring (provided, of course, we control for variables such as the pack level knob, etc. I have found that my artists tend to set their pack levels once and then not touch them again, so I could in theory collect the packs after the show while still on, and measure them at that point to get an accurate idea).
This is cool stuff - for purposes of sound exposure, we're interested in average A weighted levels (LAeq), or looking at the NIOSH dose directly as a %:
One particular concern for me would be an artist who takes out one side of their IEMs, which means turning up the pack much more to get the same perceived loudness, and that extra 6 - 9 dB means 4 to 8 times the noise exposure, not to mention what's coming in the other ear. So the ability to quantify the risk in these types of situations is a pretty cool and very welcome thing.
SPL calibration is the other area where the MiEMi is probably not the best tool for the application, as you're calibrating the mic for SPL and then fitting it with the coupler, which changes the equation, versus the TM2, where you calibrate the coupler directly. The SPL values I measured with the MiEMi were lower by 4 - 6 dB than the calibrated TM2 values depending on the IEM I was measuring, and that's far too large of a window if the goal is measuring exposure. (At just 2 dB calibration error, the uncertainty in the exposure time is longer than the duration of the show.)
Conclusions
I think these are both great tools and welcome additions to a monitor engineer's toolkit. For consistency and spot checks, they will both do the job well once you get the hang of consistent and well-sealed insertions (giggity?). Remember that the TM2 is a self-contained unit whereas with the MiEMi you'll need an EMX-7150 microphone as well. These are pretty popular in the measurement world, so you might already have one, and they're also a solid investment - SLS sells the coupler and the mic as a bundle. The additional accuracy gained by the TM2 correction file is probably not super important in most circumstances, but the accurate SPL calibration is a big selling point in my opinion. That's my two cents, hopefully it's helpful to anyone considering investing in one of these tools.
Of course I recommend going right to the source for the best information. For Audix, that's via email at info@audixusa.com. If you're in the US and interested in MiEMi, you'll want to reach out to Dennie Miller at Miller Audio Industries. Hi Dennie!