Measuring In-Ear Monitors

Measuring in-ear monitor output can be an effective tool for professionals. This is definitely overkill for the average IEM user, but if you’re an IEM pro like a monitor engineer, MON tech, church tech director, or similar, this is for you. You’re not going to become an IEM designer overnight, but you will be able to quickly diagnose faults, test loudness, and troubleshoot your entire signal chain with real data. Follow along as we demystify IEM measurements.

If reading’s not your thing we also made a video:

Why Measure?

You’ll find loads of useful information online about IEM and headphone measurements. Online content creators like Crinacle and The Headphone Show are great resources. A lot of what you see has to do with reviewing the sound of a particular product to help purchasing choices. While all of that can be incredibly helpful, that is not why we’re here. You’ll find very quickly that looking at measurements doesn’t tell the full story of the way something sounds, and with the amount of variables involved, could actually be misleading at times. We’re encouraging folks to learn about measuring IEMs specifically as a maintenance and troubleshooting tool.

A key phrase to remember is “consistency over accuracy!” This is because measuring the same unit consistently over time will be what helps you be a more effective IEM pro. Basically, once your measurement rig is built and you know how to use it, you measure/save a “golden trace” when you know that the particular IEM is in 100% working order. From there you’ll be able to measure against that “golden trace” either periodically or when an issue is presented. You’ll also be able to diagnose signal chain and processing issues the same way.

The Rig

All you need is a microphone, audio interface, and measurement software. It’s that simple. You’ve probably seen it in every FOH world being used to measure the sound in the room; same principle here just miniaturized.

Microphone

The measurement microphone is probably the most debated topic when it comes to the measurement rig. For this we’ll keep it very simple. In 1981 the International Electrotechnical Commission (IEC) specified the standards for an “occluded-ear simulator, intended for the calibration of insert earphones in the frequency range 100 Hz to 10 000 Hz in terms of the sound pressure at the eardrum“¹ called IEC 60711. This was later updated in 2010 by IEC 60318-4. “The main changes with respect to the previous edition were extension of the usable frequency range to 100 Hz – 16 000 Hz, and addition of values of maximum permitted expanded uncertainties to all tolerances.“²

These standards, referred to colloquially as the 711 coupler, are widely accepted by everyone from manufacturers to reviewers and even some of the most prolific preference research conducted by Harman International. In the building we use products made by GRAS when developing and quality controlling our IEMs. Although some of the more accurate couplers can be quite expensive there are many to choose from. We like using the Audix TM2 in the field because of its ergonomic design and the ability to be powered by phantom power without needing a power supply.

Audio Interface

The interface is the easiest bit. Mainly look for something that you’re comfortable using and that has multiple inputs and outputs; preferably a high-quality (low output impedance) headphone amp as well. The reason you need multiple ins/outs is because we’ll be using a transfer-function measurement to test the IEMs.

Software

While you can use everything from free audio analysis software to costly systems like Audio Precision, we often use software by Rational Acoustics called SMAART. All of these are what’s called an FFT measurement.

FFT audio measurements involves transforming a time-domain audio signal into its frequency-domain representation using the Fast Fourier Transform algorithm. This process reveals the frequency components present in the signal. Unlike simply looking at the waveform of a signal, FFT audio measurement provides a detailed view of sound by showing the amplitude of every frequency present in the signal, rather than just the overall signal amplitude.³

The “transfer-function” we alluded to earlier is essentially a dual-FFT mode which compares one channel with the other to show the difference. In its most simple form you use a signal like pink-noise, looped back into your interface as your reference, and measure your device playing the same pink-noise signal into the microphone. Based on what you choose as your reference signal and your measurement signal, you can get detailed measurements of your entire signal chain.

The Measurement

Once you get to measuring your first thought might be about why it looks so different. In pro-audio we’re used to seeing measurements of speakers, PA systems, and even microphones. IEMs and headphones will look a lot different, don’t let that scare you, it’s on purpose.

Acoustic Impedance of the Human Ear

We bypass the structures of the outer ear (pinna) when we use in-ear monitors. Our ears have natural resonances that contribute to the way we’re used to hearing things. There’s research dating back to the 20s, and maybe earlier, but this handy chart from 1974’s Handbook of Sensory Physiology distills it nicely. The concha and the ear canal/ear drum have peaks around 5khz and 2khz.⁴

Source: Handbook of Sensory Physiology 1974. Ch 14, pg. 468

Depending on the sonic profile of a particular IEM, you will probably see variations of these “bumps” around those two regions. This helps make the IEM sound more natural compared to the way we’re used to hearing things in natural environments.

Our Targets

Along with this acoustics research there is also the well-known preference research conducted by Harman International referenced earlier. In the early 2010s they conducted listening/measurement tests, using the 711 coupler, that resulted in their “Harman Target” that is meant to represent the preference target of at least 64% of the general public.⁵ Although this is extremely helpful, along with other preference research, this still isn’t a cheat code for designing IEMs. One major reason why it can’t be blindly followed when designing an IEM’s sonic profile is in how heavily smoothed the Harman Target is, presenting issues when trying to match it in the high-frequencies because of how much deviation you’ll typically see above 5khz or so.

Source: New Headphone Targets Defined on teh Brũel & Kjær 5128, Dr. Sean Olive 3/10/2024

The sound of 64 Audio’s IEMs take all of the research, measurement data, and predictive tools into account but ultimately it is our ears. We do have internal targets that are derived from all of this, but again it’s mostly our ears that define that internal target. You will definitely hear a “house sound” when listening through our range of custom and universal fitting in-ears. Learn more about which IEM might be right for you in this POST.

Coupler/Position Variability

All of that to say, don’t get too caught up in the shape of the graph. Remember, we’re using comparative measurements as an IEM maintenance tool rather than being able to see exactly what they sound like. Things like IEM positioning, ambient noise, and even the coupler you choose can have massive effects on the data that don’t always correspond with what you’d realistically hear.

Comparing two ear-simulators measuring same IEM.

Bonus Uses

The most practical day-to-day use of a measurement rig is actively monitoring mixes, observing processing changes, and measuring loudness.

You really have to get a handle on the mechanics of your FFT software to set yourself up for the first two of those daily uses, but for measuring loudness you’ll also need to calibrate your system with something like the Brüel & Kjær Type 4231.

Accurately measuring loudness can be one of the best ways to keep yourself and your artists within healthy listening levels which have been established by official guidelines for decades.

Comparing raw dB SPL and A-Weighted dB SPL

Tip of the Iceberg

Feel free to check out some of the references below, as well as the many AES papers behind their paywall that Sean Olive sites in his Acoustics Today article. We hope this will help you add another valuable tool to your tech arsenal and make your job, as an IEM pro, easier.