IEM Targets: One Last Word

Let’s discuss IEM frequency response targets — probably for the last time on this blog. I’ve previously touched on this topic before. Many fellow headphone fans are putting a lot of effort into making sound objective and finding the ideal frequency response target. But I think there’s one oversight that hasn’t been researched enough yet.

If you have not yet researched IEM measurements yet, I recommend you start here. In this early-2020 article I cover the basics for an easy kickstart. From there, I might recommend my approach to finding my personal sound preference in this post. I will refer to it later again.

Coupler Standards and Targets

First of all, the following is based on the IEC 60318-4 coupler, which approximates the ear canal and eardrum response — but not the pinna. The ISO 11904-2 standard tells us how free field and diffuse field neutrality measure on this coupler.

Now, I’m not sure if this is part of any “approved target” yet. Diffuse field is generally more accurate to how sound is perceived because it takes directionality out of the equation. However, I guess 99% of users still place their speakers in front of them — which, in theory, is more akin to the free field target (basically an anechoic chamber).

There are some issues with free field, though: our head is constantly moving, and our speakers are often angled, not directly in front. Even worse, every tiny adjustment of the FF measurement yields very different results. So FF cannot stand as the reference.

Blending the Targets

Ideally, we want to blend the two targets. There’s some guesswork in how they should be weighted. Directivity measurements of speakers usually show that the low-mid woofer has little influence on directivity — but maybe a bit, even down to 100 Hz. I’ll leave it to someone smarter than me to create a proper formula based on several approved loudspeakers. But a blend, assuming DF is more important but FF increases towards higher frequencies, could look something like this:

Factoring in Room Gain

So now we’ve considered the directivity of a neutral target, but we haven’t actually accounted for in-room behaviour. The lower the frequency, the more likely it is to bounce off surfaces and multiply at the receiver’s end. A good start is to consider 1 dB per octave, or a total of about 10 dB from 20 Hz to 20 kHz.

The better the room is treated, the less of a downward slope should be considered. Headphones.com adjusted this to 0.8 dB/oct, but my observation in sales is that many people prefer more warmth — maybe because they’re not used to listening in a reference studio. In this case, however, I’ll stick to a very good studio approximation, e.g. 1 dB/oct.

Introducing the HFX Target

Fortunately, my writer colleague actually did all these steps already — back in 2017. This became known as the HFX Target, named after our audio blog Headflux. Here it is plotted:

If you are looking for perceived-neutral, this might be the target curve. It works very well for full-size headphones, btw, and is not only a reference for my measurement database. Together with my mnemonic (triangle from 1kHz to 10kHz with a 10dB peak at 3kHz), I hope this makes reading raw measurements easy.

SPL and Equal-Loudness Effects

Now, there’s one more issue that needs to be addressed — and I see it mentioned far too rarely.

I think the above target would be a great starting point for studio use. But let’s not forget: studios are loud. I’ve gotten to know a couple of industry veterans and successful engineers in mixing and mastering. Every time, I’m surprised at how loud they listen to music — well, volume is needed to perceive full dynamic range, and it actually makes it easier to hear detail.

However, knowing the IEM scene, I can assure you: the majority does not listen at the same SPL! While most studios are calibrated to pink noise at 83–85 dB SPL, I’d reckon my listening volume through IEMs is closer to 70 dB SPL.

We can find data on how this affects perceived frequency response — ISO 226 offers some reference curves, though the effect might be different for sealed IEMs compared to free field. Anyway, it’s the best we have right now, so here’s the final result for listening levels at around 72 dB:

So, Is This the Ultimate Target?

This looks quite reminiscent of my personal preference curve and supports my claim that a tacked on bass boost with strong shelving should sound incoherent. And now what? Should we use this target as the reference?

Absolutely not.

At a minimum, I think warmth should be an adjustable variable instead of a fixed value, not only because of listener preference, but also to account for the mastering stage — where music is finalized for the consumer while the artist already expects some kind of acoustic distortion at the receiver’s end.

The equal-loudness contour could be pre-programmed in a DSP to take out the guesswork. But a lot of the research would have to be refined for audiophile use — and not as a one-size-fits-all approach. (But let’s be honest: if you want to make a profit from research, it makes more sense to invest in hearing disabilities than to please a niche golden-ear crowd.)

Comparisons

Referred to as “My Personal Optimal Frequency Response”, the result of constructing an audiophile reference for IEMs, here is my 2020 curve, based on equalizing several IEM to my preference, held against this more scientific approach. In my opinion, they match rather nicely.

Of course, I am not the only IEM fan and there are others who like to publish preference curves. Most noticeably, Crinacle from In-Ear Fidelity and his recent 2025 IEF Preference Target. Here are the two compared. Concerning overall balance, I think they match up well, with some distinct differences regarding the bass shelf and also the treble.

Final Thoughts

I hope this is helpful and provides an alternative to the “meta targets.” This target curve goes a little bit further than “sloped diffuse field”, but is solely based on available research data – with very little user bias. I wanted to share this a long time ago, but:

1. As mentioned, the groundwork was not done by me, though I read up on the AES papers. I was hoping for a comeback for Headflux and it would have been a good welcome post.
2. I thought I would be able to use this target myself someday. But as mentioned, I have left the audio business.

In the end, I do hope manufacturers trust their own ears more than the frequency response (especially of a 711), because:

1. We already have way too many IEMs that sound way too similar.
2. There are so many variables in a target that it would be a shame if manufacturers didn’t leverage the creative freedom of what can still be considered well-balanced.

And I haven’t even touched on HRTF variance…