FiiO kindly sent me pre-release versions of 2 of their new IEM’s, the FH11 https://www.fiio.com/fh11 (1 DD + 1BA drivers) and the FX15 https://www.fiio.com/fx15 (1 DD + 1BA + 4EST drivers). Of these the FH11 is a very bass heavy IEM at roughly $50 with, in my opinion, an odd tuning (I asked FiiO about the tuning and apparently it specifically targeting the Chinese market ) but the FX15 on the other hand is a much interesting IEM, specifically given it has 4 Electrostatic drivers per ear.

The FX15 is FiiO new flagship IEM and it’s expensive at roughly $800, but in the world of flagship IEM’s with Electrostatic drivers it’s debately reasonable priced. So I was curious would it measure well and once I got the measurements, I thought I would share them with the community here. I will also share some my opinions about it and my EQ recommendation later on as well as much technical information and photos as I can.

It has a lovely ‘smokey’ colour shell and a slightly ‘opinated’ shape and an A/B testing switch - more on the switch later on. Overall it comes in an very nice box with lots of accessories - I will also provide more pictures and details about the accessories later in this post.


Note: The cable has MMCX connectors but it is top quality with both 3.5mm and 4.4mm connectors.

FiiO FX15 Measurements

Note: These graphs are based on RAW measurements from a BK-5128 - Fiio provided the raw data and I have attached the various speadsheets for people to do their own analysis. I asked FiiO if they had a Gras coupler for a second set of measurements but they did not.

First - the RAW Left and Right channel with the switch on + with the EST ‘switch’ off and with a DF for comparison (Later on I will present a different set of FX15 measurements with a Harman Target for EQ purposes).


So, hopefully it’s clear from this that we have a somewhat V shaped sound signature with certain treble frequencies emphasised while others are recessed. I believe this is to show off the EST. Also, as we can see the A/B ‘EST on/of’ switch effectively turns off the treble. Obviously, FiiO wants to show off the EST using this switch but other than an initial test it is mostly a pointless switch so I would leave it on all the time.

In the absence of a viable way to compare with harman and normalise with BK-5128 data, I decided to borrow some other BK-5128 raw measurement data from https://www.crinacle.com and present a comparison with the FX-15 so people might get a good idea of the sound signature against more ‘harman’ compliant IEM’s. So I picked the Truthear Zero Red (with and without the bass adapter) and the Moondrop Variations. Obviously when comparing one set of measurements with those done by different people in different environments (probably using different insertion depths etc) it’s not ideal but I thought it would be somewhat useful to provide a overview of the sound signature.


This probably illustrates the V shaped signature more. Obviously picking a suitable frequency to normalise makes a huge difference in these graphs but this seems to be the fairest and nearest I could get using REW to @amirm typical 450Hz for comparative purposes.

I have a section later with my EQ recommendation but for now onward with some more measurements:

So next I thought I would look at the THD Ratio, I got 3 sets of measurements at 94DB and at 104DB for each channel sorry no 114DB:

THD Ratio

Note: I asked FIIO why the 94DB has slightly higher THD at higher frequencies and the said they noticed this but were not sure why. Given this are the frequencies that are handled by the EST drivers it is probably related. The FIIO engineers told me that EST’s start at 6.5Khz while the BA goes up until 9khz. I will share some technical ‘marketing’ material they shared later on about how the DD + BA and the EST were tuned.

So the final 2 measurements are derived from the RAW Phase response data in the attached spreadsheets:

Phase Response

Time Delay Respose

For those interested - I created a separate post here about how I generated these last few charts from the RAW data.

Features and Accessories

FiiO provide an excellent set of add-ons for this IEM within the Box. I really like the quality of the cable and the fact it has swappable 3.5mm and 4.4mm balanced connector with the cable, but it also comes with an amazing amount of tips, include a separate box full of tips with different boar sizes for tuning as well as 2 bags of tips. The leather carry case is also nice but obviously too big to put in your pocket.


Here is FIIO official photo of the accessories:


Tuning Opinions and EQ:

I briefly played around with the EST switch - but its too hard to switch when you are wearing the IEM so it can’t really be used as an A/B switch (unless you get someone else to switch it for you) but it effectively turns off the EST so it sort of a pointless switch. FiiO obviously recommend using different tips to do some fine tuning. I mainly used my regular Tips (same as I use on the Truthear Zero’s, Salnotes, Moondrops and Kiwi Ears IEM’s that I have) as those tips give me a better seal.

Having mainly listened to Neutral and Harman Target IEM’s over the past year, the tuning on the FX15 is a little bit of a shock. Given I knew I was going to be listening to an hybrid IEM with Electrostatic drivers I had initially pre-selected a playlist full of tracks I thought would challenge or show off what this IEM could do in the treble department. So for me that was lots of percussive tracks with a symbols/high hats and lots of sub-bass and in most cases these tracks on the FX15 were pretty great, e.g. Billie Eilish “When we all fall asleep” Album sounded great, as well some classic test tracks like Bjork’s Hunter or James Blakes Limit to your love, but with other tracks it was just too much, e.g. classic Portishead Dummy album Sour Times I had to stop after a while. Not fatiguing but just too much mid bass boom in certain tracks. I then went back with more normal playlist of test tracks and I immediately decided I would need some EQ as classic test tracks like Tracy Chapman Fast Car had slightly too much bass other tracks were tonally different enough for me to feel this isn’t right. I do think the DD providing the bass is very good quality, it mainly the BA tuning that is problem IMO.

So onto EQ, I decided to leave the sub-bass and the treble along and just fix the 2 glaring issues with the mid-bass and the mid-range - this squig.link comparing the FX15 with


Filter 2: ON PK Fc 180 Hz Gain -2.9 dB Q 0.600 Filter 3: ON PK Fc 1388 Hz Gain 2.4 dB Q 1.300 Filter 4: ON PK Fc 2400 Hz Gain -4.4 dB Q 3.800

So with these 3 filters, I believe I fixed the V-Shape to get the best out of this IEM still leaving its best features the DD Bass and the EST treble. But this will be very subjective so it’s only my recommendation.

Some Technical information about the FX15

Note: This information is straight from FIIO technical / Marketing team - so take it as such.

First an interesting section about their measurement and test equipment:

Our facilities are equipped with a full set of advanced development and testing tools, such as an oscilloscope worth about 200k RMB, a B&K simulation head worth about 300k RMB, two Audio Precision analyzers worth about 500k RMB, and a newly-established OTA anechoic chamber and microwave shielding chamber with corollary equipment worth a million RMB in total. And I thought this was interesting about the tuning of the FX-15: The sensitivity of EST driver is low, and 4 of them in one unit only have 99dB sound pressure after being amplified through the transformer, while the average sound pressure of conventional dynamic and balanced armature drivers goes up to 110dB, much higher than that of EST drivers. Therefore, the important and main mission during the FX15’s R&D process is to balance the sound pressure and energy between the 3 different drivers. To enable the sound of the EST driver in the headphone system, we can either improve the sound pressure of the EST driver, or lower other driver’s sound pressure. Therefore, we directly chose 4 EST drivers. Since the dynamic driver and balanced armature have high sensitivity, if we use more of them, it would be more difficult to achieve good tuning and cover up the original sound of the EST driver. So we only achieve an overall sensitivity of 103dB for the FX15, which has lower sensitivity than conventional BA and dynamic driver headphones, which requires to use the FX15 with music players with higher output power to fully release its potential. The FX15 impedance is only 240Ω. Its sensitivity is 103dB/mW.


I gave the FX-15 an overall pragmatic rating of 4 its price while very reasonable for EST driver based IEM’s is expensive and only for people who really want to experience that little bit of EST treble, but it measures well and has amazing features and accessories.


The FX15 is a flagship IEM that comes in a great package with nice accessors, where the star of the show is the EST drivers. But it is an expensive as those EST’s obviously drive up the cost significantly. I think its V-Shaped sound will appeal to lots of people but it’s not quiet for me. I personally think without EQ I would only use this occasional at best, i.e. if you want to try a different tuning on a new album that I know will work well with this sound signature, but only for a few hours. But with EQ, I think its pretty great and I have been using it for a week with my EQ above and really enjoying it.