Fit and Comfort

There are subtle fit differences between the different ear tips of the same size included with the LETSHUOER S12. It's hard to see on the camera front-on, however, so I am only showing an example with the size M black bore silicone tip installed on the right ear bud and inserted into an artificial ear mold. I have average-sized ears, and the ear mold above represents my own experiences well enough as a proxy. Size M silicone tips are my go-to for testing since foam tips are not included by some, and in this case, the black bore silicone tips worked best not just in terms of a good, secure fit with decent passive isolation, but the bore and flange combination making the sound signature more balanced. The good thing is that the S12 shells themselves are quite small and ergonomically shaped, meaning they are quite likely to fit into most ears without issue. The gentle curves and smooth exterior also result in comfort over longer periods of time, and the metal chassis is cooling to the touch. On the flip side, if you are in a hot environment outdoors, it may get warm. The cable is also angled comfortably away with the pre-formed ear hook in mind, which had the cable provide another point of support for the IEMs. The shells weigh ~6.1 g each, which is still below average—as such, the mass itself is not going to cause physical fatigue, either.

Audio Performance

Audio Hardware

The minute LETSHUOER mentioned there's a new planar IEM in development and the driver was in the 14-mm size region, people got excited, thinking it may be the same driver as in the 7Hz Timeless in a smaller shell and from a brand with actual customer support. As it turns out, the LETSHUOER S12 is not using the same driver. In fact, it is even larger (14.8 vs. 14.2 mm) as far as the transducer goes. If you need a primer on how planar magnetic drivers work, let me direct you to the equivalent page in this review. The transducer size is typically what the numbers reflect, and the near 15-mm driver is among the largest for in-ear monitors. It's also in line with analogous headphone drivers being massive relative to dynamic drivers, with the larger driver size capable of moving more air back and forth as part of the sound signature itself. The transducer (diaphragm) has to be backed by the magnets and traces on it, though, and here LETSHUOER shares very little information to where the drivers are evidently sourced from a third-party factory, which is typical with such products. Another commonality with headphone planar drivers is the below average impedance of 16 Ω paired with a relatively low sensitivity of just 102 dB/mW, which makes these IEMs more power hungry than you'd imagine. A good portable DAC/amp will still suit your needs just fine when without an available 3.5 mm audio jack on the go, and I'd certainly pick one over a standard dongle adapter to make sure there's no source-based bottleneck. If you are going to use the LETSHUOER S12 with a desktop-class source, the shorter cable included with IEMs might be a potential handicap, though.

Frequency Measurement and Listening

I will mention that I have a general preference for a warm-neutral signature with a slightly elevated bass, smooth treble range, detailed mids, and good tonal separation. I also generally prefer instrumental music over vocals, with favored genres including jazz and classical music.


Our reproducible testing methodology begins with a calibrated IEC711 audio coupler/artificial ear IEMs can feed into enough for decent isolation. The audio coupler feeds into a USB sound card, which in turn goes to a laptop that has ARTA and REW running and the earphones connected to the laptop through the sound card. I begin with an impulse measurement to test for signal fidelity, calibrate the sound card and channel output, account for floor noise, and finally test the frequency response of each channel separately. Octave smoothing is at the 1/12th setting, which nets a good balance of detail and noise not being identified as useful data. Also, the default tuning was used for testing, and no app-based settings were chosen unless specifically mentioned. Each sample of interest is tested thrice with separate mounts to account for any fit issues, and an average is taken of the three individual measurements for statistical accuracy. For IEMs, I am also using the appropriate ear mold fitted to the audio coupler for a separate test to compare how the IEMs fare when installed in a pinna geometry instead of just the audio coupler. The raw data is then exported from REW and plotted in OriginPro for easier comparison.


The IEC711 is such that you can't really compare these results with most other test setups, especially those using a head and torso simulator (HATS). The raw dB numbers are also quite contingent on the set volume, gain levels, and sensitivity of the system. What is more useful information is how the left and right channels work across the rated frequency response in the LETSHUOER S12. The left channel was separately tested from the right one, and colored differently for contrast. I did my best to ensure an identical fit for both inside the IEC711 orifice, so note how the two channels are quite similar to each other. In the price range the S12 operates in, getting hand-matched drivers for the two channels is basically unheard of. This is still more than acceptable, and the only place in practice where I even spotted imbalance occasionally was in the 1–2 kHz region. Measurements taken after 30 hours of testing, which included these playing a mix of various songs as well as white or pink noise and sine sweeps, showed no difference. There was no perceived burn-in effect thus, and none was measurable, either. The response with the anthropomorphic pinna in place matches the ideal scenario in the coupler extremely well until the coupler resonance, differing only past where you need to take things with a grain of salt anyway, which is also an indicator of how good the seal was when installed in the artificial pinna.


This is the average frequency response for both channels of the LETSHUOER S12 plotted against my personal target taken from VSG.squig.link, which also gives you an idea of my personal preferences to better correlate any possible biases. The tuning of a set of headphones or earphones does not have to match my target as long as it is tuned with some direction, makes sense, and is executed well. After all, no one set will appeal to everyone, and having different options is what makes this hobby so interesting and hard to quantify. LETSHUOER set a high standard with the EJ07M tribrids, and it seems to have mostly let the S12 speak for itself given the product page is shy with marketing talk. Indeed, outside of claiming these are suited for "electronics, pop, and other genres," there's nothing for me to compare my findings against expectations set forth from the company. The good thing is that there's a factory frequency response graph on the packaging, which is far more useful for gauging the tonality of a product anyway. Based on that, it would be fair to assume the LETSHUOER S12 is going for a more U-shaped response. I will also point out that these IEMs, when properly inserted into the coupler, display resonance just past 8 kHz. Shifting the insertion depth to reduce that peak at 8 kHz also lowered the relative peaks in the treble to where I felt it was not representative of my listening experience. As such, I stuck with the measurements seen above despite the resonance past 8 kHz.

Looking at the frequency response above, it might easily be assumed that the LETSHUOER S12 is easy-going on the bass response. After all, this is still a U-shaped tuning leaning on the brighter side of things. But this is where the planar nature of the drivers enters the picture, with the actual impact in the lower frequencies more than you would think. In fact, I dare say the S12 with its ~8 dB maximum bass elevation relative to the valley centered around 500 Hz sounds like a bass cannon delivering precise shots of energy your way. Sub-bass extension is plenty too, and I can see this being quite good for electronic music, which LETSHUOER markets. There's another trick I noticed, and I wasn't the only one based on a cursory search of other owner experiences. The LETSHUOER S12 takes EQ in the bass frequencies remarkably well, without distorting, so much so that some folks have even added a steel +10 dB bass shelf on top of the existing elevation! That's as much of a YMMV move as any I have seen before, but a bass shelf of +2–3 dB from ~180 Hz down can make this an extremely exciting set with good contrast and slam in the region. As it stands, the mid-bass is executed well enough for instrumental bass notes to flow through quickly without slow decay.

It's actually the mids that are a weakness in my books, partly because the bass somewhat bleeds into the lower mids. Male vocals are forward-facing to begin with, and the bleed-over makes it worse by adding a light haze to instruments in the background. This is why I am not convinced the S12 is especially good at technicalities, with imaging being mediocre at best on tracks with mixed sound sources and soundstage being about average for closed IEMs with a relatively compact and uniform field around the ears—the S12 is more about impact inside than a sense of space outside. Instrument separation is equally in need of improvement, although I suppose music with acoustic guitars and deeper male vocals will still be fine overall. Think James Brown more than Ed Sheeran and you get the idea. The upper mids are better-executed once you realize there's a pinna gain from 1–3 kHz, and female vocals come off more naturally rendered and detailed. Eastern pop music and even opera music will sound quite nice here, but there is a hint of sibilance with some tracks that might be off-putting.

The upper mids lead to an overall brighter signature in the higher frequencies, but don't worry about that intense coupler resonance peak. In practice, it is not as accentuated and shows up at closer to 7-7.5 kHz—it can still be annoying, though. There's a plucky planar timbre with instrument notes here, especially when it comes to string instruments. Piano keys can also feel uneven in how well they are represented. Otherwise, I was happy with the treble extension with no false sense of air. I also thought a complex orchestral piece would play well here, with fundamentals and harmonics getting their due time, but can think of several other IEMs that handle instruments better and have superior tuning in the treble frequencies.

Comparisons

The obvious comparison everyone's going to make with the LETSHUOER S12 is the 7Hz Timeless, so let's talk about that. The 7Hz Timeless is one of those IEMs that does instruments and the treble frequencies much better, as alluded to, which on its own has me wonder why some say the S12 and Timeless are very similar to each other. In fact, I'd say the S12's strength is in the bass, while the Timeless is more balanced out of the box. With EQ, the differences are accentuated depending on what set you prefer. The Timeless also has better imaging, and vocals aren't as forward-facing, either. But the S12 makes up for it in physical size and comfort on top of an appreciably lower price, with build quality and accessories mostly a wash. If I had to pick a planar IEM for ~$200, I'd go with the 7Hz Timeless myself. The LETSHUOER S12 defaults to being the only option for a lower budget, and let's not even talk about the likes of the Goldplanar GL12 I was thinking of adding to this graph simply for laughs.


This time around, I shied away from using frequency-normalized graphs since the various IEMs here are tuned differently enough to where a dB-normalized graph makes more sense. Here, I have three other similarly priced sets as alternatives, including the FiiO x Crinacle FHE:Eclipse hybrid IEMs, another set of hybrids with the IKKO OH10, and a single DD set in the form of the Tanchjim New Hana whose review is coming soon. If you can stretch the budget ever so slightly, the single DD MOONDROP KATO would really be my personal pick of the lot, and it is closer to the S12 in its bass response than the others. The LETSHUOER S12 is the only planar set here, and the timbre is the biggest distinction from the rest. The strumming of guitar strings and impact of percussion mallets hitting drums is more vivid, though in a manner that might seem unnatural to some, but preferable by others. Tuning is clearly all over the place here, and none of these are technical champs either, but the S12 being a bright set is evident, almost to the point of it being harsh. Material composition is also quite varied, ranging from resin to ceramic to heavy copper, and the S12 also just got a price increase the day before I wrote this review to where it does lose out on the value proposition. This is a tougher comparison in itself, and the KATO makes it even harder. I would say that those who like to EQ will have more fun with the S12, and a planar set of IEMs is worth trying out, too.