Этот обзор на русском.

I’ve been trying to come up with something intelligent as an introduction for a long time, but decided to say the hell with it: I just liked the unusual appearance of YU9 U-556. That is, it was clear from the very beginning that it is conceptually something a la Etymotic, but judging by the unusual shape of the enclosures, it seems to be made for normal human ears. Up to a point, of course.

I bought them for $145. At the sale, they cost $116.

All but the sound

The earphones come in a plain black box.

The insides are to my heart’s content:

• the earphones themselves;
• a 3.5mm-jack cable;
• a leather case with fish imprinted on the side (YU is ‘fish’ in Chinese);
• dampers (?) for 2-pin connectors;
• a tool for changing filters;
• replaceable filters;
• 7 pairs of very different eartips;
• a clip for attaching the cable to clothes;
• a microfiber cloth.

Abundant, but with no rubbish. No fluff, indeed.

The earphone enclosures have an unusual (for a beginner in the world of in-ear headphones) and simultaneously understandable (for veteran audiophiles) shape. They are designed for a very deep fit.

The sound ducts are thin and long, and you can see a single armature driver inside the miniature enclosure; it’s Sonion 2356, as I understand. In fact, the earphones don’t have enclosures as such: the minimum number of internal electronic components is filled with resin so that it can be conveniently inserted into the ear canal. U-556 have no decorative elements — there’s simply no space for them.

On the back of the enclosures, there are connector jacks in a separate ‘module’, as well as compensation holes. Note how large these connectors look like when compared to the enclosures.

Filters are inserted into the sound ducts at the front.

The wire is not posh at all, but it does perform its function.

The kit also includes, let me repeat, minuscule rubber bands, or dampers, which are put on the connectors. In the picture below, this thing is put on the connector of the left channel.

To be honest, I have no idea what role they play and what they’re supposed to protect against.

As for the eartips, there are a lot of them, and they all are very different.

There are ordinary 2- and 3-flange silicone tips, medium-sized foam tips, and looong tips made of thermoplastic elastomer (something like AZLA have). The eartips are significantly different in diameter and size.

With transparent triple flange tips, U-556 look even more intimidating.

OK, let’s go to the main question: how do you actually stick them in your ears? And I’ll answer immediately: totally fine! For my liking, YU9 fixed the main mistake of classic Etymotic ergonomics – they made the enclosures broaden towards the entrance to the ear canal and positioned the connectors as befits the behind-the-ear position. Therefore, the earphones don’t get loose in your ears. They fit to a T whenever you turn your head or walk.

Of all the eartips, the thermoplastic triple flange tips turned out to be the most comfortable for me, although they are the longest. They smoothly slide along the walls of the ear canals (oh my word…), that is, they’re put in and out in comfort. They are perfectly soundproof, and you don’t feel them in at all after 5-10 minutes. Foam eartips are even better in terms of sound insulation, but I just don’t feel comfortable to use them.

Joking apart, this fit is extremely specific and unusual for users of ‘normal’ in–ears. It can cause itching or discomort inside the ear canal. But, upon my word, it’s really not as uncomfortable and scary as it’s described around the web.

As for the required power of the sound source in a nutshell, U-556 have a sensitivity of 96.5 dB with a self-impedance of 116 Ohms, that is, they’re significantly tighter and more demanding of the power of the sound source compared to average IEMs. Nevertheless, any portable DAC is enough and to spare for them – I tried them with CB1200AU, and everything sounds fine, let alone players.

The sound

OK, now I’m gonna explain why all this fuss is about shape, fit and so on. The point is that if we consider the auditory canal of a human ear as a model of a cylinder closed on one side and place the sound source on its open base (and direct it inside), then natural resonances will arise. They usually fall at 2.8 kHz, 8 kHz, etc.

A couple of formulas:

The fundamental frequency is F=v/4L where v is the sound speed (340,000 mm/sec) and L is the length of the auditory canal, from 25 to 35 mm. Let’s take 30. Then F= (340,000)/(4*30)= 2,833 Hz. At 25 mm of length, it’s 3,400 Hz; at 35 mm, it’s 2,450 Hz. So, the first resonance should occur in the range of 2,450-3,400 Hz.

Next, to find odd harmonics (and there are no even harmonics in a tube closed on one side), it is necessary to multiply the numerator by n where n = 1, 3, 5, etc. (respectively, the fundamental frequency is at n = 1).

Second resonance (at L=30): F2=3v/4L=(3*340,000)/(4*30)=8,500 Hz.

Third resonance: F3=5v/4L=1,700,000/120=14,150 Hz.

And so on and so forth.

The shift of the sound source towards the inside or outside of the auditory canal will lead to a change in the value of the L variable, a change in the denominator of the fraction and, as a result, a shift in the resonant frequencies. That is why the well-known peak, which we see around 8 kHz on measurements of almost all in-ears, is actually not ‘nailed down’ to this frequency, but kind of ‘floats’ from about 6.8 to 9 kHz, depending on the depth of the fit and the length of the auditory canal.

What does a very deep fit implemented in U-556 do? As you may have guessed, this is a shift of the second resonance to the very edge of the audible range. A deep fit reduces L to some 15 mm. As a result, F2=(3*340,000)/(4*15)=17,000 Hz, and I won’t even count F3.

In other words, the earphones of this design (their sound) don’t have resonances in the audible frequency range if you place them deep enough in your auditory canals.

So, here’s the frequency response of U-556:

We see exactly what we should on the graphs:

• an almost flat response in the range from 20 to 1,000 Hz;
• a nearly ideal envelope IEF curve up to 6 kHz;
• a next to none peak at 15 kHz. This is quite what’s left of 8 kHz. Why not 17 kHz, as in the calculations above? This is because these are measurements on a simulator that has a maximum earpiece immersion depth limited by a metal mesh.
• take note of a subtle emphasis at 5 kHz. This is where the reference frequency has moved: F=340,000/(4*15)=5,6 kHz.

Interestingly, the frequency response abruptly stops at 16 kHz. That’s exactly what the manufacturer promises us as the specifications literally say: 20Hz-16kHz.

The U-556 sound is neutral, with a little bias for midrange frequencies. However, in broad terms, it’s deadly neutral. Everything is shared and shared alike in the earphones’ sound, all frequencies are heard equally well, and nothing is highlighted or veiled. This can be called ‘monitor sound’, and the earphones can be used for mixing – I can easily imagine this use case. This sound can be called ‘boring’ — yes, it’s not Harman, not at all. I tend to think practically: it was very interesting for me to listen to my music library for the 125^th time and analyze how well the tracks I know sound with this sound delivery. And I can surely say that they sound great, all and sundry, even those that are poorly recorded / mixed because U-556 don’t accentuate the dangerous area of 4-6 kHz and don’t have an emphasized ‘top’ after 10 kHz and an inflated bass. I’d like to note that there are also no problems with the perception of subbass due to a very tight fit and excellent sound insulation. These earphones are surprisingly omnivorous in terms of musical content. As for the virtual sound stages, sense of space, and precision of instrument placement, U-556 will deliver this information in the same ‘neutral’ approach: if this stuff is present on the track, you’ll hear everything exactly as it should be. If not, too bad for you.

Here’s nonlinear distortion at 94 dB with the ‘Use harmonic frequency as ref’ option turned off and on. Everything fits into the range up to 1%:

Minimum phase response:

Group-delay:

Spectrogram in the ‘Burst decay’ mode:

Here we can just admire the graphs that are close to ideal.

Comparisons

Comparing these earphones with ‘ordinary’ in-ear models is pretty useless, but I’ll give it a try.

Compared to Etymotic EX2XR (I didn’t measure any other Etymotic):

It’s often said about EX2XR that these are “wrong Etymotic because of the subbass added” — and yes, the subbass identical to Harman’s has been added there. As for the rest, the sound signature is the same, although U-556 have a smaller dip at about 10 kHz.

Compared to TRI I3 MK3 (and these are some of the best neutrally tuned earphones I’ve ever tested):

There is a 5 dB difference on subbass at 20 Hz; I3 have almost no dip at 10 kHz and a slightly longer right ‘tail’, but the middle, identical in shape, is lowered by 1.5 dB. As you can guess, the I3 sound is perceived as slightly ‘colored’ compared to U-556, with an emphasized subbass and a more distinct area of high frequencies.

And, well, I ought to compare them with Truthear NOVA just because of the formal membership of U-556 in the category of $100-150:

This is Harman sound delivery against total neutrality, Black against White.

Summary

I don’t urge you to buy YU9 U-556 right now, but only because of the unusual fit, which may be comfortable not for everyone. I personally have no complaints about the sound of the model — I’m ready to pay twice as much for a similar sound in more conventional enclosures, especially because earphones with a neutral sound delivery are off trend these days.

But yes, the fit can become a problem. The second problem can actually be the sound delivery because it is so neutral that such ‘dryness’ and ‘equanimity’ of the sound is discouraging at first. But after 5 minutes, it starts to seem that this is exactly how everything should sound.

The earphones enter the known rating next to Truthear NOVA. I believe that they’re the best in-ear monitors with neutral tuning in this price category.

And I want to get the YAN model from YU9 for a review sooo much =)