Disclaimer: This pair of IEM is generously provided by Robert. Thank you, Robert!
Measurement has been carried out with a pair of stock biflange sleeves, 3 mm away from the reference plane.
Made in France, Earsonics SM64 (MSRP: €399) is a triple-driver IEM engineered with the technological know-hows the manufacturer has continously accumulated with their custom models: EM4 and EM6. Earsonics claims the IEM is capable of an "absolute control of the audio spectrum, incredible stereo image and maximum headroom, letting you crank it up without distorting the sound." Surely, the specification indicate it can output 122 dB SPL at 1 mW, even with its high impedance value of 98 Ω.
Moreover, Earsonics states, "With its new HQ crossover process with impedance corrector and drivers combination, the SM64 provides an equilibrate spectrum at any intensities that make it very musical and accurate. Having the same circumaural headphone comportment".
PRO: Nice bass & The proprietary impedance corrector retains the overall impedance characteristic quite linearly at 40 Ω.
CON: There are quite a few, thus they will be discussed separately below.
#1: The entire frequency spectrum is disconnected in half by -24 dB @ 5 kHz relative to 1 kHz, suggesting Earsonics' crucial mistake in the crossover network design. The degree of the notch is actually worse than that of Heir Audio 3.Ai, which is about -21 dB @ 3.6 kHz relative to 1 kHz.
#2: Not only there is a substantial interchannel level mismatch above 10 kHz, but also the interchannel phase is awfully mismatched above 5 kHz, as if the tweeter driver on one channel is soldered inversely, resulting with a funky & distorted soundstage. The problem here, unless this very unit is totally defective, is actually worse than that of Novodio iHX.
#3: The 3rd order harmonic distortion barely stays below my tolerance limit of 1%.
ON SECOND THOUGHT #1: Insertion depth doesn't play much important role with SM64, as the treble is nonlinear as is.
ON SECOND THOUGHT #2: Adding a serial resistance to the IEM boosts the bass response, while slightly cutting off the bandwidth.
ON SECOND THOUGHT #3: The acoustic resistance of SM64's stock white damper is 680 Ω. Whichever the damper is used, linear distortion caused by the crossover network mismatch simply kills the overall tonal balance.
ON SECOND THOUGHT #4: Above comparison has been obtained at the reference plane.
ON SECOND THOUGHT #5: All of the issues listed above are well worth a systematic recall from Earsonics, or a lawsuit from SM64 buyers, since the measured data directly contradict what the manufacturer claims. I've never even seen/heard of a headphone designer, who would introduce a -24 dB notch in the frequency range at which the human hearing becomes the most sensitive. Here's my personal rant to Earsonics:
If you don't know a damn about designing an electrical circuit, please go back to school.