Disclaimer: This IEM is owned by my friend, James. His curiosity is as good as mine!
Ocharaku(音茶楽) Inc. is a tea store located in Tokyo, Japan. Not quite sure why, but the company somehow got into the in-ear monitor industry with an unique brand name, 音茶楽Sound Customize.
According to Ryo Yamagishi 山岸亮, the representative & the director of acoustic research at Ocharaku Inc., Ocharaku Sound Customize's ultimate goal is to create a gentle sound signature with high frequency well-extended, derived from a ground-breaking technological innovation.
And that is how Flat4-SUI is created, after Full-range 2-element 4-way effect, with two dynamic driver connected with an acoustic transfer tube, which is named as Phase Correction tube. The placement of the tube is claimed to be precisely calculated in order to achieve optimum performance of the IEM.
Yamagishi-san claims that Flat4-SUI's double driver configuration not only expands the low frequency bandwidth by cancellng off any anti-resonance induced mechanical vibration, but also improves the sensitivity of the low-mid frequency range. Moreover, while the proprietary Phase Correction tube offsets acoustic interference caused by closing off the ear canal, without using any acoustic resistance at the output of the IEM, an ultra high frequency reproduction is acquired.
And of course, Yamagishi-san does not end there. By utilizing a driver with no tangential grooves corrugated on the diaphragm, Ocharaku successfully reduces distortion in the mid-high frequency range. If all five of these claims happen to be for real, it is no doubt that Ocharaku would surely become one of the most technologically advanced headphone manufacturers, along with Sennheiser and Etymotic Research.
PRO: As the rear driver kicks in with time-delay, the IEM has a funky after-resonance in the bass, though the claimed sensitivity increase is not observed.
CON: First of all, the anti-resonance at 6 kHz caused by the acoustic tube sucks up sound by leaving a sharp notch, which is later claimed back with a slight time-delay. Second, the treble is extremely harsh in virtually uncontrollable manner. Third, the high frequency is surely well-extended, but it is masked by resonances at 5.4 kHz, which is the mechanical resonance, and at 8.3 kHz, which is a quarter-wavelength resonance.
ON SECOND THOUGHT #1: Please refer to this article for the assessment of Ocharaku's claims.
ON SECOND THOUGHT #2: Just like how Ocharaku's patent claims, Flat4-SUI's high frequency response becomes most linear when it is inserted shallow, of which the IEM barely blocks the ear canal; The half-wavelength resonance pumps up the notch at 6 kHz. However, due to resonance shift caused by shallow insertion, the high frequency is now cut-off above 15 kHz.
ON SECOND THOUGHT #3: Whether there is additional acoustic resistance applied or not, the harshness is still there.
ON SECOND THOUGHT #4: There are total of three acoustic vents on Ocharaku Flat4-SUI, and all of them are placed to knock off certain resonant peaks. However, blocking the front and side vents is recommended, as it would effectively boost the 3 kHz dip, while taming the bass.