It produces a crystal clear sound like no other in the Flat4 series, and unlike ordinary headphones, the sound quality and appearance evolve with the accumulation of age, providing a truly unique listening experience.


Made with Japanese Wooden Craftsmanship

The center cabinet is made of Japanese zelkova - a wooden material with the highest speed of sound in its category. Coated with a layer of natural raw lacquer, which is a key element contributing to the impressive aesthetics and the most crystal clear sound ever produced by the Flat4 series.

Each piece is specially selected and handcrafted by master craftsmen from OAK VILLAGE, a woodcrafts company established in 1974 in the forest of Hida-Takayama.

Features of Flat4 Keyaki/Akakeyaki Plus

A phase correction tube, a key technology of Ocharaku detailed later, applies vibration-resistant coating to suppress the howling noise. It realizes minimizing sound deterioration and warmer and softer high-tone range.

Applies a FURUTECH Co. non-magnetic pure copper rhodium plated 3-pole stereo mini plug, which ensures higher quality and enhances crystal clear sound.

Differences Between KEYAKI and AKAKEYAKI

Flat4 Keyaki/Akakeyaki Plus can be divided into two types, depending on the color of the phase correction tube:
Black is for KEYAKI while Red is for AKAKEYAKI.

It also can be divided into two types, depending on effective length of the ear canal: KEYAKI is set for 28mm while AKAKEYAKI is set for 30mm.

The length of the ear canal is generally referred to as 25 to 30mm as shown below, and the length set for Flat4-KEYAKI Plus is relatively appropriate for more people than that of Flat4-AKAKEYAKI Plus. Sound quality will be better if the effective length gets closer to your ear canal length.

References on the ear canal length 

The Human Ear Canal – V (
Ear canal (Wikipedia)


"FLAT4" Describes How Ocharaku Technology Works

"FLAT4" is an abbreviation of "Full range 2-element 4-way effect," and it shows how Twin Equalized Element System, one of their patented technologies, works. It realizes the wide frequency range, especially delivering the bass deeper than conventional closed dynamic headphones, as well as the spread of sound just like open-air ones.

Along with that, it dramatically proves two problems conventional canal headphones have: characteristically high-intensity noisy sound and masking effect in certain specific high-tone range both mainly caused by the closed tube resonance of the ear canal.

Followings are the details of how "4-way effect" works.  

1st effect

Placement of two elements ("2-element") opposed to each other cancels mechanical vibration due to reaction of vibration system. It realizes deeper bass tones than that of conventional closed types.

2nd effect

In low and mid range tones, parallel drive of the twin elements enhances sound pressure sensitivity.

3rd effect

The phase correction tube suppresses the closed-ear canal resonance and eliminates the masking effects, dramatically improving sounding in mid- and high-tone range.

4th effect

By eliminating acoustic resistors and applying small elements, it dramatically improves ultra-high-tone range. Also it realizes the spread of sound like open-air types.



Cabinet Front & Rear: Liquid crystal polymer / Center: Solid Japanese Zelkova coated with raw lacquer
Element 010e002 Φ10mm Dynamic Type x 2
Acoustic System Twin-Equalized-Element System
Sound Pressure Level 104 dBSPL/mW
Frequency 3.5Hz to 45kHz (High-resolution compatible)
Maximum Input 400mW
Impedance 18 Ohms
Weight 26g (approx.)
Cable 1.2m (Y type) Quadruple Conductor Cable
Plug Φ3.5mm Rhodium Plated Stereo Mini Plug


  • Comply T-200 Ear Tip x 2 (Size: M, L / M is attached to the main unit)
  • SpinFit Ear Tip x 3 (Size: S, M, L)
  • L type Cable Adapter
  • Cleaning Cloth
  • Wooden Casing (Japanese Zelkova & Paulownia Solid Wood)
  • Instruction Manual / Warranty Card

Other Features

  • Applies tangential-less vibration plates, reducing the distortion on mid- and high-tone range.
  • Strong magnetic circuit with neodymium magnet, pure iron, and yoke improves sensitivity and sound quality.
  • Applies specific CCAW voice coil, greatly improving input resistance.
  • Applies liquid crystal polymer on the cabinet, enhancing slightest lingering sound and beautiful resonance.

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