Sony XBA-4

A special thanx goes to my friend, LNO. He also sent me a bunch of other IEMs. They will be analyzed very soon.

Just like its sibling with a triple-driver configuration, XBA-3, Sony XBA-4 is one of Sony's very first IEM models equipped with their own proprietary balanced armature drivers. Since the development is vital for Sony's future IEM production, 投野耕治-san, a deputy general manager of Sony Electronics in Japan, who is also the master of Sony's MDR-series, takes charge of the whole development process.

And eventually, 投野-san comes up with total of four XBA models: 1, 2, 3, and 4.

The model names represent the number of drivers each of the products are equipped with, and the manufacturer's data indicate sound quality goes up as the number of drivers increases. As XBA-4 is equipped with triple drivers that cover the entire audible frequency range and an extra woofer, it should have a good linearity in frequency response along with extended bandwidth up to 20 kHz with extended bass.

"The ability to combine woofer and tweeter drivers in addition to the standard full-range driver, would also greatly appeal to music lovers seeking an even more intensely satisfying listening experience, with bass and treble frequency ranges reproduced in even higher-fidelity." http://www.sony-asia.com/pressrelease/asset/469743/section/regionalpressreleases

PRO: As Sony's proprietary BA drivers' directionality of acoustic transduction is on-axis, unlike that of conventional balanced armature drivers, no phase shift can be observed. Also, the extra woofer seems to be working as it's supposed to, by giving a little more output and decay in the sub-bass range.

CON: The unit itself is one of the bulkiest generic IEMs I've ever seen. Impedance characteristic is not linear at all, which indicates a need for low acoustic output impedance source for consistent damping. Also, the peak at 4 kHz is residual, and is more audible than that of XBA-3. Meanwhile, the harmonic distortion is pretty bad at 1 kHz, which is not occurring with XBA-3. This must be due to change in design in order to incorporate the extra woofer unit.

ON SECOND THOUGHT #1: Deep insertion is not recommended for Sony XBA-4. Once the insertion depth gets closer to the reference plane, the high frequency goes out of control, leaving a very sharp peak at  ~13 kHz. This is possibly due to lack of Sony's tuning expertise, as this is one of their very first IEMs with balanced armature drivers installed.

ON SECOND THOUGHT #2: The overall tonality becomes more linear, but the resonance at 4 kHz becomes quite problematic when under-damped.

ON SECOND THOUGHT #3: XBA-4's acoustic damping scheme is identical as that of XBA-3. Please refer to XBA-3 analysis.

ON SECOND THOUGHT #4: XBA-4's driver configuration is exactly identical to XBA-3, but with an extra woofer, which yields slightly more bass output and decay, added for extended low frequency reproduction. However, with the extra driver coupled, the upper frequency range gets totally skewed due to impedance change: as peaks have became more emphasized, the amount of 2nd&3rd harmonic distortion increases. In contrast to XBA-3's design, of which the drivers are fully optimized for maximum linearity, XBA-4 is an electroacoustic design gone incongruous simply due to the extra unit added in parallel.

ON SECOND THOUGHT #5: The peak at 4 kHz is so problematic, I can't think of any method that can make this IEM linear, not even at least to the degree of which XBA-3 may be turned into. Above combination is probably the best bet, and a parametic equalization of -9 dB centered at 3.6 kHz with a Q-factor of 5.656 should knock off the peak quite precisely.