Disclaimer: Yet again, Inks generously let me borrow his pair for analysis. I owe him greatly.
Measurement has been carried out with a pair of smallest Westone STAR sleeves, which are green-coded.
Released in 2009 along with UM3X, ES3X, and Westone 1, Westone 2 is a dual-driver IEM featuring the manufacturer's 50 years long experience in IEM industry. Designed for personal listening, this IEM is highly unnoticed by headphone users, despite the manufacturer's claim of "ultra-accurate ... the most dynamic sound available in a universal fit earphone".
PRO: Smooth frequency response, well-extended bandwidth, warm decaying bass, and low distortion.
CON: Inverted polarity & extremely non-linear impedance characteristic ranging from 16 Ω at the DC to 160 Ω at 7.2 kHz.
ON SECOND THOUGHT #1: Shallow insertion is the key to achieve maximum linearity in the treble with this IEM.
ON SECOND THOUGHT #2: Due to Westone 2's non-linear impedance characteristic, additional output resistance sets off the overall tonality. While deep-insertion may help offset the harshness by shifting the quarter-wavelength resonance at 9 kHz to the frequency range above 10 kHz, any impedance value higher than 33 Ω will make the IEM sound overly sharp.
ON SECOND THOUGHT #3: Acoustic damper modification is definitely an effective way of tuning the tonality of Westone 2, but it shall be noted that the damper only works as an acoustic low-pass filter, essentially cutting off the treble even with 330 Ω gray dampers.
ON SECOND THOUGHT #4: Westone's STAR sleeves, which now come with the IEM as stock sleeves, offer a great versatility when it comes down to sleeve choices; However, although users have a wide range of selection, it is important to note that the sleeves larger/thinner than the green silicone are to be completely avoided as they tend to introduce linear distortion and a bandwidth cut-off, especially when the electrical damping modification is desired for bass reduction.
Above are my recommendations for Westone 2 users. The blue curve is probably the flattest the IEM can possibly become, but it is a bit tricky to obtain such linearity as you need a pair of red Monster sleeve couplers, a pair of Grado single flange sleeves, a 33 Ω resistor connected in series, and deep-insertion. If users find Monster adapters and Grado sleeves hard to come by, they can use a pair of Shure Olives, Comply T-130, or any other sleeves with the bore opening conically-tapered instead, so that the quarter-wavelength resonance is effectively damped according to the horn effect.
|Westone 2 + Monster adapter + Grado sleeves + resistor|