I measured the Oppo PM-2s using a G.R.A.S. 43AG ear/cheek simulator, a Clio 10 FW audio analyzer, a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface, and a Musical Fidelity V-Can headphone amplifier. Measurements were calibrated for ear reference point (ERP): i.e., roughly the point where the axis of your ear canal intersects with your palm when you press a hand flat against your ear. This is a “flat” measurement; no diffuse-field or freefield compensation curve was used. I experimented with the fit of the earpieces by moving them around on the plate of the ear/cheek simulator, and settled on the positions that gave the best bass response and the most characteristic result overall.
The Oppo PM-2s’ response is similar to those of most of the planar-magnetic headphones I’ve measured, with one noteworthy difference. Most planar-magnetics are flat to about 1.5kHz, above which frequency their response begins to rise. The PM-2s’ response starts rising gradually at about 500Hz, which means that their midrange should sound +3 to +5dB fuller.
Adding 70 ohms output impedance to the V-Can’s 5-ohm output impedance, to simulate the effects of using a typical low-quality headphone amp, does not significantly affect the PM-2s’ response. Thus, its tonal balance will be the same from any source device. That’s especially good for these headphones, which are designed to be used with portable devices.
This comparison shows that while the PM-2s produce considerably less treble energy than the HiFiMan HE-560 or HE-400i headphones, compared to the PM-1s they have roughly +2dB more treble from 2 to 3kHz, and +3dB more from 6 to 8kHz. The PM-2s seem to strike the most even tonal balance of all four models.
The spectral-decay (waterfall) plot shows slight resonances around 2kHz. However, these are so low in level, at about -40dB, that there’s little chance you’ll hear them, especially considering that external sounds leaking in through the open backs of the earpieces will likely be much higher in magnitude.
Distortion at 90dBA from the PM-2s was almost nonexistent, except for a rise to 2% at 400Hz. At 100dBA -- an extremely loud listening level you probably couldn’t tolerate for long -- the distortion rises to 4% at 400Hz and 3% at 230Hz. (Both of these peaks correspond with peaks at the same frequencies in the impedance measurement.) I heard no distortion when I was testing the PM-2s.
For open-back headphones, the PM-2s deliver better isolation than average. There’s no isolation below 1.5kHz, but it’s -5 to -15dB from 1.5 to 10kHz. However, most closed-back models deliver isolation in the -10 to -30dB range.
The PM-2s’ impedance is almost flat, with a magnitude of 31 to 32 ohms and negligible phase shift. There are a couple of small impedance “bumps” that correspond to the peaks noted in the high-level distortion test.
The PM-2s’ average sensitivity from 300Hz to 3kHz at the rated 32 ohms measures 103.7dB. That’s +2.1dB more than Oppo’s PM-1s, and roughly in the average range of over-ear headphone sensitivity -- very impressive for planar-magnetic headphones.
. . . Brent Butterworth