I measured the Oppo PM-3s using a G.R.A.S. Model 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), roughly the point in space where the center axis of your eardrum would intersect with your palm if you pressed your hand against your earlobe. I moved the headphones around to several different locations on the ear/cheek simulator to find the one that produced the most bass and the most characteristic response. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed.
This chart shows the PM-3s’ frequency response, which is just a little bit atypical in that the usual peak centered somewhere near 3kHz is more like a hill. So it’s a broader boost than usual, and the peak around 7.5kHz is a little stronger than usual. Thus, this measurement suggests a fairly neutral tonal balance but a somewhat unusual sound.
Adding 70 ohms of output impedance to the V-Can’s 5 ohms, to simulate a typical low-quality headphone amp, has essentially zero effect on the PM-3s’ frequency response.
This chart compares the closed-back PM-3s (blue trace) with Oppo’s open-back PM-2s (red trace) and NAD’s closed-back Viso HP50s (green trace). The PM-2s and HP50s obviously have flatter responses; the PM-3s are likely to be perceived as sounding more trebly than either.
The PM-3s’ waterfall plot is pretty clean, with much less bass resonance than the norm. The only noteworthy resonances in the mids and treble are centered at 2.1 and 3kHz. They’re well damped, though, and die out after about 8ms.
The Oppos’ total harmonic distortion (THD) is practically nonexistent. This is one of the lowest distortion figures I have measured in a set of headphones.
In this chart, the level of external noise is 75dB SPL; the numbers below that indicate the degree of attenuation of outside sounds. This result is very good for over-ear (i.e., circumaural), passive headphones, with reductions of 10dB at 1kHz, and as much as 35dB at higher frequencies. However, it won’t have much effect on jet-engine noise, which is typically loudest between 50 and 200Hz.
The PM-3s’ impedance magnitude and phase are almost dead flat at 26 ohms -- the same as Oppo’s spec.
The sensitivity of the PM-3s, measured between 300Hz and 3kHz with a 1mW signal and calculated for the specified impedance of 26 ohms, is 99.8dB -- enough to get decent volume levels from most portable devices.
. . . Brent Butterworth