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I measured the Oppo PM-1 headphones 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), which is roughly the point where the axis of your ear canal intersects with your palm when you press your hand flat against your ear. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed. I experimented with the fit of the earpieces by moving them around on the plate of the ear/cheek simulator, settling on the positions that gave the best bass response and the most characteristic result overall.

Frequency response

The PM-1s measure much like the other planar-magnetic headphones I’ve tested. It’s common for headphones to have a peak at 3kHz, which is thought to make headphones sound more like speakers in a real room -- but this peak is mild, at about +6dB (a lot of them are more like +12dB). Another mild, and very narrow, peak occurs at 8.8kHz.

Frequency response

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, has no real effect on the PM-1s. That might not be a big deal with most planar-magnetics -- few are designed to be driven by portable devices -- but it’s important with the PM-1s. It suggests you won’t hear a change in tonal balance when you switch from a good high-end amp to a portable source device.

Frequency response

In this comparison with two other planar-magnetic headphone models, Audeze’s LCD-X and HiFiMan’s HE-6, the PM-1s have the flattest-looking response, which suggests they will probably have a fairly flat sound and won’t be an outlier.

Waterfall

The spectral-decay (waterfall) plot shows no major or troublesome resonances.

THD

The PM-1s’ total harmonic distortion (THD), at 90 and 100dBA, is generally very low, which is the norm for planar-magnetic headphones, although there is a little band of distortion between 200 and 300Hz. The important number here is the 90dBA result; that’s a usable (if quite loud) listening level, and there the distortion is just 2%, which is barely audible. At 100dBA, a level useful only for measurement comparisons, the distortion is 6%.

Isolation

For what it’s worth, the PM-1s deliver better isolation than any other open-back planar-magnetic I’ve measured: -16 to -20dB above 4kHz. Still, though, as with all open-back ’phones, there’s no attenuation of sound below 1kHz.

Impedance

The PM-1s’ impedance is effectively flat, with a magnitude of 32 ohms (same as the spec), and negligible phase shift.

The Oppos’ average sensitivity from 300Hz to 3kHz at the rated 32 ohms measures 101.6dB. That’s excellent for a planar-magnetic, although about -2 or -3dB below what typical over-ear headphones might deliver.

. . . Brent Butterworth
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