I measured the Bowers & Wilkins P7 headphones using a G.R.A.S. Model 43AG ear 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 the ear reference point (ERP) -- roughly the point in space where, with one palm pressed against your ear, the axis of your ear canal intersects with your palm. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed.
The P7s’ frequency-response curve correlates fairly well with what’s generally accepted as a subjectively flat headphone response. There’s a little extra energy at 2.3 and 7kHz, corresponding to the lower and mid-treble regions, which suggests that the treble response will be strong; you should get a heightened sense of detail.
Adding 70 ohms of output impedance to the V-CAN’S 5-ohm output impedance, to simulate the effects of using a typical low-quality headphone amp, has a noticeable but mild effect on the P7s’ response: a slight bass boost (typically, +1.5dB) when the P7s are used with a higher-impedance source device, such as a typical laptop computer or cheap smartphone. This will result in slightly more perceived bass, and probably a slightly softer, smoother treble response. But considering the P7s’ comparatively strong treble, they should sound good even with low-quality sources.
Compared to two well-regarded over-ear headphones, the ADL H118 and the Focal Spirit Classic models, the P7s have a flatter response below 1kHz, but a notably stronger treble response.
The P7s have a nice, clean decay, the only significant resonance showing up in the vicinity of 900Hz. And even that’s at -30 to -40dBFS, and lasts only about 10 milliseconds.
The P7s’ total harmonic distortion (THD) at 90 and 100dBA is generally fairly low, but, as with many headphones, it rises below 100Hz, reaching 8.5% at 20Hz at 100dBA. Note, though, that this is a very loud listening level.
In this chart, the external noise is at an SPL of 75dB; the numbers below that level indicate the attenuation of external sounds. For an over-ear, sealed-back headphone, the P7s’ isolation is a little better than average, reducing noise at 1kHz by about 14dB, and by 25 to 30dB at higher
The P7s’ impedance is basically flat, with a little rise in the bass. That’s why its bass response in the 5 vs. 75 ohms response chart shows a slight bump upward with high-impedance (75 ohm) source devices. The impedance phase response is also close to flat.
The B&W P7s’ sensitivity, measured with a 1mW signal calculated for the rated 22 ohms impedance, is 101.7dB. That’s moderate, suggesting that the P7s will play loud, but not real loud, from most smartphones and portable audio players.
. . . Brent Butterworth
brentb@soundstagenetwork.com