I measured the performance of the HP50 headphones using a G.R.A.S. 43AG ear/cheek simulator, a Clio 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 in space where, with your hand pressed against your ear, your palm intersects the axis of your ear canal, and roughly the place where the front of the headphone’s driver grille will sit when you wear the ’phones. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed. I experimented with the position of the earpads by moving them around slightly on the ear/cheek simulator, and settled on the positions that gave the best bass response and the most characteristic result overall.
The HP50s’ frequency response shows a strong peak centered at 2.8kHz (as one often finds in headphone response measurements), and a broad, strong boost between 6 and 10kHz. This is similar to the typical diffuse-field equalization employed in many headphones. 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, produces a slight increase (about 1dB) in bass response between 40 and 90Hz.
Compared with most of the headphones I’ve measured, the HP50s’ response looks pretty flat. The treble does show some boost, but it’s slight -- typically, 1 to 2dB relative to the bass -- and very broad, covering the entire band between 2 and 8kHz. So the treble should be pretty much uncolored, and that rise could have the result of making the HP50s sound a tad bright (or the bass a tad damped). Note the slight difference in bass response, probably due to the fit of the different earpieces on the ear/cheek simulator; these are the best results I was able to achieve.
When I tried increasing the source impedance from 5 to 75 ohms, to simulate the effect of using a low-quality source device such as a typical laptop computer, there was no notable change in frequency response.
You can see from the chart above how similar the HP50s are to Paul Barton’s other model of passive over-ear headphones, the PSB M4U 1. The HP50s produce a bit more low bass, but the difference you’ll probably notice most is the HP50s’ roughly -3dB dip in response around 1kHz. According to Barton, this results in not an audible dip in midrange sound, but an increased sense of the headphones sounding like real speakers in a real room. In comparison, the Bowers & Wilkins P7 headphones produce less bass but have a relatively strong peak around 2.8kHz, which means they should sound somewhat brighter, with less bottom-end kick.
The spectral-decay (waterfall) plot shows a couple of strong but very narrow (and thus probably inaudible) resonances, at 1.8 and 2.9kHz.
The total harmonic distortion (THD) is extremely low at 100dBA, and remains less than 2% at 20Hz.
The spectrum of a 500Hz sinewave shows that, even at 100dB, all of the distortion harmonics are well below -70dBFS (0.03%) and are thus inaudible.
For passive headphones, the HP50s do a great job of attenuating external sounds. They reduce external noise by -15dB at 1kHz, and by as much as -40dB at 8kHz. They won’t do much for you on a plane, though; as with most passive closed-back headphones, there’s no significant reduction of noise below 200Hz, which is where jet engines produce most of their noise.
The impedance averages 37 ohms, and the magnitude and phase are both close to flat.
The HP50s’ average sensitivity from 300Hz to 3kHz, at the rated impedance of 32 ohms, measures 106.3dB with a 1mW signal.
. . . Brent Butterworth
brentb@soundstagenetwork.com
