I measured the NAD Viso HP20s using a G.R.A.S. RA0045 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 drum reference point (DRP), the equivalent of the headphones’ response at the surface of the eardrum. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed. Except as noted, I used the HP20s’ medium standard eartips. I experimented with the fit of the eartips and earpieces by inserting and reinserting them in the RA0045, and settled on the positions that gave the best bass response and the most characteristic result overall.
Earphones don’t always sound like they measure, but the HP20s sure seemed to. (It probably helps that designer Paul Barton and I use similar measurement gear.) There’s a mild bass boost centered at 40Hz -- exactly as I heard -- and a lot of energy between 4 and 6.5kHz, which is surely why I occasionally perceived the sound as bright.
Adding 70 ohms to the V-CAN’s output impedance of 5 ohms, to simulate the effects of using a typical low-quality headphone amp, had no effect on the HP20s’ response above 25Hz. So as you plug them into, variously, your smartphone, your laptop, and your high-end headphone amp, the HP20s’ tonal character shouldn’t change.
This comparison of the HP20s with Bowers & Wilkins’ C5 and RBH’s EP-1 suggests that, at least alongside those esteemed competitors, the HP20s’ response is relatively flat, with a more even balance of bass and treble than the two other models. Note the RBHs’ extra bass, and the B&Ws’ relative lack of energy in the treble.
The spectral-decay (waterfall) plot shows a fairly strong resonance at 5kHz and a weaker one at 6kHz, both of which correlate with the response peak in the treble.
The HP20s’ total harmonic distortion (THD) at 90 and 100dBA is very, very low
The spectrum of a 500Hz sinewave suggests that if you push the HP20s really, really loud, you’ll get a roughly equal mix of second- and third-harmonic distortion. But if you play the HP20s at levels high enough to make it audibly distort, you won’t have much hearing left for long.
For reasons I can’t explain, the HP20s delivered superb isolation at the lower frequencies of the audioband, where it really matters (and where jet engines roar): from -10 to -28dB, up to 4kHz. At higher frequencies, however, their isolation was less than the norm.
The HP20s’ impedance magnitude was effectively flat at 16.5 ohms; the impedance phase was also effectively flat.
The HP20s’ average sensitivity, from 300Hz to 3kHz at the rated 16 ohms, measured 106.9dB.
All things considered, nothing in these measurements suggests even the slightest reason for concern.
. . . Brent Butterworth