I measured the NuForce Primo 8s 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 used. I used the medium-sized silicone eartips supplied.
The Primo 8s’ measured frequency response is unusual, and doesn’t correlate well with my perception of its tonal balance. The response below 1kHz is exceptionally flat, much like what I’ve measured from planar-magnetic headphones such as the Audeze LCD-3s -- most other earphones have a bump in the bass response. There’s a relatively mild peak at 2kHz, and above that the response rolls off quickly. Just goes to show that you need to be careful not to judge a set of headphones too strongly by its measured 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 a large effect on the Primo 8s’ measured response. The peak at 2kHz disappears entirely, and the treble rolloff becomes more severe by about -7dB. These earphones really demand a source with a low output impedance, at the very least a higher-quality smartphone such as an iPhone or a Samsung Galaxy S; an even better idea would be a separate headphone amp.
Compared to the Audiofly AF140 (a design with one dynamic driver and two balanced armatures) and the Sony XBA-H1 (one dynamic driver and one balanced armature), the Primo 8s show a much flatter response up to about 5kHz, but much less treble response.
The decay of the Primo 8s might look a tad messy, but this is an excellent result. First, see how clean the response is from 500Hz to 1kHz. That’s rare: Few other earphones I’ve measured can stop on a dime at frequencies between 500Hz and 1kHz. There are some long resonances at 6, 8, and 14kHz, but they’re very narrow and very low in level; there’s little chance they’d be audible.
The Primo 8s’ total harmonic distortion (THD) at 90 and 100dBA is modest. There’s a little, but this is an average to above-average result. At the loud-but-not-crazy-loud level of 90dBA, the THD below 1.5kHz is about 1%. Even at the very high level of 100dBA, THD is never higher than 3%.
In this chart, the external noise level is 75dB SPL; the numbers below that indicate the level of attenuation of outside sounds. Probably because of the occlusion of the G.R.A.S. ear simulator by the Primo 8’s relatively large driver enclosure, the NuForces deliver outstanding isolation. They should substantially reduce jet-engine noise, considering that they attenuate outside sound by -28dB at 100Hz and -33dB at 1kHz, and by even more between 2 and 10kHz.
The impedance plots of balanced-armature headphones usually show pretty wild swings, and the Primo 8s are no exception. You can see here that they peak at 59 ohms at 550Hz, then fall to just 6 ohms at 20kHz. Phase response is also extreme: flat through much of the audioband, but dropping to -67 degrees at 1.35kHz. All the more reason to make sure you mate the Primo 8s with a high-quality source device.
The Primo 8s’ average sensitivity from 300Hz to 3kHz at the rated 38 ohms measured 116.6dB. That’s very high, and indicates that the Primo 8s will play loud as hell from even the cheapest, crummiest MP3 player.
. . . Brent Butterworth
brentb@soundstagenetwork.com