I measured the NuForce HEM8s using a G.R.A.S. Model RA0045 ear simulator (plus a Model 43AG ear/cheek simulator for isolation measurements), 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. I used one of the supplied silicone eartips because it best fit the RA0045. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed.
The HEM8s’ frequency response is unusual: I can’t recall seeing another earphone with such a strong and dominant peak at 10kHz. Typical earphones have a strong peak (often 10dB or so) in the 3kHz region, and more of an emphasis in the 6-8kHz range.
This chart compares the HEM8s’ response with a silicone eartip and a Comply foam tip. The Comply tip slightly reduces overall sensitivity (perhaps it holds the soundtube slightly farther away from the RA0045’s internal microphone), and also reduces the treble by about 2dB at frequencies above 9kHz.
Adding 70 ohms output impedance to the V-Can’s 5 ohms, to simulate the effects of using a typical low-quality headphone amp, has a large effect on the HEM8s’ frequency response, with a large boost in the bass that averages 7dB. The output impedance of the source device tends to have a big effect on the frequency responses of balanced-armature earphones, but in this case that effect is extreme.
On this chart, the HEM8s (blue trace) clearly have a mellower, softer treble than the Klipsch Reference X20i (green) and the PSB M4U 4 (red), two other multidriver earphones using at least one balanced armature per ear.
The HEM8s have a clean spectral-decay (waterfall) plot, with no noteworthy resonances.
The total harmonic distortion (THD) of the HEM8s is low, barely hitting 2% even with the extremely high signal levels I use for this test.
In this chart, the external noise level is 75dB SPL; the numbers below that indicate the degree of attenuation of outside sounds. The HEM8s’ isolation is very good, largely because its body fits so deeply and securely into the ear. In this case, the Comply eartips improve isolation by 1-3dB in the “jet engine band” of 50-200Hz.
Like most earphones using balanced-armature drivers, the HEM8s have a large impedance swing, running from a high of 41 ohms at 20Hz to between 7 and 9 ohms at frequencies above 800Hz. This is the reason for the large effect of source impedance on the HEM8s’ frequency response. An accompanying phase shift in the transition region maxes out at -55° at 434Hz.
The sensitivity of the HEM8s, measured between 300Hz and 3kHz with a 1mW signal calculated for the specified 37 ohms impedance, is extremely high at 117.0dB. Any source device should be able to drive these earphones to loud levels.
. . . Brent Butterworth
brentb@soundstagenetwork.com
