Sidebar 3: Measurements
I didn't measure the MrSpeakers Æons' frequency response, as I lack the necessary simulated head and torso, and estimating the optimal response of headphones is far from simple. (Tyll's review includes a full set of acoustic measurements.) However, I used my Audio Precision System One to measure the Æons' impedance and electrical phase with the headphones mounted on my head. The impedance was a constant 13.4 ohms across the audioband, and the phase angle was 0° up to 10kHz, suggesting that each drive-unit behaves as a pure resistor up to that frequency; the slightly positive angle that increases above 10kHz is due to the driver's residual inductance.
Fig.1 MrSpeakers Æon, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
This is a low impedance; the Æons will need to be driven by a headphone output stage capable of delivering enough current into such a load. The Ayre Acoustics QX-5 Twenty D/A processor I used for most of my auditioning has an output impedance of 3 ohms and worked well almost all the time, as did the Naim Uniti Nova (reviewed elsewhere in this issue), which has an output impedance of 0.5 ohm.—John Atkinson
