Sidebar 3: Measurements
I don't measure headphone acoustic responses, as this requires a dummy-head-and-microphone system: see Keith Howard's article on this subject in the August 2008 issue. In addition, as Keith wrote, deciding what, precisely, is the optimal frequency response for a pair of headphones is neither obvious nor trivial.
I used Stereophile's vintage Audio Precision System One to measure the Audeze LCD-X's impedance and electrical phase (fig.1). The solid trace in this graph shows the impedance magnitude with the headphones mounted on my head facing my ears. The impedance in the audioband is close to 22 ohms. The electrical phase angle is also close to 0° across the audioband.
Other than a small peak at 4kHz, which at first I thought was due to my ear-canal resonance—until I noted that it was also present in a free-space measurement—the LCD-X's impedance was commendably constant with frequency. The headphone's frequency response will not alter with frequency when driven by a source with a high output impedance, like my Astell&Kern player. However, this relatively low impedance will mean that the Audeze headphones will work best when driven by an amplifier with a low output impedance. My HeadRoom BlockHead, which has an impedance of less than 1 ohm, was perfect in this this respect.—John Atkinson
Fig.1 Audeze LCD-X, electrical impedance (solid) and phase (dashed) mounted against JA's head and ears (5 ohms/vertical div.).
