Etymotic Research hf2 & hf5 in-ear headphones Measurements
I examined the electrical impedance of the Etymotic hf2 & hf5 headphones with my Audio Precision System One and with both pairs of headphones inserted in my ear canals. Fig.1 reveals that other than a small difference in the presence region, the two have identical impedances. The rise in positive phase angle with increasing frequency for both headphones is due to the armature's inductance; with earlier iPods and other players that have a high source impedance, this will boost the Etymotics' top-octave response a little.
Fig.1 Etymotic hf2 (bottom magnitude trace & top phase trace at 3kHz) & hf5, electrical impedance (solid) and phase (dashed). (10 ohms/vertical div.)
Both models have a minimum impedance of 17.2 ohms, which is close to the 16 ohm specification. This is on the low side, meaning that the Etymotics will draw more current than higher-impedance models, which may decrease the time between iPod battery charges. (See Jim Austin's sidebar on the effect of headphone impedance in the May 2010 issue, p.96.) However, the current draw will also depend on the headphones' sensitivity, as a more-sensitive headphone will draw less current to play at the same sound pressure level as a less-sensitive model with the same impedance. Fortunately, the Etymotics' specified sensitivity of 105dB SPL for 100mV input is very high, but I suspect their lowish impedance was why WP found the hf2 and '5 benefited from the use of the Ray Samuels headphone amplifier.John Atkinson