Shure SE310 in-ear headphones Measurements
As I wrote in the "Measurements" sidebar that accompanies Jim Austin's review of the Westone 3 in-ear headphones elsewhere in this issue, deciding what is the optimal frequency response for a pair of headphones is neither obvious nor trivial (see www.stereophile.com/features/808head), compounded by the fact that I don't have a microphone small enough to be inserted in my ears. The only measurement I perform on headphones, therefore, is of their electrical impedance, to determine how difficult they are to drive. To take these readings, I inserted each set of 'phones in my ears.
The Shure SE310 meets its specified impedance of 28 ohms at 1kHz (fig.1), but averages 24 ohms at lower frequencies. There is no sign of any port tuning in the shape of the impedance curve, but the impedance rises significantly in the top two octaves of the audioband, which might usefully boost that region a little with high-impedance headphone amplifiers.John Atkinson
Fig.1 Shure SE310, electrical impedance (solid) and phase (dashed). (10 ohms/vertical div.)