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Ultimate Ears 18 Pro in-ear headphones Measurements
Determining a pair of headphones' optimal frequency response is neither obvious nor trivial, and is compounded by the fact that I lack a microphone small enough to insert in my ears. The only parameter of headphones that I measure, therefore, is their electrical impedance, to determine how difficult they are to drive. To take these readings, I inserted the Ultimate Ears 'phones in my ears so that they had the correct acoustic loading.
The Ultimate Ears 18 Pros' impedance and electrical phase are shown in fig.1. The impedance meets the 21-ohm specification at 1kHz, but drops to 18 ohms in the lower midrange and bass, and to 11 ohms between 8 and 10kHz. The variation in impedance is relatively small overall, though the mid-treble might sound a little laid-back with headphone amplifiers having a high output impedance.John Atkinson
Fig.1 Ultimate Ears 18 Pro Custom Monitor, electrical impedance (solid) and phase (dashed). (10 ohms/vertical div.)