JH Audio JH16 Pro in-ear monitors Measurements
Sidebar 3: Measurements
Determining a pair of headphones' optimal frequency response is neither obvious nor trivial (see www.stereophile.com/features/808head, footnote 1). The only parameter of headphones that I measure, therefore, is their electrical impedance, to determine how difficult they are to drive. To take this reading, I inserted the JH16 Pros in my ears so that they had the correct acoustic loading.
The JH16 Pros' impedance and electrical phase are shown in fig.1. The impedance is lower than the specified 18 ohms below 800Hz, averaging 13 ohms in the lower midrange and bass. The impedance also drops to 12 ohms at 7kHz, though it rises to 24 ohms at 1kHz, and reaches a maximum value of 35 ohms at 2kHz. With sources having a high output impedance, this variation in impedance will shelve down the lower mids and bass compared with the low treble.
For reference, my iPhone 3G has an output impedance of 3.1 ohms across the band; my iPod Classic 160GB's output impedance varies from 6 ohms at 20Hz to 4.5 ohms at 1kHz and 5 ohms at 20kHz. Using the iPod Classic will shelve the JH16's low frequencies down by about 1.5dB compared with the level at 1kHz. By contrast, the Ray Samuels headphone amp, which has a source impedance of <0.5 ohm, will give the same SPL at 20Hz as it does at 1kHz. Perhaps this was why I felt the bass balance of the JH16 Pros to be enjoyable without being overcooked when they were driven by the iPod, though the sound was a little looser overall.John Atkinson
Footnote 1: On our new Web venture, InnerFidelity, Tyll Hertsens is developing a measurement program for headphones.