The D40 is our highest-volume headphone driver. The diaphragm uses a beryllium-coated composite; beryllium's high sound velocity and low density push the break-up frequency above 40 kHz, which is what produces the clean, open top end. The magnetic circuit uses an N52 neodymium magnet with a low-loss copper-clad aluminium voice coil, reaching 105 dB/mW sensitivity so even a phone can drive it to a satisfying level. The driver is available in a standard 40 mm outer diameter with several coil impedances (16Ω / 32Ω / 64Ω) to match different circuit designs.