The Audio-Technica ATH-E40 is 12Ω at 107.0 dB/mW; the Audio-Technica ATH-E50 is 44Ω at 107.0 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
In amplifier terms these two are the same headphone. The gap between what they need for 100 dB is under half a decibel, which is smaller than the step between two clicks of most volume controls.
| Specification | ATH-E40 | ATH-E50 |
|---|---|---|
| Impedance | 12 Ω | 44 Ω |
| Sensitivity | 107.0 dB/mW | 107.0 dB/mW |
| Sensitivity (voltage) | 126.2 dB/V | 120.6 dB/V |
| Power for 100 dB | 0.20 mW | 0.20 mW |
| Voltage for 100 dB | 49 mV | 94 mV |
| Current for 100 dB | 4.1 mA | 2.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.50 Ω | 5.50 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
6 sources are low enough in output impedance for the ATH-E50 under the one-eighth rule but not for the ATH-E40 (FiiO KA13, Schiit Vali 2+, Astell&Kern HC3, 3 more).
15 sources stay below the audible-hiss threshold with the ATH-E50 but cross it with the ATH-E40 (FiiO BTR11, FiiO K11 R2R, Lake People G103-S Mk II, 12 more).
280 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: Audio-Technica ATH-E40 and Audio-Technica ATH-E50. Every figure on this page comes from what Audio-Technica published, or is derived from it — what that maker publishes, counted.