The ZMF Atrium Closed is 300Ω at 96.0 dB/mW; the ZMF Caldera Closed is 60Ω at 94.0 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.
The Caldera Closed needs 2.0 dB more than the Atrium Closed to reach the same loudness — 4.0 mW against 2.5 mW.
| Specification | Atrium Closed | Caldera Closed |
|---|---|---|
| Impedance | 300 Ω | 60 Ω |
| Sensitivity | 96.0 dB/mW | 94.0 dB/mW |
| Sensitivity (voltage) | 101.2 dB/V | 106.2 dB/V |
| Power for 100 dB | 2.5 mW | 4.0 mW |
| Voltage for 100 dB | 0.87 V | 0.49 V |
| Current for 100 dB | 2.9 mA | 8.1 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 37.50 Ω | 7.50 Ω |
| Sources that reach 100 dB out of 280 | 277 | 280 |
3 of the 280 sources have the power for the Caldera Closed but not for the Atrium Closed — Apple USB-C dongle (EU), Steinberg UR22C, Sony NW-A306.
19 sources are low enough in output impedance for the Atrium Closed under the one-eighth rule but not for the Caldera Closed (Zoom AMS-22, Arturia MiniFuse 2, Arturia MiniFuse 1, 16 more).
277 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: ZMF Atrium Closed and ZMF Caldera Closed. Every figure on this page comes from what ZMF published, or is derived from it — what that maker publishes, counted.