Passive sound isolation (also called passive noise reduction or PNR) is the physical blocking of sound waves by headphone earcup materials — the ear pads, the cup enclosure, and how tightly they seal against your head. Unlike ANC, passive isolation requires no electronics, no battery, and no signal processing.
Closed-back headphones provide the most isolation — the Beyerdynamic DT 770 Pro achieves up to 35 dB of passive attenuation, comparable to some active systems. Open-back headphones provide almost no isolation — you hear your room and your room hears your music. In-ear monitors (IEMs) can achieve 25–30 dB isolation purely through the physical seal of the ear tip.
Good passive isolation is the foundation for effective ANC. ANC works best when the headphone already blocks higher frequencies passively, allowing the ANC system to focus on cancelling low-frequency rumble. The combination of high passive isolation + hybrid ANC produces the best overall noise reduction.