Better than expected for a vented dynamic. There's also Sen CT6 which is the same driver, but closed design - sacrificing a bit of soundstage for a bit better isolation.
(but this model has a faux-chrome that wears off from what I hear)
Short version: muffles all sound > 2kHz, 1.5/3 of my "sticking fingers in ears benchmark" (which is 3/3, there can be higher notes) isolation score.
At the topmost end it's about -20 dB.