That’s a good question!
I don’t normally do full sound leak tests. I test isolation by sitting in my favorite loud coffee shop, and subjectively rating my listening comfort experience compared to other headphones I’ve tried. I also sometimes look at objective measurements of isolation to help me out.
But I didn’t want to just say “I don’t know!” So I went and did a test.
I plugged the DT770’s into my portable Fulla amp at the coffee shop…chosen to simulate the noise level of the average office. It’s quiet at night, but there’s still some background murmur and light music.
Then I cranked up the volume to the loudest level I could reasonably stand with some nicely compressed pop music. Then I jammed the pads together in my hand and saw how far away I needed to be to hear the music at all.
I basically had to put my ear right next to the cups, the vent holes, or the pads before I heard my music.
So I think you’ll be good.
The DT770 has always been a very well-isolating headphone. So unless you’re using the absolute loudest volume out of a dedicated amp, or have the quietest office in the world, or there’s an issue with the pads sealing…I don’t see there being an issue!
Beyerdynamic does sell leatherette pads for the DT770, and those are even more isolating. They’re installed by default on the 32 ohm version, which has a similar sound signature to the 250 ohm model. I think the velour pads are more comfy though.
Also, I’d wager your head would probably explode from the detailed/zingy treble before you ever reached the maximum volume on these, depending on what kind of sound you’re mixing! If you need a really loud headphone, you might consider something for DJ’s like the Sennheiser HD 6/7/8 DJ series. Those have rolled off treble to help protect your ears at super high volumes.