I did some isolation testing to see how well the Stellia isolate relative to their closest relative, the Elegia, and also the most recent closed-back headphone I’ve had in my hands - the MrSpeaker’s AEON Flow Closed.
This shows the Stellia isolate better than the Elegia, although in the more audible frequencies not quite as well as the AEON Flow Closed.
The environmental background noise level was at 42 dB for this process and tests were made 1 meter from the ear-cup. The vertical axis indicates how loud the headphones had to be playing for a given frequency to be just-barely-audible to an outside listener. Thus it shows that lower frequencies are less audible - requiring a level of 120 dB for a 100 Hz signal to be externally audible, and dropping to 80 dB for a 6 kHz signal to be perceptible on the outside.