Headphone to dB meter adapter

I have no clue if this approach is proper for measuring dB level from headphones - I'm not an audio engineer, so not giving any warranty and this should not be used from ear health viewpoint because actual levels might be higher. However it gives higher and reasonable looking values than simply holding a meter next to the headphones and most of the air pressure escapes to room air. The idea is that generated air pressure from both sides has nowhere else to escape than to the hole where the meter