Headphones

Hi!

I always wanted to know If I'm listening at too high volume, and I've seen plenty of people on Reddit tell in the comments at how many dBs they usually listen to their headphones.
However, I don't understand where these people get their numbers from.
I understand that you could measure the output of your headphones by building a contraption which uses a calibrated microphone to measure the dB. However, I do not plan to buy such a mic just to measure this once.
I also know that it is possible to calculate your output level from the specs of your headphone output and your headphone sensitivity. Unfortunately, I don't know how to do this correctly, foremost because I don't know how the scaling of my potentiometer works.

Does somebody here have an idea?
If it helps, I am using Dt1990 Pro and an Audient id4 Mk1.

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Thanks in advance:)