I had mine measured a few years ago. I had given some headphones in a quiet room, with a low frequency sinus wave on it. Whenever I said I could hear the tone clearly, they increased the frequency. At one point, the tone blends with my tinnitus tone, that's the whole magic. They then measure the current volume in dB, just by increasing/decreasing the tone's volume.
I'm on 12kHz, vacuum cleaner level.