Understanding headphone sensitivity units

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Most headphone specifications I see only mention the sound pressure level in decibels and don't mention whether they're per mW or per volt. So when using a headphone power calculator such as http://www.digizoid.com/headphones-power.html, what sensitivity unit (SPL/mW or SPL/V) do I choose?

[Max_Settings]

What headphones do you have?

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2 hours ago, Max_Settings said:

What headphones do you have?

I'm asking in general, if you look at the Sennheiser GSP 300 for example it says 116 SPL but doesn't say if it is SPL/mW or SPL/V. Audio Technica, Beyerdynamic, Philips all mention SPL on their site for their headphones but don't specify if it is per microwatt or volt.

[Max_Settings]

SPL/V

https://en-us.sennheiser.com/global-downloads/file/8224/GSP_300_-_Fact_sheet.pdf