How measure tuning frequency of Helmholz bass trap?

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I built some book shelfs for pocket books and got some space over at the bottom. I enclosed that and added a small port to tune it to 70 Hz. Just a nerdy thing to absorb some of the half wave resonance between floor and ceiling. Can I measure the actual tuning frequency in any way?
 
picture of bookshelf


Below the bottom row of books there is a vented box uses as a bass trap. The back behind the books is covered with 10mm of felt.

I assume a velocity maxima in the pipe vent at tuning but how is the pressure inside? I do have microphones and signal generator.
 
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