View lower frequencies on Spectrum

I need to record a low frequency sound and then analyse it to determine the sound volume (in dB) for each frequency.
To do so, I use the ‘Plot Spectrum’ but it doesn’t show anything lower than about 47Hz.
What I want to analyse has a frequency around 15-25Hz. How can I see it ? And can I crop the frequency range to only see low frequencies ?

Also, how do I read the sound volume axis ? I should in theory find something around 130dB but everything is negative.

Thanks !

Don’t double post. See my response to your first post.