Question: Sub with Fs 54, but response remains above 95dB down to 27: How can I best build

@hsbrock I didn't use that 27 Hz figure; I just used the T/S parameters and a modelling program and estimated the boundary reinforcement from placement in or near a corner.  

I don't claim my suggestion is perfect, but it should be "in the ballpark" unless your room is unusually large and/or you have an open floorplan wherein your listening room is open into other rooms.  If such is the case, then my assumptions are incorrect. 

You mentioned using a corner hence my assumptions about boundary reinforcement, but I probably should have asked about your room.

Duke

"I've got a driver with free air Fs at 54 1w/1m, but free air response remains fairly even down to 27Hz above 95dB"

This sounds unusual to me.  What woofer is this? 

@hsbrock , do you have the other T/S parameters?   Qes, Qms, and Vas at a minimum? 

Imo 54 Hz is an unusually high free-air resonance for a 15" home audio subwoofer.  Is this a car audio subwoofer?  Or a prosound driver? 

Thanks for posting the T/S parameters, @hsbrock .

You might try 7 cubic feet net internal volume tuned to about 24 Hz. This plus some boundary reinforcement should get you down to 21 Hz ballpark in-room.

@hsbrock , I want to double-check something.  It might take me a couple of days. 

Duke

@hsbrock , the modeled response that I described up-thead checks out.  It assumes 3 dB from boundary reinforcement at 40 Hz, and 6 dB from boundary reinforcement at 20 Hz, and a lowpass filter frequency no higher than 80 Hz.  It also assumes the T/S parameters are correct, of course. 

I suggest two 4" diameter Precision Ports, each 14" long including the flares.  And if you don't have an amplifier in mind, I use the Dayton Audio SA-1000 in my commercial subwoofer system.  

Best of luck with your project.

Duke

@hsbrock , I used Bassbox Pro by Harris Technologies.  I have a couple of other progams but they are no longer available.