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

Go over to the DIY audio site. They have a whole section about building sub's!

"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? 

@audiokinesis  Its an 15" OEM sub from AliExpress, was accompanied with the impedance & FR graphs, and I liked the Bl 23.  

@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? 

The driver is 15", VC4", Bl 23, Vas 2.2 cuft., fairly stiff suspension w/ Qms 14.5, Cms 0.05mm/N, double spider, so for this reason & the Bl I was leaning towards a corner horn, not relying on excursion w/ an Xmax of 7.5mm. Primarily will be used from its low end to 150 Hz, & in this range it averages about 98dB. The above measurements are at 1W/1M, & compared to most subs I've looked at its fairly flat +-3-4dB up to about 1100Hz, where it begins dropping off or breaking up. Primarily considering a corner-horn cabinet, facing into the corner, with maybe with a passive radiator above it. What more would be helpful to know? Qes: .51, Qts. .50, Sd 881, Vd 661, so displacement is not the greatest for a 15", but the PR and corner placement should help. Thinking of a sealed cabinet w/ PR facing into corner w/ rear chamber could be 2-4 cu.ft. This is as far as I've gotten. The Qts. suggests an open back or infinite baffle, and with the stiff suspension (which I also liked) so I hoped the sealed rear chamber w/ the PR might better load the horn. ??? 

@hsbrock,

See articles below:

Mike

https://www.lansingheritage.org/html/jbl/reference/technical/1983-subs.htm

https://www.lansingheritage.org/html/jbl/plans/1979-manual.htm

@ditusa  Wow. This is a good article. I’ll be back when I finish.

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.

@audiokinesis Thank you!  And so... are you telling me that, given that the speaker has good response all the way down to 27 Hz, and that IF the right cabinet is designed, it can be possible to bring a speaker's performance all the way down to those lower frequencies even if the Fs is 54Hz?  

@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

@audiokinesis Duke, that's exactly what I intend to do - the room is of medium size and self contained, and my hope to gain 6dB with the corner-horn, so your analysis is spot on. And my incredulity is unfounded? That is, it is possible to get good response down into the 20s as long as the driver goes there, even though the Fs is so much higher?

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

Duke

@audiokinesis Thank you. I'll be here!

Just be careful to brace this 7 cubic foot box. This will be a big box, over 2 feet in all directions, if a cube. Without a lot of strong bracing you will loose a lot of driver energy to flexing the enclosure.

Thanks @koestner. I am realizing this more importantly than ever. In the past 24 hours I've read in one JBL Speaker Manual sent to me by @ditusa to use 2x4s for bracing (not just little lengths of MDF), and another technically authoritative source said that adequate bracing can make a difference of 3 dB in volume, which to me suggests the wood cabinet can absorb up to 50% of an amp's power in large cabinets, and that's absorbing half as much vibration as the voice coil is producing, which makes sense. I've never appreciated this fact before.  

This is a bit of a story, but who else but you guys would appreciate this? A while ago my friend built a big coffee table sub with an 18" down firing driver. He got it all completed and wired up. On top of the table/sub he had a large heavy glass ash-tray. When the first bass notes kicked in the ash-tray flew up in the air about 5 inches. He thought it was awesome! But I reminded him, where did he think the energy to launch that ash-tray was coming from? I told him to remove the driver and brace the box a lot. After doing that the ash-tray stayed put. The braces made a huge difference in driver output.

@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

Thank you Duke @audiokinesis This is really encouraging. I've got the space to do 7 cu.ft., corner-horns, and to be able to get those results with an Fs 54 gives me hope. I was afraid my speakers would be a bust. Is there a modeling software you'd recommend for me? Again, thanks for taking the time to help me with this. I'm discovering the guys on here are great. 

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

Thanks, Duke @audiokinesis  I'll check it out.