Originally Posted by bimmaguy
I'm not so sure... 15hz is roughly where you will start to see huge power draw from amps and the biggest excursions from the drivers. If you sell a product with a warranty, you want your product to be absolutely idiot proof to minimize claims. I've owned a few high $ sealed subs, and most have a brick wall filter around 10 - 15hz. The cones don't move at all below that.
Rythmik was the only sealed sub (non-DIY) I've ever had that would do it's best to reproduce a 1hz signal. It was actually a surprise to me.
You are correct. Many manufacturers employ High Pass Filter for protection of sealed subs.
The reasons for that are:
1. Driver coil thermal protection when the box is smaller (Qtc>0.7)
2. Driver cone excursion protection when the box is larger (Qtc<0.7) - bigger box size is not likely due to overall design requirements
3. Removing subwoofer load for ULF (<20Hz) which would give some more SPL capabilities at and above 20Hz
Power draw needed for reaching Xmax at different frequencies depends on the sealed sub box size which determines the Qtc.
Pmax and Xmax are the only (2) parameters limiting the sealed subwoofer performance. Ideally you would have them getting reached at the same time so the subwoofer can be used to it's full potential (Xmax at all frequencies for the smallest box as long as Pmax not hitting before Xmax for all frequencies below Fb).
For box size giving Qtc=0.7 the power draw P needed for Xmax remains constant no matter the frequency. With proper TS parameters you will get P= Pmax for Xmax below Fb as long as P=Pmax at Fb when X=Xmax. Above Fb power thermal limit Pmax is the restriction - the driver will hit power limit before getting to Xmax.
Just to give you a reference point - for Qtc=0.7 the Fb = (1.5 to 2)x Fs and depends on the box size and TS parameters.
When you have box with Qtc=0.7 the power will be constant for getting Xmax for all frequencies below Fb. If the box is smaller than that (when Qtc >0.7) then the power limit will limit performance below Fb - that's when you will see HPF employed for protection to limit power to the driver coil.
At box size with Qtc=0.7 the subwoofer power limit and Xmax limit work hand-in-hand and get all performance at any frequency - Xmax at constant P (or Pmax with proper TS parameters) down to 1Hz if you have the amp capable of getting that low frequency out.
Driving the subwoofer to Xmax at any frequency will provide SPL drop of 12dB/oct. That means for sealed sub with SPL 120dB @ 20Hz will give you 108dB @10Hz, 96dB @ 5Hz, 84dB @2.5Hz, 72dB @1Hz.
Type of room - size, air tightness, walls/floor/ceiling structure - would add to the above numbers as room gain which would be dependent on the frequency only, and not the type of subwoofer (sealed, ported, etc) or the Qtc value.
That's the simplified description of the process - not accounting for Le, etc.