Originally Posted by zheka
I followed these instructions
for the gain structure set up.
So what happens with extreme ULF content most subs/amps cannot come close to reproducing even if there is enough headroom at 20Hz ?
The obvious is that there is clipping.
A quote from Rod Elliot:
There is no doubt that a clipped asymmetrical waveform will generate a DC component in the output of an amplifier.
The main answer to your question is that the sound effect is (as Elliot describes above) an asymmetrical waveform and is almost always comprised of many frequencies across 5 octaves. The clipped waveform is not frequency specific, it's level specific, so all frequencies are affected. Lots of harmonic content and a DC component will be added as well.
When you buy a commercial EQ'd sealed sub, it's prevented from this severely clipped, potentially mechanically damaging scenario by use of aggressive limiters (which will prevent the amp being pushed beyond 500W) or HPF, a la the JL Audio and Epik stuff.
Here's a real world example of a sealed EQ'd system that employs aggressive limiting to keep the system healthy when the sonic booms hit. Seaton set up 4 SubMersives for an AVS member a few years back. Once set up, Seaton ran prog sweeps, measured @ the LP and posted the results, which I scaled to a REW graph:
You can clearly see how the limiters restrict the boosted low end of the system as the power demand increases. As a result, the FR changes from F3 in the low teens Hz to F3 in the low 20s Hz. The dashed traces are inserted to illustrate that such a system really has infinite responses since real program is dynamic and soundtracks are to the tune of 30dB in range, far more than the range shown in the measured sine sweeps from minimum to maximum. Seaton has since added 4dB of reserve power to the SM, so the results would reflect that if he repeated this exercise with the added power.
The alternative is to pre-empt the excessive demand down low when pushing the levels up by lopping off the low end from the start with a HPF.
With a DIY sub, you won't have either unless you plan for it and implement it properly into the system you build.
Josh's tests revealed that this driver is not designed to 'soft bottom', but that the bobbin will strike the back plate and that max in 4 cubes down low is under 1KW. I'm confused by his test results in that it appears to me that he fed it far more than 1KW with no bottoming and his measured Re doesn't look like it jives with the impedance trace, so you'd have to ping him to explain anything further about this particular driver.
If you have any further comments on this subject, I suggest you start a thread. I don't want to continue in this thread since we're OT.