Originally Posted by Mark Seaton
A clipping amplifier does the nearly the same thing. The sweep stopping in level makes no clear indication of why it stopped. It could be electronic limiting of power that varies per frequency, it could simply be amp clipping, or it could be a limiter softening the clipping of the amplifier.
Like you, I prefer to have the sealed enclosure result in a 12dB/octave roll off on the bottom such that constant Voltage input results in a plateau in the excursion below some frequency. Smaller boxes and higher Fb for a given driver = less excursion for the same power, or more power to reach that excursion plateau. In Josh measured your subs with the outdoor curve set for +0/-3dB at 20Hz
we could produce similar curves that resulted in a 12dB/octave slope at the bottom if we keep increasing the level to see what is in the tank until we maxed out above 40Hz. Now bypass the BossoBassis and again increase sweeps till we max out. The maximums will be similar, with clipping earlier on the sweeps. Your drivers are amp limited, so there won't be any driver damage.So now my question is this... Consider a case where we had a really heavy and powerful 15" woofer such that in-box it matched the response you electronically corrected your sub to for the +0/-3dB @20Hz curve in the same box size with the same sensitivity below 15Hz. If the drivers have the same Xmax/Xmech capabilities and used the exact same amplifier would there be any real world, observed performance differences between your implementation with your signal shaper and the driver which did not need any correction?
We're getting off track here by creating an impossible situation for the sake of an argument. If there was a driver that had same sensitivity <15 Hz in the same box with the same X-max and had the same FR without signal shaping, I'd be using that driver instead of hoeing the hard row.
That really isn't the question here, which is: If the signal shaped version used the sort of limiters the P'digm and most commercial subs use, would there be a real world difference? And the answer is yes. I don't see how you can deny that.
You posted the P'digm example, so let's continue to look at that before we create or look at other scenarios. We have the measurements. We know limiters are the cause of the "compression". We can't know what sort of limiters are employed from sine sweep testing, but we certainly can make an educated guess (and our guess will be right).
Looking at the last 3 sweeps, why does the distortion continue to increase despite the fact that driver excursion does not?
Would that be the case if, instead of limiters, the signal shaping was altered 3 times to mimic the last 3 curves?
I already know the answer to these questions. There is no room for debate here, except to argue one type of limiter vs another and the precise implementation for the P'digm box/drivers/amp. But, all that would be is dancing around the facts of the matter, which won't change.
Now, I'd like to offer another similar example. Look at the Velo DD-18 test results:
So, we have 5 curves, dictated by the servo limiting. Keep in mind that these curves are not the same as the curves I would affect through signal shaping and that's part of the argument, but I'll let that go for now.
A whopping 16dB of compression. Output at 15 Hz is static over the last 4 sweeps (thee were only 5 sweeps performed), yet distortion increases with progressive sweeps.
So, the servo, like the P'digms limiters, is only making sure the sub "keeps its composure" and the price is a continuously changing frequency response and loads of distortion.