Originally Posted by Linkwitz Riley
After looking at this over & over and reading through everything, I finally see where you are coming from and understand how you chose the Qtc of 1.2.
I used the True Audio chart and found that using 1.2 most closely approximate's Ricci's measured response graph. Shooting for a Q(p) of .8 results in a better curve that targeting Q(p) .71
Last 2 questions: does the fact that Josh's measurements were made in 4.2 cubes unstuffed and my plan is 3.5 cubes with 1 lb per cu ft fill affect the response curve enough to change from Qtc 1.2?
Your choice of f(p) being 20 Hz, resulting in DC gain of 9.72 dB. Is there a reason not to choose lower? For example: F(p) 15 Hz resulting in gain of 14.72 dB? Will I run out of amp power? Cone excursion? Distortion too high? I thought if gain was kept to under 20 dB you are ok and I figured that this driver could use a little more help down deep. Whaddya think?
Remember, the resulting FR you're seeing in the spreadsheet is a reflection of the effect of the L/T you've designed against the high Q roll off, which the actual sub will not have. What that means in the end is, as I mentioned earlier, that the spreadsheet will show a lower final Q than you will actually see with your sub. If you use a .71 Qp, the resultant Qp of your actual sub will be slightly higher, giving you a flatter response to a lower Hz.
The difference between the 4.2 unstuffed and 3.5 stuffed will be very slight. But, as I've always recommended, build the box, load it and measure it close mic just to have peace of mind. Besides the question of how the different boxes may effect response before the L/T, the parameters of the drivers are not so consistent from one to the next and can effect the FR as much as the slight diff in box details, IMO.
15dB of boost is problematic in 2 ways; the obvious way is that it requires 30 times more amplifier for all content below 20 Hz. That means if you are running the 8x18" system with 250W to achieve the playback levels you want while playing a movie, when the ULF content hits, the amps will be required to provide 7500W. With 10dB of boost in the same scenario, the amps will be required to deliver 2500W.
The second problem is that the power supply rails of your circuit can be clipped. I believe the recommended power supply for the L/T circuit is +15/-15V. As you add boost, you can easily exceed that PS capability to remain stable. When that recommendation was originally made, it was made by folks who had no conception of content to 3 Hz with requirement of 120dB at the seats. If you look at the boost curve, it doesn't reach full boost until at or below 20 Hz. Since the majority of content did not go much below 40 Hz and usually never below 30 Hz and that content was at a far reduced level than other content in those sources, up to 20dB of boost was typically not a problem. Although Siegfried later briefly mentions clipping of the PS rails, most everyone else did not see 20dB of boost as a problem, including Rod Elliot's page on the subject. Here's an excerpt of relevance:
Music Type Relative Level at <40Hz
Rock music - 13dB
Maria Carey Song - 15dB
Rap Music - 14dB
R&B song - 12dB
Rave track - 12dB
Second Rave Track - 21dB
Vinyl Bass Track - 11dB
Rave track with bass sweep - 9 dB
Average - 11.875 dB (12dB)
These figures would suggest that boosts of around 10-12dB are possible with the Linkwitz circuit, before any extra amplifier power is needed above that which is required for the frequencies above 40hz.
So, the consensus was the since the most boosted bandwidth was recorded down -12dB, a boost of +12dB would require little if any big jump in amplification headroom. Although a 20dB boost L/T would fit easily into the above analysis, imagine that same test looking at 10 Hz where the 10 Hz content is in 5 channels plus the LFE channel, all redirected to the SW output with 30dB peaks to average. The signal boost requirement would def clip the 15V PS severely.
Also, running flat to the teens anechoic assumes zero room gain increase vs frequency, which will not be the case.
Regarding solutions like the Mini and DCX, I also experimented with those as well.
Here is loopback data of my SEQSS, the Marchand Bassis and 2 digital solutions. Both the SEQSS and the Bassis are analog solutions and both have properly engineered power supplies and related circuitry to prevent overdriving. The Bassis is infinitely configurable on-the-fly and the SEQSS has 9 preset curves, 6 of which include insertion of HPF protection at 10 Hz or 18 Hz.
The digital solutions require configuration through a multi-use menu and, IMO, are not very user-friendly regarding L/T shaping and/or retrieval of pre-configured curves. All digital solutions require analog-to-digital and digital-to-analog conversion steps. ADC/DAC. I've mentioned this briefly before elsewhere and won't get into it here in depth, but common sense should tell anyone interested that a $150-300 multi-use solution is not somehow going to have optimal analog/digital/analog/power supply circuitry.
It would be great if someday Josh (or anyone) would test subs and then insert these popular signal shaping products and show the results for comparison.