How to Optimize the Low Frequency Response of your DIY Subwoofers 

I think so as well. I work with a number of small and large subwoofer designers and the DSP tuning is most of the work often. A lot of what they do is beyond what any DIYer can do. I’ve had two experiences where I got to hear a commercial subwoofer with proper functioning limiters vs not. Both were heard during the testing and in a room with content. Both were huge high output subs. I was really shocked at what a difference the limiters made to sound quality. One of the subs had limiters but the wrong DSP file was loaded so it was limiting incorrectly. That was audible (but once fixed it was no longer an issue). The second scenario was first with no limiters and then with. It would occasionally overload with content and made some ugly noises when it hit those limits. It was also mechanically noisier than with the limiters. But properly setup limiters were virtually inaudible. It just sounded normal. You didn’t notice the small compression that happens when it reels things in. My takeaway (which Geddes had taught me two decades ago) was that nothing is worse than an overloading speaker. The moment it starts to make inharmonic noises, it totally takes you out of it. Not getting loud enough is not nearly as big a concern as the bad noises are.

@@PoesAcoustics Agreed, and good points. When I first began exploring the limits of my dual Velodyne subs ~30yrs ago, I 𝙡𝙤𝙫𝙚𝙙 the way they eased into their limits ... without 𝘴𝘪𝘨𝘯𝘪𝘧𝘪𝘤𝘢𝘯𝘵 audible consequence! Seriously, it was so cool. It was like cheating. Similarly, I had that experience but even at a higher level of execution with Seaton's careful programming in my SubMersives. Also enjoyed countless back and forth discussions between Mark Seaton and Dave "Bossobass" on AVS Forum concerning the merits of various applications of limiting in small sealed designs. Whereas Bosso would insist that no sealed sub should have any type of limiting in its signal path. Mark would patiently explain at length his perspectives and the realities of a such protections and their merit in the SP amps in his offerings. Like many forums, AVS' content was so strong at that time. Thanks

So you have 5 18 in subs for your home theater??? What kinda amp powers that?

@@surrealistix1 thank you! Maybe need to cover this one in more depth.

Reading through the comments section here and elsewhere, it might be beneficial.

@@poleepwka the reason I haven’t in the past is that I am conflicted. When I discuss this with my professional colleagues, they discourage it. DIYers of course encourage it. The discouragement is because you can end up teaching people information that is well into the territory of dangerous if not fully understood. For example, sealed subs should use shelf filters. It’s how professional subwoofer designers get the response shape we are used to seeing on commercial subwoofers. The subs may are very high gain at low frequencies. And it may handle it just fine. But all subs hit their limit. Professionals have the tools and knowledge to identify those limits and protect. Average diy consumers do not. So what if I teach people how to properly shape the response and then a bunch of guys misuse that knowledge and blow their subs. Hurts my brand, hurts their wallet, and leads to misinformation amongst DIYers. The whole reason that PEQ boost is used on diy amps is because it’s a safer solution for those without specialized tools. It’s not as good and leaves a lot on the table. But it’s better than nothing without as high of a risk. And with Limiters, it’s actually a very complicated topic. The way a company like Kef or Perlisten would do it is to use laser measurement devices and microphones to monitor excursion and distortion rise. More than likely using a measurement trick that involves a microphone inside the enclosure. They also monitor the amplifiers output. Then doing a bunch of tests identify the limits at a given frequency and tailor the limiter settings to that data. They use a special form of limiting that is most similar to what pros call multiband limiting. Not all amp DSP supports this, it’s something often restricted to specialized DSP platforms and tools. Most will only have access to peak limiters. These are basically designed to limit how much power the amp will produce. But it’s not a brick wall limit. It’s an intentionally slow to clamp and slow to release limit and that limit speed is based on the subs capabilities. That helps allow for shorter duration peaks and longer duration thermal protection. You can set those to the thermal limits of the driver, but then it will exceed xmax (which is often sooner than the thermal limits). You could choose to set the limit to xmax but then at what frequency? Plus that means you are leaving 6-10-50dB of output on the table. If you put in a highpass filter and then set the xmax limit to some number like 10hz, and it’s a pro driver, I bet you are leaving 50dB of output on the table in the midbass thermal range. I think that’s a bad idea. But if you don’t use a high pass filter and you set it to xmax at 30hz, it will overload at 10hz. Which maybe isn’t a big deal. And if you used multi and limiting methods instead, no high pass needed and nothing is left on the table. But like I said, the tools needed to set those tight are not something the average person has. Plus you really need to set the limiters based on performance behavior of different types of stimulus. The average person would likely want to use a few types of test tones and actual content that is known to stress the system. I believe Mark Seaton even does that and it makes sense. Large companies may rely more on laser Vibrometer, xmax measurement, and distortion data alone using only test tone stimulus. I could be wrong about that.

@@PoesAcoustics I totally understand your viewpoint and also the reasons for limiting the scope of the discussion. I found the video extremely helpful and insightful. This is a good starting point for me to explore futher. I have never taken distortion measurement before, so this is something I am looking now looking into, ect. That said, testing your suggested settings has definitely improved my subwoofer’s bass response considerably in my room. Fine-tuning and more testing needed of course to get the max output while protecting the system. I assume a Linkwitz transfor will work in similar manner as the shelf?

@@PoesAcoustics, do you recommend to drop the gain on the DSP when applying the shelving filter? For exsample, if I add a 15db boost should I drop the gain on the DSP too by -15db to avoid clipping ?

I'm currently down this rabbit hole. Everytime I sit to watch a movie I always find myself tweaking the most ridiculous stuff. A 2 hour movie turns in a 4 hour tweaking session.

@@angussmall hah well I trust that those who understand it will listen. I don’t trust they will use it wisely. But hey isn’t that on you guys? Having said that, I think many other RU-vidrs oversimplify because that’s as much as they understand. The majority of experts who really understand this and do RU-vid videos on it is next to 0. Even I don’t know anywhere near as much as the engineers I know at the companies who do this for a living. The only thing making me feel more confident in my knowledge is the positive feedback I’ve received from those experts on past discussions of these topics. I didn’t learn this in college (did anyone) and I’ve never seen accurate forums on it. I learned it by talking with engineers a s reading white papers on the topic written by experts. Then experimenting myself.

Yes, its called a Linkwitz Transform. Essentially any driver, pro or otherwise, needs boosting below the knee, ... to overcome the air-spring of the small sealed box volume. ---- OTOH, an IB (infinite baffle) sub with a huge backspace volume requires no boosting, as the native Fs (free air resonant freq) isn't impacted greatly by the "box" air volume. 10 to 15yrs ago I built a quad 18" attic manifold IB that serves our primary system. The Fi drivers only require 200w to 300w per driver to hit full stroke of ~30mm. A comparable box sub would need 10x that power for 20hz! That's huge, ... all the way to the panelboard! But like everything subwoofer physics, it's not without comprimise.

I recognize your name from AVS forums, I am in the process of replacing all my current subs to all sealed diy. Would love to hear how your experience goes with applying shelf filter on the htp1 without minidsp.

@@viru4409 I opted to use the MiniDSP bc I am not yet sure how to use shelf filters to replicate a LPF. I do know it's possible though so once I do figure it out I'll probably opt to pull the MiniDSP out again.

Right I see that. I’ll ask Jeff. If you look at our measurements you will see it’s flat out past 400hz. Even the RS1 from 2020 was -3dB at 200hz. But he moved the low pass filter up after that generation. I don’t know why Jeff says that unless he moved it back down. It’s unrelated but I saw a post where someone measured it and assumed the rolloff was due to high inductance. Suggested Jeff’s subs had high inductance and poor midbass. But that was completely incorrect. It’s one of the lowest inductance subwoofers on the market. The rolloff of that generation is caused by DSP filtering.

@@PoesAcousticssorry I should have mentioned the +- that is listed on JTR’s website for the Caps specifically the RS1, RS2 have a +-3 of 157hz listed. I’m assuming that this is why he quoted that number in his question. Good to know that they in fact do go much higher

Yeah, that number that was directly from JTR website. I think in the test from Audioholics it has a very steep rolloff after 175 hz. Most likely he updated it after that. That said, updating websites might not be Jeffs number one priority.

@@poleepwka the update to 500hz was as far back as when I was still in Illinois. I’ll ask him about it. But the low pass behavior you see on the RTJ is roughly what the JTR should do now.

I'm skeptical of the REL rationale for the 2 stacks of 3 high approach, albeit is a very good presentation. Regardless, you've got six high performance, highly capable subs ... supporting a given system. So yes, it's going to be very impressive when well blended/executed!!

The time alignment is primarily to prevent cancelation of frequencies near the crossover point with other speakers, or with other subwoofers. In short time alignment is to prevent holes in the frequency response. It's not about our ears perceiving infinitesimal delays.

@@daphoosa so, why is that not appropriate to large floor standers comprising of multiple drivers?

It is

@@shining31 thank you for that lengthy explanation.

@@howardskeivys4184 You are asking if floor standers are doing time alignement. Yes they are. The subquestion is time aligning is what with what ? the woofer with the medium driver for exemple. It's the same concerns than mains speakers with subwoofer. But in this case, as crossover frequencies are low, it's very difficult to be accurate with in room measurements. So there is tricks to know :)

You're very welcome!