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Thanks for doing this! Those CVR amps definitely look attractive from a value perspective. It's nice to know your doesn't run like a furnace. Amps typically sound very similar when operating within their performance capabilities. However, differences are still possible for a few reasons: Amps typically use a high-pass filter to protect them and/or the speaker(s) from inaudible DC signals, and they vary a lot with regard to where the HPF kicks in. However, most amps don't start rolling off until 20 Hz if not much lower. This is something that should show up in the REW measurements if there's much difference. I certainly don't see it here. Another reason is low pass filtering used specifically with Class D amps, which together with speaker characteristics (response and impedance both) affect how treble frequencies are reproduced. Such differences could not only impact how the treble sounds but could also alter perception of the bass including the bottom end. However, this only comes into play when the amps are being used to play treble and not if the amps are only used for lows and/or subs. I believe these are the most audibly significant reasons. There will always be unusual amps that were either designed with a "house sound" or screwed up in the design process somewhere. However, I believe this is more common in the audiophile world than with pro stuff which is pretty much designed to sound neutral, excepting the necessary filters discussed above. Of course, all bets are off once you start pushing the amp(s) close to their limits, which for some amps needn't necessarily involve a lot of power output if working with very low frequencies. That's part of the original reason for this thread. Many amps can measure and perform great at quiet levels and can meet their published power handling specs at "1 kHz", but if you try a strong 20 Hz tone, they might not put out anywhere near their rated capability and may even malfunction. High power amps are most likely to be used to drive subwoofers, so understanding their output capability with heavy low frequencies is very helpful. On another note, have you thought about trying to improve the acoustics of that room? Or do you not use it for sound normally?

I don't know if the size and/or dimensions work, but this looks like the design you want: https://www.avsforum.com/threads/reed-exodus-anarchy-25hz-tapped-horn.1238398/ Edit to add: A caveat is that it requires a high-order (DSP) crossover. You really need the crossover to cut the output above 80-100 Hz or else it won't sound very good.

Wow. Nice looking setup! Just so you know, the frequency response / sound quality of these designs depends quite a lot on the drivers that are installed. Using drivers with weak motor strength like the cheap car audio drivers you used probably won't sound nearly as good or give you nearly as much output as using driver(s) with a strong motor like the ones recommended. If building new SKRAMs, I suggest going through the trouble of finding/buying nicer drivers for them, and if there's any possibility of modifying the old cabinets to accept better drivers, definitely consider doing that as well.

Distortion matters in two ways. First, the flexing of the panels might involve enough "excursion" to be nonlinear, which would *generate new distortion*. Second, the panel resonances may *amplify existing distortion* produced by the driver inconsistently across frequencies. These are actually very different mechanisms. For example, light stuffing may address the problem of amplification by absorbing the higher harmonics before they can leak out, but it might not be thick enough to control energy build-up at the fundamental frequency of the panel where "excursion" is greatest. And yes, things get more interesting with directional effects, so if using distortion measurements to assess cabinet resonance problems, you'd probably need to do sweeps from at least a few different angles. I admit it's not a big priority for me either. I worried about it a lot with my first speakers because some other (cheap) speakers I owned had a buzzing panel resonance. I think the panel must have been vibrating against a plastic part. The problem area was narrow in frequency, but it was nasty when triggered. I could reliably hear it even in fairly quiet sine sweeps. OTOH, I've never noticed any untold sound from the cabinets that I built, even with pretty loud sweeps. I largely stopped worrying about it.

Ahh, very interesting. If that's true, then perhaps measuring distortion might be a decent proxy to assess how well the speaker construction is controlling these mechanical resonances. Seeing the comment by @peniku8 above reminds me of our regret from last time you visited. Recall that you ran your speaker with the mid-range crossed much lower (like 150 Hz?) because it was better contained than the woofers. You specifically did this to avoid the cabinet resonance problems, but we wondered if my EQ system would fix the problem or if the non-linearity/distortion too big of an issue. Too bad we didn't manage to test that because I do wonder how much of the problem is distortion vs. linear problems that can be skillfully counteracted via precise DSP. OTOH, my EQ broadly boosted your 150-250 Hz range by 5 dB or more (IIRC), and I was worried that your tiny little mids which were crossed around 150 Hz were going to sound bad when we turned them up. They seemed to do OK though, at least at moderately high levels. I still laugh at my memory of the utterly convincing reproduction of concert grand piano coming a pair of speakers about half as big as a shoe box.

Thanks! I was not familiar with Ascendo and know nothing about them except for what's on their web page. Subs with 32" cones and larger are very rare. I expect such a large cone sub should do ULF bass very well, but a large cone is not strictly required. Multiple smaller subs can do just as well, and multiple subs in general give superior sound quality in small rooms when placed at different room locations. I think 21" is a very good "sweet spot size from a design standpoint. As drivers get bigger, you need bigger cabinets to get the most out of them, and once you go above 21", the cabinets tend to get rather huge. OTOH, super-size drivers are probably a great choice for stationary installations in large rooms like churches and full-size cinemas. Well all this depends, and there are many trade-offs involved. Some drivers are much better for wide bandwidth reproduction than others. The same is true for cabinet designs. Also, what is your application and how big is your room? If your room is large, you will need *way more* ULF output capability than in a small room. So in a small room with a lot of natural LF gain, it's probably simpler and more cost effective to use a design with a pricier wide bandwidth sub driver to cover the whole bass range, but if you require good ULF quality in a cinema or other large room, a dedicated ULF subsystem to go with the regular "subs" may be the more cost effective choice. Also a big con with two different sets of subwoofers is that they have to be crossed-over and integrated, and that can introduce both system complexity and sound quality degradations that may be difficult to prevent or adequately compensate for. If you want more specific advice, I suggest you post more details about your room and design and what you are trying to achieve. As far as equipment brands and whatnot, the other ones you mentioned don't offer a lot of "bang-for-buck" unless you like "buying the brand" for its own sake. Ascendo didn't list prices, so I can't comment on the value proposition there, but the concept of "value" becomes a bit more vague when dealing with rare products like 50" subs. They also seem to offer unique capabilities and services for custom systems integration, totally unlike the others as well. Most people here are using DIY designs. It's a great value proposition for a lot of us, but it's not for everyone.

I think @Ricci's M.A.U.L. uses a 12" pipe.

Something that might help is to put gasket tape in the areas where the metal grill makes physical contact with the wood.

Thanks for the clarification! DJing is an interesting case because you're using content that was mastered but you're also essentially doing a live show. And to make it more bizarre, it's like you're working with a different band every time you switch songs.

*jaw drop* At 30 cuft, those things are going to be huge though!

Thanks for the clarification! And it's probably best to disregard what I said above. I don't think this is the same driver as in Funk's top-of-the-line 24s, but I may be wrong. It should still perform very well with a big enough box and enough power. Is there a power handling spec?

Is your suggestion specific to live concert music presentations? I'm wondering because I believe mastered music usually has lots of tricks like that applied already. Of course, most mastered music doesn't much rely on "headroom" either. Also, my system has high headroom, and when I turn music up to high SPL, I don't usually notice mud that wasn't there before. If anything, more SPL usually helps a little bit. I reckon that mud is the cause of that audiophile trope: "those speakers really open up when played louder." Indeed! Or maybe the sound is just muddy. Likewise: "those speakers sound very good when played quietly" is probably just a nice way of saying that they are thin and become irritating at high SPL. Audiophilese.

If I had some cash lying around, I would be tempted by this. The driver looks a similar to the ones I own except that this one has a lot more Mms and maybe a tiny bit less BL^/Re. High frequency efficiency is probably -3 dB lower on this one than mine, but it should be similar or greater in efficiency to mine at low frequencies and may have less distortion too. And now you're thinking of doing the 24"? Funk's 24" are monsters! I hope you can post the updated specs.

There's a lot of unpack here, and I mostly agree with you but can't resist the opportunity to say more. I also want to expand on the idea of subjectivity and describe at least two distinct forms of it. First there is aesthetic subjectivity, which has everything to do with our preferences for music types, artists, etc. The creative process is all about subjectivity, and there's no general theory that will suffice to explain or predict this kind of subjectivity. On the other hand, there is the kind of subjective preference that tends to correlate amongst a universal audience. More likely than not, this has to do with the *sound quality*, which is distinct and likely possible to describe objectively, even if a complete explanation is not yet available. Then there's a third kind of subjectivity that arises when one must choose between several "flawed" options. We might all agree that a particular bass resonance and a particular treble resonance are offensive, but we might disagree strongly as to which is *more* offensive. IMHO there really isn't a wrong way. All that matters is if you like the way it sounds. I'm not necessarily talking about just bass balance or response shape with these comments BTW, I think at the end of the process, this is absolutely true, and no matter what other advice one is given from experts or "the data" or whatever, one's subjective preference should always rule. And this applies for both kinds of subjective preference described above. I might also note that in a live performance setting, the two domains of subjectivity blur together. The FOH engineer is working to optimize the sound both in terms of objective quality and in terms of aesthetic goals at the same time. We've all heard people wax poetic about the goal being to be transported to the live performance. This is an impossible fools errand IMO. The live performance from who's perspective? Front row, stage left, right, dead center? Most recordings these days are created with a totally fabricated sense of space and "venue" by the recording engineer anyway. A lot of the content is totally computer created. I *think* I know what you're trying to say, but as written I must strongly disagree. I know in the audiophile world there is a kind of obsessive pursuit of certain ideals in recordings and reproductions: realism, authenticity, and purity. And I agree that this is not what audio as an art-form is about at least 99% of the time. That's because we like making things sound better when we can, and we will almost always *embellish* things when we can. Some thousands of years ago, humans realized that acoustic spaces could be constructed for the purpose of dramatically altering and enhancing the quality of sound produced within them. Later we built elaborate concert halls for our performers. Then we had the recording revolution, and later digital technologies. No longer were we limited by concert hall acoustics but could explore sound in a wide variety of real and simulated spaces. So this only expanded the creative possibilities. And that's why I very much disagree with what you literally wrote. "Transporting the listener" to the live performance *to the sound-scape created by the artist* is ... absolutely not a fools errand! If it was, then it'd also be a fools errand for the artist to go through the trouble of deciding things like "who's perspective?" or what kind of "fabricated sense of space..." to create. That is part of the creation and contributes to the aesthetic value of the product. You could say that you want to recreate what the recording or mastering engineer heard but this will be very difficult too. Most studio control rooms are as variable as our own systems. I've been in my fair share of them and very few of them are totally flat on axis, with controlled directivity. The amount of room reverb and the room size vary a LOT too. Trying to recreate what the recording or mastering engineer heard---now *that's* a fools errand. I doubt it's possible to achieve sufficient uniformity between systems, but if it were, we wouldn't care what the recording or mastering engineer "heard" anyway. That's because what's actually important is *what the artists/engineers actually DID*. Creative actions arise from artistic intent, and the consequences of these actions are objectively "recorded" in the final soundtrack. Even though the artists/engineers might have made slightly different mixing choices, were they to monitor their work on more accurate speakers, the end result is still predominantly a product of choices made without needing to rely on guidance from the monitors. I want to add some jungle ambience to my mix. If I'm on Yamahas, I'll end up mixing end frogs. If I'm on JBLs, I'll mix in birds instead. Yeah right, that's totally absurd! Furthermore, the objective sound quality of a production is by no means limited by the sound quality the engineers heard while creating it. Not even close! Relatedly, I strongly disagree with Floyd Toole's "Circle of Confusion" analogy. There is no "Circle" because the relationship is completely asymmetric. Engineers don't EQ the inverse of their monitors. The real problem I see with less-than-ideal audio reproduction is not that the different reproductions are inconsistent but that in one way or another the reproductions all obscure and/or distort the objective truth---the ultimate product of artistic intent---that is "recorded" onto the soundtrack. The issue is not really about whether the bass sounds precisely as loud as the artist wanted it to sound and more about whether every possible tonal, timbrel, and temporal nuance of the bass instrument and the performer's command of it is conveyed to the listener. Mixing balance, quality and artistic expression is all over the map on top of that. If you personally like more bass, less treble, a mountain of mids, whatever...Do it. Yes, But! If the reproduction system is capable of ideally neutral reproduction,such that the objectively subjective aspects of sound quality are essentially perfect, then how much does "more bass", "less treble", etc. actually matter? One of the most surprising things I've experienced in my work is that as I seem to improve the finer-scale accuracy of my sound, the broad-scale frequency balance differences between content (between program as well as within the same program) become much less exaggerated and more uniform. This is very fascinating because many of us who study audio assume that what we perceive should correlate fairly closely related to the physical characteristics of the sound. However, what I'm consistently finding is that what I actually perceive has much more to do with *informational characteristics* of the sound. To my brain, a broad-scale response imbalance is essentially a nuisance to be adapted to in the process of extracting as much useful information as possible from my sensory inputs, and it seems that the less my brain has to concentrate on adapting to other issues in the reproduction, the more readily my brain "flattens" the broad differences. And to be clear, the broad differences don't vanish entirely, they just manifest a lot less dramatically. If these insights are ultimately confirmed to be true, they open up a lot of fascinating questions. Like, how does SPL play into all of this? If frequency response optimization causes relative level difference between bass and treble to matter much less to perception, then can people get satisfying bass using fairly menial sub capabilities? And what does a steeper broad-curve with more bass contribute to the sound? More physicality? How much does that aspect increase per dB? A lot of bass-heads probably habitually turn the sub up until right before they hear "too much bass", likely related to the amount of mud becoming intolerable. But what if the sound never reaches that point? What if a whole wall full of subs aren't enough to muddy the sound (at reasonable mid-high frequency listening levels) even with their headroom maxed out? Will the bass-heads be dissatisfied because they are able to clearly hear all the mids and highs no matter what they do?

I'd argue the situation is a bit more complicated than described here. First, I agree that there's a lot of variation between content with regard to bass balance---much more so than for higher frequencies. I'm not sure how much correlation there is in this regard within of specific genres, but I'd certainly expect to see some correlation of content produced put out by the same mixer(s) and/or mastering engineers. Second, the subjective spectral balance depends not just on the very broad aspects of the shape, but also on finer aspects and on how these different aspects interact. Where peaks at different scales coincide, there is often enhanced emphasis. Start with the spectral characteristics of the original content, before any significant EQ is applied in the production process. The spectrum depends entirely on what the sound is. The sustained part of continuous sounds exhibit very narrow spectral characteristics. The transient sounds and attack/release of continuous sounds exhibit broader spectral characteristics. (Even this description is over-simplified) Then the mixer and/or mastering engineer applies various EQ, usually of a broader nature when boosting. Then *you* play the track at home on a system with its own characteristics (some broad, some fine), which may have been altered via your own EQ (or auto EQ) settings. The end result of all of the above can be rather hard to predict. One track might seem to have wild bass in one room or one configuration but seem feeble in another. However my experience suggests that the more neutral the playback system is, the less apparent variation there is between content. One hypothesis I have is that the brain is capable of compensating for "EQ flaws", but only to a point. The processing resources are limited. So if you can minimize EQ flaws in the production process, the resulting soundtrack will likely "translate" well to a variety of flawed playback systems. And if you can minimize EQ flaws in the playback system, you can substantially increase enjoyment of a wide variety of "flawed" content at home. Likewise, a very "dry" recording of bass is likely to sound pretty clean when played back in a typical small listening room, albeit with the listening room's acoustic signature imposed on it. A recording of bass in a room with a strong small-room acoustic signature will also sound pretty clean when played back on a system with no LF reflections or one that is tightly optimized in a way that erases or neutralizes its acoustic signature, and the recorded acoustics will be reproduced faithfully. However, the same recording will likely have poorer intelligibility when played on a more typical small-room system. Perhaps the brain struggles to parse LF sound from acoustic effects when the acoustics of multiple rooms impose on the sound. One other point of mention is that almost all mastered content is processed with harmonic enhancement plugins that I believe have a "neutralizing effect" on the overall sound and likely suppress the negative effect of any EQ flaws. They may also make those EQ flaws more audible to the engineer, making it more likely that they'll be corrected before the track is finalized. I don't know for sure as this is not my area of expertise, but they definitely have an important impact on how the end result sounds.

That's a fascinating contraption. The bass appears to be directional down to very low frequencies. I wonder what happens below 10 Hz? One source says that the Ripole configuration has its effective resonance below the driver Fs, with the given example being lower by a factor of roughly 1/sqrt(2). The Ultimax specs indicate an Fs of 19.5 Hz, suggesting a Ripole resonance at 13.78 Hz, but the measurements don't show any drop below there. Hmm.

With just two subs, you almost always want positive summation in the listening area. With 3 or more subs, assuming you have enough headroom to spare, it may or may not be useful to run some of the subs out of phase from the others, at least for some frequencies. For example, the double-bass array involves configuring the rear subs using delay and phase inversion to cancel out what would be the rear-wall reflection. That is definitely not positive summation---by design! My own system is configured so that the mid-bass modules behind my sofa interfere somewhat with the sound from the (main) front subs in order to diminish the "power alley" effect which causes hotter bass in the middle seat than the outer seats. I'm not really sure if it was worth it. The result may be rather hypersensitive to small changes in level, placement, and the surrounding room environment. There's also a larger question of whether pressure response is solely responsible for bass perception and if flat and/or smooth pressure response at a particular seat is the best indicator of how good the bass will sound at that seat.

Is the room drawn to scale? It looks like a small room, and unfortunately it looks like your seats will fall right in the lengthwise null. I didn't mean to say that modal/standing-wave resonances aren't important. They usually do come into play in small rooms. It's just that other considerations are also important, and that study largely ignored those things. How response affects how the bass "sounds" is anything but simple. Putting a sub in a corner without other sub(s) in opposing corners to "cancel out" the standing waves does in fact increase the coupling of the sub to the room's standing wave resonances compared to placements away from those corners. The advantage, IMO, is that such resonances can be attenuated using EQ. Placement away from walls tends to cause suck-outs that aren't easily repaired using EQ, and these response issues may be harder to discern from measurements as well. Either way, EQing a system for the best sound is much easier said than done. It's hard to say why you were not successful applying DSP to "fix" problems heard when a sub is placed in a corner. My first suggestion (1) here is to not obsess too much over what your in-room "frequency response" shows. It's not "wrong", it's just not all that consistent with what you actually hear. Instead it offers hints. Second (2), always look at measurements from multiple locations. Features that appear in several locations within the room are more likely to be audible. Spatial averaging can help somewhat with selecting for these, but rule (1) still applies. Third (3), use EQ judiciously. You're trying to EQ out problematic features, after you've identified them, not make a curve look prettier. The data you get from measuring is part of a map, not the territory. Fourth (4), listen to the result and use your ears to make the final judgment. A prettier curve is no guarantee of better sound. So for example, you might look at your measurements and notice a narrow peak at the same frequency that appears at most locations. Then you might choose a Q or bandwidth for your filter, based on the shape that appears in a spatial average. You might further select a gain that's something like half of the spatially averaged peak, listen to evaluate the result, and then iterate on the gain until the character of the resonance can no longer be heard but not so much that the sound becomes lifeless in that frequency area. Apart from the obvious resonances, you can experiment with broad shape adjustments to improve octave-to-octave balance. This might be best done by ear, unless you are correcting some aspect of the sub that you know about from outdoor measurements or something. For example, you can add a Linkwitz transform to alter a sealed sub's roll-off/Qts to different values, albeit with very substantial effects on *power* and *excursion* requirements. If you know roughly where your subs exhibit an "inductance hump", then a broad EQ dip might be helpful here. To the extent that you are able to use EQ to reduce or eliminate audible problems, you can make significant improvements to your sound, but you will likely find that other problems still linger. The consequences of those problems are varied and complicated. Your changes might lead to much improved clarity, but you might lose a lot of "slam". Or maybe you get more impact but a "colder, harder" sound. Almost every good sound system gets hand-tweaked at the end of the process, so don't hesitate to experiment, especially with broader scale EQ adjustments. I think you are confusing *gain* with *power capability*. The volume setting on your pre-amp says nothing about how much headroom you have in your amps. It's possible that your "really loud symphonies" are actually clipping and distorting on some of the bigger hits. It can be hard to tell unless you are able to A/B the difference unless you're really overloading things badly. With that said, 83 dB is very insensitive. For a "reference" home theater experience, where you are sitting ~10 feet away, you'll probably want speakers closer to 93 dB sensitive. Mind you, a lot depends on details --- are we talking 2.83V or 1W? What do the speaker's impedance and FR look like? IMO, speaker sensitivity (assuming use of subs) is most important in the 120-700ish Hz range where you're most likely to see big suckouts due to baffle and boundary problems, especially where they coincide. Either way, I don't think 83 dB will not cut it unless you plan to listen at lower volume. The other thing you need to keep in mind is that each +3 dB "costs you" twice as much power. So while 700W sounds impressive, it's only 8.5 dB higher than 100W. You could easily see that by putting the sub in a corner vs. mid-wall. Or put another way, moving the sub to gain 6 dB would be similar to *upgrading* your 700W amp to 2800W, *and* you'd have less distortion. Do you have any way to take measurements before you design the room? That would help a lot. Number 3 sounds sketchy. They are essentially IB subs if their back-ends fire into open space that is isolated from the room. However, I have no idea what your giant MDF contraptions are going to do mechanically, and you could end up losing a lot of efficiency among other serious problems that way. It's definitely best for the sub drivers to be mounted rigidly. I lean towards your option 2.,putting the subs in the corners under the bass traps. If you are sitting more toward the back of the room, then the subs will be closer to you than the front wall, so hopefully that lengthwise null with be a fair ways in front of you. The opposing corners placement will cancel out the major width-wise standing wave. Is there any reason why both (1) and (2) only involve using 2 subs? Why not use all 4? Why not do (2) and also put two subs at the front of the hall, at the midpoint and on the ceiling if need be.

Good question! Originally, yes. Although first I should clarify that the shape of the spatially averaged response depends quite a bit on what measurements you include in the average. IIRC, I was flat in the MLP where there was a kind of mid-bass "power ally" effect, but had rise toward the bottom in the outer seats. I'm pretty sure I went back and made broad-shape tweaks to get the sound to "my liking". And by "flat", I only meant for the sub anyway. I totally agree that flat in-room FR will almost always sound too thin and bright. What you're doing for your room is essentially what Harman et.al. recommend these days, and seems to be pretty standard for "state-of-the-art" DIY system as well. These days, I believe Harman regards their latest target curve to be "secret sauce"---essentially proprietary. That's just bizarre to me. I mean, it's probably only subtly different from previous target curves they've recommended which are publicly known. But more importantly, a target curve is, at best, a fuzzy indication of where things should end up. The X-curve in cinema is essentially the same (e.g. +/- 3 dB tolerances, which are huge for SQ purposes, and pretty much gave the projectionist the lee-way to set the EQ to whatever sounds best for him/her, until we replaced them with robots that don't "hear" like humans do). The same system will "measure" very different, even in the same room, depending on where choose to measure. Does the average include more or fewer off-axis seats? Are the extra rows included? How close are the seats to the speakers, relative to the room size? Those things and have a huge impact on where your broad response shapes will fall, so the target curve doesn't end up being useful IMO. Another thing is that the content itself is not all roughly +/- 2 dB or something, broadly. Instead, there are huge *differences* in spectral balance, not just between programs, but between tracks and/or segment of the same program. In movies for example, a popular technique seems to be "mixing big", which among other things seems to using a huge "smile" EQ curve to up the impact of "big" on-screen events. These differences largely overwhelm target curve differences. What matters far more is that trying to fit in-room response (even with spatial averaging) to a smooth curve doesn't really lead to optimal results. It might clean up some of the most obvious modal/room resonances, but it tends to make a mess of the innate/anechoic FR of the source and may actually make transients sound worse than without the EQ processing. I don't really know, but I have often experienced more satisfying bass from speakers without EQ than with some kind of "room correction".

Sure! But your picture is missing a whole lot of *braces*. Other things to consider: material use is inefficient vs. a more square-like shape. And, the enclosure will develop internal standing wave resonances that are particularly low. You'll definitely want more depth of stuffing/fill at each end. Also, see my replies to your other thread about preferring corner placement for higher efficiency among other things.

I also want to add that the article and sound people in general take "frequency response" measurements captured in a room way too literally. The study seeks to minimize "mean spatial variation" while ignoring the crucial question of whether that should even be the objective. My experience suggests otherwise. The first problem is that bass perception isn't merely a function of hearing through the ears. Bone conduction and mechano-reception (via nerves throughout your skin and body) likely play major roles too, and they are able to pick up signals from a wider variety of locations and via different transmission mechanisms. Second, the brain possesses highly advanced cognitive processing that is well adapted to listening in small rooms and inferring the nature of the sound source independently from the "acoustical context". Floyd Toole and Harman have provided ample evidence that, at least for mid and high frequencies, the listener is capable of "hearing through the acoustics" to a great extent in order to ascertain the original sound source, i.e. the speaker or sub. They've shown that anechoic chamber measurements, on and off axis, correlate better with blinded listener preference than an in-room measurement (taken "literally"). I believe this remains true for bass, except that as you go lower, the speaker drivers interact with more than just the cabinet. They begin to interact with nearby boundaries as well. These boundaries effectively become part of the speaker, and their influence alters the sound of the speaker, just as different cabinet and baffle shapes do. Go low enough, and the speaker/sub will even interact with the modal /standing wave resonances in the room. Nevertheless, the Harman people seem to ignore these observations and instead argue that optimizing in-room frequency response is the best course for bass, despite it being sub-optimal for mids and highs. They believe that essentially all the usual rules "go out the window" below the "Schroeder transition frequency", and that below this point, standing wave behavior dominates the room response. I disagree with them on all these counts. First, Schroeder transition frequency is a theoretical construct that assumes large rooms with diffuse reverberant fields, not small residential listening rooms. They describe this transition as typically happening around 500 Hz, probably because that's the rough point below which in-room frequency response measurements start to look a lot "messier" with peaks and dips. In reality, this is simply a consequence of lower directivity, meaning more and stronger reflections are contributing to the measured response than for higher frequencies. Second, I don't believe most rooms exhibit standing waves until much lower in frequency, and that behavior may not even predominate for subwoofer frequencies. unless you are in a room with stiff walls. Third, the presence of standing waves does not necessarily preclude the listener's ability to "hear-through" the local acoustic effects. Where standing waves are involved, a null in SPL response coincides with a peak in particle velocity level (PVL) response. It's possible that listeners may be able to perceive aspects of PVL independently from SPL, perhaps through indirect mechanisms like hair or clothing movement. And fourth, I've tried optimizing for minimum seat-to-seat variation and then flattest possible response in my own room using practically unlimited custom DSP capability. The result looked beautiful in the REW plots, but the sound quality was far from ideal. This was most evident when I auditioned one of Harman's own speakers, the (Revel Salon 2), playing (in another room) with no EQ whatsoever. Its musical bass quality blew mine out of the water. For my purposes, this essentially disproved the claim that flat in-room frequency response is ideal, and it inspired me to develop a novel and more clever approach to assess the sound of the speaker independent of the localized acoustics. My current method seems to work just as well at 25 Hz as it does at (say) 1500 Hz, and so I'm not aware of any sort of transition frequency where the rules of perception change. The only thing that changes is how *complete* anechoic/ground plane measurements are for describing sound quality, where for bass those measurements are still relevant but room effects become at least as important. Anyway, sorry I can't give simple answers, but that's the nature of the subject! I'd suggest you blame your brain, but actually if it weren't for our brains conspiring to make everything sound better than it really does, none of us would probably have this hobby.

@kipman725: I'm pretty sure he's not talking about a DBA. Hi @arcsabre, I think I know what paper you are referring to. It's also covered here: https://www.audioholics.com/room-acoustics/optimum-locations-for-subwoofers-in-rectangular-rooms The study is a theoretical investigation of standing wave room resonance behavior, and the opposing mid-wall placement recommendation arises as the optimal placement for two subs (run together as one, with no differential delay in the signal going to each) that minimizes seat-to-seat variation of frequency response in the room interior I want to highlight a number of problems with taking that conclusion from the study and running with it. First, the study is theoretical and does not consider absorption and losses in the walls, floor, and ceiling. Unless you're inside a bunker, those losses are not trivial . The lack of symmetry due to different kinds of construction on different walls is one thing. If your walls aren't particularly rigid, you may not have much of a problem with standing waves anyway. That's because standing waves require multiple successive reflections to build up. Second, mid-wall placements typically have poor output efficiency and suffer more from boundary interference effects from the adjacent walls. My own experience is that these problems can be severe. Depending on your room, such interference problems may predominate over standing waves for much of the response. In general, subs are most efficient in a corner because the corner allows more pressure to build near the cone, which improves energy transfer. This efficiency boost is very important because it reduces the amount of equipment and cost required to reach a particular output goal. Also because the sound energy spreads through a smaller section of the unit sphere ("pi/8 space" vs "pi/4 space", the directivity is effectively higher, meaning you get high SPL for the same power output too. Even the paper notes that mid-wall placements tend to have poor output vs. corner placements, but they are focused on minimizing seat-to-seat variation regardless of cost or compromise. Third, the study treats the subwoofer system completely independently of the mains speakers. The trouble here is that the subs do eventually cross over with the mains, and that crossover region is quite broad even with fairly steep high order. For an 80 Hz crossover 4th order LR, it might be 60-110 Hz, which is nearly an octave, and covers frequencies that are crucial for enjoying all bass content. IMO, a good crossover with the mains is likely to be *more important* than avoiding standing waves at lower frequencies, especially if your focus is music. My suggestion if you are building two subs is to put one in each front corner of the room. This puts them relatively close to the mains, so they will be more likely to combine with the front channels at multiple room locations. These placements will also cancel out some of the standing waves along the width of the room, if that's a problem for you. It will not cancel out standing waves across the length of the room. This is most likely to be a problem for in the very middle of the room (length-wise) or a little further back, and if it's possible to move those seats even a couple feet, it might make a big difference. If you can build 4 subs, then you can put one in each corner. Though this might make integrating with the front mains channels a bit trickier---or not. A lot depends on specifics.

In the end, you absolutely have to build and test. I generally agree with most of what you say here, but I would not agree that the issue has been figured out for decades. It'd be great if all we had to worry about was making the flow area big enough, but we often have to balance many competing concerns in the overall design, most especially cabinet size. And for any given design, there may still be 1 or 2 dB of performance that can be eked out with a tweak here or there. Of course a lot depends on what you're trying to do. Right now I'm experimenting with a dirt cheap 12" mid-woofer driver with a kind of whimpy motor compensated for by a very low Mms and soft suspension. It's got quite a bit of Xmax for what it is though, so it can make some bass in an appropriate cabinet. That cabinet needs to be relatively big to do it though, such that it's trivial to find a vent with a large enough flow area. This can make a decent, very affordable living room speaker, but the output density is poor. In contrast to this consider a Skhorn loaded with two 21" IPALs. With enough power, it's easy to overload the vents on that thing, and there's no easy way to expand the vents without making the cabinet bigger --- probably a lot bigger. The Skram, relying on a single 21", has proportionately more flow area than the Skhorn, but even it can be overloaded. Pretty much any kind of vent-based design in which one's trying to optimize output density is going to require difficult compromise involving flow area. Often this involves doing stuff like adding 90 degree bends to make the pipe or slot fit into the cabinet, and it's helpful to know how those features affect compression and whether the trade-off is worth it vs. shrinking the flow area to get the length to fit.

Good guessing can save a lot of time and money that would be spent building physical things that don't work.

Can you post your EQ settings? Also, can you post any measurement data you have (preferably before EQ is applied)? Data for measurements taken at multiple locations is even better. In my experience "frequency response" measured in-room, especially at only a single location, is not a reliable direct indicator of subjective sound quality. It's not that frequency response isn't important. In fact, FR matters a lot! The problem is that the in-room FR at the seat is not really the FR that you hear. EQ that makes in-room FR appear "flatter" can actually degrade sound quality. Measurements at multiple locations can help reveal and distinguish modal room resonances from interference effects. Modal resonances are audible and likely to benefit from EQ treatment. Interference effects are less audible and much more difficult to treat using EQ. Other than that, I suggest experimenting only with broad (low-Q) changes, mostly cuts rather than boosts. Often times, cutting bass in the right place (i.e. where cut is actually needed) can paradoxically make it seem louder and more intense. Your 5-10 dB peaking filter at 60 Hz is likely to just emphasize whatever FR problems exists in that region, which may make things better or worse. Applying a broad dip centered at the "inductance hump" for your sub may be helpful if you have any outdoor measurement data to refer to, but this hump is hard to see in an in-room measurement. At the end of the day, the sound quality of the result is what's important, not how the FR picture looks. Always listen to the result of any EQ you apply and consider playing with the gain to see if you can find a balance point. ==== Sealed subs don't necessarily need a HPF like vented subs do. Below their tuning frequency, vented subs essentially behave like leaky boxes and excursion tends to rise very rapidly for the driver, even though it's not making much useful sound. Driver motion in sealed subs is constrained by the air cushion. As for what you get with pro-style drivers vs. UM18s, the answer is complicated. The differences are as follows: (1) Headroom/output capability: This is important if you are running the system hard and are constrained by output capability. The pro drivers will have quite a bit more mid-bass output than the UM18s. In sealed cabinets, they will have less deep bass and ULF output. With that said, you seem to have a lot of output capability in there, so unless you're really hardcore, you're probably not constrained by output capability. (2) Innate/baseline frequency response differences: This affects the sound without EQ and also with EQ to the extent that you don't completely compensate for the difference in response shapes. Pro style drivers will typically give an FR with more mid-bass and less deep bass. A sealed sub will give up even more deep bass, but may allow ULF output to lower frequencies than your current vented subs are capable of. Pro style subs often (but not always) have better management of inductance, which reduces "humping" that often shows up in the 40-60 Hz range and can overemphasize this region at the expense of everything else. Realize that, in principle, innate frequency differences can be compensated for using EQ, but in practice this can be difficult. So even when using EQ, it might be advantageous to start with a baseline that has more mid-bass emphasis and maybe less severe inductance hump centered at a higher frequency. (3) Non-linear effects, in terms of distortion and compression: Different types of drivers exhibit different non-linear behaviors. The is more of a driver-by-driver thing, but inductance non-linearity may be quite important here. So drivers that manage inductance better are also less likely to exhibit inductance non-linearity. Note that non-linear compression is not necessarily the same at all frequencies, so non-linear effects can involve frequency response changing with driver level. For example, the "inductance hump" might be diminished at high drive levels relative to low drive levels. EQ (which acts linearly) cannot compensate for these non-linear effects. So to the extent these things matter (and unfortunately, there's no simple answer here), drivers with lower distortion and better managed inductance are likely to sound better. ==== So probably the first thing you want to figure out is whether you are constrained by output capability. With 6 ported 18s in there, I rather doubt it, but if you are in fact constrained in the mid-bass and have extra headroom in the deep bass and ULF, then moving to more pro-style subs is likely to help. If you have enough headroom, then you can try experimenting with EQ (or with turning it off! ) to see if you can improve things that way. If not, then a move to pro-style subs could still help for reasons (2) and/or (3) above. FWIW, I do lean toward recommending pro-style subs in general because small listening rooms tend to provide substantial boundary gain to compensate for the relative deficiency of bottom-end output with such subs and because they almost always handle the higher mid-bass frequencies better. Pro-style subs have also become quite popular in the DIY community, and a lot of people swear that they sound better or "slam" harder than traditional home-theater style subs. This is likely due to some combination of (2) and (3) above, and I mostly lean toward (2). Of course, you've already invested a lot of time and money in your existing setup, so IMO it's worth trying to get the best out of it before changing it. Good luck with whatever you do!