The ear simply does not have the time-domain resolution to hear a slight timing mis-match in the subwoofer region. The ear is UNABLE to even detect the presence of bass energy from less than one full wavelength, compared to which small timing errors are insignificant. What the ear IS very good at, is detecting SPL differences in the bass region. This is implied by the bunching up of equal-loudness curves south of 100 Hz. So the ear can indeed hear something going on as you fine-tune the phase control, but what it’s hearing is the effect on the frequency response. If the main speakers are a bit muddy in the upper bass, you can have the subs overlap them a bit and then use the phase control to dial in some cancellation, thus improving the clarity. I’ve had many customers claim to hear an improvement in clarity from the addition of subs without highpass filtering the mains, and I’ve heard it too, but cannot explain it, apart from the occasional situation described in the preceding paragraph. Of course there is also the improvement in clarity that can come from highpass-filtering the main speakers to reduce cone excursion, in which case the transparency of the highpass filter can be a factor. Regarding adding a sense of depth or sense of envelopment/immersion, if you have two subs, you can place one at either side of the listening position and dial in 90 degrees of phase difference. This will tend to synthesize hall ambience. Credit to David Griesinger for this idea. The 90 degree phase difference ("phase quadrature") will also tend to improve the modal smoothing. Sometimes adding a subwoofer to a tonally well-balanced system tips the spectral balance to the dark side. When that happens, we can bring balance to the force. We can add a rear-firing tweeter to just nudge the spectral balance back to normal, without messing up the imaging by adding another source of first-arrival sound. Duke |
Jetter, I don't know how to reliably predict in advance what would be the ideal, so some trial-and-error is probably called for. You might even find that something in between, like 45 and 135 degrees, works best. Or you may find that it makes little difference. And it doesn't have to be 90 degrees on the dot. I've gone a bit over 90 at times. Phase quadrature will result in something closer to semi-random-phase summing of the outputs of the two subs in-room, which implies partial cancellation, so you might need to increase the level of the two subs just a little bit. Is "Jetter" said with a French pronunciation? Duke |
@avanti1960 wrote: "Adjusting the continuous phase control provides much more than a slight amount of timing adjustment and my ears were quite capable of hearing when I had the correct phase setting." And I totally believe that you are quite capable of hearing when you had the correct phase setting. I just think you are most likely hearing the frequency response improvement that arises from your phase optimization. Whatever the case may be, thank you for throwing a spotlight on the improvements possible from adjusting the phase control. Duke |
Millercarbon, my suggestion is this: Once you have placed the subs, first adjust the gain, then the frequency, then the phase on the sub amp. Expect to cycle through gain-frequency-phase several times. The word "timing" seems to me to imply "arrival time". The ear/brain system is not very sensitive to arrival time in the bass region, but it is sensitive to decay time, because decay time translates to frequency response in the bass region. Let me explain: Because speakers + room = a minimum-phase system at low frequencies, the frequency response and the time-domain response track one another. When you have a time-domain problem, you either have a peak or a dip. The good news is, when you fix one, you simultaneously fix the other. So bass traps reduce the decay time and thereby fix the frequency response along the way. Likewise when a distributed multisub system fixes the frequency response, it also fixes the time domain response along the way. Peaks take longer to decay into inaudibility than the rest of the signal, which is why they make the bass sound "slow". Back to "arrival time" for a minute. I read an AES paper several years ago where they played signals of less than one wavelength at bass frequencies through headphones. The listeners did not detect any sound. They could not detect the presence of bass from less than one wavelength, and had to hear multiple wavelengths before they could detect pitch. This implies that relatively small timing differences - "small" relative to the wavelengths in the bass region - are not going to make an audible difference in and of themselves. But in the bass region, the ear actually has a heightened sensitivity to differences in sound pressure level. This is shown by the way equal-loudness curves bunch up south of 100 Hz. A 6 dB difference at 40 Hz is subjectively comparable a 10 dB difference at 1 kHz. This is why we can hear the bass so much better when we turn the volume up. This is also why it takes so long to dial in the correct gain setting on a subwoofer amp - if we're off just a little bit, it's distracting. Finally this implies that the subjective improvements from smoothing the response in the bass region are greater than we would think simply from eyeballing before-and-after curves. So I think that where the phase control makes an audibly significant difference ends up being in the frequency response domain, especially in the region where the sub is blending with the mains. I believe that the correct phase control setting is the one that results in the smoothest frequency response, but this is found in conjunction with the gain and frequency controls. They all work together... gain makes the biggest difference so we adjust that first, then we adjust the lowpass filter frequency, and then the phase, and then we cycle back through fine-tuning at least once. Imo, ime, ymmv, etc. Duke
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@noble100 , thank you for taking the "leap of faith" in the first place, in spite of your totally reasonable skepticism. And thank you for posting your experiences here. I try to promote the CONCEPT of a distributed multi-sub system (which I learned from Earl Geddes), rather than specifically "my way" of doing it. Obviously I put some thought into "how to do it well" at my chosen price point, but a distributed multi-sub system is a cat which can in more ways than one be skinned. Anyway I think it's great that you find Jim Romeyn's DEBRA to exceed your expectations and continue to make the magic happen for you over the years. His subs arguably have a bit more practical footprint than mine do, as his hug the walls better. Duke
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Millercarbon wrote: "Hard to overstate how disappointed I am that so few people are interested." The following is just my guess: One of the things that is an awful lot of fun about high-end audio is, pride of ownership in a piece of equipment that has astonishing and enthralling characteristics. In a subwoofer, "astonishing and enthralling characteristics" usually have to do with how powerful it is, how deep it goes, how much air it moves, and how impressively it renders highly challenging program material such as cannon shots, pipe organ, depth charges, and dinosaur footfalls. That stuff is as engaging and entertaining as understanding and appreciating what’s under the hood of a sports car and how it translates into raw, adrenaline-jolt performance. With a distributed multi-sub system, your budget is spread across (typically) four subwoofers, so the individual subwoofers will fall well short of what you could have in a single sub for the same price. It’s like you can have four Toyotas or one Lamborghini. And it is not at all obvious that those four Toyotas used properly might be able to do something really cool, something even better than that one Lamborghini. The IDEA that four (relatively) small subs can successfully address what is arguably the most important issue (room interaction) rather elegantly is competing against far more widely-accepted, and frankly far more self-evident, ideas about "what really matters" in a subwoofer system. Thank you for helping it to do so. Duke |
Vtvmtodvm wrote: "The "distributed array" concept is worthy with one notable exception: It generally (?) recommends leaving the main speakers Unfiltered." Whether or not filtering the main speakers is desirable depends on the specifics of the situation. The same is true with ANY subwoofer system. The amplifier included with the Swarm and DEBRA systems has a fixed 80 Hz 2nd order line-level highpass filter built in, so the option is always there. If there’s a worthwhile benefit to the main speakers from using it, then by all means use it! If not, then why not leave it out of the signal path? Duke designer of the Swarm
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"To OPTIMALLY separate the (low bass) subwoofer passband from the (mid-bass) main woofer passband, it’s necessary to use a full 4th order (24dB/octave) active Linkwitz-Riley crossover." How do you know? Are you taking into account the rolloff inherent in the main speakers, and its accompanying phase shift? If so, what frequency and what slope is it? How much phase shift does it contribute? Well the answer is... IT DEPENDS! The real world is far more complicated than your "one size fits all" claim implies. I certainly do not agree with your claim that a 4th order LR crossover is "necessary". And out in the real world, my second order highpass + the speaker’s inherent rolloff will usually (but not always) come closer to a 4th order acoustic slope than what you are suggesting (4th order highpass + the speaker’s inherent rolloff). "A simple (generally passive) 2nd order (12dB/octave) crossover is simply not sufficient; the overlap is excessive." If there does happen to be some overlap, you have more than enough adjustability in the amp’s lowpass filter section to deal with it: Phase, frequency, gain, and if necessary a parametric EQ. Low-frequency speaker systems in rooms are minimum-phase systems, which means that once you get the amplitude response right, the time-domain response is also right. And the amp has the tools needed to do that in the crossover region. "Is a 12dB/octave crossover better than nothing? Well, probably; but why compromise unless there’s a cost-related cause." Yes in the Swarm I’m trying to keep bang-for-the-buck fairly high. There is always some improvement that can be made for more money, so these sorts of things are judgment calls. I don’t think it would be a wise investment for me to have custom subwoofer amps made, but if I did, I’d look into parts quality in the highpass section before I opted for a steeper slope (or ideally, a variable slope). Nobody is obligated to buy the subwoofer amp that I offer. Anyone can buy just the subs and provide their own amplifier and filtering. Duke |
Jnorris2005 wrote: "One thing to be aware of is that the sub will not add clarity to the main speakers unless the sub or the preamp provides a high pass filter to them, blocking out the bass that is now to be handled by the sub. Otherwise you're just adding murk to the low end." That is certainly a very reasonable statement, but I don't think it's an absolute truth, logical as it may be. In my experience it is possible to clean up the bottom end with subs even without a highpass filter in series with the main speakers, and depending on the specifics of the situation, that may even be preferable. I can cite examples if you would like. "Also, if your room is small, chances are you don't need a subwoofer at all. The addition of one will muddy up the sound and probably activate room nodes that will devastate the imaging and sound balance of your speakers." Agreed! The smaller the room, the worse it is from a bass mode standpoint. Chances are a single subwoofer in a small room will sound boomy, tubby, and bloomy, unless you turn its level down very far below the level of your main speakers. That being said, imo there is hope for good quality deep bass even in a small room. This is somewhat counter-intuitive, but smaller the room, the more it benefits from the modal smoothing effect of using a whole bunch of intelligently-distributed bass sources. Duke |
"Xilica XP-3060 digital XO... isn't detrimental in regards to transparency" Thanks for posting this, as I like to have options. Looks like it's between two and three times the price of the subwoofer amp that I use, but that would be okay in some situations. Duke |
"Getting that done WELL, but with subs, IMHO is for experts." Then there must be a lot of experts out there, because an awful lot of people have done it. And somewhat counter-intuitively, the more subs the easier it is, because the location of any one sub becomes non-critical. Once you've initially positioned the subs, time to adjust the controls. First the gain, then the frequency, then the phase, in that order. Then cycle through a few more times. Don't be surprised if you continue to make fine adjustments, particularly to the gain, over the course of the first few weeks. Duke |
noble100 wrote: " Duke somehow compensates for this so that the in-room bass response is flatter and more accurate but I'm not certain of exactly how he does this." According to Martin Colloms and several others, "typical" room gain is about 3 dB per octave from 100 Hz on down. Imo the implication is that a subwoofer's response should gently roll off by 3 dB per octave across the same region. This is too shallow to achieve with an unequalized sealed box even if the Qtc is very low. However it can be approximated with a vented box with the right woofer parameters, box size, and tuning. The Swarm units come very close to this target curve from 80 Hz to about 20 Hz, and since I don't normally recommend lowpass-filtering them any higher than 80 Hz, that seems to work fairly well. There is another related effect going on with a distributed multi-sub system: The subs are spread far enough apart that their outputs are combining in semi-random phase at the top of the bass region, but down at the bottom of the bass region the room size may be too small relative to the wavelengths for that to be the case, and their outputs end up combining approximately in-phase. The transition from semi-random phase combining of the outputs to approximately in-phase combining of the outputs has the effect of once again boosting the bottom end relative to the top end of the bass region by maybe 2 or 3 dB. If using only one amp, reversing the polarity on one of the subs usually restores balance to the force. If using two amps with the phase controls set for phase quadrature, that phase setting addresses the issue. Other ways of dealing with either too much or too little "room gain" include plugging and un-plugging ports, engaging the "bass boost" switch on the back of the amp, and using the parametric EQ. The goal is to end up with smooth bass, and all of these things are just a means to that end. Duke |
Hi Tim, Thank you for posting your experiences! Very interesting! I haven’t come across any insurmountable integration issues, but there have been occasional challenges. Integrating with a system that was horn loaded allegedly down to 80 Hz did present a challenge, in that the horn-loaded midbass section actually shelved down significantly at about 150 Hz and then rolled off rapidly south of 80 Hz. So we used two amps, one for the pair of subs near the mains, and the other for the pair of subs closer to the listening area. The front pair was lowpass-filtered to blend well with the 150 Hz "shelf" region, and then the rear pair joined in south of 80 Hz. I did encounter one customer’s room, open into the rest of the house, where the primary issue was simply getting sufficient low-end extension. The subs the customer already had on hand could do that just as well as the Swarm, and I didn’t feel the modal smoothing of the Swarm offered any significant benefit in his room. It made more sense for him to stick with his current subs and just crank in the maximum amount of bass boost. So the Swarm is not a one-size-fits-all solution. I like to find out as much as I reasonably can about the specific situation so I can hopefully make a good call as to whether the Swarm makes sense. Duke |
