My bottom line is that when I optimize my subs for smoothest FR in a co-planar arrangement it sounds no better (or worse) than when the subs are FR optimized for placement 2 1/2 feet behind the mains...I'm pretty confident (can't be 100% sure, though) that no one would characterize the bass that I'm currently producing as either bloated or slow... So, my experience just runs counter to your theory. I don't doubt your experiences, Marty, and I thank you for sharing them. FWIW, the theory I proposed in the OP wasn't intended to be an exhaustive account of the variables that determine good subwoofer integration. I have no doubt that there are many things I have failed to consider. In fact, that was the principal reason I started the thread - to explore some of those things with people who, like me, use subs in their 2 channel systems. And FWIW, I'm not trying to convert anyone to a theory that I only tentatively believe. I apologize if I appear that way. Bryon |
Bryon,
Certainly no need to apologize - particularly since there is a clean chain of logic to your theory. I've just found that, in my experience, varying the listener to sub distance doesn't produce any audible issues TO MY EAR, provided that FR is appropriately EQ'd. Of course, that last bit about MY EAR always requires the YMMV disclaimer.
It is always possible that other listeners are more sensitive to the phenomenon than I am, so my responses should be treated as a single data point in the discussion, and that is the purpose for which they were intended.
Marty |
Marty, you mention in your response that you had your speakers/subs coplanar. That might not actually result in optimal time alignment depending on a number of factors. In particular, if there is EQ on the subs, that can delay the signal by anywhere from one to several milliseconds. That means your subs might need to be one to several feet closer to your listening position than your speakers for proper time alignment. If you have a means to delay your mains, that's easiest, but otherwise you're stuck moving the subs. There may be other sources of delay, too. But even if there is no delay, the subs may need to be a bit forward or back from the mains. In my case, the optimal position put the subs an inch or so in front of my mains.
Some people may already know this, but there's a neat trick for getting your time alignment close: You flip the polarity of either your mains or your subs (whichever is easier), then play a tone at the crossover frequency (e.g., and 80 Hz tone for an 80 Hz crossover). You then adjust the delay (or sub position) to *minimize* the SPL at the listening position. (You're effectively maximizing destructive interference.) When you're done, you flip the polarity back to normal, and you should be very close to time aligned. From there you can listen to a music passage that spans the crossover frequency and move the subs an inch or so at a time and see what sounds best. If you've moved your subs a lot from their original position, you should probably re-EQ before this last step.
It's a fun Saturday project, even if you can't possibly leave the subs in their final position. And you can let your ears tell you if time alignment matters for your system. In my case, it was a huge difference. As Marty says, YMMV. |
Cbw,
Good point re: time coherence and co-planar siting, and an interesting test excersize that makes some sense. Never tried it, but - if I get the energy - I will.
I spent well over a year screwing around with my subs daily (during a long early retirement/extended unemployment stretch) which allowed for pretty much unlimited indulgement (other than wife imposed constraints) in such activities. Now that I'm working again, time constraints are a limit, but the idea is appealling and I hope to give it a go at some point. |
Marty and CBW: Interesting posts. On Jan 23rd, I mentioned above:
"I . . . run two channel fronts and I have a Paradigm Signature Servo subwoofer. This Paradigm sub has its own Class D amp to power the woofer and three controls knobs to control phase, cut-off and loudness.
Because I only have one sub, I also bought a Paradigm X-30 crossover unit. The X-30 has the same controls and also allows me to combine left and right channels without shorting my pre-amp Mains. I feed my main fronts off the preamp's Main 1 output to the power amp. I run my fronts straight out. I feed the X-30 off of the pre amp's Main 2. output As stated, I use the X-30 to combine the two channels into one channel for the subwoofer.
My front speakers are Paradigm Signature 8 v2. The fronts roll off at roughy 35-40HZ. Of course room placement also affects base contour. However, as a general matter, I cross the sub over at 40-50HZ. Based on my ear, that sounds about right. Too high a cross-over and the bass sounds boomy. Too low, and bass is thin.
I also adjust phase by ear. I listen for the best sounding bass response. Loudness often varies by source material, but as I got used to working with the sub controls, I don't move the adjustment controls around too much."
Does this sound like a reasonable approach to adjusting the subwoofer? Also, as mentioned, I can adjust phase from 0 to 180 degrees. My sub is placed about 6 inches behind the fronts. For some reason, it seems like a 40 60 degree phase adjsutment sounds best to my ear.
Would you guys explain the relationship of (a) phase degree adjustments (i.e., 0 to 180 degree) to (b) coplanar placement and (c) time coherence??? My sense is that in a perfect world, if the sub and mains are placed equidistant on a fron plane with the mains from the listener, there is no need for a phase adjustment. However, if the sub is moved forward or behind the mains, or placed along a different wall, then a phase adjustment is needed.
One other complication: As stated, I can adjust phase, cut-off and loudness from BOTH the sub and the X-30 control box. Because it is more convenient to use the X-30 control box, I set the sub controls at zero degrees for phase; 3/4 gain for loudness; and 180 HZ (max) for crossover cut-off. However, as stated, I use the X-30 box to control the foregoing adjustments. Phase: 30-40 degrees; Crossver 35-40 HZ; Loudness -- whatever sounds about right, less is generally more. Does this approach make sense to you??
Thanks for your advice. BIF |
BIF - IME, the two simplest and most effective techniques for subwoofer placement that require no equipment other than a SPL meter are: 1. Optimizing frequency response by placing the sub at the listening position, then walking around the room and listening for the location where the bass is *consistent* across low frequencies. You can do this by ear with music, or you can use a SPL meter and low frequency test tones. Once you find the location in the room that has the smoothest frequency response, you place the sub in that location. Now the bass response at the listening position should be in pretty good shape. 2. Optimizing time alignment by maximizing constructive interference at the crossover frequency. This technique was described by Cbw in his post on 1/27: You flip the polarity of either your mains or your subs (whichever is easier), then play a tone at the crossover frequency (e.g., and 80 Hz tone for an 80 Hz crossover). You then adjust the delay (or sub position) to *minimize* the SPL at the listening position. (You're effectively maximizing destructive interference.) When you're done, you flip the polarity back to normal, and you should be very close to time aligned. Both of these techniques are simple and highly effective. Unfortunately, IME, they don't always give you the same answer about the best location for the sub (which is the point I've been trying to make throughout the thread). So, if I were you, I would start with the first technique to find a general location for the sub, then use the second technique to refine the placement. Hope that helps. Bryon |
Personally, I'd recommend an RTA unit, preferably with on-board PEq.
Adjust the PEq and phase for flat on-axis FR at and around your usual listening position. There may be some variation as you move the mic, but -in my case- there was little since my room is set up for solo listening and "window" is narrow.
Screw around a bit with the PEq bands (i.e. use them as tone controls) and/or placement - changing one variable at a time - and try to note all correlations between audible results and the shape of the RTA's FR output. If you have control of the sub's Q, adjust that to different settings as well.
Eventually, you'll figure out what the best sound "looks" like on the readout, and you'll be able to re-locate the subs and dial in a close replica of your preferred response very quickly. Tweak from there by ear.
For me, the best SQ is yielded by flat (or flat to +/- 35hz with slighly and gently rising output below) with subwoofer Q set for medium damping. This has been pretty much true, regardless of where the subs have been physically placed.
I find the SQ remarkably consistent (and remarkably good) when using my Rythmiks/NHT and Velo SMS-1 when dialed in with this technique, which basically holds on-axis FR constant. I would, however, be the first to acknowledge that, for other listeners, differences may be more audible.
Marty |
I agree with Marty that, for the greatest control over subwoofer integration, you will need some way of measuring and correcting the frequency response of the system. There are both hardware and software approaches to measurement/correction. Both can yield excellent results. Personally, I use a software approach to measurement, namely Room EQ Wizard, and a hardware approach to correction, namely Meridian Room Correction. IME, the ability to measure and correct the system's frequency response dramatically increases your control over sub integration. But if your system doesn't allow for this, the techniques I described in my previous post will get you a good part of the way toward excellent bass. Good luck. Bryon |
I rarely post on tweaks, but I just discovered one that is relevant to this thread: I placed 3 Black Diamond Racing Jumbo Pucks under my sub, which is resting on a three inch maple platform. This, in effect, replaced the soft feet of the sub with extremely hard feet. The result: An audible increase in bass articulation. FWIW. Bryon |
04-21-11: Dbphd
One of the most reliable phenomena in psychoacoustics is what is know as the masking level difference (MLD). Present a mid-freuquncy sinusoid in correlated noise to both ears and adjust the level until it becomes inaudible; flip the phase of the sinusoid in one ear, and the signal pops up as much as 15 dB, depending on frequency. Dbphd - My understanding is that BMLD is a measure of the difference between the thresholds of detection for… 1. Auditory stimuli in which the signal and noise are the same phase and level. ...and… 2. Auditory stimuli in which the signal and noise are different in phase and/or level. It’s unclear to me how the existence of BMLD affects my observations about subwoofer time alignment. Maybe you can elaborate. At the time, the data suggested the auditory system doesn't preserve timing as such up the neural chain, but may convey such information by the more central areas excited. Again, it’s unclear to me how this bears on my observations about subwoofer time alignment. I’m not saying it doesn’t. I just don’t see the connection. Maybe you can say more. Having said that, I do have a few thoughts... As you are probably aware, there has been a great deal of neuroscientific research over the past 20 years using fMRI and PET scans. Much of that research has been directed at correlating various brain regions with perceptual, linguistic, and motor abilities/deficits. A significant amount of that research has been devoted to auditory perception, including the brain regions that correlate with temporal processing and temporal resolution. I’m under the impression that the neuroscientific research on temporal resolution has discovered a number of neural correlates for auditory temporal resolution, corroborating earlier measurements of temporal resolution obtained through psychoacoutic experiments (e.g. gap detection). Here is an excerpt from an article in the journal Cerebral Cortex: We used positron emission tomography to examine the response of human auditory cortex to spectral and temporal variation…Results indicated that (i) the core auditory cortex in both hemispheres responded to temporal variation, while the anterior superior temporal areas bilaterally responded to the spectral variation; and (ii) responses to the temporal features were weighted towards the left, while responses to the spectral features were weighted towards the right. These findings confirm the specialization of the left-hemisphere auditory cortex for rapid temporal processing, and indicate that core areas are especially involved in these processes. Here is an excerpt from a paper by researchers at UCI and University of Toronto: Our findings of M100 modulation by the shortest gap (2 ms) tested are also in good accord with animal studies of auditory cortical temporal acuity, where gap detection thresholds have been measured using electrophysiological methods to record activity in single or cluster units. A key result of those studies is that the firing patterns of neurons in auditory cortex reflect minimum detectable gap thresholds that are similar in scale (at 2-10 ms) to thresholds measured psychophysically in human [4, 5, 13]. Our MEG findings reported here provide evidence for a similar level of temporal resolution to brief (2ms) discontinuities in sounds in the synchronized neural response of tens of thousands of neurons in secondary auditory cortical fields [10], reflecting neural response properties at the population level in auditory cortex. Research like this leaves me with the impression that human temporal resolution is highly sensitive, not only when measured psychoacoustically, but also when measured neurologically. Of course, it does not *necessarily* follow that subwoofer time alignment is audible. But it does seem to indicate that my statements about subwoofer time alignment are not *disproved* by the current state of neuroscientific research. Bryon |
Was wondering with all this talk about time alignment
of subs with the mains, are their any concerns say if
your running your sub & speakers both off of the speaker
level connections of your amp, and the speaker cables are 2 feet longer than the sub speaker cables, are there any issues
with alignment this way?? Probably a dumb question but!?!?!? |
Telescope - The speed at which electricity travels through a wire is so fast that it makes differences in cable lengths inaudible, since the high speed makes any differences in latency (elapsed time) vanishingly small.
So you have nothing to worry about.
Bryon |
Hi Bryon,
That's what I figured, but hey!? :o)
Thanks a bunch! |
