Axiom Home Page Forums General Discussion Technical Questions 2 Subs / Stereo Frequency Response. A phase thing

In an attempt to fill out a couple of semi "dead" spots in our great room I recently took an older M&K MX-200 and added it to my M80 / EP 600 system. The results were at first some what surprising as initially I found that if anything the bass went DOWN in all listening positions. Well finally I get to see if the phase switch actually does anything and presto! Flip the switch and it's night and day different. Rocks you out of your socks. The bass extension is incredible. Not that the EP 600 was really under performing or anything but I had the other sub so why not.

Anyway, this situation got me to researching this scenario a bit and I found this:

http://www.roger-russell.com/truth/truth.htm#twosubwoofers

For those that don't know the guy worked at Mcintosh almost his whole career and is considered quite the audiophile guru....but I diverse.

The question (s) that I have and would like to find answers to or see discussed are:

If in fact good recordings are using 2 or more microphones placed in different positions for sub frequencies, say 100 hz or below, and as per the article which makes sense to me, then why don't manufacturers give us 2 sub outs? One for each channel? Instead of blending both into 1. I know what folks will say about not being able to localize etc etc but...as it does make perfect sense to think that in a real world environment that there will be harmonics and wave forms that would reach your ears at different times. When blended into one signal these would cancel each other out. It would seem a fairly simple task to leave low frequency signals to the left and right channels separate instead of combining them into 1. But even the hgih end equipment, Macs, Classe, etc all give us 1 channel sub out.

I suppose one solution is to go with 2 subs and hook each up to their speaker level in running the pre amp at say "large" for the speakers and allowing the subs crossover to do the work. However I have always been of the assumption that this will degrade quality.

The other solution I am thinking of trying, and would love to know if any one has done this, is to "bi-wire" both the mains and the sub. In other words run two sets of wires from the amp. One to each of the mains set at "large" and one to each sub's speaker level in on each channel. Is this a good idea?

Even a browse through Axioms and other company’s pages doesn't reveal much. Most 2 sub situations that I can find talk about daisy chaining them together from the sub out. Once again a mono signal potentially suffering from being out of phase.

I'm particularly interested in this because I have a dedicated home theater room in the works and have always planned on 2 subs but never gave it much thought till now.

If any one has experience with this, or any thoughts / ideas I would sure like to hear them.

Thanks.

Robert, I haven't really given this a lot of attention, but the consensus appears to be that it isn't a significant problem. For example, Tom Nousaine, in an article in the January, 1995, Stereo Review(apparently not available online, but some libraries have the back issues), discusses optimizing bass reproduction. Among other topics he discusses "stereo bass" and describes blind listening tests he conducted in which the listeners couldn't distinguish between the stereo and mono options when the crossover was at 80Hz. Keep in mind that the harmonics of even bass tones with a fundamental around 50Hz or so are played through the main speakers primarily, not the sub, and it's these harmonics which allow a string bass, etc., playing a low note to be localized although the fundamental itself is low enough in frequency to be omnidirectional.

Sure, you can experiment with two subs along the lines that you suggest, but there doesn't appear to be any real difference in what you term an "other solution" from your preceding one. Running parallel sets of wires to mains and subs isn't any different than wiring to the subs and straight through them to the main speakers(if that's what your first suggestion is). The sub's internal crossover just rolls off the sub above the selected frequency; it has no effect on the mains, which continue to run full-range.

John, thanks for the reply. I'm kind of confused by a few things though. It sounds like you are saying that if I use the subs speaker level through put that it sends a full signal to the mains. I may be totally wrong but I've always been under the assumption that the subs crossover was there to cut off lower frequencies and send them to the sub, and only allow the upper frequencies, say 80 hz or what ever, and above to reach the mains. Albeit rolled off at 6dB as to allow for a smooth transition. If I'm wrong about this I feel foolish not knowing something so fundamental after all these years. I'm almost positive that this is the way it worked with my M&K's. Maybe the EP 600 doesn't do this?? Though I drive it through the XLR. I realize that good speakers all have internal crossovers that roll off at what ever given frequency but I'm almost certain that my M&K 150's would have been blown long ago had they been run full spectrum. Also if this is not the case then why do so many manufactures advise against running speakers at full range? I’ve heard lots of speakers bottom out that couldn’t handle the lower end. One other thing that I can think off right now is that so many things that I have read advise against running through the subs crossover when it comes to musical clarity. (With full range speakers) Why throw another crossover into the mix is what I’ve heard over and over again. Once again I'd love to know if I'm completely wrong about this because it would bring up a completely different solution to what I'm thinking.

Back to original subject though. The argument of stereo bass that was made in that article that made sense to me was that if we are all trying to seek that magical musical nirvana of depth, clarity, soundstage etc…..then taking into consideration the fundamentals of a live performance it makes perfect sense that say an organ, orchestra, or electric deep bass something at 30hz or so would have waves that would be reflected and there by reach your ears at different times and angles. If these “stereo” bass frequencies are combined by the pre amp into one channel then something is lost. Hence my thinking of two runs of speaker wire to each of the left and right channels. One for the mains and one for the subs.

Taking your string bass analogy one step further. What you say about 50hz being primarily on the mains left or right with the harmonics on the sub makes perfect sense until I started to think about 2 or 3 or more microphones added into the recording equation. Lets say your sitting at a concert hall and the note is struck on that bass. If there is only one microphone on that bass right in front of it then yes that analogy covers it quite well. However if we put another, or several other microphones into the mix that are scattered across the stage, then we have harmonics bouncing off walls that are all being received by the listener at different times from different directions. Maybe or maybe not being able to localize them but lost none the less if we are using one mic and one sub through one channel.

I have researched extensively on the other boards too about this subject but it doesn’t seem to be covered. Thought there is a lot of discussion of dual subs this particular issue isn’t addressed at all that I can find. I was hoping some one around here might have had some experience with this that they might be willing to share.

If any one is running dual subs I’d love to know your experience with any of this, and how they are hooked up, especially if you have gone the 2 channel route.

Robert, I've got dual subs running in a two channel set up. First, let me say I've never been a huge fan of trying to integrate a sub into a music only system. Always preferred just the speakers to handle everything. Well, recently I acquired some bookshelf speakers that I love, but they are a bit thin on the bottom end, so I felt a sub was needed.

One of the things I like about these speakers is their wide soundstage. When I added a single sub I got the bottom end, but somehow the width and depth kinda collapsed on me. I bought a long RCA cable and tried multiple different placements and fiddled with the gain, crossover, and phase, but still couldn't get satisfactory results. Went back to listening to the speakers by themselves.

Soon, I came across a deal where I got two matching subs that had what I was looking for in terms of size and hook-up options. My preamp has two sets of outputs. I run one set of outputs to my amps with a 70Hz high pass filter attached. This cuts off all frequencies below 70Hz going to the speakers. This is supposed to ease the load on the amp, but I feel it also cleans up the crossover area where the sub integrates. I am by no means an expert, but this is what my ears tell me. Kinda the reason why I never really like subs with music--always felt the crossover was the weak point.

Anyway, from the second set of outputs on the preamp I run RCAs to the line level inputs on the subs. The subs have both a left and right input. I keep both subs right next to the speakers, but set them so they are assymetrical to each other. I think one of the key things to running dual subs is to have a various phase dial--not a two way switch, but a continously variable dial. I don't have any test equipment, so it took a lot of listening and adjusting to get the subs to work together.

As far as the sound, I much prefer them to a single sub. I not only get more extension, but stll retain the wide soundstage. I have a small room (11.5 x 11.5) and you'd think running two subs would be overwhelming, but it's not. I wasn't looking for lots of vision-blurring bass, but a three dimensional presentation to the music. Something I felt could not be achieved with a single sub. Maybe with a lower crossover you can get away with a single sub with less localization problems. Well, there are probably lots of variables like room size and shape, listening distance and location, sub placement, etc.

Robert, this is just my opinion. Lots of people on the board run one sub and are perfectly happy with it. I'm not trying to say one way is better than the other, just offering my take on it. What JohnK said about listeners not being able to discern a difference in a blind test is interesting. I believe psychoacoustics play a huge part in our interpretation of what we hear, and it is so subjective it is often hard to say what is correct and what is not. Sometimes the best solution is to try something and see for yourself. No harm, no foul.

JohnK, believe me, this is not an underhanded attack on you. You have a lot more knowledge than me on. . .well, just about everything . Just trying to give Robert my take on dual subs.

One of the problems I'm running into at this point is that I'm trying to mix apples and oranges. The 12 year old M&K MX-200 just doesn't quite cut it when it comes to trying to keep up with the EP-600. What I need to do is buy another EP-600!

Seriously though I've been listening / researching / experimenting all day with different settings and hook ups. I’ve got an Outlaw 990 feeding this set up. One thing I found is that the balanced XLR is several db’s higher than the rca sub pre out. While running the balanced to the EP-600 and an rca out to the M&K it’s all the M&K can do to even be heard. I then tried a “Y” connector from the XLR to both the subs, still very low output on the M&K. However when I changed to running a left and right rca from the main left and right pre outs instead of the single rca sub out on the 990….shazam! Night and day difference. For what ever reason those pre outs (left and right main rca’s) are several db’s above the sub rca pre out.

I have run one sub for years and always been happy with it. Most of my listening is music. 90 /10 music / movies. However I had the M&K just sitting around collecting dust so I thought I would give it a try. Let me tell you, now that I have experienced 2 subs I don’t think I could ever be happy with one again. Dual subs has allowed me to attain a much wider soundstage as well as depth. I like the sound of the M80’s at “large” rather than set to “small” and using the 990’s crossover. Finding just the right balance of crossover and phase though is a bit tricky especially considering I’m incorporating the subs crossing over into the M80’s territory. Once I hit it though it sounds great. What seems to sound best is the EP600 set to 60hz and the M&K set to about 80hz. I think that the M&K has quite a steep roll off. Something like 32db’s so it needed to be stepped up a bit. Sounds great though! Really this is all a learning experience so that I’m better educated for the future. Some day there will be a dedicated theater / media / music room in our house and I want to do it right from the start.

I did find in one of the M&K manuals actually where they suggest for dual subs running the left and right pre outs to each individual sub. Even using a “Y” splitter if they are your only pre outs to your amp. This makes a lot of sense to me considering the whole stereo bass issue that I’ve been pursuing here. Right now I’m running all XLR except for the M&K. Funny thing is though in reading through all the Velodyne literature that I can find, all they talk about is daisy chaining their subs together. Seems that most of the world is locked into blending both channels into one for subs. I would have thought that a lot of the so called "audiophiles" would have taken this to task by now. Maybe they have and I'm just not finding it in my research.

What would seem to make sense is to have pre amps with two sets of crossovers that were both adjustable. One for the mains and one for the subs. With 2 sub pre outs. That way we could all tweak things to our own liking. Right now it seems that the majority of processors have 1 crossover. Say at 80hz. It may be adjustable but that’s all you get. So if you’re in a situation where you would like to run your mains at say 40 or 50 hz….which is where I think the M80’s sound best….and you want to bump up your subs to say 50 or 60hz then you can’t do it. Unless of coarse you run them from your pre outs full and use their crossovers which is what I do now. Yes there are external crossovers and such now but then we’re getting into more signal degradation. Maybe I’m way off base and this would cause more confusion than it’s worth but it’s a thought anyway. Ah well.

It would be nice to have a variable phase switch instead of a toggle but that will have to wait. Right now I’m considering a pair of B&W 800D’s and 2- Velodyne DD-15’s for the music room. However for now I'm very happy with the sound of the Axioms. They are by FAR the best value for the $$ out there. If only the EP600 had a variable phase and remote.

Boy, lots of stuff. Robert, as was said, the internal sub crossover has no effect on the mains, even if they're wired through the sub. It's simply a variable low pass filter on the sub below the selected frequency. Some subs have an entirely separate fixed(nominally about 100Hz)high pass filter on the speaker level outputs to the mains, but most, including the Axioms don't, and their effectiveness isn't held in too high regard. In general the mains connected to the sub speaker level outputs run full-range, so this isn't any different electrically than running parallel speaker wire to sub and mains .

Sean, I didn't consider your report to be criticism, because it isn't. I merely gave Nousaine's test results as relevant info that I was aware of; any disagreement is between his results and yours, I've done nothing like the experiments that you have. A point on terminology here is that the question isn't single sub vs dual subs, but rather dual mono subs vs dual stereo subs. As was said, Nousaine's blind tests for two mono subs vs two stereo subs indicated no benefit for the stereo configuration regarding directionality or spaciousness when the crossover was 80Hz or lower.

Another example of the "consensus" (which may be wrong, of course)on this issue is the often-cited Harman report on multiple sub positioning, which used a mono configuration for the analysis and expressed doubt(pp.4-5)about stereo bass.