Dual-Mono Versus Stereo Subs: An Experiment

For the fun of it, I recently embarked on a quest to acquire the necessary equipment in order to conduct listening tests to determine if I could tell the difference between a dual-mono and a stereo sub set-up, with stereo source material.

In my main HT (see link below) I already have a multichannel / multi-sub set-up. However it is a multi-mono / LFE set-up.

Within that treated room I was running two stereo systems as well (taking advantage of the already existing listening room and source electronics), each of those having a pair of speakers and a single/mono sub.

One of those stereo systems consisted of a pair of “floor standing” Koss CM/1020 speakers, and an SVS 20-39PC+ subwoofer (with a version 12.3 woofer). Quite some time ago I decided to acquire a second matching sub in order to create a dual sub stereo system. Late last year I was finally able to do so, however it took several weeks to get the “new” sub fit for duty: it came with a version 12.1 woofer which eventually got replaced with a version 12.3; and the original BASH amp had issues and was replaced with a brand new SVS-provided SLEDGE amp. Now the two subs differ only in those amps; both now have one port plugged (16 Hz tuning), and both have 20 Hz high pass filters (the old sub’s BASH amp selected as such; the new SLEDGE amp comes with a fixed 20 Hz filter.)

Any number of source components can be used to provide a signal from the HT’s Onkyo PR-SC886 pre/pro’s ZONE 2 stereo output. That output goes first to a just-acquired Outlaw Audio ICBM-1 external crossover / bass management unit. From there the L/R main signals go to a NAD 2200 power amp which drives the Koss CM/1020 speakers, and the L/R sub signals go to their respective SVS sub.

Since I already owned an ICBM-1 (and use it for bass management in the HT’s multichannel system), I knew that another one was exactly what I needed to complete a planned stereo sub set up. I completed the physical moves (photos below) - which included adjusting the positioning of the matching subs (basically moving them forward along their respective side walls) - and the installation of the ICBM-1 in my equipment shelves. I went through a calibration process (first matching each sub’s output to the other via their respective gain controls, and then blending their output to the mains, using a RS Analogue SPL Meter, test disc, and correction charts) and then spent time listening to a number of my go-to demo tracks.

The set-up is about as perfect as one could get in order to compare and contrast the use of stereo, versus dual-mono, subs: a symmetrical set-up in a treated room. The icing on the cake is the ICBM-1's rear panel "mono/stereo" sub switch. Given that my gear shelves are fully-accessible from the rear, it's easy to switch between the two modes - or have someone else do it so that you get an instantaneous change at the listening position.

In my experience there's very little music available that has true stereo mixing in the lowest frequencies ie. where there are obvious differences between left and right positioning of ultra-low content. In my music collection, I've only come across one: Tove Lo's Talking Body. Whether by design or accident, the positioning of ultra-low synth tones can, with headphones capable of reproducing them, be heard in distinct spatial positions with that track. (For example, it can be very clearly heard with Denon AH-D5000 headphones; somewhat less so with psb M4U 2 headphones; and not at all with AKG K240 headphones – to name just three pairs that I have at my disposal.)

So could this same effect be heard with dual stereo subs in my newly-created system designed to answer that question? Nope. Not at all. The synth notes are easily reproduced by the quite-capable SVS subs, but in stereo sub mode no distinct left-right positioning cues can be discerned and the ultra-low synth notes do not move about the front sound space. Stereo and dual-mono modes sound the same.

I'm crossing things over at a relatively-high 80 Hz (for the Koss CM/1020 - which have a claimed -3 dB point of 31 Hz), which should put considerable bass content into the subs' hands, and those subs are placed much wider apart than the speakers which they support, and somewhat closer to the main listening position. So if ever could be heard a difference, this would be it. Regardless, there's no discernible difference between dual-mono, and stereo.

When I was using a single mono sub in that system I could sense (rather than feel) the location of the sub (to my left) when those synth notes hit. I was feeling the acoustic pressure with my body - feeling a difference in pressure from that left side. But with either dual mono or stereo subs, with that Tove Lo track with true stereo ultra-low frequency content, I could not discern a direction for that content.

Up until now I’ve been talking about a particular track, with ultra-low bass stereo content, with a crossover point of 80 Hz between the subs and the mains. What about “regular” bass and higher crossover points?

In stereo sub mode, with non-music content (a 20-250 Hz frequency sweep from a test disc), playing only in one channel, or the other, I could clearly feel the difference and tell whether it was the left or right sub that was active - at the lowest frequencies – and hear the difference at the higher frequencies (which were coming from the sub’s main companion). That changed when in dual-mono sub mode: at the lowest frequencies the “feel” was non-directional (ie. all around me) because both subs were active. Once things crossed over to the mains the directionality was the same in both sub modes – as one would expect.

With the wide variety of other music that I played (rock, jazz, classical, pop, etc.) with an 80 Hz crossover setting, the positioning did not noticeably change when the ICBM-1 was changed between dual-mono and stereo sub settings. While I did not notice a change when I switched to a 100 Hz and then 120 Hz crossover setting, intuitively I would have expected a difference (in terms of positioning), between the dual mono and stereo settings, of the lowest frequencies, because we should be getting up to frequencies that are now directional. Perhaps I’ve just not yet come across any tracks that will provide the necessary positioning cues.

There are of course other characteristics / benefits to consider in a stereo set-up that incorporates subwoofer(s). The most obvious one is that in almost all cases you will get extended bass response with the addition of a sub. Dual subs tend to give a more even in-room bass extension (and yes, I am generalizing based on my own experience). But so far I’ve not been able to hear any difference in bass extension or evenness between dual-mono and stereo subs.

(But to repeat an earlier point: mine is a perfectly symmetrical set up. Things might be quite different in an asymmetrical room with asymmetrical positioning of mains and subs.)

Anyhow I'm sure I'll continue to experiment with other tracks, and play around with crossover settings, but for now if anyone asks me which I prefer: dual-mono or stereo subs in a stereo set-up. The answer will be both.

Jeff

Photos

Front of room. The Koss CM/1020s are sitting on Velodyne subs, and below Koss CM/530 speakers.



Left front showing one SVS 20-39PC+.



Right front showing the other SVS 20-39PC+.

 

After changing around the other system shown in the picks above (namely the Koss CM/530s) I took another stab at the dual stereo sub set-up, and specifically the crossover setting. I'd been leaving it at 80 Hz but there was a nagging feeling that all was not perfect, the more and more I listened to it over the weeks.

I concluded that, unlike with the main multi-sub HT system that's the real focus in that room, there were some bass peaks and troughs in the room with the stereo dual-sub system. Specifically there was a trough that appears to run right down the centre of the room. It was noticeable when I got up and moved around the room. That doesn't happen with the main HT system. I attributed this to having the subs positioned on the side walls, and I considered moving them both to the (already crowded) front wall.

However this would have entailed moving the Koss CM/530 system's sub and associated electronics, which I like right where they are.

So I tried one thing first: I changed the crossover setting on the ICBM-1 from 80 Hz to 40 Hz and re-calibrated. And for reasons not fully known, the apparent centre line trough went away and the bass response now seems very even as I walk around the room. The CM/1020 have pretty outstanding bass response (claimed -3dB point of 31 Hz, but an old Stereo Review article says it's much lower than that: "The combined response curve of the port and cone radiation was almost perfectly flat down to 20Hz - well beyond Koss's 31 Hz rating.") So unlike other speakers where a sub is really needed to handle a lot of the bass load, the Koss CM/1020 can handle a lot of it on their own, and a 40 Hz setting should not be an issue.

I'd initially used the 80 Hz setting to make sure that the subs were getting brought into the picture, in order to investigate a dual sub set-up (conclusions above.) However now it's clear that, in my specific case, it needed an even lower setting in the interest of overall sound. In doing so, and smoothing out the room response, I now notice much tighter bass in the ultra-low range. On one playing of that Tove Lo track I might have, just maybe, been able to convince myself that I could hear those ultra-low synth notes moving off centre. I plan repeated listening sessions over the next few days.

Not much else to do during the pandemic!

Jeff

 

So far still not able to distinguish stereo subs from mono subs?

It was mentioned this has stereo sub:

Are you able to stream in CD quality from Amazon music, etc., and see if you can hear a different?

 

First off, thanks for pointing me to Glass Animals. I don't know whether or not I should be embarrassed to say that I've not heard of them before.

I can access both their albums off Spotify (at 320MP3 - which will have to do for now.)

I listened to "Wyrd" off the Zaba album that you pointed to. I did so using my Denon AH-D5000 headphones - which are my go-to for all things revelatory when it comes to bass. And headphones because - well it's should be obvious if we're trying to detect even a smidgen of "stereo" placement in the deep bass regions. I ran through it three times and while it is quite bass-heavy, none of it exhibits any channel separation in the deepest regions where a sub (or subs) would come into play. I place it smack dab dead centre and it doesn't vary from there. Now at 1:35 into the track I thought I might have heard something when everything but a bass note drops out momentarily, but after listening to it over and over again I'm convinced that any audible directional / placement cues are coming at frequencies well above the fundamental, and they would be heard through the mains, and not the subs anyhow (unless the crossover frequency to the subs was set pretty high).

I also had hopes that I might detect something from 3:32 onwards. There seems to be the deepest bass "grumblings and rumblings" at the end of the track, but again I'm not hearing (or feeling) anything that would make this track a candidate to explore the difference (if any) between running stereo or dual mono subs.

While I don't expect to hear anything different when I run it through the full Koss/SVS combo, I will let you know if I do. It will have to be tomorrow when I have the house to myself and can listen at levels that will be both revealing, and not upset my better half!

And thanks again for bringing this music to my attention. I'm liking it.

Jeff

ps. I'm neither a musician nor a recording engineer, so I'm using descriptive terminology that makes sense to me, and I hope to folks reading this as well. Sorry if the way I say it puts anyone's nose out of joint - or if it's not clear.

 

Haha, no worries. I can't play loud too unless I have the house all to myself.... and with kids at home and wife works from home, not too often during these few months. ; )

I was told that I can hear stereo bass more distinctly with CD quality stream. I tried CD quality using Amazon HD and indeed I can sense dual bass. But like you said, it may be higher up above the fundamental, like 80 Hz and up, that provides the clues of dual bass.

 

Btw, I believe between 20 seconds to 40 seconds is where you hear the most dual bass. You may want to keep going back to those time when checking your SVS setup. : )

My preamp can only do dual mono out and my crossover is at 80 hz. I believe the sensation of dual bass have a lot to do with my speakers handling bass above 80 Hz and giving me the dual bass clues.

@oskar , would you please confirm if my observations of 20 to 40 seconds is correct? Thx!

 

Yes you're right.
Today I checked this song with both subs set to mono, end the effect is gone.
Sorry, but if You cant hear it in Your system, something is wrong.

 

Please describe the "effect" and exactly what one should be listening for. As a best guess, what frequencies (or range thereof) are coming into play?

What crossover frequency are you using for bass management, if at all?

If you are doing bass management, are you using both LPF and HPF? If so what are their respective slopes?

Thanks.

Jeff

 

I don't use any bass management. Crossover is set at about 80Hz.
The effect is. Two big drums playing one on the left, one on the right. And You hear their membranes vibrate simultaneously. (I'm talking about the vibration and decay after the drum is hit)But One makes my room wobble on the left and other distinctly on the right.

When i switch to mono i hear just one big drum (bas freq) in the center but two drum hits on the sides. (hi freq)

 

Curious, how do you switch between stereo and mono? Your preamp have stereo and mono out capabilities? What preamp do you use? Would you please add your setup information in your profile information page when possible? Thanks!

 

I use Schiit Freya preamp. I just changed from using left right output to using both left ones.

 

I've now spent some time listening to the track in my main HT. I am unable to reproduce / detect the "wobble" that you have described.

One thing that I have noticed is that while the track (like the whole album) is bass-heavy, there's really not a lot going on at the deepest frequencies that might require a sub to reproduce (depending, of course, on the ability of the mains). This was the case when I listened yesterday on headphones, and today with the Koss CM/1020 / SVS 20-39PC+ system - whether in dual mono or dual stereo modes.

I was able to cycle through a range of bass management crossover frequencies, including full bypass, and the deepest bass was not at all affected. Given the performance of the Koss in those nether regions, I suspect that all of the bass therefore resides from the low '30s upwards.

Just for the heck of it I then played the track through the 7.1 multi-sub system in stereo mode first to confirm there was nothing deeper to be heard (the system is dead flat to 15 Hz) and then I bypassed all the subs and ran the F L/R Koss CM/1030 stacks full range. There was no reduction in bass output. The Koss stacks alone are solid down to the high '20s. I then took them for a run, so to speak, and brought the average SPL up to 105 dB. While there was all the stereo separation that I'd expect, there was nothing at those deepest frequencies (that would have required a sub, or dual subs, to reproduce - and hence, at least in my experience, there's nothing about this track that would serve to highlight a difference between dual subs in mono, or stereo, modes.)

I can't think of an obvious reason why you are hearing something that I am not.

Jeff

 

When using both left, what is your crossover point? Do you cross your mains? Thanks!

 

Interesting, so even when running your speakers full range in stereo, you can't sense left bass and right bass? For me, from 20 to 40 seconds, the right one is the strongest and the left one not as strong.

Are you streaming in CD quality? I used Amazon HD music streamed via Blusound Node 2i to my preamp.

I think you can try Amazon 3 months free trial.

 

Following...

 

The same 80Hz, I did it only to check. I never use it like that.

 

Interesting, I thought your preamp can't do crossover. How do you cross your mains?

 

I mean the crossover on the subs. My mains are without hi pass.

 

I see. Using just your mains, you can still hear dual bass, just not as obvious with the subs, am I correct?

 

No i just checked, I don't hear it without the subs. But my mains are quite bass shy.

 

Interesting. My preamp's room setup program determined that in my room, my speakers crossover point should be around 60 Hz. I switched to running all 5 speakers full range without subs and listen with "multi Ch Stereo" and still able to get excellent dual bass effects. Your speakers are really bass shy!

 

It seems my Yggdrasil was lying to me in the bass department.
Now checking this track on Denafrips Terminator I have a different image.

There is one big drum slightly on the left, hit with 2 mallets. And 2 smaller drums hard panned to the left and right side.

The stereo bass is still there.

 

Nice! So now you are hearing big drum and smaller drum effects like I initially commented in another thread. Do you hear also differences in back and front positions of drums? Regarding 2 mallets and 2 smaller drums, hmm, nice to know. I thought when using 5 speakers full range, I was getting extra details, but was only checking if I could get bass since you said your speakers were bass shy.

 

Great thread!
I am interested in adding dual subs to my stereo setup and need as much knowledge as possible.
I have the luxury of using my preamp (NAD M12) to handle all of the bass management. It can do high and low pass crossovers in either stereo or mono for two subwoofers.
My upgraded main speakers (Vandersteen Treo CT’s are on the way!) do not go as low as my current speakers (Vandersteen 2CE Signatures). This has me researching which subwoofers, settings, placement, etc.

 

Well this thread is....inconclusive at best. Which isn't a surprise, considering the two pop song test tracks that have been used so far.

If you want to hear the advantage of a true stereo bass setup, check out this album


On track number 3, Mother Earth, after about the half minute mark there is an upright bass positioned on the left and right side of the room and they're both playing a different passage.

My setup is: Klipsch Cornwall IVs powered by an audiolab 6000a Play and a Windows PC running Audirvana feeding a Schiit Modius DAC as a source.

I also have two cheap Klipsch subs (R-12SW). The audiolab doesn't have any bass management capabilities, it only has pre-outs. So the right sub is fed by the R pre-out and the left sub by the L pre-out and the CWIVs are running full range. The subs are crossed over at 40hz, which means they're basically just picking up where the Cornwalls fall off.

I don't normally even use these subs for music, I feel the CWIVs produce more than enough bass for music and it's tight, textured and detailed. And I feel like having these cheap subs hooked up cheapens the whole system lol. But they performed well enough for this experiment, and actually sounded pretty damn good IMO. Hell I may start using them more.

But yeah guys, if you want to test stereo bass, you need to use music with instruments arranged on a stage.