Stereo Bass using subwoofers

[Duke]

Wouldn't subs being out-of-phase by 180deg result in cancellation?

I like @j_j's response better than mine.

Resorting to an anecdote: The first time I tried it, it was by mistake. I had delivered a little four-sub system to someone and we set them up arrayed around his Maggies sort of like half a Stonehenge. But I had I mistakenly hooked up the two subs on one side in reverse polarity. We listened to an FM radio broadcast of a classical concert (I think that was the only source functional at the time) and the sense of envelopment was surprising and enjoyable. We didn't notice the lack of bass impact, probably in part because our expectations for FM radio were not very high. We later fixed the wiring mistake and the low-end thumpage was much better but the sense of envelopment was reduced.

With the subs hooked up out-of-phase there was definitely more bass than from the Maggies all by themselves, so the cancellation was partial rather than total.

 

[bmc0]

As we probably can't hear any directional cues at those low frequencies between 40 and 90 Hz in small rooms, what is it that we will experience with a good recording made in a good venue?

Re-reading this and j_j's reply, it appears that I misinterpreted your question a bit. In my mind, I think of envelopment as "directional" in some sense, even though the "direction" is "all around". It's true that one doesn't generally perceive low frequency content in music as sharply localized like higher frequencies can be.

Griesinger states that LF envelopment is due to chaotic fluctuations in interaural level and phase (directional cues) occurring at a rate of ~3Hz-20Hz.

 

[j_j]

Well your insight has turned my understanding of measurements upside down. Until now, I believed that if you measure a null, you hear a null. But you don't hear a null, because your body converts that null back (i.e. maximum velocity) back into pressure. Instead, you hear god-knows-what depending on the size of your body.

Geez that complicates things. Now I see why some people measure from their listening position with dummies to simulate bodies. And I am starting to see why BACCH's ORC (which uses in-ear microphones) makes sense.

I am in UTTER agreement with your last paragraph, you bet it complicates the <bleep> out of the issue. In-ear, however, has another wart, in that you will then perhaps EQ out your HRTF and comfuse things that way.

 

[j_j]

Re-reading this and j_j's reply, it appears that I misinterpreted your question a bit. In my mind, I think of envelopment as "directional" in some sense, even though the "direction" is "all around". It's true that one doesn't generally perceive low frequency content in music as sharply localized like higher frequencies can be.

Griesinger states that LF envelopment is due to chaotic fluctuations in interaural level and phase (directional cues) occurring at a rate of ~3Hz-20Hz.

It's interesting to hear that. Last time I talked to David I did not hear him say the bit about fluctuations, but I can comfortably say that I think it's a bit more complicated than that, and that head movement is involved, but that it's not by any means wrong, either, perhaps just incomplete.

 

[Keith_W]

Hmm. I think I have a tutorial on that tucked away somewhere. I'll have to go look.

When you find it, please post it. Your last few posts have been world changing for me, and I would love to have something a bit more formal to read.

 

[bmc0]

It's interesting to hear that. Last time I talked to David I did not hear him say the bit about fluctuations, but I can comfortably say that I think it's a bit more complicated than that, and that head movement is involved, but that it's not by any means wrong, either, perhaps just incomplete.

I'm sure my understanding of his position is not complete. Regarding ILD and ITD fluctuations, he said (source):

ITD and ILD can be detected for tones, and in a free field these sounds can be localized in the horizontal plane. The perception of spaciousness and envelopment arises when ITDs and ILDs are detectable but not stable. Room reflections arrive at the listener chaotically from all directions, causing the ILDs and ITDs at the listener’s ears to fluctuate randomly at rates that depend on the frequency of the music and the size and reverberation time of the space. Fluctuations lower than about three Hertz are heard as motion – a kind of wander or swirl. Rates faster than about 20Hz are heard as a kind of tone or buzz. But between 3Hz and 20Hz these fluctuations are perceived as a sense of enveloping space.

 

[levimax]

Great thread and a lot to think about. I am working on a 4 sub integration project with my system and originally I summed to mono but after some expiramenting after reading this thread I am preferring stereo subs but I can't really call the results "controlled testing".

A couple of questions:

1. How does "hearing through the room" fit in with all of this?

2. Does group delay come into play with AE? 2 of my 4 subs are old school DIY large infinite baffle enclosures with almost no GD and 2 are new small enclosures with large drivers and lots of DSP and lots of GD ( ~ 125 ms @ 20 Hz). This is making integration challenging but it seems like it might mess up AE as well?

 

[EJ3]

Not that I don't believe you, but I am wondering why "less punchy bass" should be the result of setting up subs in stereo?



Wouldn't subs being out-of-phase by 180deg result in cancellation?

Less punchy BASS? Something is not right.
Less enveloping?;YES!
I do not have phase controls on mine, so I have to rely on placement 100%. Since mine are floor firing, under the mains seems to work fine for me.

 

[HiMu]

Not that I don't believe you, but I am wondering why "less punchy bass" should be the result of setting up subs in stereo?

Set up properly, that shouldn't be the case.

That is a fair question. Less punchy in a similar very subtle way as the enveloping effect, as with most acoustics related phenomenas it is hard to try to describe the effect with words. I had a few friends visiting and most of them couldn't hear any difference in the bass whether it was mono or stereo, but the guy who was the most skeptical due to technical knowledge from pro audio and music production heard the difference quite clearly.
Room layout for clarification.

 

[j_j]

Great thread and a lot to think about. I am working on a 4 sub integration project with my system and originally I summed to mono but after some expiramenting after reading this thread I am preferring stereo subs but I can't really call the results "controlled testing".

A couple of questions:

1. How does "hearing through the room" fit in with all of this?

Bass is the hardest thing to overlay in a different room. The "more the merrier" is about the best you can do. You won't be able to overwrite bass modes, but you may be able to make the original environment (if such exists) more audible than the listening room.

2. Does group delay come into play with AE? 2 of my 4 subs are old school DIY large infinite baffle enclosures with almost no GD and 2 are new small enclosures with large drivers and lots of DSP and lots of GD ( ~ 125 ms @ 20 Hz). This is making integration challenging but it seems like it might mess up AE as well?

Yeah, that is a LOT of delay. Is this group delay (varies with frequency) or is it time delay, actually? It could be either, and "lots of DSP" suggests "lots of latency" is a possibility. There's no way for me to tell from here, I fear.

 

[HiMu]

I'm sure my understanding of his position is not complete. Regarding ILD and ITD fluctuations, he said (source):

For anyone interested in testing whether their soundsystem and room is capable of reproducing low frequency envelopment, on page 44 of that source there is dropbox link to test tones provided by Griesinger.
Had not tested this before with such test tones but I had noticed the similar effect with music, I can turn my head in any direction and the sound seems to come from all directions.

Quote from the paper:

I played these five tracks in my studio using the two lateral
subwoofers described previously. The sound was moving
and spacious in all five bands. I then switched on the main
speakers and their subwoofers. The lowest four bands were
just as spacious as they were without the front speakers, but
the highest band, at 100°Hz, was not spacious. It seems the
7ms delay in the side channels of L7 has an important
function.
The five test bands are in my opinion a successful method of
checking a system for its ability to reproduce envelopment.

 

[levimax]

Yeah, that is a LOT of delay. Is this group delay (varies with frequency) or is it time delay, actually? It could be either, and "lots of DSP" suggests "lots of latency" is a possibility. There's no way for me to tell from here, I fear.

I am pretty sure it is group delay, see my measurements below with red being my DIY subwoofer (big sealed enclosure) and the green line being my SVS SB-3000 (small sealed DSP). Below that is a 3rd party measurement for the SVS sub that is close to what I got. Obviously I am having some trouble integrating these 4 subs (2 of each) together due to the group delay but I don't want to derail this thread with that rather I am curious about how GD could potentially affect AE if at all. I also am thinking that the main driving force for subs these days is "smaller at all costs" and it seems to me that one of these costs is sound quality even if they do manage to get the "marketing specs." to look good.

 

[RayDunzl]

Anyone have experience running 2 subs in a stereo vs mono configuration?

Yes.

My subs are adjacent to the mains (and the little "Daily Drivers".

When the music is mono in the bass I get a big hole in the bass around 48Hz due to standing waves from the asymmetrical rear of the toom.

When the music is stereo in the bass the difference in phase in that frequency area moves that null away from the listening position.

Either way, I don't really hear the hole, didn't know it was there till measured. I suppose I either "hear" the direct wave or imagine the bass from the context and harmonics.

 

[gnarly]

That is a fair question. Less punchy in a similar very subtle way as the enveloping effect, as with most acoustics related phenomenas it is hard to try to describe the effect with words. I had a few friends visiting and most of them couldn't hear any difference in the bass whether it was mono or stereo, but the guy who was the most skeptical due to technical knowledge from pro audio and music production heard the difference quite clearly.

I also think multi-sub deployments are more likely to be less punchy as sub count goes up.
I think rooms mask this phenomenon, especially small ones. Bigger the room, the less the masking.
Tactile feel that is more in the mode of steady vibration, like hair tingling, pants flapping, or chest resonating......multi-works just fine ime.
But punch, the hit in the chest, the bass drop that sends loose beers cans flying, comes from a strong vector...which is usually easiest to obtain from a single sub cluster.

Over the last dozen years I've amassed a near ridiculous amount of subwoofer power. (I wanted to partner with some folks for live sound, but found I don't like the late night hours gigs often require.)
Anyway, I've built a lot of subs...sealed, reflex, front-loaded horns...and bought a few more. I've compared then indoors and out, in single, stereo, and multiple sub fashion.
Subs definitely have a directional punch ime. And with multiples it's a vectored punch.

Heck, even a single reflex sub has a directional punch, depending on where the ports are in relation to the cone.
I've built double 18" subs with the ports surrounding the drivers in the front baffle like typical, with ports firing to to floor, with a slot port facing outward between two opposed drivers facing each other clamshell style, and drivers in a 90 degree frontal angle with a large port below them. They all hit a little different, and it's almost track by track which sub will hit hardest on it. (They are were tuned maximally flat to within a few Hz, using the same 18" BMS drivers.)

Anyway again, I know this thread is really in reference to home audio rooms, but it don't think it takes that big of a room, to feel the hit difference from a single sub (with balls) vs multi-subs (also with balls).

 

[bmc0]

I am pretty sure it is group delay

The measurements seem to suggest lots of excess group delay. Here's what I get for the estimated minimum phase group delay using Erin's SB-3000 frequency response measurement:

I'm not entirely sure what SVS might be doing to cause the group delay to be ~5 times higher at 20Hz.

 

[levimax]

The measurements seem to suggest lots of excess group delay. Here's what I get for the estimated minimum phase group delay using Erin's SB-3000 frequency response measurement:
View attachment 433055
I'm not entirely sure what SVS might be doing to cause the group delay to be ~5 times higher at 20Hz.

I am pretty sure the excess GD is from the heavy handed DSP required to get close to flat response at 20 Hz with a 13" driver in a very small box. Playing around with these I also notice some "pre-ringing" in the impulse response which again points to DSP. If I knew what I know now I would not buy DSP subs.

 

[j_j]

Yes.

My subs are adjacent to the mains (and the little "Daily Drivers".

When the music is mono in the bass I get a big hole in the bass around 48Hz due to standing waves from the asymmetrical rear of the toom.

When the music is stereo in the bass the difference in phase in that frequency area moves that null away from the listening position.

Either way, I don't really hear the hole, didn't know it was there till measured. I suppose I either "hear" the direct wave or imagine the bass from the context and harmonics.

Or your body interferes with the volume velocity at 48Hz to some extent. Not "lots" but perhaps some. With stereo signal in bass, yes, you'll move the zeros around the room, too.

No, this is not easily calculated.

 

[j_j]

I am pretty sure the excess GD is from the heavy handed DSP required to get close to flat response at 20 Hz with a 13" driver in a very small box. Playing around with these I also notice some "pre-ringing" in the impulse response which again points to DSP. If I knew what I know now I would not buy DSP subs.

Yeah, that's due to the lowpass system that flattens the drivers. That is a lot of group delay, frankly I don't think that the audibility of that has been tested, but yeah, it's a lot, enough to give pause to me.

 

[j_j]

The measurements seem to suggest lots of excess group delay. Here's what I get for the estimated minimum phase group delay using Erin's SB-3000 frequency response measurement:
View attachment 433055
I'm not entirely sure what SVS might be doing to cause the group delay to be ~5 times higher at 20Hz.

Huge IIR filters doing frequency shaping into the driver will do that.

 

[bmc0]

I am pretty sure the excess GD is from the heavy handed DSP required to get close to flat response at 20 Hz with a 13" driver in a very small box.

I agree it's related to their processing, but DSP does not necessarily cause excess group delay (ignoring latency since it's pure delay). In fact, for simply extending the LF response of a sealed system like this, it seems to me that it would be much easier to not cause lots of excess group delay.

Huge IIR filters doing frequency shaping into the driver will do that.

Perhaps I'm missing something, but I don't see why "Huge IIR" filters would be needed in this case. Since this is a sealed box sub, it should exhibit a 2nd order highpass response, which can be trivially corrected to any desired corner frequency and Q using a minimum phase digital biquad filter. As both the sub and filter are minimum phase, the result must also be minimum phase.