Stereo Bass using subwoofers

[dshreter]

Running dual subs in stereo is probably a bad idea.

Most of the time you are going to have a similar bass recording in both channels - for the same reason bass is difficult to localize, if a performance is recorded with stereo microphones, they are likely to "hear" pretty much the same thing during recording. So if R & L channels are similar, running in stereo will be functionally similar to running mono.

In the case where the right and left channels are different, it would be actively detrimental. Let's set as an extreme example that in production a bass guitar track is panned hard left and vocals are panned right. The bass frequencies of the bass guitar would all come out of the left subwoofer in that case. However, the primary reason you purchased dual subs was to even out the in-room frequency response. If all of the bass is coming out of the left subwoofer, you'll be left with the same peaks and nulls in room response as if you had only one subwoofer, and defeated the purpose of what you initially set out to correct.

Because bass is difficult to localize and not part of "imaging" so to speak, we can take advantage of that attribute. That lets us choose the sub location more flexibly based upon the in-room frequency response or deploy multiple subs for even further gains in response, without deteriorating the imaging performance of the system.

 

[dshreter]

In the case where bass management software is used, then dual-mono if better, as it allows smoother response and better null cancellation.

Wow, your response is so much more succinct than mine, yet still said everything I did with three paragraphs.

 

[digitalfrost]

I get the point, and you're not wrong, but I still prefer stereo subs. I can hear the room info. Things are lost if you go mono. If smoothing of response is really an issue, you could add additional mono subwoofers to the stereo subs I guess.

I never thought I'd be one of those people, but I can hear it, and it's not even a subtle difference. It could easily ABX this with statistical significance, but I don't have to because it's that obvious.

You're theoretical points are all fine, and I'm not arguing against theory. But really guys. Try stereo subs if you can.

 

[RayDunzl]

Stereo Bass, and black holes:

Red is the left and right speakers individually.

Green is left and right sounding the same signal simultaneously, with a mono signal (same signal to both sides).

Neither left nor right has a severe problem, but when combined, in this asymmetrical room, the resulting phase at the listening position craters in the 40 to 50 Hz range.

It isn't particularly noticeable, so, it is what it is.

When the bass in this area actually is in stereo, with different levels and phase of signal between the channels, the problem area is less severe.

Mono Phase is near 180 degrees difference (cancellation) between left and right in this area

 

[Biblob]

I guess a good usecase for running stereo with two subs, is when you want to EQ and/or delay them independently. So could you tackle room-modes more effectively and integrate them better with the mains.

I must say I haven't had experience with Room EQ yet so I don't know if this Is actually true. So correct me if I'm wrong.

 

[Pio2001]

My general understanding is that you want to run two subwoofers because having two evens out the in room frequency response due to cancellations

It may even out the room frequency response.
It may also worsen it. It all depends where they are located, what room modes are excited, and if these room mode have an effect or not at the listening position.

Is it possible that you had the stereo subwoofers out of phase?

Depending on the room modes and room gain from the walls, the relative phase of the waves coming from the two subs can be random.
Remember that even if the subs are at the same distance, there is no such thing as "direct sound" at these frequencies. Since we are talking about very slow oscillations, the reflections from the walls arrive at the listening position before the signal has completely left the CD player line output (still playing a waveform that is starting to rise from zero) !

Below around 80Hz you cannot localise the direction the sound is coming from, so stereo subs (one sent left and one sent right channel) are a tad pointless.

Actually, it is possible, even below 80 Hz. But it is indeed pointless, as recordings with useful stereo information in low frequencies are nearly non-existent.

Thanks Alan, I'm familiar with sub basics
Maybe I should have posted in the Topping DX7PRO forum, for others that use it in this way. Maybe a better question is in the recordings - is there separate info recorded below 80 hz in the L and R channels of most music? If so, this would help explain the discrepancy - each channel is adding to the sum whole.

The best example of stereo information below 80 Hz is the ambiant background noise of large spaces (churches, concert halls...) when it is recorded with a wide AB pair of microphones (more than two meters away from each other). These frequencies are decorrelated.
They are however nearly mono if they are recorded with a more traditional pair of microphones close to each other.
With an AB pair of microphones, you will also get a very different signal from music instruments in the low frequencies, but this is mostly a random effect that brings nothing good to the recording at these frequencies, because they will destructively interfere at some frequencies, while higher frequencies reproduction is correct.

In the french forum homecinema-fr, we experimented with recordings containing stereo low frequencies with Igor Kirkwood's system, that has 4 subs (left, right, front, back), that can be configured in a lot of experimental configurations with a QSC processor.
Link to the discussion (it deals with experimental M/S matricing, trying to improve envelopment effect in low frequencies, and it features measurements of decorrelation from the listening position with an artificial dummy head) : https://www.homecinema-fr.com/forum/post180106785.html#p180106785

In classical music, we tried Faure's requiem, that is recorded with a widely spaced pair of microphones. Analyzing the recording, even in this configuration, stereo information below 80 Hz is very rare. Actually, there is not much sound at all below 80 Hz.
But we located a note that was played below 80 Hz, rather on the right side. The effect is difficult to hear, even with headphones, because most of the note is composed of harmonics above 80 Hz.
The background acoustic rumble of some recordings is indeed stereo. But this rumble is not something that we want to hear to begin with.

In pop music, stereo frequencies below 80 Hz are also very rare, and when they are present, it is almost impossible to hear it, even with headphones.

The only very clear examples I could find were Front 242's track "Television Station", that has a very low frequency sound on one channel only (even on the vinyl !), and a track from an unknown artist, that features a decorrelated low frequency pink noise as introduction.
It's not worth buying two subs just to have a slight difference with these two tracks only.

If there is a difference in bass level in left and right channel you will always get more bass in scenario where bass is summed up to mono and then sent to both channels vs when it is played as stereo.

Not always. At some frequencies, the signal coming from both subs can be out of phase. At these frequencies, a stereo signal might sound louder than a mono one.
Below are some measurements made on my system by Jean-Luc Ohl. They are made from the listening position, without room correction, and there are only two small KH-120 speakers (no sub at all).
The interesting curve is the "low frequency coupling" (middle right). Everywhere the pink curve is above the blue, the out of phase component of the signal is louder than the mono component.

There is also another reason for having two subs, that is often forgotten : you have twice more power than with one sub !

 

[Pio2001]

By the way, here is a simple stereo test for low frequencies :

Pink_80_abcd.flac

This is a pink noise lowpassed at 80 Hz. Nothing at all is left above that frequency. The gain is -3 dB at 80 Hz, and -100 dB at 100 Hz.

It plays in stereo (decorrelated noise), then in mono, then on the left channel, then on the right channel.
If you can hear the difference, then you can hear stereo below 80 Hz.

 

[Pio2001]

Stereo Bass, and black holes:

Red is the left and right speakers individually.

Green is left and right sounding the same signal simultaneously, with a mono signal (same signal to both sides).

Neither left nor right has a severe problem, but when combined, in this asymmetrical room, the resulting phase at the listening position craters in the 40 to 50 Hz range.

Your message arrived when I was writing mine.
Very good exemple !

 

[Kal Rubinson]

In classical music, we tried Faure's requiem, that is recorded with a widely spaced pair of microphones. Analyzing the recording, even in this configuration, stereo information below 80 Hz is very rare. Actually, there is not much sound at all below 80 Hz.
But we located a note that was played below 80 Hz, rather on the right side. The effect is difficult to hear, even with headphones, because most of the note is composed of harmonics above 80 Hz.
The background acoustic rumble of some recordings is indeed stereo. But this rumble is not something that we want to hear to begin with.

As a somewhat fanatic about Faure's Requiem, can you tell me which recording you used?

 

[Pio2001]

As a somewhat fanatic about Faure's Requiem, can you tell me which recording you used?

Unfortunately, I don't know. It comes from David Griesinger, who was using it to test his theories about the stereo effect in low frequencies.

Here is the sample :
Fauré, original version.flac
Fauré with mono bass.flac
Fauré, only below 80 Hz.flac

 

[March Audio]

1 to 4 subs in my room. No dsp. Small BW asw608.

With dsp

 

[Kal Rubinson]

Unfortunately, I don't know. It comes from David Griesinger, who was using it to test his theories about the stereo effect in low frequencies.

Here is the sample :
Fauré, original version.flac
Fauré with mono bass.flac
Fauré, only below 80 Hz.flac

Thanks. I will get a chance to really hear them in the morning.

 

[AudioJester]

Bear with me, a novice question here.
Bass really goes upto 300-400Hz. Most bass in music is >80Hz. Subwoofers only work (without localisation issues) <80Hz.
So multiple subs are only fixing "sub bass" issues. What do we do from 80-400Hz where most of the bass in music is presented? Is this purely EQ/DSP?

 

[dshreter]

Bear with me, a novice question here.
Bass really goes upto 300-400Hz. Most bass in music is >80Hz. Subwoofers only work (without localisation issues) <80Hz.
So multiple subs are only fixing "sub bass" issues. What do we do from 80-400Hz where most of the bass in music is presented? Is this purely EQ/DSP?

From 80Hz+, we play that through our stereo main speakers. Besides that, above 80Hz it’s much more attainable to avoid significant room modes, while below 80Hz they must be managed in almost all ordinary rooms.

 

[Mnyb]

Bear with me, a novice question here.
Bass really goes upto 300-400Hz. Most bass in music is >80Hz. Subwoofers only work (without localisation issues) <80Hz.
So multiple subs are only fixing "sub bass" issues. What do we do from 80-400Hz where most of the bass in music is presented? Is this purely EQ/DSP?

Room EQ below circa 300 Hz for most rooms ( is it called the transition frequency? )

 

[DonH56]

https://www.linkwitzlab.com/rooms.htm
https://en.wikipedia.org/wiki/Room_acoustics

 

[Krunok]

Stereo Bass, and black holes:

Red is the left and right speakers individually.

Green is left and right sounding the same signal simultaneously, with a mono signal (same signal to both sides).

Neither left nor right has a severe problem, but when combined, in this asymmetrical room, the resulting phase at the listening position craters in the 40 to 50 Hz range.

It isn't particularly noticeable, so, it is what it is.


View attachment 46980

When the bass in this area actually is in stereo, with different levels and phase of signal between the channels, the problem area is less severe.

Mono Phase is near 180 degrees difference (cancellation) between left and right in this area

View attachment 46982

As phase difference is originating from putting SW's at different positions of the room it is SW related, not signal.

Only when amplitude of the signal is not equal in both channels in stereo scenario the cancellation will be reduced, but if the amplitude would be equal you will end up with the same canellation as with mono scenario.

This can be solved with phase correction at LP.

 

[Krunok]

Bear with me, a novice question here.
Bass really goes upto 300-400Hz. Most bass in music is >80Hz. Subwoofers only work (without localisation issues) <80Hz.
So multiple subs are only fixing "sub bass" issues. What do we do from 80-400Hz where most of the bass in music is presented? Is this purely EQ/DSP?

Jazz bass guitar goes down to 32Hz, pipe organ down to 20Hz. With electronic music bass can go as deep as musician want.

 

[Krunok]

Room EQ below circa 300 Hz for most rooms ( is it called the transition frequency? )

Yep, transition or Schroeder frequency. But it is not a hard limit so the room affects amplitude up to 600-700Hz, but progressively to the lesser extent.

 

[Krunok]

1 to 4 subs in my room. No dsp. Small BW asw608.

View attachment 47002

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With dsp

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Wow - well done and beautifully presented!