Subs in mono when phase problems arise

[alaios]

Hi all,
I am making this post so I can better understand when sub phase problems typically arise in space. I am talking about cases that speakers are connected in "mono" (by that I mean that there is one rca cable getting out of the amplifier being split to two rca cables that end up to the subs)

The example I have created here shows two setups that according to my friend (he is not an expert thus the post here) claims that positioning two subs, either in the two A positions or either in the B positions can create phase issues.
According to my friend for not having phase issues there are two options:
a. having both subs on the furniture
or
b. having both subs at the side of the furniture (try to avoid the temptation to discuss the topic that subs should not be atop of furniture, this is discussed more here: https://www.audiosciencereview.com/forum/index.php?threads/measurements-with-a-tiny-sub.42413/)

but I kindly ask you to help me understand what might cause any phase misalignemnts. To my brain I find that both the two As and the two Bs will not have any phase issues since distances to ear are quite close. We are talking about listening positions in the order of 1.5 to 3.5 meters.

When do phase problems withing two subs really arise? You might also want to note that those tiny subs only have two options for phase alignment, it is either 0 or 180 degrees so fine tuning them is not possible.
Thanks a lot
Have a nice weekend.

Picture follows below

 

[jsilvela]

Phase and subwoofers are tricky. The short and unsatisfactory answer is that you're going to have phase issues no matter how you place your subs. Well, unless you stack them.
The interference patterns are frequency related. For example, at 80Hz, the half-wavelength would be about about 2.1m.

Even with a single sub, you'll have strong "phase" issues because typical room dimensions are comparable to half-wavelengths for low frequency sounds, and reflected sound will interfere with direct sound.
More troubling, room modes at low frequencies will wreck your frequency response.

If you haven't done so yet, I suggest that you get a calibrated mic and start playing with REW.
REW has an amazingly useful room simulator where you can drop your two woofers and predict the response as you change their locations.

You could also look at the work of Welti on multi-sub placement. There's no shortage of threads at ASR on this.

 

[jsilvela]

BTW since you specifically mention mono: a lot of the AVR's have mono subwoofer output, even using two separate RCA outs.
There's quite a bit of agreement that "stereo" subs don't really make sense, though it is useful to be able to adjust phase/delay on each.

 

[alaios]

Phase and subwoofers are tricky. The short and unsatisfactory answer is that you're going to have phase issues no matter how you place your subs. Well, unless you stack them.
The interference patterns are frequency related. For example, at 80Hz, the half-wavelength would be about about 2.1m.

Even with a single sub, you'll have strong "phase" issues because typical room dimensions are comparable to half-wavelengths for low frequency sounds, and reflected sound will interfere with direct sound.
More troubling, room modes at low frequencies will wreck your frequency response.

If you haven't done so yet, I suggest that you get a calibrated mic and start playing with REW.
REW has an amazingly useful room simulator where you can drop your two woofers and predict the response as you change their locations.

You could also look at the work of Welti on multi-sub placement. There's no shortage of threads at ASR on this.

thanks a lot for the detailed reply
My first reaction on your reply was that I would need at the end to invest to a dsp to start fixing such issues. It really starts to look unavoidable..

 

[jsilvela]

thanks a lot for the detailed reply
My first reaction on your reply was that I would need at the end to invest to a dsp to start fixing such issues. It really starts to look unavoidable..

So, you're using an integrated amplifier, and a pre-out for the subs?

Either a DSP box or an AVR will be required, yes, unless you're very lucky with your room acoustics, or you're willing to do a lot of tests.
With two subs, in a relatively symmetrical/ rectangular room, you might give a try to two configurations proposed by Welti:
Mid-front and mid-back; or mid-left and mid-right.
The Welti configurations use physics to try to reduce room modes with symmetrical sub placement.

Getting good low frequency requires some work, definitely, but is worth it.
I'm a beginner myself.

 

[jsilvela]

Is the little white sub a REL TZero?

 

[DVDdoug]

I am talking about cases that speakers are connected in "mono" (by that I mean that there is one rca cable getting out of the amplifier being split to two rca cables that end up to the subs)

The bass on most recordings is "mono" (identical in both channels).

The one thing you should NOT do is use a Y-splitter in reverse to combine left & right signals together. You should never "short" two outputs together.

My first reaction on your reply was that I would need at the end to invest to a dsp to start fixing such issues. It really starts to look unavoidable..

That's OK and it's "easy" but DSP can only fix half of the problems.... It can cut frequencies that are resonating ("bumps" in the frequency response) but it can't fix "dips" where the bass is canceled because that takes "infinite" power and "infinitely large" woofers.

Acoustic treatment (bass traps) can help with both problems, smoothing-out the bass by reducing the reflected frequencies that are added and frequencies that are subtracted. So in perfect world you use acoustic treatment first and then DSP to fix any remaining issues.

...I'm not saying any of this is "necessary". I don't have dedicated DSP or acoustic treatment. I ran the automatic adjustment/calibration that's built-into my AVR... And I'm not sure exactly what it's doing, and I still adjust the subwoofer level to-taste by-ear.

 

[alaios]

Is the little white sub a REL TZero?

yes it s

 

[alaios]

The bass on most recordings is "mono" (identical in both channels).

The one thing you should NOT do is use a Y-splitter in reverse to combine left & right signals together. You should never "short" two outputs together.


That's OK and it's "easy" but DSP can only fix half of the problems.... It can cut frequencies that are resonating ("bumps" in the frequency response) but it can't fix "dips" where the bass is canceled because that takes "infinite" power and "infinitely large" woofers.

Acoustic treatment (bass traps) can help with both problems, smoothing-out the bass by reducing the reflected frequencies that are added and frequencies that are subtracted. So in perfect world you use acoustic treatment first and then DSP to fix any remaining issues.

...I'm not saying any of this is "necessary". I don't have dedicated DSP or acoustic treatment. I ran the automatic adjustment/calibration that's built-into my AVR... And I'm not sure exactly what it's doing, and I still adjust the subwoofer level to-taste by-ear.

I am not sure what you mean by not "shotening" two outputs. This is what I am using directly after my amplifier output so I can feed the two subwoofers. Is it wrong?

https://www.amazon.de/dp/B07V9P5MSC?psc=1&ref=ppx_yo2ov_dt_b_product_details

and why you keep adjusting the suwoofer level ? I guess once you configure it once you should be happy not? I have it at the level where main speakers seem to be reproducing bass

 

[alaios]

So, you're using an integrated amplifier, and a pre-out for the subs?

Either a DSP box or an AVR will be required, yes, unless you're very lucky with your room acoustics, or you're willing to do a lot of tests.
With two subs, in a relatively symmetrical/ rectangular room, you might give a try to two configurations proposed by Welti:
Mid-front and mid-back; or mid-left and mid-right.
The Welti configurations use physics to try to reduce room modes with symmetrical sub placement.

Getting good low frequency requires some work, definitely, but is worth it.
I'm a beginner myself.

what is the input and output of the DSP unit? The input is the RCA from the amplifier and the output is 1 or two RCAs?

 

[jsilvela]

what is the input and output of the DSP unit? The input is the RCA from the amplifier and the output is 1 or two RCAs?

That could work, yes. Here is an example https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd

BTW going to comment on your earlier questions:

- The "short" was meant for the case you were using an RCA "Y" cable to combine to sources to a single output, thereby creating a kind of "short circuit". As far as I can tell this is not your case. You're using the "Y" to feed two subs with a single source, right?

- You'll need to adjust the sub's gains for 1) matching to the Left-Right, 2) like many here, you may find it's nice to have a non-flat frequency response where the low frequencies are boosted a few decibels. After having found your desired setting, the gain would not need to be changed though.

 

[FrankW]

BTW since you specifically mention mono: a lot of the AVR's have mono subwoofer output, even using two separate RCA outs.
There's quite a bit of agreement that "stereo" subs don't really make sense, though it is useful to be able to adjust phase/delay on each.

There is some disagreement about mono sub bass only, making sense https://www.audiosciencereview.com/...than-digital-right.37657/page-20#post-1513869

 

[FrantzM]

Hi

I tend to take phase issues of mono bass as more of an academic issue than of audible/perceptual/enjoyment. IOW, these are barely perceptible and if ever, not enough for most people (as in 99.9999999999%) of the world audiophile population, to bother.
I have from time to time tried to listen to subwoofers alone and in many cases, the output is low,; even on some music pieces that are supposedly "bass heavy", the subwoofers output is faint...

My own conclusion: Mono bass under 80 Hz (IME under 100 Hz) and its phase problems with respect to the mains are not an issue.
I wait to be educated on my position.


Peace

 

[alaios]

very interesting point Frantz lets see how we can discuss those more. Do we really need dsp for subs for normal listening?

 

[RayDunzl]

but I kindly ask you to help me understand what might cause any phase misalignemnts.

Picky Opinon:

Any reflective surfaces that are not symmetrical with the speakers.

I have a bottomless 48Hz pit because the left rear corner of the otherwise rectangular room opens to the kitchen/breakfast area.

With stereo bass the hole at the listening position fills in, but it surely appears some other place in the room, which becomes a "don't care".

 

[JohnnyNG]

Picky Opinon:

Any reflective surfaces that are not symmetrical with the speakers.

I have a bottomless 48Hz pit because the left rear corner of the otherwise rectangular room opens to the kitchen/breakfast area.

With stereo bass the hole at the listening position fills in, but it surely appears some other place in the room, which becomes a "don't care".

"Whack-a-mode".