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Collapsing reverb in mono

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Collapsing reverb in mono

Postby apaclin » Thu Jun 03, 2021 10:16 pm

Hey gais!

Check Hozier's song "Almost" in mono and in stereo. Andrew Scheps made the reverb exist in stereo, but it collapses when you switch to mono.

https://www.youtube.com/watch?v=JJ9IX4zgyLs

Can you describe how do you do that?
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Re: Collapsing reverb in mono

Postby Hugh Robjohns » Thu Jun 03, 2021 10:51 pm

Not sure what you mean by 'collapsing', and I have no means of listening to the YT track at present... But if you mean the reverb largely disappears then thats quite common. Reverb -- both natural and artificial -- usually has a high degree of stereo incoherence -- there are a lot of 'out-of-phase' components which gives it that spacious effect, and they cancel and disappear when you sum to mono, so the reverb naturally tends to be noticeably drier in mono than in stereo.

The better artificial reverbs allow the coherence to be adjusted, to enable the mono/stereo compatibility to be tweaked.

The same adjustment can be effected by optimising the stereo mic techniques when recording in real acoustic spaces.
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Re: Collapsing reverb in mono

Postby blinddrew » Fri Jun 04, 2021 9:52 am

What Hugh said ^^^. Toggling between mono and stereo that sounds like a fairly typical 'reverb collapse' to me.
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Re: Collapsing reverb in mono

Postby Hugh Robjohns » Fri Jun 04, 2021 10:13 am

Had a listen to the YT track.

Pretty much all the reverb is carried out at the edges of LR stereo image and so is only in the Sides channel -- something that's easy to check by listening to the stereo difference signal and comparing it to the mono signal. The reverb is therefore in opposite polarities in the left and right channels and so inevitably cancels and disappears completely when listening in mono.
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Re: Collapsing reverb in mono

Postby apaclin » Fri Jun 04, 2021 11:31 am

Just reversing the phase of left or right channel of the reverb doesn't cancel it out.

Let me demonstrate how it sounds.

Stereo, reverb audible:
https://www.dropbox.com/s/69dagv2d6nv80 ... o.mp3?dl=0

Mono, reverb disappeared:
https://www.dropbox.com/s/zm04u9o8bdhqj ... o.mp3?dl=0
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Re: Collapsing reverb in mono

Postby Wonks » Fri Jun 04, 2021 11:50 am

Are those your examples? If so I'm not sure what you are doing there, you'll need to explain in a bit more detail. If you've reversed a reverb channel phase, then a non-reversed example would also help.

But what you do compared to what they did is all going to depend on the reverb they've actually used and how they've processed it (compared to what you used). They've probably fed the reverb channel into a m/s processor and pushed the sides out a lot wider, or used a reverb with a built-in m/s function.

if the reverb doesn't collapse in mono, it's because there is very little sides element to it. You need to create a reverb with a lot of sides element to it, then those will 'collapse' in mono.
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Re: Collapsing reverb in mono

Postby Hugh Robjohns » Fri Jun 04, 2021 12:35 pm

apaclin wrote:Just reversing the phase of left or right channel of the reverb doesn't cancel it out.

I fear some misunderstanding!

Reversing the phase of one channel and summing to mono -- to listen to the 'stereo difference' signal -- will reveal JUST the reverb.

Summing to mono cancels virtually all of the reverb out because the reverb is largely incoherent (opposite polarities on each channel).

That's just the nature of the reverb used in that particular Hozier track.

As a general rule, most reverbs -- real and artificial -- contain a lot of incoherent signal and so collapse fairly substantially when summed to mono. But that's certainly not the case for all reverbs and some artificial reverbs are intentionally designed to suffer fairly minimal cancellations when summed to mono.

Let me demonstrate how it sounds.

Both your clips are mono. Are they meant to be?
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Re: Collapsing reverb in mono

Postby apaclin » Fri Jun 04, 2021 7:56 pm

Hugh Robjohns wrote:
I fear some misunderstanding!

Reversing the phase of one channel and summing to mono -- to listen to the 'stereo difference' signal -- will reveal JUST the reverb.


Never mind what I wrote about the phase. Just turning reverb into mono doesn't make disappear, but in "Almost" example it does and I wanna know how.

Hugh Robjohns wrote:
Both your clips are mono. Are they meant to be?

No, they are not both in mono, they are just as written.
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Re: Collapsing reverb in mono

Postby Hugh Robjohns » Sat Jun 05, 2021 12:11 am

apaclin wrote: Just turning reverb into mono doesn't make disappear, but in "Almost" example it does and I wanna know how.

...and I explained. The reverb is highly incoherent.

It may have been the natural acoustic and recorded in a specific way to achieve that effect.

More likely it's an artificial reverb designed or programmed to sound like that. The better hardware Lexicon reverbs allow the coherence to be varied in program mode, for instance.

In extremis, generate a mono reverb, duplicate it, pan the two reverb tracks hard left and right, and polarity invert one or them. You'll then have a big spacious reverb in stereo, and no reverb at all in mono. Its not great for 'mono compatibility' of course, but if that's the sound you want, that's one simple way to get it.

No, they are not both in mono, they are just as written.

Not here...
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Re: Collapsing reverb in mono

Postby James Perrett » Sat Jun 05, 2021 12:22 am

Hugh Robjohns wrote:
No, they are not both in mono, they are just as written.

Not here...

The first one is stereo and the second one is mono here. And mono-ing the first one gives the second one. The strange thing is that the reverb doesn't sound very spread out to me (in fact it sounds almost mono) but it completely cancels out in mono. In the old days cheap reverbs would often invert the polarity of one output in order to make the sound seem more spread out and make you think they were stereo - I don't know if that is what is going on here.
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Re: Collapsing reverb in mono

Postby Tim Gillett » Sat Jun 05, 2021 2:19 am

James Perrett wrote:
Hugh Robjohns wrote:
No, they are not both in mono, they are just as written.

Not here...

The first one is stereo and the second one is mono here. And mono-ing the first one gives the second one. The strange thing is that the reverb doesn't sound very spread out to me (in fact it sounds almost mono) but it completely cancels out in mono.

Perhaps the reverb source is basically mono, and has been flipped on one channel, but the right side reverb signal is a little higher in level than the right. So in stereo the reverb signal appears (to human perception) pushed further to the right than panning alone would. Like the old trick of making a source seem to emanate from further right than the right speaker's actual position, we need not only the 180 deg inversion but also a little level difference.

In monoed I think I hear just a little reverb which could be this small L/R level difference signal.
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Re: Collapsing reverb in mono

Postby Hugh Robjohns » Sat Jun 05, 2021 10:05 am

apaclin wrote:
Hugh Robjohns wrote:
Both your clips are mono. Are they meant to be?

No, they are not both in mono, they are just as written.

Apologies -- checked again afresh this morning. You're quite right. I had a funny routing setup here. Mea culpa.
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Re: Collapsing reverb in mono

Postby apaclin » Sat Jun 05, 2021 2:46 pm

Thanks for the answers! Seems like I found the way. Taking the mid component out of the reverb and then just monoing it cancels it out.
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Re: Collapsing reverb in mono

Postby Hugh Robjohns » Sat Jun 05, 2021 5:12 pm

Yes, that's a neat way of doing the same as previously described -- ensuring complete incoherence, and moving all of the reverb into the Sides (stereo difference) channel.
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Re: Collapsing reverb in mono

Postby Trackers » Sat Jun 05, 2021 7:28 pm

Tim Gillett wrote:
Perhaps the reverb source is basically mono, and has been flipped on one channel, but the right side reverb signal is a little higher in level than the right.

Tim Gillet is right. Reverb MONOisation then phase inversion is the smartest way to go.

With this trick, your music sounds completely different when played on mono sound systems. I love it !

I am quite sure Andrew Scheps used this method. I’ve just tried several approaches and this one sounds just like Hozier’s song :


1. As usual, create a stereo auxiliary channel and load a reverb.

2. As usual, feed this reverb aux channel (via aux send) with your piano, vocals, etc.

3. Make your reverb mono (e.g. by inserting a utility plugin just after the reverb plugin and summing the left and right signal).

4. Invert the phase on one side of your aux reverb track (e.g. by adding a second utility plugin on your reverb aux channel and inverting the left or the right signal).

There you go. The reverb channel is now 100% out-of-phase.
When you listen to your track in mono, the reverb is completely cancelled. It sounds as dry as the desert. :-)
Listening to your track in stereo, you may feel that your reverb is like heavily panned, btw just like in Hozier’s song mentioned above.

5. If you want to keep a little reverb when you play your track in mono, slightly pan your reverb channel left or right. Slight panning will also help you to better integrate your out-of-phase reverb in the stereo field of your mix.


Working with Mid-Side tools and keeping your reverb channel stereo throughout the whole process also works fine but it isn’t as easy and radical as the MONOed-antiphased reverb method.

I want my next project to be all about phase cancellation. It’s like magic. :-)

I wonder what happens with portable BT speakers like Boom and stuff. Do they sum L and R? I guess they don't but, since the L and R sources are so close, some phase cancellation should occur. I'll check that... :-)

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