Flanging on acoustic recording redux

[Stig Ø]

A few weeks back I posted a question here regarding an acoustic steel string recording, which I've linked to below for context. In short, I thought I heard a comb-filtering, or flanging effect on the recording, which none of the people here who listened to the file noticed. I have now recorded two short acoustic pieces, one centred around A major played in the open position, and one roughly sticking to E flat major, played with a capo on the third fret. Based on the tips I received in the linked thread, I have adjusted my miking technique; my LDC is now placed 30cm from the twelfth fret and pointed at the soundhole. The basic raw result sounds promising, so yay - learnt something!

The tune in Eb sounds good - it needed some EQing to cut some low end and shape the midrange, but otherwise I'm happy with it. For some reason the impression of subtle flanging is back in the piece in A major, but - and this is the strange bit - it's only noticeable when listening through monitors! With two sets of IEM/headphones there is no sign of the flanging, but switch back to monitors and it reappears!

Does anyone have an idea what might be causing this? The monitors are KRK VXT4. I was wondering if maybe the key of A major had some notes landing right around the crossover frequency, and that this was causing some phasing issues? I haven't been able to find information about what the crossover frequency actually is - the documentation isn't extensive to put it mildly.

The recording is fine, so it's not a problem that has to be solved - there is no flanging on the recording. I'm just trying to understand what's going on with my monitoring. Does anyone have any thoughts?

The original thread:
https://www.soundonsound.com/forum/view ... hp?t=93058

[Stuart79]

Room reflections.

[Sam Spoons]

A single mic recording? Two speakers playing identical sounds into the same space will cause comb filtering because each ear will hear sound from both speakers but you won't experience the same effect with headphones because the sound from the right transducer won't be heard by your left ear and vice versa.

You could try listening to the recording on a single monitor and see if the flanging goes away?

Adding stereo reverb may cause it to go away too as the signals feeding the left and right monitors will no longer be identical.

[Wonks]

It may be best if you can tell us exactly how many mics you’re using (one or two) and what you’re doing to the track(s) in your DAW.

If you are using two mics, have you tried aligning the two tracks so the transients match up, or have looked to see how far apart time-wise they look to be?

[Stig Ø]

Thanks for the replies, everyone. It's a single-mic recording with a Røde LDC placed 30cm from the 12. fret, pointing at the soundhole. The room is treated with some acoustic panels to cut down on flutter echoes, which has been quite effective. The dimensions of the room are not ideal, but the room is filled with amplifiers, various furniture, and guitar pedals, so it's not a naked, ringy room.

If it were room reflections (which I don't think, as the room is quite dead), I would expect to hear the phantom flanging on everything I played over the monitors. This does not happen.

If it were the result of the two monitors playing the same source, thereby causing phasing of some kind, I would expect to hear it on every source. This is not the case, either.

Both recordings are with the same mic and the same setup, recorded in a fairly dead room. I have EQed, compressed and added reverb to both tracks, but the "phantom flange" was always just heard on the A major piece, even before processing.

I would post the recordings, but I don't see the point as the flanging is not on the recording.

Thanks for reading and posting - I understand that this is tricky to diagnose. Maybe it's just one of those things one has to work around. I haven't gotten around to trying playback with only one speaker, but I will at some point. I'll report the results here for anyone interested.

[Tomás Mulcahy]

Get the trial version of Sonarworks Sound ID reference, and a measurement mic, and see what it measures for your room. It could be you have strong reflections at the frequency of the offending notes.

Or put in more/better absorbers especially at mirror points. Better usually means broadband rockwool with quarter wavelength gap, not foam stick on ones- but that's me taking a total guess at what you have in the room. These are at the sides and also the ceiling.

[Drew Stephenson]

If the problem is not on the recording (or audible on headphones) then it must be to do with either the speakers or the room. What happens if you turn off one speaker at a time? Does the flanging sound persist when you've only got one source playing?

[Sam Spoons]

If it were room reflections (which I don't think, as the room is quite dead), I would expect to hear the phantom flanging on everything I played over the monitors. This does not happen.

Not necessarily. And bear in mind your 'quite dead' room will still have a surprising number of reflections (a completely dead room (anechoic chamber) is very difficult to cope with, most people can only stand it for a few minutes).

If it were the result of the two monitors playing the same source, thereby causing phasing of some kind, I would expect to hear it on every source. This is not the case, either.

No only on mono sources, and maybe only sources that have particular frequencies in them as suggested by your A major piece exhibiting the problem while your Eb piece does not?

Both recordings are with the same mic and the same setup, recorded in a fairly dead room. I have EQed, compressed and added reverb to both tracks, but the "phantom flange" was always just heard on the A major piece, even before processing.

AIUI adding stereo reverb would reduce the effect as the signals reaching the monitors are no longer entirely mono/identical.

I would post the recordings, but I don't see the point as the flanging is not on the recording.

As Drew says if the flanging is not in the source recording it must be caused by something downstream, the only point where the two channels interact is after the sound emerges from your monitors so the problem must be related to your monitors, your room and/or your ears (not so far fetched as it might appear, ears have resonances too).

Thanks for reading and posting - I understand that this is tricky to diagnose. Maybe it's just one of those things one has to work around. I haven't gotten around to trying playback with only one speaker, but I will at some point. I'll report the results here for anyone interested.

Please do, I could easily be wrong (not uncommon) but I think listening on one speaker will remove the effect proving it to be caused by the room/monitors. Keep us updated.

[Stig Ø]

OK, I did some more testing/listening/swearing and this is what I found: muting one speaker didn't remove the flanging completely, but it didn't seem to be as prominent. I turned up the volume to compensate for the missing speaker (by ear). Did a control with headphones, and found out that what has caused it probably is a trait of the tonality of that guitar, combined with some reflections from the desk I'm sitting at (I guess).

I've never had these kind of problems before, so I must have been lucky. Mixed quite a bit of music on these monitors, and always felt I had a pretty good idea about what was happening. In this case I ended up redoing some EQ adjustments, and that helped. I also checked against a suitable commercial release. I think I'm OK with it now.

Final question: I usually aim for -14 lufs when mixing "band" songs (just me, but drums, bass etc.); what is a suitable level for acoustic solo guitar? I aimed for -18 the last time around, but thinking it could use a little bit more volume. Are there any rules of thumb regarding this? I think spoken word is -16(?), and thought that might be OK? I know that listeners (not many of those in my case) can turn up/down, but the established volume "limits" have been very helpful guides for me, so if anyone knows I'd love to hear about it.

Thanks again

[Drew Stephenson]

There are no rules obviously because it will depend on the material that's around it and whether it's sitting as part of an album or as a single.
That being said, double-checking my notes from my last couple of things, I find that more stripped back arrangements will often sit comfortably 2-3 LUFS below a typical band arrangement.

[Sam Spoons]

One last suggestion then, can you cover the desk with a duvet to see if that cures the 'flanging', if it's reflections off the desk that should pinpoint where the problem lies?

[Martin Walker]

One last suggestion then, can you cover the desk with a duvet to see if that cures the 'flanging', if it's reflections off the desk that should pinpoint where the problem lies?

I was about to suggest this as well, but if desk reflections turn out to be involved, another solution may be to angle your desk (I put 3-inch wood blocks under the two back legs of mine, and it noticeably improved the room sound at my ears, by shifting the reflections below them.

[Hugh Robjohns]

^ These two ideas!

I was going to suggest laying a folded beach towel over the desk to kill any reflections, but a duvet works too.

If the computer screen is forward of the line between the speaker baffles, then try covering that with a towel or duvet too, as you can get reflections and diffraction off the edges.

And if the towels/duvet fix the flagging effect, then try tilting the desk as Martin describes, and/or pushing the screen back behind the speakers.

[ajay_m]

Towels? Duvets? Surely some audiophile company has a solution made from the virgin threads of the Amazonian Orb Spider, hand-woven by dedicated artisans to create the ultimate reflection barrier, enhanced by quantum effects courtesy of our proprietary neutron-enhanced crystal coating.

I mean, seriously, if there are audiophile fuses, not to mention that magic cable with, I assume, individually hand-curated electrons precisely adjusted for mass and energy, I'd be so disappointed having to put a humble duvet on my desk.
And I'd want it to be black. Not just black, I want it to be black like Vantablack, a black hole absorbing every photon of light that hits it along with every soundwave from my monitors.