Left = flattish. Right = flattish. Both = a mess

[Jas0_0]

Hi all,

Today I've been trying to EQ my system using a UMIK, REW and Fabfilter Pro Q3. The 'after' plots are posted below.

I've measured and EQed each channel separately for a reasonably flat response (plan is to add a tilt from low to high frequencies later). Left is admittedly better than right. But when I measure both channels together it all goes wrong, with big dips around 190 and 380Hz.

Why is this happening? I haven't touched the mic or any settings. Are the speakers somehow cancelling each other out at those frequencies?

Many thanks,

James

 

[amirm]

That is what happens. Your two speakers measure slightly differently and when summed (with phase which you are not seeing) there can indeed be these cancellations/variations.

 

[tuga]

Should sound less "bad" than it looks with stereo that you imagine. Mono on the other hand...

 

[andreasmaaan]

Is your room and the locations of speakers and listener in it asymmetrical? Perhaps you could post a quick floorplan?

These dips could be caused by just the speakers, but that would be some pretty gross phase differences between them if the only cause. Much more likely the room is involved here IMO.

 

[Jas0_0]

That is what happens. Your two speakers measure slightly differently and when summed (with phase which you are not seeing) there can indeed be these cancellations/variations.

Thanks - is there anything I can do about it?

Is your room and the locations of speakers and listener in it asymmetrical? Perhaps you could post a quick floorplan?

These dips could be caused by just the speakers, but that would be some pretty gross phase differences between them if the only cause. Much more likely the room is involved here IMO.

Yes the room is quite asymmetrical. Here is a floorplan. The speakers are either side of a chimney breast. The door next to the listening position is kept closed. Unfortunately the domestic situation means I don't have much option to move the speakers.

 

[andreasmaaan]

Yes the room is quite asymmetrical. Here is a floorplan. The speakers are either side of a chimney breast. The door next to the listening position is kept closed. Unfortunately the domestic situation means I don't have much option to move the speakers.

I see. Presumably the asymmetry between the open double-doorway(?) on the left-hand side and the bay window on the right is what's doing it.

It can likely be improved with the use of channel-independent all-pass filters.

How are you implementing your filters currently? Would it be possible to add phase correction to your filters?

 

[thewas]

Thanks - is there anything I can do about it?

In this case usually only moving the loudspeakers or trying with some FIR filter based DRC, although the second often fails there too in my past experience.
If you plot the phase responses with some windowing like FDW with 5-15 cycles you will see that due to room influence the phases of both speakers rotate and at the positions the sum makes a dip they are usually in opposite phase. Better use also "var smoothing" so you can see if this dips are narrow or wide, in the first case they usually cause no big audible problems.

 

[ernestcarl]

... more likely the room is involved here IMO.

Reflections may be causing the phase in that specific area to veer too much at the measured listening position.

I’d measure each speaker nearfield and overlay their phases together to see if the issue lies within the speakers themselves or the room to try to isolate where it’s coming from.

 

[Jas0_0]

It can likely be improved with the use of channel-independent all-pass filters.

How are you implementing your filters currently? Would it be possible to add phase correction to your filters?

I don’t really know if I can do phase correction. I’m using a Mac and performing EQ using Fabfilter Pro Q3 plugin in Audirvana and Audio Hijack. Is there an Audio Units plugin that allows for phase correction?

Also sorry for my ignorance, but what is a channel-independent all-pass filter?

 

[sigbergaudio]

The best approach here is to understand and accept that a flat response in a normal living room situation is very unlikely. It's perfectly normal, and typically sounds much better than the graph looks. I would also advice against applying anything other than very few and broad filters. Especially in the higher frequencies just moving the mic (or your head) a few inches will completely change the response, so trying to "fix" individual peaks with EQ is futile.

Also, you could try reducing the resolution of your graph, then it will look better. Looks like you're within +/-6dB across the band, that's not half bad. Lastly, it looks a bit too flat, a bit more roll-off in the upper frequencies are usually preferrable. You only appear to have about -3dB at 10khz, you will probably find a more pleasant sound if you do at least -6dB.

For reference, here's my living room, 1/6 smoothing (probably with a bit more bass than some would like), notice the downwards slope towards the highs.

 

[Jas0_0]

In this case usually only moving the loudspeakers or trying with some FIR filter based DRC, although the second often fails there too in my past experience.
If you plot the phase responses with some windowing like FDW with 5-15 cycles you will see that due to room influence the phases of both speakers rotate and at the positions the sum makes a dip they are usually in opposite phase. Better use also "var smoothing" so you can see if this dips are narrow or wide, in the first case they usually cause no big audible problems.

Thanks. I have to admit I've never really measured for phase. Is there a tutorial somewhere of how to do this? And then how would I correct for the phase issues once identified?

Here are the VAR-smoothed plots:

 

[thewas]

Thanks. I have to admit I've never really measured for phase. Is there a tutorial somewhere of how to do this?

When doing sweep measurements with REW, the phase is measured also.
Good reading resource is the REW manual https://www.roomeqwizard.com/help.html

And then how would I correct for the phase issues once identified?

With non-free DRC tools like Acourate, Audiolense etc. or with free ones like Rephase, mind you this is not simple stuff and something learnable in a day.

Here are the VAR-smoothed plots:

They look quite fine, in that case I wouldn't really worry unless you hear audible issues that you don't like.

 

[RayDunzl]

Are the speakers somehow cancelling each other out at those frequencies?

I have a phase-induced bottomless pit around 48hz in my room at the Listening Position.

Neither left nor right is terrible around that frequency, but because the left rear corner of this otherwise rectangular room is open to the kitchen area, and deeper than the right side, something happing.

You can see it in the RTA of music playing earlier this evening (it's not morning until I wake up)





Sweep Measurement:
Red, left and right
Green, both

180 degree phase difference at the crosshairs:

I didn't know it was there until measured, so despite the appearance, well, it doesn't seem to be bothersome.

Assuming I'm interpreting the Psychoacoustic Smoothing correctly, it's only a 5dB dip, perceptibly.

 

[ernestcarl]

In a nearfield setup clear of significant boundaries with favorable placement, the traces will look "nicer".

Frequency dependent windowing (FDW) will help clear out some of the ripples/reflection induced phase rotation.

SPL & Phase view

Messy without smoothing and/or FDW.

 

[ernestcarl]

channel-independent all-pass filter?

https://en.wikipedia.org/wiki/All-pass_filter

Something like miniDSP or other DSP software/hardware may use this type of filter to rotate or alter the phase(?). Haven't had the need to use it myself yet.

 

[andreasmaaan]

Also sorry for my ignorance, but what is a channel-independent all-pass filter?

It just means a filter that applies different phase correction to each channel

I don’t really know if I can do phase correction. I’m using a Mac and performing EQ using Fabfilter Pro Q3 plugin in Audirvana and Audio Hijack. Is there an Audio Units plugin that allows for phase correction?

I'm not 100% familiar with your software there, but I believe Fabfilter is a VST plugin, is that correct? And does that mean you can quite easily insert any VST plugin in the signal chain?

 

[Jas0_0]

It just means a filter that applies different phase correction to each channel



I'm not 100% familiar with your software there, but I believe Fabfilter is a VST plugin, is that correct? And does that mean you can quite easily insert any VST plugin in the signal chain?

Thanks for the definition. Yes Fabfilter is VST/audio units.

I wondered if something like this might help fix phase issues?

https://www.meldaproduction.com/MFreeformPhase

Though it seems the general consensus is that it’s not too much to worry about so I might not bother. My neighbours are probably getting tired of sweeps coming through the walls. My girlfriend certainly is!

 

[andreasmaaan]

Thanks for the definition. Yes Fabfilter is VST/audio units.

I wondered if something like this might help fix phase issues?

https://www.meldaproduction.com/MFreeformPhase

Though it seems the general consensus is that it’s not too much to worry about so I might not bother. My neighbours are probably getting tired of sweeps coming through the walls. My girlfriend certainly is!

Haha, yeh I'm not sure I'd bother with it given the mild extent of the problem here

But anyway yes, that plugin looks like it will do the job. Another option would be to do your entire equalisation (amplitude and phase) in an FIR filter generator like RePhase and to then use a VST convolver plugin like this one.

 

[Jas0_0]

Haha, yeh I'm not sure I'd bother with it given the mild extent of the problem here

But anyway yes, that plugin looks like it will do the job. Another option would be to do your entire equalisation (amplitude and phase) in an FIR filter generator like RePhase and to then use a VST convolver plugin like this one.

Thanks, that seems like an attractive option, though I think RePhase and the convolver plugin are PC only. Do you know of any Mac software that would do the same thing?

 

[andreasmaaan]

Thanks, that seems like an attractive option, though I think RePhase and the convolver plugin are PC only. Do you know of any Mac software that would do the same thing?

Ah shit, I'm a PC person here

So perhaps you don't want VST plugins, but rather AU plugins for your software. Is that right?