# Average of left/right not equal to left+right?



## Nonlinear (Jun 9, 2012)

I apologize if this has been asked before but I don't know how to search this - I took a room measurement with only the left channel speaker playing then another measurement with only the right channel speaker playing. I then created a plot averaging those two readings. Finally, I turned both speakers back on and let REW run its normal left-then-right routine.

The "average" plot looks completely different then the plot that REW produced running its normal left-then-right routine. 

Please see attached file. "Average" is the manual average of the separate left/right measurements. "Both" is when I let REW run its normal routine (combines both channels). 

Why are the two results different and which is actually correct when both speakers are playing?


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## Sieglander (May 6, 2011)

What is REW's normal left-then-right routine?


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## Phillips (Aug 12, 2011)

Hi i find the best way is to measure

Left main seperate
Right seperate
Then together (both)

By Averaging the two speakers what are you trying to achieve?

Normally i see that "Averaging" is more for measuring Multiple Listening Positions


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## Nonlinear (Jun 9, 2012)

Sieglander said:


> What is REW's normal left-then-right routine?


Oops, my mistake. I have been using DRC Designer also and it runs separate left/right sweeps.

REW 5 runs both speakers at the same time and generates 1 plot for the combination. Why, then, does it give you a choice of mono or stereo filter impulse export? Either way it's mono, right?


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## Nonlinear (Jun 9, 2012)

Phillips said:


> By Averaging the two speakers what are you trying to achieve?


I was wondering if it would give a better approximation of what I actually hear. The sharp nulls that show up in measurement plots may not be as audible as they look because we have two ears - only one ear may be in that null the other not.

Probably a better approach would be to run both speakers and then average two measurements taken at, say, 8 inches apart, to simulate our ear spacing. Still wouldn't be exactly right but might be a closer approximation to what we actually hear than a single point measurement or average of many random points.

This is for a mixing desk - relatively small working area in a medium sized room. I don't need the entire room flat just a relatively small "sweet spot" for proper mix balancing and EQ.


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## HifiZine (Feb 7, 2013)

Hi, running both speakers together will result in destructive interference at different frequencies, whereas when doing an average you are looking at the magnitude only i.e. phase information is lost. The Nyal and Hedback white paper on acoustic measurement recommends running left and right separately above 250 Hz and both together below 250 Hz.

* http://blog.acousticfrontiers.com/w...-standards-for-stereo-listening-rooms-pu.html


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## Nonlinear (Jun 9, 2012)

HifiZine said:


> The Nyal and Hedback white paper on acoustic measurement recommends running left and right separately above 250 Hz and both together below 250 Hz.
> 
> * http://blog.acousticfrontiers.com/w...-standards-for-stereo-listening-rooms-pu.html


Thank you for that info.

Now, how would one generate correction filters using this approach with REW? And how do you handle the transition region at 250Hz?


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## HifiZine (Feb 7, 2013)

If you mean using the REW automatic filter generation, I'm not sure but perhaps the place to start would be to do the left and right independently, then measure both to see how the result looks < 250.


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## AudiocRaver (Jun 6, 2012)

Nonlinear said:


> Thank you for that info.
> 
> Now, how would one generate correction filters using this approach with REW? And how do you handle the transition region at 250Hz?


It will be a partially manual operation (I have not tried this, but it should work):

Generate filters for left speaker and save.
Generate filters for right speaker and save.
Run a measurement sweep with both speakers that only goes to 250 Hz.
Generate low-frequency filters and save.
Now manually inserted low-frequency filter values from the last step into the first two filter sets.
Remeasure each speaker (& both together below 250) to verify the results.
If the curves above and below 250 failed to line up, manually generate a house curve to compensate for the error and rerun the process.


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## Nonlinear (Jun 9, 2012)

AudiocRaver said:


> It will be a partially manual operation (I have not tried this, but it should work):
> 
> Generate filters for left speaker and save.
> Generate filters for right speaker and save.
> ...


OK, so basically what you are saying is to generate stereo correction filters above 250Hz and mono filters below 250Hz? Sounds like a lot of work that could create a real mess if not done properly! LOL

The only thing I'm not sure about in that case is the interaction above 250Hz when both speakers are playing the same tones. As I mentioned in one of my other posts here, I have been experimenting with "DRC Designer" which does create stereo correction filters using separate left and right sweeps. However, in my experience so far, the mono filters created by REW sound better. Probably because they account for the interactions.

Nevertheless, I will try out what you suggested and see what happens!


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## AudiocRaver (Jun 6, 2012)

Nonlinear said:


> OK, so basically what you are saying is to generate stereo correction filters above 250Hz and mono filters below 250Hz? Sounds like a lot of work that could create a real mess if not done properly! LOL
> 
> The only thing I'm not sure about in that case is the interaction above 250Hz when both speakers are playing the same tones. As I mentioned in one of my other posts here, I have been experimenting with "DRC Designer" which does create stereo correction filters using separate left and right sweeps. However, in my experience so far, the mono filters created by REW sound better. Probably because they account for the interactions.
> 
> Nevertheless, I will try out what you suggested and see what happens!


I didn't say it was a great idea, just a way to go about it.:bigsmile:

You are absolutely right to want to preserve the phase relationship between the two speakers. How much that gets disrupted by having different filters for left and right depends on how how much filtering is used and how different the two sides are. With a single filter, the phase goes away from zero and back again, so only that small band of frequencies is affected. Then there is the difference in frequency response left versus right to be considered, that is not good either, so which is worse? But if frequency response left versus right is very far off, then the phase response left versus right is probably pretty messed up too, so don't worry about it, fix what you can, the frequency response.

I tried DRC designer last summer and it gave me weird results, never figured out why. The left correction impulse looked great and the right correction impulse look totally weird, so I dropped it, never could get it to behave. I did figure out how to get initial measured impulses using DRC designer, run them through DRC manually, not a trivial task. I think DRC is really cool and well done, but it is NOT easy to use. Anyway I was very happy with what DRC came up with, but only using the absolute minimalist amount of correction. Using greater amounts of correction ended up in overcorrection and it sounded kind of strained, not great. The phase correction that DRC gave was almost perfect, even with the minimalist amount of correction.

There is my two cents worth. Let us know what ends up working for you.

EDIT: There is a fixed/converted version of DRCDesigner available, no new function, just converted to an .exe instead of a .jar because of Java issues. It works great. Follow this link for a full explanation and link to the converted version.


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## Nonlinear (Jun 9, 2012)

First of all, I got rid of DRC Designer! As you said it gave me strange results. It certainly changed the sound but it didn't sound "right".

My approach is this:
1) Apply reasonable amount of room treatment with primary goal of *reducing nulls* (flattened overall response being secondary side benefit).
2) Apply REW filters to help level out the remaining fluctuations
3) Get back to recording music
4) Let the mega-buck mastering studios with their perfect rooms tweak the final recording


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## Nyal Mellor (Feb 24, 2011)

Hi, Nyal here 

There is some point at which the L/R combined response starts to resemble more of what you hear than the individual L/R responses. This will vary by content (is the low frequency info in the music mono or stereo?), room size, speaker placement (at what point do the speakers start to 'couple' acoustically) etc.

A good starting point for measurements is the L/R combined response. You can use that to see the overall frequency response and easily identify modal issues which are generally the biggest problems in the bass. It's unlikely you would make a decision about acoustic treatment looking at the individual L/R responses unless your placement relative to room boundaries was very assymetrical, and then it would only apply to speaker boundary interference cancellations anyway.

For EQ it's a slightly different story. My current thinking is that for EQ decisions with stereo you want to measure the L/R separately and then also the L/R together. Broadly speaking I would suggest that >100Hz the L/R individual responses are more important than the L/R combined and below 100Hz the opposite. However it's not cut and dry, it's more of a transition, and probably only <50Hz can you ignore the individual L/R responses. 

I generally do not recommend using EQ > 100Hz as you should be fixing any response issues in this region acoustically not electronically. As you apply EQ at higher frequencies it applies to a smaller and smaller area due to the decreasing wavelengths. Once you get to the point where your EQ at one mic position does not apply 6" to the left / right / up / down then you have gone too far unless you have your head in a vice when you listen!

Hope that helps :sn:

Nyal


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## AudiocRaver (Jun 6, 2012)

Nonlinear said:


> First of all, I got rid of DRC Designer! As you said it gave me strange results. It certainly changed the sound but it didn't sound "right".
> 
> My approach is this:
> 1) Apply reasonable amount of room treatment with primary goal of *reducing nulls* (flattened overall response being secondary side benefit).
> ...


A wise approach. I especially like step number three, keeping your priorities straight.:T


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## Wayne A. Pflughaupt (Apr 13, 2006)

I’ve never had a problem equalizing the upper frequencies, and I’ve never seen it result in the “head in a vice” situation that many talk about. In fact, I can’t say I’ve ever seen anyone on this Forum who’s equalized their main channels ever report that they ended up with that problem (here’s but one example). Due to its location in a cabinet, my center channel speaker requires a substantial cut filter at 4.7 kHz to restore balanced response. Switching the filter in or out I can hear its effects even 45 degrees off-axis.

The only issue I’ve ever seen EQing the main channels is that specific (stereo) filtering for left and right speakers can whack out the imaging. My early experiments (back in the mid 90s when I first acquired some high-end equalizers) showed that I had to use matching filters from the high end down to 300 Hz. Below that point I didn’t notice any adverse effects on imaging using separate filters for the left and right speakers. However, I was using 1/3-octave equalizers at that time. Parametric EQs are infinitely more precise, and I currently have one speaker with a cut filter at 620 Hz with no adverse effect. 

So, even if their upper frequency response is different, it’s best to equalize the left and right speakers in tandem with matching filters. At some point down the frequency range you should be able to “break out” to independent filters, if needed. At what frequency that happens will depend on the capabilities of the equalizer and possibly the characteristics of speaker and/or the room itself. Just be mindful of your imaging – if employing a specific-channel filter makes things sound weird in that frequency range, not better, then your transition to per-channel filtering needs to happen at a lower frequency.

Regards, 
Wayne


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## AudiocRaver (Jun 6, 2012)

Wayne A. Pflughaupt said:


> The only issue I’ve ever seen EQing the main channels is that specific (stereo) filtering for left and right speakers can whack out the imaging.....
> 
> So, even if their upper frequency response is different.....


A minor point: In my experience, if the frequency response is much (how much?) different between the two speakers in a stereo pair, imaging is usually not very good to start with. I have experimented with MINOR frequency response adjustment of one speaker relative to the other in order to tighten up the imaging, with good results, using minimum-phase parametric filters. I definitely agree that if the imaging is already pretty good, you are better off applying the same EQ to both channels to keep it that way.


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## Nonlinear (Jun 9, 2012)

Yesterday I created two different sets of filters. I used the same smoothing and target settings for both. The first filter I generated from REW with both speakers playing to generate a common filter for both channels. The second filter I created by generating and exporting separate left and right filters and then combining into a stereo file with a wave editor.

When I played the two filters with music material they give very different results. The mono filter sounds a bit "scooped" but solid with punchy low end and sparkling highs. The stereo filter sounds a bit animated with a pronounced midrange that seems to enhance the stereo image.

So now I have two different filters - generated from the same measurement data using the same filter settings - that sound very different. One corrects the speakers independently, the other as a whole.

Which one is "right"?


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## JohnM (Apr 11, 2006)

Probably would be better to generate filters across different correction ranges for each. Could try making a filter set for the region up to 200Hz or so for the measurement with both speakers playing, allocating some number of filters to correct that region by disabling those filter positions you will later use for individual corrections. Then make filter sets from 200Hz upwards for the independent left/right measurements, leaving enough filter positions disabled to be able to afterwards add in the filters you produced with the combined measurement before exporting the filter settings.


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## Nonlinear (Jun 9, 2012)

JohnM said:


> Probably would be better to generate filters across different correction ranges for each. Could try making a filter set for the region up to 200Hz or so for the measurement with both speakers playing, allocating some number of filters to correct that region by disabling those filter positions you will later use for individual corrections. Then make filter sets from 200Hz upwards for the independent left/right measurements, leaving enough filter positions disabled to be able to afterwards add in the filters you produced with the combined measurement before exporting the filter settings.


That's basically what AudiocRaver suggested above. I didn't get that complicated but just wanted to see what difference stereo vs. mono correction filters would make. It's a lot.

So, I guess what is "right" would be a combination of the two using ~200Hz as the transition point - mono for the lows, stereo for the mids and highs. I'm afraid that transition region, though, might create a bigger mess than what I started with if not done right.

I have a stereo width plugin (in Cubase) that can "monoize" stereo signals below a threshold frequency. There might be some way to press this into service here. Will have to think it out.


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## natehansen66 (Feb 20, 2011)

Nonlinear said:


> Yesterday I created two different sets of filters. I used the same smoothing and target settings for both. The first filter I generated from REW with both speakers playing to generate a common filter for both channels. The second filter I created by generating and exporting separate left and right filters and then combining into a stereo file with a wave editor.


What software are you using to do the convolution? AFAIK vst convolver based setups support configuration files, so you don't have to do the wave editing. Here's a link detailing how the config files are setup: http://convolver.sourceforge.net/configegs.html. I'm not sure what's involved to make the stereo wave file but the config setup is super easy once you get it figured out. It also allows you to do multi-channel convolution if your player supports that. Good for surround setups or you can use it for active crossovers.......


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## Nonlinear (Jun 9, 2012)

natehansen66 said:


> What software are you using to do the convolution?


I'm using a VST plugin called SIR. It will accept any wave file as an impulse file.

To create the stereo filter I generated and exported the left/right mono filters separately then merged into one stereo file using Cubase. Simple!

Now, how to combine those filter sets with a transition from mono to stereo at ~200Hz is going to be a little more tricky. I need to think about that for a bit!


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## JohnM (Apr 11, 2006)

Nonlinear said:


> how to combine those filter sets with a transition from mono to stereo at ~200Hz is going to be a little more tricky


Not really. Start with your combined measurement and generate filters for it using 200Hz as the upper limit of the match range. Allocate (say) 10 of your filters to that by unticking the boxes on filters 11-20 on the filters window so those positions are disabled. Save the filter set using the button at the top of the filters window. Select your left-only measurement and open the filters window. Load the filter set you just saved, then untick the filters from your low frequency set so they are disabled and leave positions 11-20 enabled. Generate a filter set using 200Hz as the lower limit of the match range. Now re-enable all the filters and you have a combined set for the left channel. Repeat for the right. You may need to do a little manual tweaking of the filter settings if there are filters close to 200Hz in either set. To save as a stereo file click the Stereo button in the REW filter impulse response export dialog and select the left and right measurements from the list.


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## Nonlinear (Jun 9, 2012)

JohnM said:


> Not really. Start with your combined measurement and generate filters for it using 200Hz as the upper limit of the match range. Allocate (say) 10 of your filters to that by unticking the boxes on filters 11-20 on the filters window so those positions are disabled. Save the filter set using the button at the top of the filters window. Select your left-only measurement and open the filters window. Load the filter set you just saved, then untick the filters from your low frequency set so they are disabled and leave positions 11-20 enabled. Generate a filter set using 200Hz as the lower limit of the match range. Now re-enable all the filters and you have a combined set for the left channel. Repeat for the right. You may need to do a little manual tweaking of the filter settings if there are filters close to 200Hz in either set. To save as a stereo file click the Stereo button in the REW filter impulse response export dialog and select the left and right measurements from the list.


REW will actually export a stereo file with different left and right filters? Wow, I thought the "stereo" option just created two identical channels with a stereo header. Awesome!

Great - thank you - will give it a try this evening!


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