# Dealing with the dips!



## ALMFamily (Oct 19, 2011)

I have started the process of using REW to get some measurements of my room. After moving the subs to a few different locations, I think I found the spots that give me a response I can work with.

However, once I started measuring the mains, I am seeing a huge dip in the 90-100 Hz range on both speakers (as well as the CC). I have the crossover set to 80 for both the mains and the center. The stated frequency range for these speakers (the Song Series from Salk) is in the 40s for both the L/R and the CC, but when I set the crossovers that low, I get a really poor curve. I am just wondering what the potential issues could be that would cause a dip - after a bit of research, the only things I have been able to turn up as possibilities are boundary interactions and phasing issues.

However, I think phasing would not be it because I am measuring the mains by themselves which seems like it would take phase out of the equation.

That leaves boundary interactions unless there is something else I am not taking into consideration. I am not sure how you would go about dealing with boundary interactions - the mains are pretty much stuck where they are going to be so moving them is not feasible.

Any other suggestions I should consider / research?


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## bpape (Sep 14, 2006)

It can still be phase related - just not between mains and sub. Can be the mains to each other coupled with a difference in path length between direct sound and reflected sound from the side walls or ceiling - or the wall behind the speakers even. 

Bryan


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## ALMFamily (Oct 19, 2011)

Does the best way to figure out if that is happening involve putting up 703 at potential reflection points and remeasuring? I did bring in a couple sheets of 703 and leaned them against the wall where I thought a reflection point would be (side walls only) and remeasured - no change of course.

Unfortunately, I am so early into the REW process that there very well could be one of those graphs that shows me exactly what is happening and I am just not familiar enough with it......


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## ALMFamily (Oct 19, 2011)

OK, more reading done - I read the SBIR effect piece at GIK's site and now understand better what you were referring to Bryan.

I also found an article that provided an equation to determine what frequency you may have issues at - it was:

affected frequency = speed of sound / (2*difference in path traveled)

If this is the case, given that my affected frequency is right about 100 Hz and the speed of sound is roughly 1126 ft/sec, the distance I am looking for for the reflection is roughly 6' longer than the direct path.

Does this make sense?


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## bpape (Sep 14, 2006)

Well, it depends on the frequencies in question. It could be that the material simply wasn't thick enough or it could be that it's from the front wall or the ceiling.

Bryan


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## ALMFamily (Oct 19, 2011)

I checked another article and it had a simpler equation - with the dip at @95-100 Hz, it suggests that the distance is right about 4 feet. (distance=frequency / 86) - distance is in meters.

Lo and behold - that is almost exactly how far the speakers are from the front wall. I have 2" 703 across the bottom and 1" 703 behind the screen right now, but as you said, that may not be enough.

I will play around some more tonight - perhaps I will place the speakers real close to the front wall and see what happens.

As always, thanks for your insight Bryan!


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## jim1961 (Apr 8, 2011)

ALMFamily said:


> I checked another article and it had a simpler equation - with the dip at @95-100 Hz, it suggests that the distance is right about 4 feet. (distance=frequency / 86) - distance is in meters.
> 
> Lo and behold - that is almost exactly how far the speakers are from the front wall. I have 2" 703 across the bottom and 1" 703 behind the screen right now, but as you said, that may not be enough.
> 
> ...


I am following your thread in part because I have my own lull problem.

The wavelength of 100hz is about 11.25 feet. i am wondering what article or calculator you used to get to 4 feet as the distance in question, given 4 feet is not a common multiple of 11.25.

1" or 2" OC703 is not effective at all at 100hz.


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## ALMFamily (Oct 19, 2011)

jim1961 said:


> I am following your thread in part because I have my own lull problem.
> 
> The wavelength of 100hz is about 11.25 feet. i am wondering what article or calculator you used to get to 4 feet as the distance in question, given 4 feet is not a common multiple of 11.25.


It was an SBIR article from Paul Spencer at Red Spade:

http://redspade-audio.blogspot.com/2011/02/sbir-speaker-boundary-interference.html



jim1961 said:


> 1" or 2" OC703 is not effective at all at 100hz.


Excellent point - I do also have corner traps as well as using my soffits for absorbtion from 20-200 Hz (design suggested by Bryan) and should have probably mentioned that right up front.

I guess what I was thinking is that if I put those close to the front wall, the corner traps may be more effective. Of course, that is an uneducated guess. 

I was thinking the front walls 1" and 2" was more for higher frequency reflections from the rear surrounds - correct?


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## bpape (Sep 14, 2006)

1 and 2" is more for mid and highs and correct, at a direct angle of incidence, it's not going to hit 100Hz. Now at a random angle, (which is what you're more closely dealing with here), it can be somewhat effective at 2".

0.17 0.86 1.14 1.07 1.02 0.98

Not great certainly at .17 but something - and something that in real world rooms people can tolerate. 6" is tougher to live with losing that much space but better performing. 

Moving to 3" takes the coefficient at 125 to just over 0.5 - again, though, many people won't/can't give up that much space.

Bryan


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## jim1961 (Apr 8, 2011)

ALMFamily said:


> It was an SBIR article from Paul Spencer at Red Spade:
> 
> http://redspade-audio.blogspot.com/2011/02/sbir-speaker-boundary-interference.html
> 
> ...


Every room / situation is different. But personally, I wouldn't use 1" or 2" OC703 except on small surfaces or when the intent was to only address high / mid frequencies. 









http://www.stanleyhallstudios.co.uk...1=21000&s41=2&d41=100&v41=21000&s42=1&d42=100

The graph model illustrates nicely what generally to expect. Even at 4" thick with a 4" gap, the absorption coefficient is only .5 @ 100hz.


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## jim1961 (Apr 8, 2011)

bpape said:


> Not great certainly at .17 but something - and something that in real world rooms people can tolerate. 6" is tougher to live with losing that much space but better performing.


I usually aim for .75 or better when gauging effectiveness and / or flatness when thinking on broadband absorption. At .5, your going to get coloration, and at .17, I doubt one could tell the difference.





bpape said:


> Moving to 3" takes the coefficient at 125 to just over 0.5 - again, though, many people won't/can't give up that much space.
> 
> Bryan


The figure of .5 at 125hz for 3" I think is a bit optimistic. I dont get that in any model simulations nor have I experienced 4" OC703, much less 3" OC703, to be fully effective at 125hz. If we are talking about a bass trap where the only aim of the panel is low freq absorption, then if the trap is large enough, a difference will be realized. But a wall or ceiling panel where you want broadband, you will notice the difference in a .5 and .9 absorption coefficient in the form of coloration.


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## ALMFamily (Oct 19, 2011)

To be honest, when I run measurements with the entire front stage, it probably is not a bad response when I crossover at 80. I do have a dip at 95ish, but with the sub response, it is not really that pronounced. I was just trying to do my due diligence and try to get the best repose curve I can.

I will run those other measurements and report back with what I come up with.


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## bpape (Sep 14, 2006)

Jim. I agree mostly with what you're saying. Just remember that the graph you posted is for impedance tube measurements - which ONLY account for waves hitting the absorber direct in a straight line which is not what's happening on side and ceiling reflections which are at much shallower angles of attack. The coefficients and numbers I mentioned are for random angle of incidence which is more closely tied to what happens and what you can expect in terms of reflections and overall decay time.

Sure - ideally, all reflection panels would be 4-6" thick with an air gap behind them. Realistically, as I said before, many people simply don't have the space and/or don't have the desire for something that thick and losing that much space. It's always a compromise to do the best you can with the parameters you have to work with.

Bryan


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## jim1961 (Apr 8, 2011)

bpape said:


> Jim. I agree mostly with what you're saying. Just remember that the graph you posted is for impedance tube measurements - which ONLY account for waves hitting the absorber direct in a straight line which is not what's happening on side and ceiling reflections which are at much shallower angles of attack. The coefficients and numbers I mentioned are for random angle of incidence which is more closely tied to what happens and what you can expect in terms of reflections and overall decay time.
> 
> Sure - ideally, all reflection panels would be 4-6" thick with an air gap behind them. Realistically, as I said before, many people simply don't have the space and/or don't have the desire for something that thick and losing that much space. It's always a compromise to do the best you can with the parameters you have to work with.
> 
> Bryan


Not to be nit picky, but if you look, the graphs were modeled with random incidence :T

And for those that DO have the real estate, I am currently using 16" Pink Fluffy which seems to do much better than 4" OC703 with a 4" gap.

http://www.stanleyhallstudios.co.uk...11=21000&s12=1&d12=100&s21=2&d21=400&v21=5000


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## bpape (Sep 14, 2006)

Yes- but it's still an impedance tube type of test where it's just more a single angle, not all angles.

Bryan


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## ALMFamily (Oct 19, 2011)

I started doing some measurements last night, but I was having an issue with my laptop where the fan was introducing noise and completely skewing the results. By the time I got it all cleaned out and quieted down, it was late so I do not have any updates yet.

Bryan - if you have a chance, could you email me that form once again with the questions. My old laptop took my previous copy down with it when it decided to give up on me.


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