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-   -   Dip at 125hz (http://www.avsforum.com/forum/113-subwoofers-bass-transducers/1594706-dip-125hz.html)

dftkell 07-07-2014 02:21 PM

Dip at 125hz
 
Hi,

I've got my two subs in what appears the be the best configuration in my room. It's an irregular room and there are some peaks and valleys below 300hz that are unfixable with positioning. Audyssey helped but my biggest concern is a dip at 125hz. Is this floor bounce? My couch can't be moved forward or backwards to any significant degree. Would some room treatments/insulation-bass trapping help? Or is this just something I have to live with?

Thanks
Dan

falzpozitive 07-07-2014 02:23 PM

Quote:

Originally Posted by dftkell (Post 25559250)
Hi,

I've got my two subs in what appears the be the best configuration in my room. It's an irregular room and there are some peaks and valleys below 300hz that are unfixable with positioning. Audyssey helped but my biggest concern is a dip at 125hz. Is this floor bounce? My couch can't be moved forward or backwards to any significant degree. Would some room treatments/insulation-bass trapping help? Or is this just something I have to live with?

Thanks
Dan

What's your crossover set at??

Bill Fitzmaurice 07-07-2014 02:47 PM

Quote:

Originally Posted by falzpozitive (Post 25559362)
What's your crossover set at??

+1. Subs shouldn't be run that high. FWIW a dip at 125Hz is most likely caused by the LP being 4.5 feet from the wall behind the LP or the speaker being 2.25 feet from the wall behind it.

dftkell 07-07-2014 04:49 PM

Quote:

Originally Posted by falzpozitive (Post 25559362)
What's your crossover set at??

The crossover is set at 80hz.

dftkell 07-07-2014 04:54 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 25559986)
+1. Subs shouldn't be run that high. FWIW a dip at 125Hz is most likely caused by the LP being 4.5 feet from the wall behind the LP or the speaker being 2.25 feet from the wall behind it.

I just realized this probably wasn't a post for the subwoofer section. But since I was setting up my subs, that's where I posted.

So the dip isn't from my subs since they're crossed over at 80hz. My room has an angular shape with an open stairway. You can see it in my Little Loft signature. The speakers and sub in those photos have been upgraded, hence I'm remeasuring and positioning.

My couch is 6.5 feet from the flat part of the rear wall. But considering the angle of the way, you're rule of thumb probably doesn't apply. Playing with the distance from the front wall to the front speakers is an option though. I had them setup so that they were equidistant to the LP with the center speaker ala Dolby guidelines. But I'll pull them out a bit and push them back a bit and see if the dip starts to flatten out.

Thanks!
Dan

Bill Fitzmaurice 07-07-2014 06:56 PM

Quote:

Originally Posted by dftkell (Post 25562946)
Playing with the distance from the front wall to the front speakers is an option though.

What probably is happening is Allison Effect. When the speaker is 1/4 wavelength from the wall behind it there's a cancellation notch caused by the reflection off the wall meeting the original wave 180 degrees out of phase. The cure is don't have the speaker 1/4 wavelength from the wall. With an 80Hz crossover than means more than 3.5 feet. Or have the speaker flush mounted, so that there is no reflection.

dftkell 07-07-2014 07:31 PM

Thanks Bill. Neither of those is an option, but it's nice to learn something new. I did experiment with a 60hz crossover on the mains instead of an 80hz and it appears to be a slightly better FR. Just a bit better, but I'll take it.


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tvuong 07-07-2014 07:44 PM

^^ perhaps, you can leave it at 80hz crossover and try to reduce or increase the sub(s) distance from your avr? It's hard to do without seeing your FR graph though

dftkell 07-07-2014 09:35 PM

5 Attachment(s)
Quote:

Originally Posted by tvuong (Post 25566826)
^^ perhaps, you can leave it at 80hz crossover and try to reduce or increase the sub(s) distance from your avr? It's hard to do without seeing your FR graph though

Here are some graphs < 300hz:

Red is crossing the mains at 50hz. I also used one band of PEQ on the Dayton plate amps to boost 20hz.

Green is crossing the mains at 80hz with the same boost.

Blue is engaging Audyssey MultiEQ XT and using an 80hz crossover for the mains. I don't particularly like what Audyssey does above 300hz so I'm inclined not to use it.

Also, I've included a comparison of crossing the mains at 50hz vs 80hz. And a comparison of a 50hz cross vs. using Audyssey with an 80hz cross.

Thanks for looking.
Dan

Bill Fitzmaurice 07-08-2014 05:12 AM

Take a measurement with the sub turned off. Also, take the measurement at 1/12 octave smoothing. A boundary sourced dip will be much narrower and deeper than what you're seeing at 125Hz, but 1/6 octave smoothing hides how narrow or deep it really is.

dftkell 07-08-2014 06:00 AM

5 Attachment(s)
Quote:

Originally Posted by Bill Fitzmaurice (Post 25572825)
Take a measurement with the sub turned off. Also, take the measurement at 1/12 octave smoothing. A boundary sourced dip will be much narrower and deeper than what you're seeing at 125Hz, but 1/6 octave smoothing hides how narrow or deep it really is.

Hi Bill,

If it's worth anything, here are the previous measurements at 1/12. Fuzzmeasure allows me to convert easily. I'll give it a try with the sub off as well. Unfortunately, I packed everything away last night. I've measured so many positions, so many times over the past few months that I just got exhausted. I'll put everything back in place to measure later today or tomorrow without the sub. Just curious, if my crossover is at 50hz for example, how would that affect a dip at 125hz?

P.S. Is there a way to have my images appear large in my post instead of attachments you have to click on?

Thanks
Dan

dftkell 07-08-2014 06:13 AM

4 Attachment(s)
Also, if this helps any, here are some pics of the room and current configuration.

The couch is 12' 7" front the front wall.
There is about 6' 5" from the back of the couch to the rear wall.

Pics came out a little dark due to backlighting.

Thanks!
Dan

Bill Fitzmaurice 07-08-2014 07:26 AM

Quote:

Originally Posted by dftkell (Post 25573673)
if my crossover is at 50hz for example, how would that affect a dip at 125hz?

Crossovers are by no means brickwall. Since the dip is in the range created by the mains you want to remove the sub from the loop and concentrate on the mains, which, BTW, I'd say are too low to the floor. When they're right add the subs. You also have huge expanses of hard flat surfaces, which can't help matters at all.

dftkell 07-08-2014 08:57 AM

Quote:

Originally Posted by Bill Fitzmaurice (Post 25575713)
Crossovers are by no means brickwall. Since the dip is in the range created by the mains you want to remove the sub from the loop and concentrate on the mains, which, BTW, I'd say are too low to the floor. When they're right add the subs. You also have huge expanses of hard flat surfaces, which can't help matters at all.

Okay, I'm going to take a set of measurements without the subs in an hour or so.

I know the speakers are low but I tend to slouch down with my feet up when I'm on the couch and that actually puts them close to ear height. Plus, I didn't want the coaxial drivers of the mains to be too high relative to the center. That being said, I'll try raising them and giving that a go.

I know there are a lot of hard surfaces. I do not plan on treating the side walls. Toole recommends that it is not necessary when using speakers with good directivity. (I know you know this and it's common knowledge around here. Just pointing out why there's nothing there.) If I were to treat anything else, what would you recommend?

Measurements to follow…

Thanks
Dan

dftkell 07-08-2014 10:37 AM

6 Attachment(s)
Here are more measurements at 1/12 smoothing.

The black is taking a baseline again. Same setup, a 50hz crossover with a slight peq boost around 20hz.

The red is the same but with both subs turned off. Same 50hz crossover.

You can then see the comparison, black vs red.

Green is where I then elevated the speakers 11 inches higher so they are now on 30" stands vs the current 19". I measured with subs on and same crossover.

Blue is where I kept the speakers elevated higher and then turned the subs off.

And then you can see green vs blue.

Thanks for looking. I appreciate it.

Dan

Bill Fitzmaurice 07-08-2014 11:11 AM

I like the last chart the best. Now I'd leave the cabs elevated the eleven inches and try moving them closer to the side walls. I'd also not have them the same distance to the side walls. Visual symmetry looks pretty, but it plays havoc with response.

dftkell 07-08-2014 12:22 PM

Quote:

Originally Posted by Bill Fitzmaurice (Post 25582393)
I like the last chart the best. Now I'd leave the cabs elevated the eleven inches and try moving them closer to the side walls. I'd also not have them the same distance to the side walls. Visual symmetry looks pretty, but it plays havoc with response.

Thanks Bill. Fortunately, one side wall is further away from the other already. The "stands" were me putting my poor-man's DIY Sarin tables (Bose 901 stands with a round birch tops painted white) with buckets underneath. So I'm going to build something new that is higher. This will place the tweeter at ear height. Hopefully it will still sound cohesive with the center. I can always go back to the shorter stands if need be.

I'll try placing them a little closer to the side walls.

Thanks again. Very much appreciated.
Dan

NicksHitachi 07-08-2014 12:40 PM

Your sitting in the low pressure region of the 125Hz width mode as it looks to me. Is your room approx 10' wide?

Also move your rear sub out of the corner to the center of the rear wall like the front sub. If your not corner loading all subs its best not to corner load any of them.

Room treatments would be a good start, If your really worried about the 125Hz dip membrane panel absorbers adjacent to the listening position tuned to the frequency of interest would be the best start IMO.

However looking at the rest of the FR I would start with treating the primary reflections with your choice of treatments and go from there. I don't see the 125Hz standing out as that much worse than the rest of the FR.....

dftkell 07-08-2014 01:45 PM

Quote:

Originally Posted by NicksHitachi (Post 25585561)
Your sitting in the low pressure region of the 125Hz width mode as it looks to me. Is your room approx 10' wide?

Also move your rear sub out of the corner to the center of the rear wall like the front sub. If your not corner loading all subs its best not to corner load any of them.

Room treatments would be a good start, If your really worried about the 125Hz dip membrane panel absorbers adjacent to the listening position tuned to the frequency of interest would be the best start IMO.

However looking at the rest of the FR I would start with treating the primary reflections with your choice of treatments and go from there. I don't see the 125Hz standing out as that much worse than the rest of the FR.....

My room is 11' 6" wide with the open stairway.

I did try placing the rear sub at the center of the rear wall. There was no significant difference which surprised me. I'm sure the asymmetry of the room plays a part. So I put it in the corner as having it right next to the crawl space door makes it awkward to get things in and out of there.

It's not that the 125hz dip is a huge concern. And it certainly isn't' the only dip happening. I've just been tweaking and tweaking to try to get things as smooth as possible.

My next step will be to raise the mains and spread them out a bit more towards the side walls as Bill suggested. After that, I may eventually upgrade my receiver to an Anthem with ARC which allows me to only EQ below the Schroeder frequency. Audyssey does some good things below 300hz, but I don't particularly like what it does above 300hz so I'm not using it. An Anti-mode may be a possible upgrade too. But my problem seems to be more with dips than mountains.

Thanks
Dan


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