Mid and upper bass dips and nulls - AVS Forum
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post #1 of 14 Old 08-14-2012, 01:30 AM - Thread Starter
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upperbass.jpg 137k .jpg file

Attached is the REW frequency response from 60Hz to 350Hz. The sweep was done with subwoofer and the mains together. The variable is the crossover frequency of the mains...adjusted from 80,100,120 & 200Hz to see the effect.

Not shown is the 20-60Hz range, which is good enough for me (variation within 5dB)...thanks to my dual subwoofer setup.

What bothers me is the serious nulls across the mid and upper bass range. What could possibly be the reason? What are the fixes? Simple bass traps?
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post #2 of 14 Old 08-14-2012, 07:26 AM
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Play around with subwoofer placement, and run sweeps again. I would also turn you mains off and do just the subwoofer to work on getting the flattest subwoofer response without the mains.

I used superchunk style bass traps in my room to help with the midbass response. I used 2" 703 and cut them in 17"x17"x24" triangles and stacked them in the corners.
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post #3 of 14 Old 08-14-2012, 09:19 AM
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Quote:
Originally Posted by Skylinestar View Post

upperbass.jpg 137k .jpg file
Attached is the REW frequency response from 60Hz to 350Hz. The sweep was done with subwoofer and the mains together. The variable is the crossover frequency of the mains...adjusted from 80,100,120 & 200Hz to see the effect.
Not shown is the 20-60Hz range, which is good enough for me (variation within 5dB)...thanks to my dual subwoofer setup.
What bothers me is the serious nulls across the mid and upper bass range. What could possibly be the reason? What are the fixes? Simple bass traps?

You seem to have nulls at 120, 180, 240, and 300 Hz. Looking at the null at 120 Hz, the path length to the speaker and back must total to be 1/2 of the wavelength of 120 Hz which is about 9 feet for a full wavelengh, 4.5 feet for a half wave. What is 2 1/4 feet from the speaker? Damp it! My first guess is a bounce off the floor or ceiling. Most things that are used to damp ceilings and floors are too thin to have much effect at these frequencies....
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post #4 of 14 Old 08-14-2012, 11:00 AM
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Quote:
Originally Posted by Skylinestar View Post

What bothers me is the serious nulls across the mid and upper bass range. What could possibly be the reason? What are the fixes? Simple bass traps?

Welcome to reality. biggrin.gif

All domestic size rooms have a response like this. The cause is reflections off the walls, floor, and ceiling, combining with the direct sound from the speakers and with each other. Yes, you need bass traps, though you should also experiment with speaker placement. You'll never get a perfectly flat response, so the more bass traps you have the closer you'll get. It's that simple.

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post #5 of 14 Old 08-14-2012, 12:07 PM
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Does the rolled insulation work as well as the "superchunk" style traps? Someone said the rolled insulation rolled in 16-24" cylinders and stacked in the corners provide great results, maybe even better than the triangle traps.
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post #6 of 14 Old 08-14-2012, 08:30 PM - Thread Starter
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upperbass2.jpg 208k .jpg file

Attached image is the sweep done on my mains alone (crossover setting on my AVR at 40Hz so that the speakers play as low as they can...and the subwoofers are switched off). Just wanna see the SBIR effect with various speaker placement. The graphs (black, dark red & light purple) with lower SPL at the lower end are due to plugging the ports on the back of the speakers with foam.
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Originally Posted by arnyk View Post

You seem to have nulls at 120, 180, 240, and 300 Hz. Looking at the null at 120 Hz, the path length to the speaker and back must total to be 1/2 of the wavelength of 120 Hz which is about 9 feet for a full wavelengh, 4.5 feet for a half wave. What is 2 1/4 feet from the speaker? Damp it! My first guess is a bounce off the floor or ceiling. Most things that are used to damp ceilings and floors are too thin to have much effect at these frequencies....
2-1/4' could be the distance from the rear port of the speaker to the front wall. The speakers are on 24" dedicated stand. Floor to ceiling height is 9 feet. Construction is solid brick wall, concrete tiled floor and suspended plaster ceiling.
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Originally Posted by Ethan Winer View Post

Welcome to reality. biggrin.gif
All domestic size rooms have a response like this. The cause is reflections off the walls, floor, and ceiling, combining with the direct sound from the speakers and with each other. Yes, you need bass traps, though you should also experiment with speaker placement. You'll never get a perfectly flat response, so the more bass traps you have the closer you'll get. It's that simple.
--Ethan
I know flat is impossible for a cheap system like mine but I'm trying to improve as much as I can. I can relocate the speakers to fix the big dip at 100-120Hz region but the speakers won't be at the ideal 30 degree angle anymore. The new position will be about 20-23 degree. The speakers with their stands will be standing in front of the TV cabinet (old position is flanking on the side of cabinet), which certainly have issue with WAF. I hope the bass traps can do magic. tongue.gif
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post #7 of 14 Old 08-15-2012, 05:58 AM
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Quote:
Originally Posted by Skylinestar View Post

upperbass2.jpg 208k .jpg file

Attached image is the sweep done on my mains alone (crossover setting on my AVR at 40Hz so that the speakers play as low as they can...and the subwoofers are switched off). Just wanna see the SBIR effect with various speaker placement. The graphs (black, dark red & light purple) with lower SPL at the lower end are due to plugging the ports on the back of the speakers with foam.
Quote:
Originally Posted by arnyk View Post

You seem to have nulls at 120, 180, 240, and 300 Hz. Looking at the null at 120 Hz, the path length to the speaker and back must total to be 1/2 of the wavelength of 120 Hz which is about 9 feet for a full wavelengh, 4.5 feet for a half wave. What is 2 1/4 feet from the speaker? Damp it! My first guess is a bounce off the floor or ceiling. Most things that are used to damp ceilings and floors are too thin to have much effect at these frequencies....
2-1/4' could be the distance from the rear port of the speaker to the front wall. The speakers are on 24" dedicated stand. Floor to ceiling height is 9 feet. Construction is solid brick wall, concrete tiled floor and suspended plaster ceiling.
Quote:
Originally Posted by Ethan Winer View Post

Welcome to reality. biggrin.gif
All domestic size rooms have a response like this. The cause is reflections off the walls, floor, and ceiling, combining with the direct sound from the speakers and with each other. Yes, you need bass traps, though you should also experiment with speaker placement. You'll never get a perfectly flat response, so the more bass traps you have the closer you'll get. It's that simple.
--Ethan
I know flat is impossible for a cheap system like mine but I'm trying to improve as much as I can. I can relocate the speakers to fix the big dip at 100-120Hz region but the speakers won't be at the ideal 30 degree angle anymore. The new position will be about 20-23 degree. The speakers with their stands will be standing in front of the TV cabinet (old position is flanking on the side of cabinet), which certainly have issue with WAF. I hope the bass traps can do magic. tongue.gif

Can you move speakers further away from the wall? It always helps. If you can't, hang thick (at leat 4", better 6") absorption panels right behind speakers. It helped in my case.
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post #8 of 14 Old 08-15-2012, 09:46 AM
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Interesting that the original dips are at harmonics of 60 Hz. I wonder this is a clue. Since graph stops at 60Hz, I wonder if there is a dip around 58 Hz and the dips are harmonics of 58 Hz.


I also wonder about the absorbsion efficincy of large artwork such as rugs, quilts, blankets with nice designs. I don't think all absorbers have to be panels that offend the other-half's sense of decor.
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post #9 of 14 Old 08-15-2012, 11:13 AM
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Quote:
Originally Posted by Skylinestar View Post

Construction is solid brick wall, concrete tiled floor and suspended plaster ceiling.
I know flat is impossible for a cheap system like mine but I'm trying to improve as much as I can. I can relocate the speakers to fix the big dip at 100-120Hz region but the speakers won't be at the ideal 30 degree angle anymore. The new position will be about 20-23 degree. The speakers with their stands will be standing in front of the TV cabinet (old position is flanking on the side of cabinet), which certainly have issue with WAF. I hope the bass traps can do magic. tongue.gif

For one the room sounds like a reflection nightmare. Big cause of your problems.

Why are you trying to solve so many problems by just creating new ones? You want you speakes to be placed for proper imaging first. Run a sweep with just subs and work on getting a flat response with your subs, then set your mains for proper imaging, then address the nulls/peaks with room treatments. Panels, rugs, thick curtains, to tame the reflections and bass traps to help smooth out the bass/midbass and lower midrange. After that I would use Audyssey to bring down the peaks.
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post #10 of 14 Old 08-15-2012, 11:56 AM
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Quote:
Originally Posted by Larry L 


I also wonder about the absorbsion efficincy of large artwork such as rugs, quilts, blankets with nice designs. I don't think all absorbers have to be panels that offend the other-half's sense of decor.

In general all those things have limited to no effect at low frequencies, like the ones where you have most of the dips.
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post #11 of 14 Old 08-15-2012, 08:12 PM - Thread Starter
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Quote:
Originally Posted by Larry L View Post

Interesting that the original dips are at harmonics of 60 Hz. I wonder this is a clue. Since graph stops at 60Hz, I wonder if there is a dip around 58 Hz and the dips are harmonics of 58 Hz.
Generally, I'm happy with the subwoofers output from 20-80Hz.
20-80.jpg 123k .jpg file
Quote:
Originally Posted by gtpsuper24 View Post

For one the room sounds like a reflection nightmare. Big cause of your problems.
Why are you trying to solve so many problems by just creating new ones? You want you speakes to be placed for proper imaging first. Run a sweep with just subs and work on getting a flat response with your subs, then set your mains for proper imaging, then address the nulls/peaks with room treatments. Panels, rugs, thick curtains, to tame the reflections and bass traps to help smooth out the bass/midbass and lower midrange. After that I would use Audyssey to bring down the peaks.
OK. Looks like room treatment is the only solution here.
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post #12 of 14 Old 08-15-2012, 11:34 PM
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Quote:
Originally Posted by Skylinestar View Post

Generally, I'm happy with the subwoofers output from 20-80Hz.

OK. Looks like room treatment is the only solution here.
If those sharp drops only appear when you play multiple speakers at once, then messing with the room boundaries is not a solution to that. The sources are simply interfering with each other. Also, your hearing does not have sufficient resolution to pick up the super narrow troughs. Change the filtering to 1/12 and see if they go away. If they do, I would not chase them.

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post #13 of 14 Old 08-16-2012, 04:55 AM
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Quote:
Originally Posted by Skylinestar View Post

upperbass.jpg 137k .jpg file

Attached is the REW frequency response from 60Hz to 350Hz. The sweep was done with subwoofer and the mains together. The variable is the crossover frequency of the mains...adjusted from 80,100,120 & 200Hz to see the effect.

Not shown is the 20-60Hz range, which is good enough for me (variation within 5dB)...thanks to my dual subwoofer setup.

What bothers me is the serious nulls across the mid and upper bass range. What could possibly be the reason? What are the fixes? Simple bass traps?

Apply a little smoothing to this graph and repost.  Mains FR is a little easier to interpret with a little smoothing, especially with as many graphs overlaid as you have there.  Just from what I can see the 80Hz after Audyssey looks like good starting point.  You can adjust the distance setting on your AVR for the subs to tweak delay/phase mains/sub interaction around crossover, see Audyssey FAQ thread for a how to.

 

In practical HT applications I've found you can't move the speakers far enough away from the front wall to mitigate the front wall bounce, I've found it easier/better to move the speakers as close to the front wall as possible and use 4-6" treatment with 4" air gap behind the acoustic center of the speaker. 

 

What is the published FR of your speakers and and tuning?  Try plugging the ports of your speakers, especially if they are tuned close to crossover which would mean considerable port output in that region and re-run Audyssey.

 

If your running the sweeps all channels stereo, consider running mono for center channel.  Center intelligibility is paramount IMO for HT since there are so many channels its next to impossible to get them all flat for one seat let alone more than one.

 

As stated that region is tough in small room HTs.

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post #14 of 14 Old 08-16-2012, 09:22 AM
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Quote:
Originally Posted by gtpsuper24 View Post

Does the rolled insulation work as well as the "superchunk" style traps?

Yes, that works well too. What matter most is total size and depth.

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