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Discussion Starter · #1 ·
I have a 12 X 15 room that opens in one corner to stairs and another room. Otherwise it is a perfect rectangle. Eight foot ceiling. Due to WAF and layout, I can't move things around. The "front" is on the 12 foot wall. Anyway, i'm curious as to why bass frequencies are the strongest in the middle of the room, towards the ceiling. I sit down (head at three feet) and the bass drops by at least 5 dB and as much as 8 db. (Measured) Mostly in the 50-100 Hz range. Is it typical to have this happen? Like I said, I can't move things or install treatments. I just want to know the physics behind it. However, I do have a spot for a second sub on the opposite end of the room. If that would make a noticeable difference.

Thanks!
 

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Extra subs will make a definite difference while increasing the probability of getting a linear response
 

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I have a 12 X 15 room that opens in one corner to stairs and another room. Otherwise it is a perfect rectangle. Eight foot ceiling. Due to WAF and layout, I can't move things around. The "front" is on the 12 foot wall. Anyway, i'm curious as to why bass frequencies are the strongest in the middle of the room, towards the ceiling. I sit down (head at three feet) and the bass drops by at least 5 dB and as much as 8 db. (Measured) Mostly in the 50-100 Hz range. Is it typical to have this happen? Like I said, I can't move things or install treatments. I just want to know the physics behind it. However, I do have a spot for a second sub on the opposite end of the room. If that would make a noticeable difference.

Thanks!
It's due to standing waves. Did you take a lot of science classes in high school and college? Part of Physics is Wave Propagation and this problem has to do with the energy reflecting from one or more surfaces and colliding with the energy from the souce which, in this case is the speakers. Depending on where this collision occurs, the energy will be constructive, destructive or because the energy at that point is halfway between its maximum and minimum, it won't cause much of a problem.

These may help, although there are other links and articles-

http://en.wikipedia.org/wiki/Room_modes

http://www.audioholics.com/room-acoustics/listening-room-acoustics-1
 

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Discussion Starter · #4 ·
I'm slapping my forehead with the palm of my hand. I posted late and was VERY tired! I just wish I could do treatments in my room. I had some classical music playing as I was cleaning up for the night. Moving up and down and was constantly reminded of these modes. Lots of bass. Little bass. lots of bass and so on. I pulled out my cheapy Radio Shack meter and got some rough numbers. Another reminder of how important room treatment is.
 

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I'm slapping my forehead with the palm of my hand. I posted late and was VERY tired! I just wish I could do treatments in my room. I had some classical music playing as I was cleaning up for the night. Moving up and down and was constantly reminded of these modes. Lots of bass. Little bass. lots of bass and so on. I pulled out my cheapy Radio Shack meter and got some rough numbers. Another reminder of how important room treatment is.
From all the data I have read, room treatments for LF would have to be extremely thick. The lower the FR, the thicker the panel. The typical panels you see wont do much of anything in the lower range of the sub woofer.. Anyone is welcome to correct me if I am wrong.

And yes, there is always spots that have more sound. This is a problem with people that has a real home theater room with 2 or 3 rows of seats. This is another reason why its a good idea to have 2 subs instead of 1.
 
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Discussion Starter · #6 ·
This is why I wish I had a room in my house where I could dedicate it to sound. Sound management. And, of course, fun!
 

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It's due to standing waves.
Not so much standing waves as boundary effects, which are related, but aren't the same thing. When the spot where you're listening coincides with where the direct wave and a reflected wave are 180 degrees out of phase at a given frequency there will be a response null at that frequency. When you move the distance relationships between the sub, the room boundaries and you shift, and along with them the frequency where a null will occur, if one occurs at all. Adding more subs tames nulls, because a null zone created by one sub's position will be offset by the output of another sub with different placement.
 
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Not so much standing waves as boundary effects, which are related, but aren't the same thing. When the spot where you're listening coincides with where the direct wave and a reflected wave are 180 degrees out of phase at a given frequency there will be a response null at that frequency. When you move the distance relationships between the sub, the room boundaries and you shift, and along with them the frequency where a null will occur, if one occurs at all. Adding more subs tames nulls, because a null zone created by one sub's position will be offset by the output of another sub with different placement.
I should have worded the first sentence differently.

In addition, more subs changes the distribution of the intersections, placing them more evenly through the space.

Corners that are open- hardwood floor, walls covered with drywall/plaster/paneling are big offenders, too. Placing absorptive materials in the corners helps to smooth the bass response.
 

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From all the data I have read, room treatments for LF would have to be extremely thick. The lower the FR, the thicker the panel. The typical panels you see wont do much of anything in the lower range of the sub woofer.. Anyone is welcome to correct me if I am wrong.

And yes, there is always spots that have more sound. This is a problem with people that has a real home theater room with 2 or 3 rows of seats. This is another reason why its a good idea to have 2 subs instead of 1.
The thickness depends on the density of the material- if you look at different types of insulation, you'll see a chart for NRC (Noise Reduction Coefficient) and this will show the effectiveness at various frequencies, thickness and sometimes, it will show different installation methods. If the frequencies being treated are low, thick and lower density work but when the material is more dense but still flexible, it can work as well with less material. Sometimes, treating both walls in a corner can substitute for using a wedge, which almost nobody will like to see. Columns work for this- a hollow column filled with absorptive material, with open back and a gap at the sides, is effective, too.

Lots of ways to treat these problems. I need to RTA my system again- I'll post the results with and without the treatments- the 80Hz dip was extreme and now, it's completely gone.
 

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It's due to standing waves.
Not so much standing waves as boundary effects, which are related, but aren't the same thing. When the spot where you're listening coincides with where the direct wave and a reflected wave are 180 degrees out of phase at a given frequency there will be a response null at that frequency. When you move the distance relationships between the sub, the room boundaries and you shift, and along with them the frequency where a null will occur, if one occurs at all. Adding more subs tames nulls, because a null zone created by one sub's position will be offset by the output of another sub with different placement.
One questuon that has been troubling me:

So once you get 4 subs, an easy way to change when a wave reaches a certain point in space compared to another wave is simply to chnage the delay. So in other words, have slightly different delay settings for each sub.

One would think that setting all sub delays "correctly" based on distance would give you a nice tight "time aligned" sound. Would using delay settings as a tactic to achieve linearity make the sound feel less "tight?".

Is the delay or even the phase a reasonable thing to tweak on subs and what algorithm would you use to go about it? Adjust phase and then delay or vice versa?

Taming peaks with a peq seems intuitive to me. Taming nulls without specialized bass trapping and using other tactics is more nebulous.

What are our best expectations for bass linearity at main listening position without trapping and without moving subwoofers? Are phase and delay adjustments just a gimmick which will worsen something else?
 

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Would using delay settings as a tactic to achieve linearity make the sound feel less "tight?".
The issue there is the same as trying to use EQ alone to fix room response. It will work, but only within a small listening position, as what fills in a hole at one spot in the room results in creating a peak in another spot.
 

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One questuon that has been troubling me:

So once you get 4 subs, an easy way to change when a wave reaches a certain point in space compared to another wave is simply to chnage the delay. So in other words, have slightly different delay settings for each sub.

One would think that setting all sub delays "correctly" based on distance would give you a nice tight "time aligned" sound. Would using delay settings as a tactic to achieve linearity make the sound feel less "tight?".

Is the delay or even the phase a reasonable thing to tweak on subs and what algorithm would you use to go about it? Adjust phase and then delay or vice versa?

Taming peaks with a peq seems intuitive to me. Taming nulls without specialized bass trapping and using other tactics is more nebulous.

What are our best expectations for bass linearity at main listening position without trapping and without moving subwoofers? Are phase and delay adjustments just a gimmick which will worsen something else?

Delay is useful for having the original (non-reflected) waves from subs reach the listener at the same time, but it won't help with reflected waves. And it generally won't help much with a null. The best way to battle nulls or peaks is to get more subs or move the subs you have.
 

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The thickness depends on the density of the material- if you look at different types of insulation, you'll see a chart for NRC (Noise Reduction Coefficient) and this will show the effectiveness at various frequencies, thickness and sometimes, it will show different installation methods. If the frequencies being treated are low, thick and lower density work but when the material is more dense but still flexible, it can work as well with less material. Sometimes, treating both walls in a corner can substitute for using a wedge, which almost nobody will like to see. Columns work for this- a hollow column filled with absorptive material, with open back and a gap at the sides, is effective, too.

Lots of ways to treat these problems. I need to RTA my system again- I'll post the results with and without the treatments- the 80Hz dip was extreme and now, it's completely gone.
Cool, I would like to see a before and after, and see how far the FR you can see any change.
 

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Cool, I would like to see a before and after, and see how far the FR you can see any change.
It wasn't a subtle change. IIRC, the dip was about 30dB.

If you have REW in a laptop and an AVR with Audyssey, fire up REW and go into the speaker setup part of the AVR's setup, then set the distance increments to .1 feet. Once that's done, change the distance of one speaker one step at a time. Listen for the changes and watch the response as you do this. You don't need to use pink noise, wide bandwidth music works great. I did this with a Denon AVR, but I would think the others would operate similarly.
 

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It wasn't a subtle change. IIRC, the dip was about 30dB.

If you have REW in a laptop and an AVR with Audyssey, fire up REW and go into the speaker setup part of the AVR's setup, then set the distance increments to .1 feet. Once that's done, change the distance of one speaker one step at a time. Listen for the changes and watch the response as you do this. You don't need to use pink noise, wide bandwidth music works great. I did this with a Denon AVR, but I would think the others would operate similarly.
You're describing the "Subwoofer Distance Tweak." This has been well documented.

https://dl.dropboxusercontent.com/u/9653266/Images%20for%20Forum%20Posting/Audyssey%20Sub%20Distance%20Tweak%20Procedure%20Oct%202013.pdf

Here is my graph of the CC/subs with the *only* difference between the 2 graphs being the subwoofer distance setting:




Here's another where the change is much more subtle:



The change in sound with the first graph was astonishing. The bass went from weak and thin to strong and powerful.

The change in sound with the second graph was more subtle, but definitely noticeable.

Craig
 

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Delay is useful for having the original (non-reflected) waves from subs reach the listener at the same time, but it won't help with reflected waves. And it generally won't help much with a null. The best way to battle nulls or peaks is to get more subs or move the subs you have.
That pretty much sums it up.

You may be able to affect frequencies down to 80 Hz, as has been claimed, with lots of absorption. But that is about the practical limit. Below that, you're stuck with the structural boundary effects.
 

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You're describing the "Subwoofer Distance Tweak." This has been well documented.

https://dl.dropboxusercontent.com/u...sey Sub Distance Tweak Procedure Oct 2013.pdf

Here is my graph of the CC/subs with the *only* difference between the 2 graphs being the subwoofer distance setting:




Here's another where the change is much more subtle:



The change in sound with the first graph was astonishing. The bass went from weak and thin to strong and powerful.

The change in sound with the second graph was more subtle, but definitely noticeable.

Craig
Actually, I did this without the sub, just the front speakers. Their alignment is just as important as the sub/main interface.
 

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Nice graphs, nice points. I think I'll try some of these ideas and see if I get me anywhere "better" than where I am now.

I did a lot of changes to seating and other factors so it is time to "re-do" everything anyway.

I predominantly want the main listening position to be "correct" above all other locations. The reality is that I am the only one in the household that is going to notice the exact bass response. Everyone else can have "imperfect" bass and they wouldn't care.
 

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Columns work for this- a hollow column filled with absorptive material, with open back and a gap at the sides, is effective, too.
Interesting point. I had one of my two subs about four feet from a corner. We had a book case there, flat against wall. This weekend I angled it in that corner because it looks better, the sub wasn't on my mind. Immediately I noticed my subs were sounding better. I bet this column principle is why. Heck, maybe I sould go get another bookcase for the opposite corner!
 

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Actually, I did this without the sub, just the front speakers. Their alignment is just as important as the sub/main interface.
I suggest you do this only with the subwoofer distance. When you start changing the speaker distances, you start manipulating the imaging, particularly the "phantom imaging" that happens between a pair of speakers. This is related to the Hass Effect, (aka the "Precednce Effect.") When two speakers are playing the same content, and they both arrive at the ears at the same time and level, they are perceived by the brain as coming from in between the two speakers. This effect is most noticeable with 2-channel systems where voices are mixed so they "image" from the middle of the two speakers. However, even in multichannel content, sounds can be phantomimaged between adjacent speaker pairs. So, you can hear sounds that image between the L and C, and between the C and R. Phantom images can even exist between the fronts and surrounds or between the sides and rears.

If you change the arrival times of the speakers, you throw off these effects. The sounds that arrive earlier takes "precedence", and becomes the predominat sounds. The brain perceives these sounds as coming from the direction of the earlier arriving speaker. In the case of voices mixed to be in the center, the image will collapse to sounding like it is originating at the earlier arriving speaker.

If you instead keep the speakers all at their optimal distances, and manipulate the subwoofer distance, you can keep the proper imaging of the speakers while still getting the sub properly timed with all the speakers.

When you manipulate the speaker distance to optimize the bass response you potentially compromise the imaging. Better to manipulate the subwoofer distance. This has the same effect on the bass response with no impact on the imaging.

Good luck and I hope this helps.

Craig
 
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