You can cancel standing waves using subwoofer and speaker placement, rather than absorbing away the very bass you spent money on subwoofers to create.
DIY Room Accoustics for Home-Theatre
- Thread starter shelley
- Start date
shelley
New Member
@sanjay
What you are telling is absolutely wrong...You are not removing bass by bass traps...
Myth:
Bass traps absorb low frequencies, so putting them in a room reduces the amount of bass youll hear. Absolutely wrong and a myth as suggested by Ethan Winer..
In truth, bass traps lower peaks and also raise nulls, so they make the response flatter, rather than add or remove bass. In most small rooms, the main problem is deep nulls caused by reflections from the wall behind you combining out of phase with the direct sound from the loudspeakers. Adding bass traps therefore increases the perceived level of bass. But in some rooms, especially those that are square or cube shaped, peaks can dominate the response. In that case, adding bass traps reduces the peaks, again making the response closer to flat.
"The most effective place for bass traps is in corners where bass waves tend to gather, though other locations are also viable. Note that rectangular rooms have 12 corners: 4 where each wall meets another wall, 4 where each wall meets the ceiling, and 4 more where each wall meets the floor. After treating as many corners as possible, the front and rear walls are good candidates for even more bass traps.
After bass traps are added, the response becomes not only flatter and more full-sounding but also tighter and clearer because the decay times are reduced."
What you are telling is absolutely wrong...You are not removing bass by bass traps...
Myth:
Bass traps absorb low frequencies, so putting them in a room reduces the amount of bass youll hear. Absolutely wrong and a myth as suggested by Ethan Winer..
In truth, bass traps lower peaks and also raise nulls, so they make the response flatter, rather than add or remove bass. In most small rooms, the main problem is deep nulls caused by reflections from the wall behind you combining out of phase with the direct sound from the loudspeakers. Adding bass traps therefore increases the perceived level of bass. But in some rooms, especially those that are square or cube shaped, peaks can dominate the response. In that case, adding bass traps reduces the peaks, again making the response closer to flat.
"The most effective place for bass traps is in corners where bass waves tend to gather, though other locations are also viable. Note that rectangular rooms have 12 corners: 4 where each wall meets another wall, 4 where each wall meets the ceiling, and 4 more where each wall meets the floor. After treating as many corners as possible, the front and rear walls are good candidates for even more bass traps.
After bass traps are added, the response becomes not only flatter and more full-sounding but also tighter and clearer because the decay times are reduced."
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john_k_antony
Well-Known Member
Just thought of adding my personal experience. I just finished finishing the acoustics and I installed floor to ceiling bass traps in all the corners. Each trap is 6" deep and are 2' wide running from floor to ceiling. It did reduce the extra booming of the sub woofer. However there are still very noticeable nulls and peaks for bass in my room when I walk around.
I did get a much better bass response by moving my sub woofer from corner of the room to the center of the room as suggested by Sanjay.
Just my 2 cents and experience.
However what I have found after doing the acoustics is that the speaker placement also plays a very very significant role. I followed lots of Ethan Winer principles. But I also read in some international forums that Ethan Winer is trying to add more bass traps as he runs a company that sells bass traps
. Don't know how much of that is true. However I have read in numerous forums that the corner bass traps are not that effective in controlling bass in a room. That is the reason, I didn't invest much money on bass traps.
Now I am faced with a very unique issue, I have done exactly identical acoustic treatment on all the four walls, ceiling and floor. Still my left channel is sounding wider, and feel like I am getting the sound emanating from the wall where for the right channel, the sound is emanating directly from the speaker.
This has nothing to do with the bass response though.
I made some very small speaker position changes and the sound stage changes significantly. Its pretty sad that getting the acoustics is very sensitive procedure.
Now I am going to see if there are any mess up with my room treatment. I am going to try the REW software to see the actual frequency response in my room. REW - Room EQ Wizard Room Acoustics Software
Thanks,
John.
I did get a much better bass response by moving my sub woofer from corner of the room to the center of the room as suggested by Sanjay.
Just my 2 cents and experience.
However what I have found after doing the acoustics is that the speaker placement also plays a very very significant role. I followed lots of Ethan Winer principles. But I also read in some international forums that Ethan Winer is trying to add more bass traps as he runs a company that sells bass traps
. Don't know how much of that is true. However I have read in numerous forums that the corner bass traps are not that effective in controlling bass in a room. That is the reason, I didn't invest much money on bass traps.Now I am faced with a very unique issue, I have done exactly identical acoustic treatment on all the four walls, ceiling and floor. Still my left channel is sounding wider, and feel like I am getting the sound emanating from the wall where for the right channel, the sound is emanating directly from the speaker.
This has nothing to do with the bass response though.
I made some very small speaker position changes and the sound stage changes significantly. Its pretty sad that getting the acoustics is very sensitive procedure.
Now I am going to see if there are any mess up with my room treatment. I am going to try the REW software to see the actual frequency response in my room. REW - Room EQ Wizard Room Acoustics Software
Thanks,
John.
Then what are they trapping? Or are you claiming that bass traps don't work by absorption?
shelley
New Member
@John
Do you have photos of your room...I tested bass traps using wood wool(double thick) and polyfibre on 3 corners and bass boom has reduced considerably..Need to fix it now..Waiting for carpenter..
I treated the side walls only upto 13' from front wall. Front Wall completely treated. Now I get crisp clear mid's and high's and i am highly satisfied..
@Sanjay, please read my above post carefully..Suggested by experts, not me
Do you have photos of your room...I tested bass traps using wood wool(double thick) and polyfibre on 3 corners and bass boom has reduced considerably..Need to fix it now..Waiting for carpenter..
I treated the side walls only upto 13' from front wall. Front Wall completely treated. Now I get crisp clear mid's and high's and i am highly satisfied..
@Sanjay, please read my above post carefully..Suggested by experts, not me
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john_k_antony
Well-Known Member
Shelly,
Here is the page from my home theater build thread that has some pictures of the room am also attaching a short video of the room.
http://www.hifivision.com/home-theater/46258-yet-another-dedicated-home-theater-build-4.html
Video
20130907012843 - YouTube
I only added sound absorbers on both side walls up to 5' from floor as the ear level is around 3' from the floor when seated. The bass traps are however from floor to ceiling and I have kept bass traps inside the false ceiling at all the four corners.
The false ceiling is Anuton Subtex with 0.9 NRC.
My flooring is laminated wood, but I have placed a thick rug that is 4' wide and 15' long at the first reflection point of the front speakers.
Thanks,
John.
Here is the page from my home theater build thread that has some pictures of the room am also attaching a short video of the room.
http://www.hifivision.com/home-theater/46258-yet-another-dedicated-home-theater-build-4.html
Video
20130907012843 - YouTube
I only added sound absorbers on both side walls up to 5' from floor as the ear level is around 3' from the floor when seated. The bass traps are however from floor to ceiling and I have kept bass traps inside the false ceiling at all the four corners.
The false ceiling is Anuton Subtex with 0.9 NRC.
My flooring is laminated wood, but I have placed a thick rug that is 4' wide and 15' long at the first reflection point of the front speakers.
Thanks,
John.
I'll ask again: what are bass traps "trapping"?@Sanjay, please read my above post carefully..Suggested by experts, not me
Earlier you said: "What you are telling is absolutely wrong...You are not removing bass by bass traps..."
If I'm absolutely wrong, then it should be easy to provide the right answer. If bass traps do not work by absorbing bass, then how do they work?
There are only 3 things you can do when a sound wave hits a wall: reflect, diffuse, absorb. Which one is the bass trap doing?
john_k_antony
Well-Known Member
Sanjay,
You are absolutely right, sound waves can be reflected, absorbed or diffused. And as far as I know, all the acoustic treatments can only control any of the three attributes mentioned above.
I think, with my limited knowledge on this topic, what the bass traps are "trying" to do is to absorb the low frequency sound waves and reduce the intensity of the reflected sound from the walls so that the reflected waves doesn't contribute in producing very deep nulls and very sharp peaks. However, the nulls and peaks will be there but, the depth of nulls and height of peaks are reduced.
Another way to deal with bass is what Sanjay already mentioned, you can "move" the nulls and peaks to a different locations in the room by moving around the sub-woofer in the room so that you don't get nulls/peaks at your primary listen area, but it will be shifted to some other points in your room where you can still experience them.
When you use bass traps, what are you trying to do is to reduce the intensity of the reflected wave that is causing the nulls and peaks. So here, you still have the nulls and peaks at the same location as before, but the intensity is reduced. But you cannot get rid of them complete unless you have traps that are 40' wide.
So with combination of bass traps and position of your sub-woofer, you can try to reduce the effects of nulls and peaks to a good extent and get a good reproduction of bass.
Again, this is just my understanding and experience.
Thanks,
John.
You are absolutely right, sound waves can be reflected, absorbed or diffused. And as far as I know, all the acoustic treatments can only control any of the three attributes mentioned above.
I think, with my limited knowledge on this topic, what the bass traps are "trying" to do is to absorb the low frequency sound waves and reduce the intensity of the reflected sound from the walls so that the reflected waves doesn't contribute in producing very deep nulls and very sharp peaks. However, the nulls and peaks will be there but, the depth of nulls and height of peaks are reduced.
Another way to deal with bass is what Sanjay already mentioned, you can "move" the nulls and peaks to a different locations in the room by moving around the sub-woofer in the room so that you don't get nulls/peaks at your primary listen area, but it will be shifted to some other points in your room where you can still experience them.
When you use bass traps, what are you trying to do is to reduce the intensity of the reflected wave that is causing the nulls and peaks. So here, you still have the nulls and peaks at the same location as before, but the intensity is reduced. But you cannot get rid of them complete unless you have traps that are 40' wide
.So with combination of bass traps and position of your sub-woofer, you can try to reduce the effects of nulls and peaks to a good extent and get a good reproduction of bass.
Again, this is just my understanding and experience
.Thanks,
John.
shelley
New Member
Sanjay,
I did not mean to hurt you, buddy...It is not my statement..Ethan Winer said it...Please read the "Myth" statement quoted earlier and below..
Absorbers work by converting acoustic energy to heat, due to friction as the sound waves pass into and through the porous material. Some absorbers meant only for bass frequencies use a vibrating wood panel or vinyl membrane, rather than porous material like foam or fiberglass. But acoustic energy is still converted to heat as part of the process. The sound decay times are reduced and hence standing waves are reduced, which results in reduced bass boominess...
If converting sound energy to mechanical energy and heat is what you meant by absorption, you are correct..
Quoting Ethan Winer from his renowned book "The Audio Expert"..Worth reading
Myth:
Bass traps absorb low frequencies, so putting them in a room reduces the amount of bass youll hear. Absolutely wrong and a myth as suggested by Ethan Winer..
"In truth, bass traps lower peaks and also raise nulls, so they make the response flatter, rather than add or remove bass."
Sorry, I am not a professional..
But Ethan Winers DIY tips did work for me
The carpenter came and fixed wood wool in a slanting position at 3 corners (from floor to roof) as in the image below:
The cavity at the corners were fiiled with fluffy Polyfibre and the bass boom has been reduced significantly ...I am very much satisfied with the result
A simple MP3 file to test low frequency sounds without any software or meters...
http://www.audiocheck.net/Audio/audiocheck.net_frequencychecklow.mp3
A -6 dbFS sweeping sine tone, from 10 Hz (supposedly inaudible) to 200 Hz). On the top of the test tone, a voiceover tells you which frequency is currently playing.
(File -> Save As) to save to your system...Test in your HT and locate the frequency at which bass boom appears...An easy way...
I did not mean to hurt you, buddy...It is not my statement..Ethan Winer said it...Please read the "Myth" statement quoted earlier and below..
Absorbers work by converting acoustic energy to heat, due to friction as the sound waves pass into and through the porous material. Some absorbers meant only for bass frequencies use a vibrating wood panel or vinyl membrane, rather than porous material like foam or fiberglass. But acoustic energy is still converted to heat as part of the process. The sound decay times are reduced and hence standing waves are reduced, which results in reduced bass boominess...
If converting sound energy to mechanical energy and heat is what you meant by absorption, you are correct..
Quoting Ethan Winer from his renowned book "The Audio Expert"..Worth reading
Myth:
Bass traps absorb low frequencies, so putting them in a room reduces the amount of bass youll hear. Absolutely wrong and a myth as suggested by Ethan Winer..
"In truth, bass traps lower peaks and also raise nulls, so they make the response flatter, rather than add or remove bass."
Sorry, I am not a professional
..But Ethan Winers DIY tips did work for me
The carpenter came and fixed wood wool in a slanting position at 3 corners (from floor to roof) as in the image below:
The cavity at the corners were fiiled with fluffy Polyfibre and the bass boom has been reduced significantly ...I am very much satisfied with the result
A simple MP3 file to test low frequency sounds without any software or meters...
http://www.audiocheck.net/Audio/audiocheck.net_frequencychecklow.mp3
A -6 dbFS sweeping sine tone, from 10 Hz (supposedly inaudible) to 200 Hz). On the top of the test tone, a voiceover tells you which frequency is currently playing.
(File -> Save As) to save to your system...Test in your HT and locate the frequency at which bass boom appears...An easy way...
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You're not hurting me, buddy. I have no problem admitting I'm wrong (like about the testing on rats) IF you can show me where I am wrong.
If you are converting some of the bass energy to heat, do you end up with less bass, more bass or the same amount of bass as before?
Actually, I was talking about cancelling the room mode (standing wave) so that those peaks & nulls are severely reduced, if not eliminated. Once the room mode has been cancelled, that means the peaks & nulls no longer exist, not just for a particular location but for the entire room.
Here is a snippet from one of Floyd Toole's papers showing how to cancel peaks & nulls with subwoofer placement:
rdksrk2013
Active Member
Actually, I was talking about cancelling the room mode (standing wave) so that those peaks & nulls are severely reduced, if not eliminated. Once the room mode has been cancelled, that means the peaks & nulls no longer exist, not just for a particular location but for the entire room.
Here is a snippet from one of Floyd Toole's papers showing how to cancel peaks & nulls with subwoofer placement:
Thanks for the posting. Its interesting. Does this mean two subwoofers in a room is better than having one?
or it is not as simple as it sounds and depends upon the room dimensions!!
Exactly, depends on room dimensions.
For example, in your 11-foot wide room, your first 4 width modes will be around 51Hz, 103Hz, 154Hz and 205Hz.
If you could visually see the behaviour of the modes across the width of your room, this is what they would look like:
So if you played a test tone at your 1st width mode (51Hz) and walked across the width of your room, the sound would be loud at the left wall, become very quiet at 50% (midpoint) of room width, and become loud again at the opposite wall. (see first graph in diagram above)
When playing sounds that fall at your second width mode (103Hz), it would be loud on your left wall, become very quiet at 25% of room width, get loud again at 50% of room width, become quiet again at the 75% mark, and loud again at the opposite wall. (see second graph in diagram above)
Of course you're not going to be walking across your room when watching a movie, but if those peak & null points fall at/near seating locations, that is a problem. Imagine the person at the middle of your sofa hearing certain frequencies very quietly while the listeners on either side of him are complaining that those same sounds are too loud.
Placing your source of sound pressure (subwoofer) where sound pressure is lowest (null) will cancel that mode. Toole's example above can cancel the first 3 width modes of a room. But that is not useful in your room because your subwoofer won't be playing at 103Hz & 154Hz (your 2nd & 3rd width modes), since it will be crossed over lower (80Hz?). If the subs aren't playing sound at those frequencies, how can they have any effect (let alone cancel modes) at those frequencies.
However, your sub will be reproducing sound at 51Hz, so you can use it to cancel your 1st width mode by merely placing it at the null of that mode (hence my suggestion in your other thread). To get the same result using absorption would require very thick bass traps.
shelley
New Member
Re: DIY Room Acoustics for Home-Theatre
"In most circumstances two subwoofers will perform better than one. While you might assume this is for added SPL, the greatest benefit will actually be smoother bass response. Two subwoofers are easier to place and result in a flatter frequency response and creation of a much larger sweet spot for everyone in the room to hear smoother and more consistent bass."
For maximum output, some experts suggest that you put a single subwoofer in a corner for maximum output and place a second one in a less reflective area to smooth out the response.
Dr. Toole suggests that in a rectangular room you should put one subwoofer close to the front wall in the middle, and another subwoofer at the back of the room in the same relative position. THX recommends placing them in the middle of the left and right walls.
"In most circumstances two subwoofers will perform better than one. While you might assume this is for added SPL, the greatest benefit will actually be smoother bass response. Two subwoofers are easier to place and result in a flatter frequency response and creation of a much larger sweet spot for everyone in the room to hear smoother and more consistent bass."
For maximum output, some experts suggest that you put a single subwoofer in a corner for maximum output and place a second one in a less reflective area to smooth out the response.
Dr. Toole suggests that in a rectangular room you should put one subwoofer close to the front wall in the middle, and another subwoofer at the back of the room in the same relative position. THX recommends placing them in the middle of the left and right walls.
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john_k_antony
Well-Known Member
I have put a single sub-woofer in the middle of the front wall / screen wall and it gives me a smoother bass compared to the initial placement of the sub-woofer near the front right corner of the room.
-John.
-John.
rdksrk2013
Active Member
Sanjay, Thanks a lot for the best of explanations for me to understand. Picture conveys a lot.
Can I first finish my HT room, play a bit with placement of sub-woofer and then plan acoustic material?
How does the audessey calibration come into picture in all these scenario's?
This was my question in the thread I had started in ths section but did not get any convincing answers. The thread I had come across in avsforums and link posted here has comments by you. Interesting! i understand you had done lot of research. Happy to have you on this forum to give advice.:signthankspin:
Sorry for diverting this thread of Shelley. He is doing a good job keeping us posted on his experiences.
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What you describe is the order I would do it in.
First use placement go get the best frequency response possible with greatest seat to seat consistency. Doesn't have to be perfect, just better than you had before (some improvement is better than no improvement). Then, use treatment to fix things that placement alone couldn't fix, like long decay times.
Finally, run Audyssey to catch things that placement and treatments couldn't fix. Equalization is tricky, because fixing a problem at one location can create a problem at another seat. So it is better to use Audyssey as a finishing touch rather than relying on it alone to fix all your problems.
Once you finish your HT room and do the best you can with placement, then you can start targeting the treatment. For each piece of treatment you put up, ask yourself why you're using that type of treatment (absorption, diffusion) and why you're putting it at that location.
If you can't answer those questions, then you don't have a good reason to proceed. Just as you wouldn't place your speakers randomly in a room, likewise don't place treatments randomly.
If you look at the graph of modal behaviour in my previous post, you'll notice that you ended up combining high sound pressure (your sub) with low sound pressure (the null at middle of room width) so that those two problems (peak & dip) cancel each other out and result in smoother bass.
So if placing a subwoofer at a room null can minimize or eliminate a room mode (standing wave), then the same thing can be done with your front speakers.
Look again at the diagram and notice that the 3rd width mode has 3 null locations. What a coincidence that you happen to have 3 speakers up front. Based on your experience with subwoofer placement, what do you think would happen to the frequency response if you placed your L/C/R speakers at the nulls of the 3rd width mode?
shelley
New Member
Re: DIY Room Acoustics for Home-Theatre
in a rectangular or square room, the center of the room is the worst position for a subwoofer. For most rectangular rooms, the subwoofer should be located at least 25 percent away from the corner walls to best avoid stimulating any of its first three harmonics.
With one sub and careful placement, you can get decent bass response for one position in the room.
With multiple subs, you can get more even and smoother bass across the room for almost all positions.
Resources for speaker tests:
FREE TEST TONES
http://www.realtraps.com/test-cd.htm
http://www.cleansofts.com/softdownload/411/29899/Test_Tone_Generator.html
http://www.nch.com.au/tonegen/index_b.html
TEST SUBWOOFERS AND SPEAKERS
http://www.realmofexcursion.com/downloads.htm
2D High Definition Trailers (HD)
http://www.demo-world.eu/trailers/high-definition-trailers.php
Waiting for gypsum ceiling works to start..currently the workers are busy on another project..
in a rectangular or square room, the center of the room is the worst position for a subwoofer. For most rectangular rooms, the subwoofer should be located at least 25 percent away from the corner walls to best avoid stimulating any of its first three harmonics.
With one sub and careful placement, you can get decent bass response for one position in the room.
With multiple subs, you can get more even and smoother bass across the room for almost all positions.
Resources for speaker tests:
FREE TEST TONES
http://www.realtraps.com/test-cd.htm
http://www.cleansofts.com/softdownload/411/29899/Test_Tone_Generator.html
http://www.nch.com.au/tonegen/index_b.html
TEST SUBWOOFERS AND SPEAKERS
http://www.realmofexcursion.com/downloads.htm
2D High Definition Trailers (HD)
http://www.demo-world.eu/trailers/high-definition-trailers.php
Waiting for gypsum ceiling works to start..currently the workers are busy on another project..
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IMO, The speaker/sub placement will vary based on different room condition.
e.g, My room is rectangular and fully closed on screen end, and i have a door on left side on the projector location and a long window on right side to the projector location. (Rear wall).
Yesterday, i have tested a small speaker with various locations. (My current room is heavy in echo and untreated) to indentify the variations in echo.
Found the following :
1. Keep/face the speaker parellel to floor [Pependicular to PJ wall]. (Medium echo)
2. Keep/face the speaker partially/slanding towards roof. (Too much echo)
3. Keep/face the speaker slanding downwards to floor. (Lot of reduction in echo)
The 3rd one is some thing like, it is matching to cinema theater placement, (Hooking above your head and facing downwards).
And identified another point that, My door & window helping to pass the reflection through it to out of the room. The same way i am planing to face my SUB to door, but crossing the audience location. (Not center (or) Not corner)
IMO, Theories we can take it at some level, not exact. (SOUND is definite [theory] in a stand alone mode. It is INFINITE when it combined with air and particles around it).
But, I am sure this thread is helping lot of people like me [even i am not a technical guy in sound].I am learning new things from this thread. I thank to all the experts paticipating in this and helping others.
@RDKSRK: IMO,Better you place your spekers and fix the location with natural sound observers [Doors/Windows...etc] and then apply treatement whereever required, If we need to go for a cost effective method. [I am going on this way]
e.g, My room is rectangular and fully closed on screen end, and i have a door on left side on the projector location and a long window on right side to the projector location. (Rear wall).
Yesterday, i have tested a small speaker with various locations. (My current room is heavy in echo and untreated) to indentify the variations in echo.
Found the following :
1. Keep/face the speaker parellel to floor [Pependicular to PJ wall]. (Medium echo)
2. Keep/face the speaker partially/slanding towards roof. (Too much echo)
3. Keep/face the speaker slanding downwards to floor. (Lot of reduction in echo)
The 3rd one is some thing like, it is matching to cinema theater placement, (Hooking above your head and facing downwards).
And identified another point that, My door & window helping to pass the reflection through it to out of the room. The same way i am planing to face my SUB to door, but crossing the audience location. (Not center (or) Not corner)
IMO, Theories we can take it at some level, not exact. (SOUND is definite [theory] in a stand alone mode. It is INFINITE when it combined with air and particles around it).
But, I am sure this thread is helping lot of people like me [even i am not a technical guy in sound].I am learning new things from this thread. I thank to all the experts paticipating in this and helping others.
@RDKSRK: IMO,Better you place your spekers and fix the location with natural sound observers [Doors/Windows...etc] and then apply treatement whereever required, If we need to go for a cost effective method. [I am going on this way]
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