DIY Room Accoustics for Home-Theatre

[sdurani]

What is your explanation for "2nd order mode is already at 100%" at mid point (magenta) in the Graph. and how it correlates with smoother bass..sir

Asked and answered earlier: by getting rid of 2 of the 3 problems (nulls that a receiver cannot fix) you are left with only one problem (peak that a receiver can fix). No complicated.

 

[sdurani]

Sir, is Acoustic Sciences Corporation and Arthur Noxon wrong ?? LOL...

Toole apparently disagrees:

However, a corner is a logical starting location. For many simple installations, an unequalized single woofer in a corner can work very well indeed. In fact, I am listening right now to just such a system in my office/den.

Which expert are you going to believe?

 

[shelley]

However, a corner is a logical starting location. For many simple installations, an unequalized single woofer in a corner can work very well indeed.

That was for bass response. Bass will be the highest at corners but it it is not the optimal position for sub-woofer placement if we consider all room modes...

After so much of discussion...you are still off topic :annoyed:

Smoothening bass by placing subwoofers in correct position taking into consideration room modes and bass at corners are entirely different things...

That is why I told you earlier that you are clever at diverting discussions by posting irrelevant and unrelated matters...

Dr.Toole is referring to bass response in one paragraph of his article
http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White Papers/LoudspeakersandRoomsPt3.pdf

and

Arthur Noxon is discussing about sub-woofer placement after taking into consideration the room modes in the following article: (which is relevant to the discussion)
http://www.asc-home-theater.com/htvol3.htm

Toole apparently disagrees:

Can you show us from where you got the information that they disagree ??? :licklips:

It is not Dr. Toole that disagrees with Arthur Noxon, it is you sanjay..

So Arthur Noxon a great physicist and state licensed acoustical engineer and the inventor of the TubeTrap and the owner, working president, and director of engineering of Acoustic Sciences Corporation is also wrong according to you ..: LOL...

I pity you man...hyeah:

About Arthur Noxon:
The author of the article on optimum sub-woofer placement available at:
http://www.asc-home-theater.com/htvol3.htm

Arthur Noxon is a Eugene-based Acoustical Engineer with Master of Science degrees in Mechanical Engineering/Acoustics and Physics. A professional engineer since 1982, Mr. Noxon is state licensed to practice engineering in the public domain with a specialty area of acoustics. Acoustic Sciences Corporation, was the company he founded in 1984. inventor of the TubeTrap..

http://www.acousticsciences.net/art-noxon-pe.htm

About Acoustic Sciences Corporation:
http://www.acousticsciences.com/about

@rdksrk2013

In all these correspondences we forgot that Shelley is sharing his HT build and we would love to keep hearing his progress.
Great going. Let us help shelley to realise his dreams. :yahoo:

Thank you buddy...I will post progress and photos as usual.. I am waiting for ceiling guys to start their job...Here it is onam Holiday
I am stopping further discussions about acoustics as I have already provided links and opinions from experts......Let the users decide after reading all those stuff, rather than indulging in pointless discussions to hijack this thread :thumbsup:

 

[sdurani]

Dr.Toole is referring to something else and Arthur Noxon is discussing about sub-woofer placement after taking into consideration the room modes

That didn't stop you from using a Toole quote about bass quantity in a discussion about bass quality, as I demonstrated earlier. But when I bring up a Toole quote, suddently "Dr.Toole is referring to something else". Can't have it both ways.

The fact remains that Toole never mentions placing a subwoofer between 25% and 50% of room width. For a single sub, his recommendation has always been to start with corner placement. It's not like he's unaware that corner placement will excite every room mode. But that is still his first choice for placement.

You don't agree with that recommendation and I don't agree with it either. And that's fine. It is OK to disagree with experts because they disagree amongst themselves. Experts don't have a monolithic mindset.

What I don't understand is why a different view, such as mine, upsets you to the point of name calling over and over again (even Manoj pointed out your behaviour). If experts can understand that there will be differing viewpoints amongst themselves, then why do you take a differing viewpoint so personally?

 

[manoj.p]

@ shelley,
I think its right to put out a chronology of posts here because you are taking it quite seriously. Granted you are editing your posts afterwards, but it would be a good summary so everyone can see how these things unfold and how you are switching you views and twisting words.

It was Sanjay who said first that putting subs at quarter position (25% so you won't accuse me of mathematical or some other thing and do that stupid LOL) in this post

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:

He again answered some more explanation about it in this post

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.

To which you countered below in this post You claimed based on google search that center position is the best as Dr. Toole & THX recommends.

"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.

Then in subsequent post, John K Antony says he got results in center position as per Sanjay's recommendation and rdksrk praises Sanjay.

Few posts below, you take 180 degree turn and say center is the worst position in this post

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
RealTraps - Test Tone CD
Test Tone Generator Free Download
Tone Generator Software - Create Audio Test Tones, Sweeps or Noise Waveforms

TEST SUBWOOFERS AND SPEAKERS
Realm of Excursion

2D High Definition Trailers (HD)
2D High Definition Trailers (HD) - Demo World

Waiting for gypsum ceiling works to start..currently the workers are busy on another project..

To which Sanjay asks why you think that is in the next post

In a rectangular or square room, placing a subwoofer at the centre of the room will avoid stimulating the first length harmonic AND avoid stimulating the first width harmonic, both at the same time. Why are you telling people it is the "worst position for a subwoofer" when it prevent the two largest nulls from occurring in a room?

Then you post a big tutorial by "Big Daddy" at blu-ray.com forum here Not gonna quote it because it's too long. But the gist is you have now found from google search that best position is not center (as found by another google search by you earlier) nor 25% (as posted by Sanjay) but between 25% to 50%.

After that, you started personal comments which are uncalled for in a truthful discussion. Especially if you don't know the other person and that really shows the character and maturity. Anyway - the discussion continues and Sanjay keeps saying that sub needs to be placed at null position to get rid of it and in-between. By the way that finding is not by him by Dr Toole, in the very first post linked here by Sanjay again

After lots of name calling by you, you quote one paper by Dr. Toole where he says the center position is the worst for "quantity" (not quality) of sub. That's quote in isolation and twists the truth. When confronted with it, you dropped it and found google to come to rescue and follows in this post quoted below. What you don't understand though it was the point made by Sanjay first of putting the subs at 25% location.

Another expert opinion from Acoustic Sciences Corporation
ASC Home Theater Acoustics

Arthur Noxon, physicist and state licensed acoustical engineer, also the inventor of the TubeTrap, continues still today as the owner, working president, and director of engineering of Acoustic Sciences Corporation.(USA)

The link above is knowledgeable for every user of this forum to understand sub-woofer placement. Clears all doubts..

Please view the first image for combined first, second and third harmonic modes..



Source:
@sanjay, read the article to clear your distorted ideas and notions: (Acoustic Sciences Corporation)
ASC Home Theater Acoustics


This is the crucial point to remember:


A pressure zone effectively extends about one quarter of the distance between adjacent pressure zones and the speaker should not be located inside the effective pressure zone space. That is how the pressure zones overlap...
Open your eyes widely ....Watch the pressure bands in the diagram above and sub-woofer positions




READ THIS IN THE ABOVE ARTICLE:



Sanjay sir, what does one-quarter mean ??? is it 1/4th or 25% ??? OOOOH....maths again

Sir, is Acoustic Sciences Corporation and Arthur Noxon wrong ?? LOL...

=======================================
The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is.
Winston Churchill

Total about turn from "center is the best position" to "center is the worst" to "Best position between 25% to 50%" to "25% or a quarter". Not surprising for a man on mission.

Anyway, the truth is - there are different theories and views. Every room is different and subwoofer placement and results differ. Otherwise subwoofer placement would have been so easy as the front LCR and surround speakers. The experts do understand those and acknowledge those as well depending on room.

As for you, asking for original to show whether center location is good? Look at his paper by Todd Welti at Harman (presented at AES) http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White Papers/multsubs.pdf
Go to page 28 where the conclusion is and see the sub location recommended by him. First two positions are center locations.

So much for the "Google Warrior".

Some disclaimer - The quote functionality in the forum software removed quotes within quotes. That's why I linked the quotes so you won't divert topics. But that won't stop you from calling names. I know that very well. Bring it on "Buddy".

 

[manoj.p]

By the way, the above post is not to defend Sanjay. Not that any truthful view needs defending, but just want to bring out what's happening here and how the words are being twisted, positions changed and name calling etc.

It's one thing what Shelley does in this room. It's all his prerogative and preference and I have full respect for that. But it ends where he starts propagating that as ultimate science fact, starts pushing it and then abuse others.

 

[shelley]

To which you countered below in this post You claimed based on google search that center position is the best as Dr. Toole & THX recommends.

total about turn from "center is the best position" to "center is the worst" to "Best position between 25% to 50%" to "25% or a quarter".

Just make me laugh again and again hyeah: hyeah:

Here is the extract of my post: (post 94)

Originally Posted by shelley
"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.

With multiple subs, you can get more even and smoother bass across the room for almost all positions.

I was referring to what Dr.Toole and THX recommends about placing 2 sub-woofers in a room...Room modes and harmonics for Two sub-woofer and 1 subwoofer in a room are different..Understand that basic principle first..

Are they the same ?????? Think man..use your brains..

Look at his paper by Todd Welti at Harman (presented at AES) http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White Papers/multsubs.pdf
Go to page 28 where the conclusion is and see the sub location recommended by him. First two positions are center locations.

Todd Welti is referring to multiple sub-woofers placed in a room...Here is the screenshot of page 28...See the subwoofer positions in the picture...He is talking about 2 and 4 subwoofer placements..The url itself is multsubs.pdf

Quoting Todd Welti in above article:

One subwoofer at each wall midpoint is the best in terms of Std, Max-ave and Max-min but does not support low frequencies particularly well.

What is the meaning of "EACH" ????

You are getting confused at the following sentence in the article..

One subwoofer in each corner also has good low frequency support, but does not perform quite as well as one subwoofer at each wall midpoint, in terms of Std, Max-ave and Max-min. If cost and aesthetics are considered,
subwoofers at 2 wall midpoints is preferred.

Todd Welti is referring to "One subwoofer in each corner "...He is referring to multiple sub-woofer placement...What is the meaning of "EACH" ????


Poor reading and comprehension skills on display once again..

*** That is why I told you earlier...you and your friend are poor at comprehending information

Repeating again:
Room modes and harmonics for Two or four sub-woofers in a room and 1 subwoofer in a room are different..Understand that basic principle first..

Some disclaimer - The quote functionality in the forum software removed quotes within quotes.

See below: A quote within quote..

Testing quote within quote. This is the first quote

This is a quote within quote

If i can create a quote within a quote you too can hyeah:

Please do not comment on matters which you are not aware of and display your ignorance to the world as you have been doing in all your posts:...Lot of users are watching this thread...See the views...
Most of them are intelligent and can understand facts...hyeah:

By the way, the above post is not to defend Sanjay.

(An indirect confession) hyeah:

Everybody in this forum is aware of it

Remember once again that I was referring to articles by experts explaining single sub-woofer setup in a room taking into consideration all room modes which will produce smoother bass..
http://www.asc-home-theater.com/htvol3.htm
http://forum.blu-ray.com/showthread.php?t=48286

You and your friend are referring to a single article by Todd Welti relating to multiple sub-woofer placement in a room and confusing it with single sub-woofer placement.. and are yourselves getting confused and in the process repeatedly displaying your ignorance...:yahoo:

 

[sdurani]

I am stopping further discussions about acoustics as I have already provided links and opinions from experts....

You finally posted something I was willing to believe. Alas, even that turned out not to be true.

 

[Swamy_HT]

All,

I have just pass through a thread. (A manual from Yamaha YHT not recomending placing sub in the center)

http://www.hifivision.com/home-theater/28044-purchased-yamaha-yht-196-today-20-999-a-5.html

 

[manoj.p]

All,

I have just pass through a thread. (A manual from Yamaha YHT not recomending placing sub in the center)

http://www.hifivision.com/home-theater/28044-purchased-yamaha-yht-196-today-20-999-a-5.html

Yes, that exactly will happen if you put the sub in the center of the wall and seat in the center. See where it points about standing waves. In any case, center of the room is the worst position for listening. If you put a sub in corner then center of room has null at first mode, 100% booming at second mode and null again at 3rd mode. If you put the sub in the center of wall, it will take of care of nulls at first mode and third by making them disappear. Now, all 1st, second and third modes will be at 100%.

This works to the listeners advantage, if your listening position is not in that center zone. As for all the three peaks, users can use the equalizer to bring those down. Remember, we can bring down the peaks, but we can't bring nulls up by equalization. So its important to make those nulls disappear by positioning.

 

[shelley]

If you put the sub in the center of wall, it will take of care of nulls at first mode and third by making them disappear. Now, all 1st, second and third modes will be at 100%.

Now, all 1st, second and third modes will be at 100%
This has turned into a comedy show hyeah:

@Swamy_HT

All,

I have just pass through a thread. (A manual from Yamaha YHT not recomending placing sub in the center)

Just ask him what happened to second room mode at 100% if subwoofer placed in middle and its consequence ???
(second room mode at 100% at middle position, (magenta line)which is not optimal)

The speaker has to be located somewhere, but not at either end, not at the middle, and definitely not at the third waypoints.

A listening room can be approximated as if composed of three intersecting pipes. These pipes would lie along the three room axes -- front to back, side to side, and floor to ceiling. This means that the subwoofer location for best, non-resonant playback will be about one-quarter of the ceiling height off the floor, one-quarter the width of the room off the side walls, and one-quarter the room length off the front or back wall

Source:
Arthur Noxon, Acoustic Sciences Corporation
http://www.asc-home-theater.com/htvol3.htm

 

[sdurani]

A manual from Yamaha YHT not recomending placing sub in the center

Instead they recommend placement at/near the side wall, which will make the sub louder by exciting all the room modes. Same reason Toole starts with corner placement: when you have just one sub to pressurize the entire room, some (like Yamaha and Toole) will prioritize quantity over quality. User has to decide which result is more important to them, maximizing levels or mode cancelling.

 

[manoj.p]

@Swamy_HT

Just ask him what happened to second room mode at 100% if subwoofer placed in middle and its consequence ???
(second room mode at 100% at middle position, (magenta line)which is not optimal)

You tell me what will happen to 2nd mode. Let's see if you have understood that.

 

[john_k_antony]

Just thought of sharing this

Velodyne blog: 5 Tips for Optimal Subwoofer Placement | Velodyne Blog. What they talk about in this blog is very simple and less technical.

I have 2 Velodyne Subs in my home. One is Living Room and Other in my dedicated HT Room. EQ-Max10 in Living room and EQ-Max12 in HT room.

The one in Living room is kept at the corner, for exactly the same reason cited in the above blog point #4 . And the quality of bass in my Living room is no where in the league of the quality bass of I get in my dedicated room.

Quote from the above blog.

No One Puts a Velodyne in the Corner Unless loudness is your top priority, we dont recommend corner placement for the sub. Subwoofers placed in a corner will excite the room easier, making the sub more directional (i.e. you can tell where it is) and it will tend to emphasize the bass at certain frequencies. This may make the overall bass performance more boomy or muddy. Then again, if you want to impress your neighbors with your subs earth-shaking volume, the corner is the best place.

However, another funny thing is that if you read the placement guide from Velodyne, you see contradicting statements. There they say, place the sub woofer in the corner for "crisp" response. Please read the below document.

http://velodyne.com/pdf/guides/sub_placement_guide_reva.pdf

Reading these contradictions, I realized that YOU have to spend time experimenting the best position for your sub-woofer. Because each room and the sub-woofer is different. At the same location in the same room, two different branded sub-woofer's will sound totally different. A bigger sub-woofer will sound totally different from a small sub-woofer at the same location in the same room. These are purely based on my experience.

Now coming to the position of the Sub in my dedicated HT room, it is kept at the following location.

4ft from the screen wall and at the center of the side wall.

I get a very good bass response for my liking. I wouldn't say earth shattering, but a very balanced bass that goes well with rest of my speakers and gives me a very pleasant movie experience. Another user might find it less "punchy" or "heavy". Again, I am not in that league of people who wants their sub woofer to create "pounding in chest". However, initially when my sub woofer was placed close to the corner, I was getting too much of boomy bass.

With all these, what I have learnt is that you and your ears are the primary judges for the right placement of your sub woofer for the kind of bass you like. Room treatment tries to reduce some of the imbalances caused by reflection and gives you few more placement options.

My 2 cents,

Rather than complicating things further with technical details and expert opinion, concentrate on your room and the sub woofer you have and experiment with yourself. There is lots of fun in doing these experiments yourself rather than relying too much on technical details and expert opinions. Technical details and expert opinions will definitely help you start your experiments and make some starting assumptions. But I will never rely 100% on anything else other than my own pair of ears .

Thanks,
John.

 

[sash]

Hey Shelly, any progress on the build? That part was also quite interesting by the way :b
Please update the thread as and when you have updates.

 

[Swamy_HT]

All,

First thing: I am not the sound techincal guy.

I think, there are 2 points collapsed with each other here.

Point: 1. In a closed room, NULL will be on center of the room on 1st and 3rd mode.
[My understanding: It will cancel actual wave from sub on 1st and 3rd mode at the NULL position (or) The original wave will reach the center position 100% on 1st and 3rd mode which will fill the NULL position - Which one is correct?]

Point: 2. Best place to keep the SUB.

In these two points,

Point#1 - is the pure science. I hope there is no difference of opinion between all of you. Please correct me if i am wrong.

Point#2 - is the debate going on between all of you. IMO, It is personal taste by physically moving around various locatons according to room accuastic.! Please continue your debate with additional supporting points/visual representations if required (or) Please move to next interseting points such as placing speakers,disctance...etc.

I have a question on point#1 to understand more, this statement is valid only if the two end is closed (Well treated ? (or) Reflected ? Please clarify).
Most of the google i found a pipe as an example, But nowhere it is mentioned that, this pattern accured due to placing the sound source in the "center of the pipe (or) corner of the pipe'? (OR) we are considering 2 opposite straight line walls as a pipe? If anyone through more light on this would be helpful .

e.g. If i place a sub in the corner (or) 20% from wall...etc, and facing the speaker plane perpendicular to oppsite wall. (Straingt line), It that case, it will not create that NULL pattern and cancel the original wave? (OR) we are talking the placement including the tilting angle of the sub front horizontally.?

Looks interesting topic/thread...Thanks to all.:licklips:

 

[manoj.p]

All,

First thing: I am not the sound techincal guy.

I think, there are 2 points collapsed with each other here.

Point: 1. In a closed room, NULL will be on center of the room on 1st and 3rd mode.

Bass sound waves go in all directions. In a rectangular room, the modes will be lengthwise, width wise, height and then oblique. because all users are seating more or less same height, the height mode is ignored. Same with oblique mode. So, that leaves two important ones, length mode and width mode. If the sub is in the corner, it will have nulls of 1st and 3rd order at center of the room (which is incidently the cross point where width and length modes intersect)

[My understanding: It will cancel actual wave from sub on 1st and 3rd mode at the NULL position (or) The original wave will reach the center position 100% on 1st and 3rd mode which will fill the NULL position - Which one is correct?]

If you have a sub at front wall mid point - then it will cancel 1st and 3rd width mode. Length mode will still be present.

If you put sub at side wall mid point, it will cancel same modes length wise.

Personally, I would take care of width modes first. That will ensure that all the listeners in the same row will get same bass response.

I have a question on point#1 to understand more, this statement is valid only if the two end is closed (Well treated ? (or) Reflected ? Please clarify).
Most of the google i found a pipe as an example, But nowhere it is mentioned that, this pattern accured due to placing the sound source in the "center of the pipe (or) corner of the pipe'? (OR) we are considering 2 opposite straight line walls as a pipe? If anyone through more light on this would be helpful .

Not sure of the pipe experiment. But all the talk of standing waves can be mathematically predicted only if the room is rectangular and completely closed, meaning no openings etc. My room is trussed ceiling and has a big opening at the back. So, all the calculations are way off. The only way in that case, is to measure and play with placements to find an optimum response.

e.g. If i place a sub in the corner (or) 20% from wall...etc, and facing the speaker plane perpendicular to oppsite wall. (Straingt line), It that case, it will not create that NULL pattern and cancel the original wave? (OR) we are talking the placement including the tilting angle of the sub front horizontally.?

Looks interesting topic/thread...Thanks to all.:licklips:

Not able to visualize this. Can you please include a diagram?

 

[sdurani]

Point: 1. In a closed room, NULL will be on center of the room on 1st and 3rd mode.
[My understanding: It will cancel actual wave from sub on 1st and 3rd mode at the NULL position (or) The original wave will reach the center position 100% on 1st and 3rd mode which will fill the NULL position - Which one is correct?]

There is a small distinction between preventing a mode from being excited in the first place vs actively cancelling a mode. What makes the distinction even smaller is that you end up with the exact same result.

Imagine a swing in a playground. Not touching that swing means that it won't move (resonate). But having two people on opposite sides of the swing, pushing equally hard, at the same time, will also result in the swing not moving (two opposing forces cancelling). In both cases, you get the exact same result: the swing doesn't move.

Which is why everyone uses the term "mode cancelling", even when it is technically 'preventing' a problem rather than 'curing/fixing' a problem. The only people interested in that small distinction are folks like me who are curious about how this phenomenon works. For everyone else, when a mode is no longer there due to subwoofer placement, then they consider it cancelled.

The Art Noxon site that Shelley is fond of quoting says the following: "However, if the speaker were moved to the exact middle of the pipe, the first resonance would not sound out. Nor would the third resonance, the fifth, and so on. Odd numbered resonances cannot be stimulated in a closed pipe when the speaker is located in the middle of the pipe."

"Cannot be stimulated" means won't occur to begin with. The mechanism by which this works is by combining the highest sound pressure (sub) with the lowest sound pressure (null). I guess you could think of that as cancelling or filling in a dip with a peak (filling in low sound pressure with high sound pressure).

But that's different from the type of mode cancelling that Toole is talking about with a pair of subs. Modes have polarity, so placing a pair of subs symmetrically on opposite sides of the centre line will cause the two sound waves (positive and negative) to meet and cancel each other's peaks & dips. When I say opposite sides of the centre line, I mean the subs could be anywhere (even on opposite side walls) as long as the separation is symmetrical from the midpoint of room width.

Each sub is still causing modal excitation, but in opposite phase, which results in them cancelling each other's peaks & dips. Neither one of the subs has prevented modal problems, but both combine to actively cancel (fix/cure) the problems.

If you look at the same diagram I posted previously, you'll notice the small plus and minus signs showing polarity. The odd-order modes (1st, 3rd, etc) are in phase (plus sign) on one side of the room but when the sound wave reaches the opposite wall it is out of phase (minus sign). Since modes have polarity, you can take advantage of this for mode cancelling (different from preventing).

So your original question was like asking whether there is a difference between not touching the swing vs two people on opposite sides pushing on the swing simultaneously. The former is like placing a sub in a null to prevent a standing wave from occuring, the latter is like using two subs to cancel each other's standing waves. Same exact result either way: odd modes gone.

Make sense?

 

[Swamy_HT]

Based on both Sanjay & Manoj, As a layman what i have understood is, if we place the sub in center location (Front/Side wall), the odd number modes will get cancelled (ie, You will not able to listen the flat (or) actual produced by the sub on odd number modes, Intead you will hear the room modes/boomy effects on odd number modes). [Applicable to single sub in a rectangular closed room].

If my understanding is correct, we should avoid placing sub in the center position (side/front).I thought shelly also said the same.

If my understanding still not correct, Please ignore my technical understanding and Just let me know, if i have a single sub, in a closed room, which is the best location to place sub? (Corner, 20% off from corner, center, (or) as per individual taste...?)

The attached image i have taken from Manoj setup. (He has one more sub on Right side)
The sub is placed on left side, but it is angled straight compared with the near by speaker. Why not i can turn the sub also align to left speaker? Is there any specific reason?

 

[manoj.p]

Swamy,

If you put the subwoofer in null position, then it gets rid of the null for any mode at that position. So putting the sub in center will get rid of odd order modes like 1 and 3. That's what Sanjay means by his analogy of "not touching the swing". You are getting rid of that mode completely.

Here is a real life example. http://www.avsforum.com/t/1449924/s...nd-how-to-interpret-graphs/1050#post_22944510

first graph is with corner placement and then center position. Second graphs is audyssey correction + corner position vs center position with no room correction. This shows how much important positioning is to get the bass right.


About turning sub - Frequencies below 80 hz are non directional and cannot be localized. Won't make a difference if you turn it around.