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# Room mode calculator and speaker placement

I downloaded an Excel room mode calculator after a helpful forum member started giving me speaker 'distance a part' placement suggestions based off calculations. I got curious as to room modes, calculators, etc.... and started reading a bit. Ended up with the calculator.

What I don't understand is how you calculate the speaker distance based on the results from the calculator. Here's a pic of the calculator with my room dimensions.

How do I get the speaker placement for width based on the info below. Actually, I have the info as the helpful forum member gave it to me, but I like to learn too.

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Quote:
Originally Posted by pdxrealtor

What I don't understand is how you calculate the speaker distance based on the results from the calculator.

You don't. Mode calculators are meant to help find good dimensions for a new room being built. They are not intended to help you treat an existing room. For that you can measure the room's response, though even that is not truly necessary. This short article is mainly about home recording, but all the same principles apply to hi-fi and home theater too:

Acoustic Basics

--Ethan
I've read several times that a room mode calculator provides a good place to start. Did I read that wrong?
Since I opened this can o'worms in another thread, I might as well finish the explanation.
Quote:
Originally Posted by pdxrealtor

What I don't understand is how you calculate the speaker distance based on the results from the calculator.

Calculating modes is easy: divide the speed of sound by a room dimension. 1130 ÷ 16 = 70.6 (round up to 71). But 71Hz is not your first width mode because half a wave can bounce back and forth between your side walls and complete a wavelength. So your first width mode is at 35Hz, and multiples thereof (71Hz, 106Hz, 141Hz, etc).

Imagine a ragged landscape with a mountain and a valley. If you slice off the mountain peak and flip it over into the valley, you'll get a flatter landscape. Likewise, if you place your sources of pressure (subwoofers, speakers) at room locations that have the least pressure (nulls), then you can get a flatter response by nixing peaks & dips that were a result of modal problems.

Looking at your width mode chart, notice the numbers at the middle bottom:

Placing a subwoofer 8 feet from your left wall (midpoint of room width) will mitigate the 35Hz width mode. Those numbers are right there in black & white (well, blue & grey). If you've got 2 subwoofers...

...Placing them at 4 feet and 12 feet from the left wall (1/4 of room width from the side walls) will mitigate the 35Hz and 71Hz modes. Above the crossover point, you'll have to use the pressure generators (woofers) in your 3 front speakers, the locations for which are...

...Right there on your chart, at the 3 null points of your 3rd width mode, with the L/R speakers 2.75 feet from the side walls.

BTW, this isn't some theoretical exercise. If you played a 106Hz tone and walked across the width of your room, you would actually hear very noticeable peaks & dips at the locations shown in your chart. Place those 3 speakers at those 3 locations and those peaks & dips would be gone.

You can likewise use your length mode chart to choose seating position. If you're ever tempted to place your seating at the midpoint of room length...

Notice that the frequency response will have some severe peaks & dips.

By comparison, if you place your seating at one of the third divisions of room length...

...Notice how much closer together those frequencies are, giving you a smoother overall response. Even the 3rd lenght mode (in red) is relatively close in level to the other frequencies. Not perfect, but better than other locations in room length.

If you've noticed a pattern, it appears that even divisions (halves, quarters, sixths) work better for speakers while odd divisions (thirds, fifths) work better for seating. Pretty easy rule of thumb to rememeber.
One example would be noticing the width mode section for the 16 foot dimension--
The second width mode has a dip at both the 4 ft and 12 ft distance. That would be an 8 foot spread between dual subs deployed so as to NOT excite the second width mode.

Although not indicated on the calculator is the fact that modes have phase. In particular the phase of a mode on either side of a dip is flipped 180 degrees. Gain matched subs placed symmetrically on either side of a null (dip) will effectively provide what is known as modal cancellation.

Armed with this information, you may notice that subs deployed at the 1/4 points along the front wall will cancel the first 3 width modes. Simple as that.

The calculator(s) is/are based on a set of ideal assumptions which may or may not play out in the real world of "as built" rooms but can certainly be a good starting point.

edit--writing when Sanjay posted so "never mind "
Quote:
Originally Posted by jamin

edit--writing when Sanjay posted so "never mind "
Not "never mind", since you bring up some good points that I didnt cover here.
Quote:
Originally Posted by jamin

The calculator(s) is/are based on a set of ideal assumptions which may or may not play out in the real world of "as built" rooms but can certainly be a good starting point.
Right, the real-world physical room might not match the idealized virtual room of the calculator, so results might not be perfect. As you say, still the basis for a good starting point.
Quote:
Originally Posted by jamin

Armed with this information, you may notice that subs deployed at the 1/4 points along the front wall will cancel the first 3 width modes. Simple as that.
Had talked about that subwoofer configuration cancelling the first 3 width modes in the OP's other thread, but I doubt he will be crossing his subs above 106Hz. So in this thread I limited the discussion to using the subs to cancelling the modes below the crossover point (35Hz, 71Hz).
Quote:
Originally Posted by sdurani

..........Had talked about that subwoofer configuration cancelling the first 3 width modes in the OP's other thread, but I doubt he will be crossing his subs above 106Hz. So in this thread I limited the discussion to using the subs to cancelling the modes below the crossover point (35Hz, 71Hz).
Yup, I just tossed it in for some completeness.
Quote:
Originally Posted by pdxrealtor

I've read several times that a room mode calculator provides a good place to start. Did I read that wrong?

Or maybe someone else wrote it wrong. Such misinformation is rampant all over the Internet. Mode calculations can be off by 20 percent or even more, due to real-life rooms having boundaries that are not 100 percent reflective. For example, if a wood floor on joists resonates at a modal frequency, that can shift the mode frequency downward. This is why measuring is the most reliable way to assess a room's behavior.

--Ethan
Quote:
Originally Posted by sdurani

Since I opened this can o'worms in another thread, I might as well finish the explanation.

Calculating modes is easy: divide the speed of sound by a room dimension. 1130 ÷ 16 = 70.6 (round up to 71). But 71Hz is not your first width mode because half a wave can bounce back and forth between your side walls and complete a wavelength. So your first width mode is at 35Hz, and multiples thereof (71Hz, 106Hz, 141Hz, etc).

Imagine a ragged landscape with a mountain and a valley. If you slice off the mountain peak and flip it over into the valley, you'll get a flatter landscape. Likewise, if you place your sources of pressure (subwoofers, speakers) at room locations that have the least pressure (nulls), then you can get a flatter response by nixing peaks & dips that were a result of modal problems.

Looking at your width mode chart, notice the numbers at the middle bottom:

Placing a subwoofer 8 feet from your left wall (midpoint of room width) will mitigate the 35Hz width mode. Those numbers are right there in black & white (well, blue & grey). If you've got 2 subwoofers...

...Placing them at 4 feet and 12 feet from the left wall (1/4 of room width from the side walls) will mitigate the 35Hz and 71Hz modes. Above the crossover point, you'll have to use the pressure generators (woofers) in your 3 front speakers, the locations for which are...

...Right there on your chart, at the 3 null points of your 3rd width mode, with the L/R speakers 2.75 feet from the side walls.

BTW, this isn't some theoretical exercise. If you played a 106Hz tone and walked across the width of your room, you would actually hear very noticeable peaks & dips at the locations shown in your chart. Place those 3 speakers at those 3 locations and those peaks & dips would be gone.

You can likewise use your length mode chart to choose seating position. If you're ever tempted to place your seating at the midpoint of room length...

Notice that the frequency response will have some severe peaks & dips.

By comparison, if you place your seating at one of the third divisions of room length...

...Notice how much closer together those frequencies are, giving you a smoother overall response. Even the 3rd lenght mode (in red) is relatively close in level to the other frequencies. Not perfect, but better than other locations in room length.

If you've noticed a pattern, it appears that even divisions (halves, quarters, sixths) work better for speakers while odd divisions (thirds, fifths) work better for seating. Pretty easy rule of thumb to rememeber.

The illustrations are great and help me to really understand your explanation. Thank you! You've been helping me a lot in my other thread, so I didn't want to overwhelm you when I could most likely do some leg work and figure it out. But.... looks like you weren't overwhelmed.
Quote:
Originally Posted by jamin

One example would be noticing the width mode section for the 16 foot dimension--
The second width mode has a dip at both the 4 ft and 12 ft distance. That would be an 8 foot spread between dual subs deployed so as to NOT excite the second width mode.

Although not indicated on the calculator is the fact that modes have phase. In particular the phase of a mode on either side of a dip is flipped 180 degrees. Gain matched subs placed symmetrically on either side of a null (dip) will effectively provide what is known as modal cancellation.

Armed with this information, you may notice that subs deployed at the 1/4 points along the front wall will cancel the first 3 width modes. Simple as that.

The calculator(s) is/are based on a set of ideal assumptions which may or may not play out in the real world of "as built" rooms but can certainly be a good starting point.

edit--writing when Sanjay posted so "never mind "

What if there were two subs each crossed over at 50hz. Then where would they be ideally placed? Next to each other at the 8' mark?

Am I correct to say at some point the logic of crossing speakers over at certain frequencies will prevent a situation where for instance subs are crossed at 50hz and L/R speakers crossed at 110hz, leaving a 'null' that would be hard to fix with a speaker?

How would you correct these nulls with only one sub. Seems that both subs are taking care of quite a few dips.

Quote:
Originally Posted by sdurani

Not "never mind", since you bring up some good points that I didnt cover here.
Right, the real-world physical room might not match the idealized virtual room of the calculator, so results might not be perfect. As you say, still the basis for a good starting point.
Had talked about that subwoofer configuration cancelling the first 3 width modes in the OP's other thread, but I doubt he will be crossing his subs above 106Hz. So in this thread I limited the discussion to using the subs to cancelling the modes below the crossover point (35Hz, 71Hz).

Quote:
Originally Posted by Ethan Winer

Or maybe someone else wrote it wrong. Such misinformation is rampant all over the Internet. Mode calculations can be off by 20 percent or even more, due to real-life rooms having boundaries that are not 100 percent reflective. For example, if a wood floor on joists resonates at a modal frequency, that can shift the mode frequency downward. This is why measuring is the most reliable way to assess a room's behavior.

--Ethan
I'm not understanding you, I'm sorry. It seems it's a good tool. Not trying to say measuring isn't superior in every way, but it seems you're saying a mode calculator is not a good starting point and that I got misinformation from the internet, which does not seem to be the case.

If I had the subs crossed at 80hz and L/R crossed at 110hz, would this be the correct placement?

Stars are subs. Arrows are L/R
Quote:
Originally Posted by Ethan Winer

You don't. Mode calculators are meant to help find good dimensions for a new room being built. They are not intended to help you treat an existing room. For that you can measure the room's response, though even that is not truly necessary. This short article is mainly about home recording, but all the same principles apply to hi-fi and home theater too:

Acoustic Basics

--Ethan

So that's the theory and explains why it's so hard to place a sub -- or more than one sub.

At a more practical level in an existing room, I remember reading to put my operating sub in the main seat (position) for listening. Then walk around the room listening for the best low frequency response. That's where you want to place your sub. Same principles applied in reverse. This worked very well for me in my really odd and large listening area.

When I got a second sub, I repeated the exercise with my first sub operating in place. The spot identified for the second was impossible -- so I went with the next best sounding location. I've had outstanding sound for 15 years.

BTW, you may well be surprised about where the best location for you sub ends up being.

Rich
Quote:
Originally Posted by Ethan Winer

Or maybe someone else wrote it wrong. Such misinformation is rampant all over the Internet. Mode calculations can be off by 20 percent or even more, due to real-life rooms having boundaries that are not 100 percent reflective. For example, if a wood floor on joists resonates at a modal frequency, that can shift the mode frequency downward. This is why measuring is the most reliable way to assess a room's behavior.

--Ethan

If all your walls are similar in terms of construction then these calculators can be pretty accurate. Worst case you might find a null or peak to be 1ft away from where it should be.

If your walls are dissimilar e.g. one wall is brick another sheetrock the modes can be shifted a lot. I had one case where the peaks and nulls were 4ft from where predictions said they would be.
And large furniture, alcoves, windows, etc. can affect the results...

The "sub crawl" has been oft-dicussed here as a means of finding the optimum sub placement. It is not a bad thing, but I am not sure it as useful as folk seem to think. It might help find the loudest place for one frequency, but that could just as well be a harmonic you can hear rather than the deeper sound you "feel". I prefer mic and a meter.
Quote:
Originally Posted by Nyal Mellor

If all your walls are similar in terms of construction then these calculators can be pretty accurate. Worst case you might find a null or peak to be 1ft away from where it should be.

If all the walls (and floor and ceiling) are similar construction, then the mode ratio will probably be similar to what's predicted. When a boundary absorbs a given frequency, that frequency is shifted lower by some amount. So thin drywall on the front and rear, with cinder block left and right, results in the width mode being accurate but the length mode being lower than predicted. There may also be some interaction between such a length and width, though I haven't tested that so I can't say for sure.
Quote:
I had one case where the peaks and nulls were 4ft from where predictions said they would be.

Exactly, that's my main point. This is why measuring is the only way to know the true response at a given location in a room. Measuring also helps to find the best location for speakers and subwoofers. As Don mentioned, the "crawl" method is not reliable. One big reason is it's usually done while playing music, but music contains bass frequencies that may or may not align with a room's modes. And the bass notes keep changing. Versus room measuring software that tells you exactly what you have in five seconds.

--Ethan
Removed and posted in a new thread
Edited by pdxrealtor - 5/30/13 at 5:11pm
Quote:
Originally Posted by pdxrealtor

If I had the subs crossed at 80hz and L/R crossed at 110hz, would this be the correct placement?

Stars are subs. Arrows are L/R
Looks like you're confusing room modes with crossover frequencies. Not sure why you would want to limit the subs to reproducing sound below 80Hz and limit the speakers to reproducing sound above 110Hz. Is there a reason you don't want the content between 80Hz and 110Hz reproduced?

Besides, I don't think most A/V receivers will allow you to do that. Think of crossovers as slicing the frequency response in two: if you set it to 80Hz, then content below that frequency goes to the subwoofers while content above that frequency goes to the speakers.

Also, your proposed placement above won't work as well for modal cancellation as what you had previously decided on. Not to mention that reducing your front speaker spread to 5 feet will result in a tiny soundstage. Is this chart for a different room, since it says 22' for width?
Quote:
Originally Posted by sdurani

Looks like you're confusing room modes with crossover frequencies. Not sure why you would want to limit the subs to reproducing sound below 80Hz and limit the speakers to reproducing sound above 110Hz. Is there a reason you don't want the content between 80Hz and 110Hz reproduced?

Besides, I don't think most A/V receivers will allow you to do that. Think of crossovers as slicing the frequency response in two: if you set it to 80Hz, then content below that frequency goes to the subwoofers while content above that frequency goes to the speakers.

Also, your proposed placement above won't work as well for modal cancellation as what you had previously decided on. Not to mention that reducing your front speaker spread to 5 feet will result in a tiny soundstage. Is this chart for a different room, since it says 22' for width?

Oh no, those were purely examples, some realistic some not, to see how to place speakers based off different room width calculations.

The above example is for a room that is 22' wide. No room in specific, and certainly not mine. Again it was an example to see if I'm understanding speaker placement based off room mode measurement results.

Where would you put the speakers on the above example of a room that's 22' wide?
Quote:
Originally Posted by pdxrealtor

Where would you put the speakers on the above example of a room that's 22' wide?
Same as your room: each of the 3 speakers would be placed at one of the 3 nulls of the 3rd width mode; each of the 2 subs would be placed at one of the 2 nulls of the 2nd width mode. If I were using 1 sub, it would be placed at the only null of the 1st width mode. The locations of these nulls are in your chart, colour coded by mode (1st, 2nd, 3rd, 4th).
Quote:
Originally Posted by sdurani

Same as your room: each of the 3 speakers would be placed at one of the 3 nulls of the 3rd width mode; each of the 2 subs would be placed at one of the 2 nulls of the 2nd width mode. If I were using 1 sub, it would be placed at the only null of the 1st width mode. The locations of these nulls are in your chart, colour coded by mode (1st, 2nd, 3rd, 4th).

What I don't understand is the 3rd null is 77hz, and if you place your main L/R at the 3rd null and it's crossed at 80hz, or heck let's say 100hz to get further away from the roll off, it won't 'pressurize' the null because 77hz will not be coming out of those speakers.

Is that right or wrong? I'm probably over thinking it....

Is it as simple as always placing speakers at the 2nd and 3rd null points? Regardless of crossover?
The speakers do put out sound at 72 Hz, of course. The nulls are places the direct and reflected sound cancel since they are of opposite polarity. Hand-wavingly, placing a speaker at that point adds more power to the "direct" sound to circumvent the null.
Edited by DonH50 - 5/30/13 at 6:28pm
Quote:
Originally Posted by DonH50

The speakers do put out sound at 72 Hz, of course. The nulls are placing the directed and reflected sound cancel since they are of opposite polarity. Hand-wavingly, placing a speaker at that point adds more power to the "direct" sound to circumvent the null.

Craftfully said - hand-wavingly. I get it.... as much as anyone can in the beginning.

I got got caught up on hz when they started talking about my subs and being crossed over at 80hz.
Thanks. Duh, "places", not "placing", time to return to my English as a second language class...
Quote:
Originally Posted by sdurani

Same as your room: each of the 3 speakers would be placed at one of the 3 nulls of the 3rd width mode; each of the 2 subs would be placed at one of the 2 nulls of the 2nd width mode. If I were using 1 sub, it would be placed at the only null of the 1st width mode. The locations of these nulls are in your chart, colour coded by mode (1st, 2nd, 3rd, 4th).

This is why I'm confused- You say above in your original explanation that- ::: --
Placing them at 4 feet and 12 feet from the left wall (1/4 of room width from the side walls) will mitigate the 35Hz and 71Hz modes. Above the crossover point, you'll have to use the pressure generators (woofers) in your 3 front speakers, the locations for which are...

In my example of a 22' wide room placing the 2 subs at the 1/4/ and 3/4 points (as in my real life room) the subs would mitigate the first three modes, because the 3rd mode is 77hz. Where as my real life room the third mode is 106hz, above the crossover point.

What effect does this 3rd mode have on L/R speaker placement.

Does the L/R speaker sit best at the 3rd mode points or in my hypothetical since the sub is crossed at 80hz and the third mode is 77hz would the sub now take care of one, two, and three modes, leave the option to widen or shorten my sound stage with my L/R speakers while at the same time taking care of the 4th mode?

Am I making sense?
Quote:
Originally Posted by pdxrealtor

If I had the subs crossed at 80hz and L/R crossed at 110hz, would this be the correct placement?

Stars are subs. Arrows are L/R

Quote:
Originally Posted by jamin

.....Gain matched subs placed symmetrically on either side of a null (dip) will effectively provide what is known as modal cancellation.
The dual subs or L&R that are deployed symmetrically on either side of the 1st width mode null will provide modal cancellation for all odd order width modes. if the speaker pairs do not happen to receive energy at those modal frequencies oh well, no harm no foul.

In your above example the subs will have modal cancelation for the odd order modes but will excite the 2nd and 4th order modes. Of those modes, the 103 Hz mode is probably sufficiently above an 80 Hz crossover that it could be ignored. Not true for the 51 Hz 2nd mode.

The L&R in your example above will also have modal cancellation in effect for the odd order modes. The 4th order is effectively cancelled by placement at the nulls. The 2nd mode could be excited except the 51 Hz mode is sufficiently below an 80 Hz crossover so that you can ignore it.

Placement of the L&R at the, ostensibly, 3/8 & 5/8 width dimension would probably make for a small sound stage unless you sit close. The same modal cancellations could be observed by placement at the 1/8 & 7/8 positions which are the other farther out nulls for the 4th order mode.

Spacing between the sub at the 1/6 point and an L speaker at the 1/8 point is much smaller than the placement spacing that you could realize with the subs at 1/4 points and mains at the 1/6 points.

Please realize that we are talking about axial width modes only and those for a rectangle type space. As mentioned, "as built" rooms may differ from the model but generally this model can be a good starting point. Measurements will confirm.

Moving to an L shaped space will throw some kinks in things. Just so you know.
Quote:
Originally Posted by pdxrealtor

Quote:
Originally Posted by sdurani

Same as your room: each of the 3 speakers would be placed at one of the 3 nulls of the 3rd width mode; each of the 2 subs would be placed at one of the 2 nulls of the 2nd width mode. If I were using 1 sub, it would be placed at the only null of the 1st width mode. The locations of these nulls are in your chart, colour coded by mode (1st, 2nd, 3rd, 4th).

This is why I'm confused- You say above in your original explanation that- ::: --
Placing them at 4 feet and 12 feet from the left wall (1/4 of room width from the side walls) will mitigate the 35Hz and 71Hz modes. Above the crossover point, you'll have to use the pressure generators (woofers) in your 3 front speakers, the locations for which are...

In my example of a 22' wide room placing the 2 subs at the 1/4/ and 3/4 points (as in my real life room) the subs would mitigate the first three modes, because the 3rd mode is 77hz. Where as my real life room the third mode is 106hz, above the crossover point.

What effect does this 3rd mode have on L/R speaker placement.

Does the L/R speaker sit best at the 3rd mode points or in my hypothetical since the sub is crossed at 80hz and the third mode is 77hz would the sub now take care of one, two, and three modes, leave the option to widen or shorten my sound stage with my L/R speakers while at the same time taking care of the 4th mode?

Am I making sense?

Edited by jamin - 5/30/13 at 10:26pm
Quote:
Originally Posted by pdxrealtor

Is it as simple as always placing speakers at the 2nd and 3rd null points? Regardless of crossover?
That's a good rule of thumb when using 2 subs and 3 speakers, since that combination will cancel the first 5 width modes.
Quote:
Originally Posted by pdxrealtor

Placing them at 4 feet and 12 feet from the left wall (1/4 of room width from the side walls) will mitigate the 35Hz and 71Hz modes. Above the crossover point, you'll have to use the pressure generators (woofers) in your 3 front speakers, the locations for which are...

In my example of a 22' wide room placing the 2 subs at the 1/4/ and 3/4 points (as in my real life room) the subs would mitigate the first three modes, because the 3rd mode is 77hz. Where as my real life room the third mode is 106hz, above the crossover point.
In both cases, placing subs or speakers at the quarter points can cancel the first 3 width modes. But it depends on whether those devices are producing sound at those frequencies.

In your room, the subwoofers can only address a problem at 106Hz if you allow them to play sound at 106Hz. People prefer not to cross their subs at such a high frequency because the subs can become localizable (male vocals will sound like they're coming from those locations and your centre speaker simultaneously). To keep that from happening, most folks transition their subs to their speakers around 80Hz, where sound is still non-directional. But if your subs are rolling off at 80Hz, they won't be producing much sound at 106Hz to help with a problem at that frequency. That's why I suggested using your subs to cancel the first 2 modes.

In the 22-foot width example, the 3rd mode (77Hz) is below the typical 80Hz crossover, so a pair of subs at the quarter points will cancel the first 3 modes. Might want to move the crossover point a little higher, like 90Hz, so that the subwoofer-to-speaker transition isn't happening close to one of the modal frequencies.
Here's a question that popped into my head today while looking at subs.

SVS has the cylindrical subs- sub faces down - ports face up.

Would the cylindrical subs have the same mode cancellation affect as a typical sub facing towards the seating area?
Quote:
Originally Posted by pdxrealtor

Would the cylindrical subs have the same mode cancellation affect as a typical sub facing towards the seating area?
At frequencies that low, the radiation pattern is spherical. For the purposes of mode cancellation, it doesn't matter which way the drivers are facing (up, down, forward, etc).
hmmm.... Ok.

You're pretty familiar with my room and setup. Acoustically, would port firing up at ledge height against a wall provide any disadvantage over port firing forward into the seating area?
Some people advise pointing subwoofer drivers and ports away from the listeners to reduce the audibility of higher order distortions. But you can do that with a box sub by rotating the driver and port 90° from the listener. For mode cancelling it won't matter, since subwoofer location (not orientation) is key.
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