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PostPosted: Thu Oct 07, 2004 9:34 am 
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Now, steps 4 and 5 - as you can see, there are few places that won't be less than ideal in a room. I've ignored center mode problems from left/right, as you need to be centered in that axis for stereo imaging. Just be aware of the width modes when you look for reasons for a particular peak or null, and try to stay out of similar nulls/peaks in the other two axes.

For front to back placement of your head, keep in line with the green blocks - that way, you won't end up in a double null, or a peak and null for two different frequencies.

The way I do this is to set a line (the purple ones) at about 1/3 the total height of the graph, and draw colored rectangles that coincide with the intersection of each colored graph with the purple line - this establishes the non-ideal area for each color (harmonic) in each axis. I also do this for each color at peaks, as shown - then, I use the Fill with Color tool set to green and fill the areas that are NOT included in any of the other colored rectangles - this is shown in the last drawing.

Finally, to figure out how this relates to your room, you need to remember what "pixel scale" you chose to make your drawings, and look at the x/y coordinates at the bottom right of the paint window -

I drew this fast, so it's not totally accurate - also, you'll have better luck figuring out where your room works well if you put the graph on the BOTTOM of the diagram, instead of the top - that way, your x/y coordinates can be directly translated to feet/inches.

As time-consuming as this is, it still beats moving furniture 35 times and STILL not getting close -

You'll find you get better readability by keeping your drawings in the 24 bit BMP mode, as long as you're not posting them - for posts, JPG keeps color info better.

Hope this helps - if you're confused, just ask and I'll try to clarify the steps... Steve


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PostPosted: Sat Oct 09, 2004 9:56 am 
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Tom, did I lose you or are you busy moving furniture?


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PostPosted: Tue Oct 12, 2004 12:18 am 
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Steve,

Sorry for the delay getting back to you, had to take our remote truck out to shoot a football game. With a "crew" of 12, all but 1 is new, which makes for a long long day!!!!

Spent most of yesterday staining thresholds.

After reading through everything, I'm not sure moving my speakers will change all that much. My favorite speakers are rear ported. I'm in a "green square" now, though I can move closer to the front wall and still be in the next "green square". I do plan to move everything forward. I hope to find the time this week. I'll let you know what I find.

Thanks again,
Tom
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PostPosted: Tue Oct 12, 2004 4:35 am 
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Tom, take your time and don't apologise; I'm not rushing you, just wanted to make sure I hadn't confused things to the point that you went away. Trust me, I have enough to keep from getting bored :? Whenever you're ready... Steve


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PostPosted: Thu Nov 04, 2004 2:08 pm 
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Could someone please post distances as fractions or percentages of room dimensions?

Thanks,

John[/img]


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PostPosted: Thu Nov 04, 2004 7:52 pm 
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John, as long as you have Excel on your machine there's a really quick way to do this; just download the spreadsheet linked above, take any one of the dimensions and make it 100 feet 0 inches - that way, all the nulls at the bottom of that graph will read in percentages. From there, copy that graph into paint, draw a horizontal line thru the graph at around 35-40% of total graph height, drop red lines from where each null intersects with the new horizontal line, and avoid the areas between the red lines. Kind of like this - Then, if you like you can copy your graph into paint, make it the same length in pixels as your room drawing, and paste the graph alongside the room. This lets you see where the good/bad spots will be relative to your room... Steve


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PostPosted: Tue Nov 09, 2004 1:27 am 
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fbars,

I don't know if this will help you or not. Based on where you and your loudspeakers are currently located, it appears that 60 Hz could indeed pose a "suck-out" problem. Here's a graph:

Image

Note that this is an undamped predicted, amplitude (frequency) response. Therefore, anything above a certain frequency corresponding to your treatments will be smoothed out. That said, it may be that your rear-wall trap is either not large enough, or is simply not "enough" to tame the dip you see around 60 Hz.

Any chance you could measure the response? PM me if you don't have the means to do this, but would like to. I might be able to help.

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PostPosted: Sat Apr 02, 2005 2:15 am 
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At knightfly:

You lost me here.....
this means you can never sit in the middle of of two walls
because your always in a null of the lowest frequency between those two walls.

When you want to have the biggest volume(?) of that frequency you must shift to the left or the right.
doesn't this effect the optimal stereo position in a negative way ?

perhaps I'm very stuppid right now :-)
but can you please explain this to me.
Many thanks ....

chris

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PostPosted: Thu Apr 14, 2005 3:37 am 
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Chris, sorry I missed this;

While it's true that sitting halfway between two walls will put you directly in nulls of First and Third harmonics and Peaks of Second and Fourth harmonics of Axial modes, for proper Stereo balance there isn't much choice; IF you shift sideways in the room, you upset Stereo imaging by changing the left channel's effective frequency response as compared to the right channel.

This is what I would call a "damned if you do, damned if you don't" situation. All I know to do about this is to be aware of the problem, and do all you can to treat your room to MINIMISE these response problems in the ENTIRE room with treatments (corner absorbers, clouds, deep rear traps, diffusion (if the room is deep enough)

For the OTHER two dimensions, you CAN do something; don't place speakers or your head in nulls (or peaks) of either length or height, and don't place speakers in peaks or nulls of Width (adjust your speaker width/head position until ALL these criteria are met)

And, you're not at all stupid; this "conundrum" has bothered me for years, it wasn't until just recently that I finally realized I couldn't do anything about it other than what I outlined above... Steve


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PostPosted: Fri Apr 15, 2005 4:37 pm 
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It is ok Steve, I thank you for your comments.
I think have have to look the other way to correct the problems.
Like clouds, depth etc.

When I finished my room in that 3d program I'll show it to you.....

May this be a problem I'll let you and the others know.

Chris

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PostPosted: Fri Apr 15, 2005 11:15 pm 
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The conventional wisdom in control room design is to position the listening position at 1/3rd the length. This puts you on the peak of the 3rd harmonic, but you'll notice that the difference between the 1st, 2nd, 3rd and 4th harmonics at this point is actually a minimum. It's actually a good spot to listen, and you'll also notice that if you move forward just a little, you get into one of the green areas on knightfly's chart (i.e. it's a pretty big sweet spot). The fact that you're on a peak means that the subjective output at this point is higher...i.e. your monitors don't have to work as hard because you have modal reinforncement. This means lower overal SPL levels, and happier neighbors too!

Here's another way to interpret the results of this mode chart...take your listening position, and see what null is closest. What frequency is the harmonic, and is it caused by the length, width, or height. Now, you can tune a bass trap to this particular frequency. This will have the effect of adding damping to the mode, which would be like reducing the 'Q' of that resonance...which makes the sweetspot bigger. Voila, there's how bass traps can increase the subjective bass level in a room.

Of course, we want to sit in the center of the room for imaging purposes (the mode chart tell us nothing about early reflections, and doesn't even consider stereo...it's just concerned with <300Hz), which puts is right in the null of the first harmonic of the axial mode along the width of your room. So, you'll definitely need bass trapping tuned to that frequency (and, mount it on the side walls).

Hope this helps...

Cheers.

Kris


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PostPosted: Sat Apr 16, 2005 8:35 pm 
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All good points, Kris, thanks... Steve


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PostPosted: Wed Apr 20, 2005 4:24 pm 
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it realy makes sence and shows that it is not that difficult to understand.
reading the book , controlroom, studio and listeningroom design helpt also very much.

again many thanks..
I think I can work out the problem zones now :)

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PostPosted: Wed Apr 20, 2005 6:27 pm 
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Glad we could help; please keep in mind, however, that this is a VERY SIMPLIFIED method that's intended to sort of "get you in the ballpark" - it doesn't even touch on the other two types of room modes; tangential modes, which occur between 4 surfaces (either two walls, floor, ceiling or four walls) and oblique modes, which occur using all 6 surfaces in a room. These greatly UN-simplify the process in some cases, to the point that once a room has been set up to rough dimensions it's sometimes easier/quicker to just run measurements/listening tests than it is to try and calculate every possible move for speakers/ears... Steve


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PostPosted: Wed Nov 30, 2005 12:53 am 
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Which "Newell" book talks about the rear wall that seemed to work so well here? I've got "Recording Studio Design" and on page 171 I've got a discussion about walls made out of rock and stone.

len


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