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PostPosted: Wed Oct 06, 2004 6:46 am 
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I think my speakers are in a null. Briefly, the room is 11ft.8in. wide, 16ft. 4in. deep, with a ceiling of 8ft. In the front corners are 34in. "superchunks", and the rear wall is a 2 foot deep "trap" if you will, from page 171 of Newell's book.

The room actually sounds amazing and the imaging is wonderful! You can hear some REALLY low notes with clarity, for example, Lyle Lovetts "She's Already Made Up Her Mind has great low end. I some how feel that the response around 60hz is down several db.

The center of each woofer is 39 inches from the side walls and are 62 inches apart. The front of each woofer is 24 inches from the front wall and I'm seated 6 feet 2 inches from the front wall.

Steve [knightfly], Eric, Wes. Am I in the WRONG spot??

Thanks,
Tom Durso
First Bass Audio


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PostPosted: Wed Oct 06, 2004 8:51 pm 
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If your room's length includes the depth of the rear trap, then your speakers would be in a null front to back that's the 4th harmonic @ 138 hZ; Width-wise, your speakers are as good as they can get - you forgot to list the height of your speakers or your head, so I can't say about any nulls in the vertical axis.

Your head is in another null of the same 4th harmonic of length, or 138 hZ - you should try moving your head rearward by about 4-6", and your speakers forward by about the same.

Heightwise, neither your ears or your woofers should be at 48", they should be either 40-42" high, or at 56-58" high. It's likely that the 40-42" will be more practical.

Hope that helps... Steve


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PostPosted: Wed Oct 06, 2004 11:55 pm 
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Steve,

Thank you for your reply.

First let me apologize, I gave the wrong dimensions. It was the day from hell here [I'm the Chief Broadcasting Engineer at a university], maybe the drugs wore off, a brain fart, could be because I'm 51 and I don't HAVE to remember anything, or as you say, "the gators got me".

Now for the correct dimensions, actually measured last night with an actual tape measure!!!

Each woofer is 44 inches from the wall.
The center of each is 51 inches apart.
The center of each is 45 inches from the floor.
The woofer is 36 inches from the front wall.
I am sitting 6' 2'' from the front wall(this was the only measurement I had correct).

And, by the way,using "Newell"s rear wall design, the sound is amazing. Everyone that has been in the studio to listen has commented on the fact that no matter where you sit, it sounds good everywhere!

Steve, I must thank you for recommending the "Newell Wall", all credit goes to you.

Thank you sir!!

Tom Durso
First Bass audio


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PostPosted: Thu Oct 07, 2004 9:20 am 
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"Steve, I must thank you for recommending the "Newell Wall", all credit goes to you." - nah, if that were true it'd be called the "Knightfly Wall"... :?

Instead of my first approach, I'm (more wisely) going to "teach you to fish" -

First, you need to download this

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=5801

You'll need Excel on your computer to run it, hopefully that's not a problem.

Open several instances of Paint, and copy these steps as I've done but with your own dimensions and scaling -


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Last edited by knightfly on Thu Mar 16, 2006 6:44 pm, edited 1 time in total.

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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
First Bass Audio


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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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It is easy enough to be friendly to one's friends. But to befriend the one who regards himself as your enemy is the quintessence of true religion. - Mahatma Gandhi


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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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