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PostPosted: Sun May 14, 2006 6:36 am 
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Is there a program/website that will show room nodes and anti nodes such as on page 283 of the Alton Everest book, "Master Handbook of Acoustics"? I like how it shows the location of the nodes and the resultant warping when the same area is nonrectangular.

What program did he use to do that? Anyone know of a program, preferrably freeware, that can do the same?


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PostPosted: Sun May 14, 2006 6:50 am 
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This is the closest thing I've seen.

http://www.hunecke.de/english/calculato ... modes.html

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PostPosted: Sun May 14, 2006 7:03 am 
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Yeah, that is close to what i'm looking for, thanks David. That will help visualize the lower frequencies. I just wish it showed up to 500 Hz and would show a 2d map as well.

As an alternative, where can I find the information to calculate and draw them by hand? There are just a few specific ones I need to know the location of for my room.


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PostPosted: Sun May 14, 2006 7:11 am 
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I can help you with that. Which modes?

You certainly don't need to be looking at node/antinode locations for 500 Hz modes; This is getting to the point where modal behavior is starting to break down, and even basic treatment will stamp them out of existence.

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PostPosted: Sun May 14, 2006 7:35 am 
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Thanks for your help, it's greatly appreciated. So you think I don't need a resonator at above 450Hz perhaps?

To get an idea of my situation here are my room designs and plans:
http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6194

I decided to not worry about the height modes because the pressure points will be on the ceiling and floor, and I will have suspended 3.5'' fibre glass panels on the ceiling, not edge to edge, but spaced apart a few inches (gaps in between all sides). Taking into account a 4'' or 6'' gap between ceiling and traps I think that should help reduce them significantly.

So, these are the modes I would like to know the locations of to play around with for the position of the slat resonators: I'm even thinking of going 3 stacks instead of just 2 to hit a broader range. I'm wondering if I should skip the modes where I see a dip in my LF graph or if a dip should be treated as well?

Length
230.61
276.73
322.86
368.98
415.10
461.22

Width
275.61
330.73
385.85
440.98
496.10


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PostPosted: Sun May 14, 2006 8:35 am 
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I think you have misunderstood a good deal about modes. Have a look at this animation I did for RO:

Modal Pressure Animation

I hope this will make some things clear. When you said that you didn't worry about height modes becuase their pressure points will be on the ceiling , you worried me. Pressure variation as per my animation exists for axial modes between any parallel surfaces, not just in the length, but in the width and height as well.

Your clouds will have some effect on the modes in the height dimension and in other dimensions as well, but they won't work nearly as well as porous corner absoprion or membrane/resonator traps on the surfaces; see this animation of mine and its associated thread:

Cloud Effects Animation

Cloud Effects Thread

Finally, there is no need to know the position of nodes and antinodes for frequencies above 200 Hz or so. They are just too weak and too easy to treat. Really, there's little need for most anyone to know the exact locations of nodes and antinodes for any frequencies when the goal is the same: use plenty of lwo frequency absorption.

I really hope I have helped you here.

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PostPosted: Sun May 14, 2006 10:37 am 
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Yeah, that has helped actually. It's caused me to question whether I have this down or not. I will read some more and then come back to this. I wish I could see the waves for a few minutes, that would clear up a lot of the confusion.

I am going back to the beginning to do some more reading about sound waves and pressure so I can understand modes better. The way I understand it is, the mode will create a concentration of pressure at it's anti-nodes. This pressure moves with the sound wave and oscillates mostly between the anti-nodes. The height modes would bounce between the ceiling and floor, they would touch every surface, but the main pressure areas would be located at anti-nodes or slices between the two parallel surfaces. The narrow edge of these slices would hit the length and width walls.

The location of the nodes and anti-nodes of the harmonics of the fundamental wave seems to follow a recognizable and distinct pattern but a formula to locate their location would be nice. Did some searches for one and pulled up nothing. I think it's something like D/n. n is an integer 1,2,3,4,5,6,7,etc. D=distance of the two parallel survaces. I just have to shift and squeeze the curve in based on this. That's how it looks anyway, I have to review my sine maths. :P

I learn much better when I see or hear things, so i'm also searching for more animations and pictures on the internet as well. I'll be back later, thanks for pointing those things out.


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PostPosted: Sun May 14, 2006 11:00 am 
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So where are the nodes for each mode? It's really quite simple.

First, the two parallel walls are always antinodes.

The nodes for a given mode are at percentage distances from a wall which are odd multiples of 1/(N x 2) where N is the mode number. The even multiples are the antinodes. An example:

Q: If you have parallel walls that are 10 feet apart, where are the nodes and antinodes for the third mode in this dimension?

A: By the formula, you would have a mode of 1/(3 x 2) or 1/6th of the distance between the walls. The odd multiples of this are also nodes, so that's 1/2 and 5/6 of the distance as well. In a 10 foot dimension, this would be 1.66 feet, 5 feet, and 8.33 feet. The antinodes are at the even multiples of 1/6, so that's 1/3 and 2/3, or 3.33 feet and 6.66 feet.

Hope this helps.

Look for the Harman Kardon white paper Lourspeakers and Rooms Part 3 for a nice description of what modes really are.

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PostPosted: Sun May 14, 2006 11:38 am 
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Thanks, that's the right one! :)

I will check those white papers out. I would like to understand, as you do, why 200 Hz is a good range to stop worrying about the modes.


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PostPosted: Sun May 14, 2006 1:43 pm 
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Several resons:

When you get that high, the distance between node and antinode becomes pretty small, so small movements change everything.

Absorption is very effective at frequencies this high, so they will completely disappear.

Above a certain frequency near 200 in smallish rooms, sound waves start behaving in a different manner.

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PostPosted: Mon May 15, 2006 3:13 am 
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I spent some time just studying the way a wave moves. There are some great sites with nifty animations if you type "sound animations" in google.

I then read the first chapter of "Master Handbook of Acoustics" and it all made more sense after looking at the animations.

I've been playing guitar all morning but now will go on and read the white papers you mentioned. I would like to know more about how the behavior of the waves change after 200 Hz. I'm really interested in knowing exactly what is happening. Thanks David for your guidance in my critical stage of building.

Here are the animations:

http://www.ltscotland.org.uk/5to14/reso ... /index.asp

http://www.kettering.edu/~drussell/demos.html

http://www.kettering.edu/~drussell/Demo ... otion.html

http://www.physicsclassroom.com/mmedia/waves/tfl.html


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PostPosted: Mon May 15, 2006 5:06 am 
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Here's another must-see animation that mokes standing waves very clear.

Plane Wave Superposition

Research 'Schroeder critical frequency'.

I'm very happy to hear that I have helped you. 8)

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PostPosted: Mon May 15, 2006 5:12 am 
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There is an easier frequncy than the Schroeder, the Davis frequency. 3x the wavelength of the shortest room dimension. With an eight foot ceiling, a little over 200 Hz. Hm. :D

Andre


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PostPosted: Mon May 15, 2006 6:54 am 
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The 'Davis' critical frequency, from Sound System Engineering by Don and Carolyn Davis, p. 168, is actually written as (3 x speed-of-sound) / smallest dimension, and for an eight foor ceiling smallest dimension, this would be 423.75 Hz, well over 200 Hz.

My recommendation that Lex study the Schoeder critical frequency was designed simply to point him in the right direction for understanding what heppens at this frequency, not to provide him with the best way of calculating it.

There is also the equation (3/2) x speed-of-sound / MFP where MFP (Mean Free Path), the average distance a ray travels before reflecting is equal to 4 x Volume / Surface Area. I can't for the life of me remember whose formula this is, and it's really pissing me off! Anybody?

All methods are mere guidelines, and there is no one frequency where the rom suddenly becomes diffuse. I personally don't sweat it. If you really want to get into it, here's yet another approach.

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PostPosted: Tue May 16, 2006 1:55 am 
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David French wrote:
The 'Davis' critical frequency, from Sound System Engineering by Don and Carolyn Davis, p. 168, is actually written as (3 x speed-of-sound) / smallest dimension, and for an eight foor ceiling smallest dimension, this would be 423.75 Hz, well over 200 Hz.


Yep, that is it. I do not my copy handy. I remembered (and I do not that accurately) the denominator as being one half the smallest dimension, taking into account the have wavelngth effect of the wave being reflected.

If I am wrong, it won't be the first time this week. :D

Quote:
My recommendation that Lex study the Schoeder critical frequency was designed simply to point him in the right direction for understanding what heppens at this frequency, not to provide him with the best way of calculating it.


Understood. Agreed with studying the area of change from wave to statistical modeling of acoustics.

Quote:
There is also the equation (3/2) x speed-of-sound / MFP where MFP (Mean Free Path), the average distance a ray travels before reflecting is equal to 4 x Volume / Surface Area. I can't for the life of me remember whose formula this is, and it's really pissing me off! Anybody?


Rettinger presents it in Acoustics and Noise Control. I seem to remember ( see above) it being from Olson, but I may be wrong

Quote:
All methods are mere guidelines, and there is no one frequency where the rom suddenly becomes diffuse...


Excellent summary!

Andre[/i]


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