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PostPosted: Fri May 12, 2006 6:30 am 
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Here's my plan based on this elegant design by John:

So far I've completed the Bass Hangers.

I've decided to make 12 resonating chambers based on the 3 main triangle sections. Each triangle is split into four sections, once vertically and once horizontally at about halfway between. Each chamber is designed for a specific axial mode derived from my room dimensions.

Does anyone know how low I can design these chambers for and expect them to actually work for that frequency? If I design the largest chambers to hit the 70.6 and 92.2 axial modes will they actually absorb down to these extremes? Should I start at a higher frequency mode instead such as 110.2 and work my way up from there instead?

Is changing the slot depth, (thick slats) the most effective way of reaching the low frequencies or are all variables equal in effect?


Last edited by lex on Sat Jul 29, 2006 2:41 am, edited 1 time in total.

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 Post subject: Axial Mode Anti Nodes
PostPosted: Fri May 12, 2006 12:12 pm 
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Steve is this what you meant by placing the resonators at the anti nodes of the target frequencies?

So for example, I should design part of the top right slats(the yellow area) to absorb the 220 Hz Axial Mode since that is the major pressure point?


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PostPosted: Sat May 13, 2006 11:47 am 
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Here are some sweep tests done at approx -9DB OK setting. I did the machine gun shots and altered the gain on the mic till I saw minimal change in the graph.

These tests are done using a Radio Shack 33-3030 Dynamic Omni-directional. The range is stated as 100-12k Hz response. Therefore I don't know how precise these measurements actually are. I found however that the extremes coincide with the predicted Axial Modes. The mic was placed very near the upper left ceiling corner pointed up and speakers/woofer in opposite bottom floor corner pointed to wall(approx 6 inches distance). I changed the distance of the woofer to wall but got almost identical results.

These are some of the best consistent shots I obtained that show the typical values I was getting:

After doing these tests I see I should target the Axial Modes as planned with Slat Resonators and perhaps Panel Traps for the lowest modes. I was thinking Broadband in the upper right and lower left by way of false slat resonator sections. They will look like slat resonators but they won't be sealed and will be filled halfway with Fibreglass. This will act as a broadband and reflect some of the highs instead of absorbing them all at once. The front of the false slat sections will have slats to reflect some highs, with 3 cm gaps in between perhaps.

I will make a new drawing of my new plan now and post it up here. Any thoughts on the new data and my plan to counter the problem freqs? Thank you.


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 Post subject: New Plan
PostPosted: Sat May 13, 2006 12:59 pm 
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Here is an updated and modified version of the original plan: Any thoughts/criticisms? :?


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PostPosted: Tue May 16, 2006 7:36 am 
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Here are some frequency graphs of a recording I made after making the hanger traps and having 8 Fibre glass traps in ceiling and wall corners(1 in each corner). It looks very ragged to me.


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PostPosted: Wed May 17, 2006 8:03 pm 
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It sounds to me like you're working at this harder than necessary; if you do indeed need a slat resonator for your second axial mode due to length ( 92 hZ then it's in the right place; halfway between the two ends of that mode is where the high pressure would be.

However, I can't help thinking that first, your room isn't all that crappy looking and second, I'd try just standing up a bunch of MW in all the corners, a few randomly spaced patches on the walls, and probably some on the ceiling - this is much easier, and much less likely to backfire on you. Tuned treatments aren't generally for the faint of heart, nor are they normally needed in decently proportioned rooms.

About the only part of your room that seems to need help is around the 92 hZ region, and that's partly because there's not a lot of modal activity there with your dimensions, in ANY of the three types of modes.

I believe you'd be better off absorbing broadband in an attempt to bring down all the OTHER modes in the room, and if that doesn't smooth things out enough THEN build a slat tuned to around 92, and placed dead center of your long wall... Steve

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PostPosted: Thu May 18, 2006 3:30 am 
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I've been doing a lot of reading but I haven't gotten to the part yet that may explain why I'm getting the huge dip at 92 Hz. Do you think this is a case of destructive interference? Is the same mode or harmonics of it canceling it out somehow do you think? By treating the 92 Hz will I actually bring up the amplitude instead of decreasing it, as a result of breaking the cause of the destructive interference?

My goal is decent reverberation, avoiding a dead room, avoiding a multitude of early reflections into the mics, and relative equilibrium of the frequency decay and amplitudes. So whatever will get me there is fine by me. I just have this idea of wood slats around the room so it won't sound dead and will shift the parallel walls. However, like you said, I want to avoid making the room worse. That would be horrible, all that work and material to make it sound worse. I will keep reading till I think I understand this enough to start building. Thanks very much for your suggestions, they will help me.


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PostPosted: Thu May 18, 2006 4:26 am 
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IF you're a member of Studiotips, log in and go here

http://forum.studiotips.com/viewtopic.php?t=497

I use Jeff's ModesV2 spreadsheet regularly, check it out - Steve

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PostPosted: Thu May 18, 2006 8:21 am 
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That's a great program, thanks for the link. :) I entered my dimensions and made a print out. So I'm on the right track, that's good.

I'm still confused about the dip at 92 Hz. It seems like that should be a rise and not a dip. I'm wondering what the best thing to do for dips at modes is. What exactly causes such a dip at an Axial Mode? I'm not sure if harmonic frequencies or the 92 Hz frequency itself is the cause of the dip. Why don't any of the other modes dip so drastically?


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PostPosted: Thu May 18, 2006 11:25 am 
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http://www.surroundpro.com/articles/pub ... _230.shtml

I found an explanation for the 92 Hz dip. If you use the find function in your browser and search for dip you will find this paragraph:

Quote:
Room modes are low frequency resonances caused by standing waves that occur because of the geometry of the room and its boundary surfaces, with the most prominent modes occurring between opposite surfaces. If response peaks due to these modes are not corrected, mixes can be bass-shy because you tend to overcompensate for what may sound like too much bass in the program. Conversely, room modes can cause a lack of bass at the mix position that can lead you to add too much bass into the mix. In most rooms, the response rise due to the dominant mode will usually be in the range from about 40 Hz to 60 Hz, with response dips due to floor reflections from 50 Hz to about 90 Hz.


Describes exactly my situation, which I see now is not so strange but to be expected. :)


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PostPosted: Thu May 18, 2006 8:29 pm 
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Few things -

How can you get about 117 dB on your ETF plots at about 40 hZ, when your mic only goes to 100 hZ (and is probably 5-6 dB down at that frequency)?

What is your speaker response rating at LF, and within how many dB?

How can you get "nulls" (nodes) when both speaker and mic are in opposing tri-corners which, by definition, are the anti-nodes of ALL frequencies?

To answer these for you; you can't.

I have no idea what all that "loud" crap is below 100 hZ, but with the setup you described it's NOT real; noise? Gremlins??!?

IT's very possible that the 92 hZ "dip" you're seeing is just Radio Shack's "creative specifications" - looks more like they decided that if you could hear anything at all then it should be included in the frequency response, but without the disclaimer of "plus or minus 20 dB"...

The worst place in your room for a 92 hZ null/node would be at 37" from either short wall - your mic isn't there nor is your speaker, so I'm pretty sure this leaves equipment/software problems; before you go any further with treatments of ANY kind I would recommend finding out what's wrong with your test gear/software - this just isn't making any sense to me as yet... Steve

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PostPosted: Fri May 19, 2006 3:57 am 
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Maybe a gremlin infestation. :lol: Where can I get some gremlin traps?

Hmmm I was thinking the microphone was just underated but perhaps not. The only way to know for sure, I think, is to get a good microphone that is rated to 20 Hz and see the difference.

Quote:
IT's very possible that the 92 hZ "dip" you're seeing is just Radio Shack's "creative specifications"
:lol:

It seems all the major extremes are under the microphone's rated response so I think you are right... I should do this with a better mic before I start adjusting the room.

Quote:
The worst place in your room for a 92 hZ null/node would be at 37" from either short wall.
I was thinking this too. Looking at where the nodes "should" be this made no sense to me too. So I'm glad this makes no sense to you too. :)

Compared with the recording I did with a proper recording mic the 90 Hz dip isn't present there with the same room treatments. Thanks Steve, I will check out the specs. on this sub and speakers and replace the mic.

http://www.performanceaudio.com/cgi/pag ... tID=004602 (this goes to 20Hz and similar to the ETF test mic.)


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PostPosted: Fri May 19, 2006 7:47 am 
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The mic ETF recommends has only an RCA on the rear, and won't go to 140 dB or even close; and it costs close to the same. Sounds like a good deal to me... Steve

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PostPosted: Fri May 19, 2006 1:22 pm 
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I went with the Audix mic after all. Can't wait to see how they compare.


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PostPosted: Tue Jul 25, 2006 9:49 am 
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Well I finally got around to doing these tests with the Audix mic and this is what I found. I did some machine gun tests after these to confirm that these were accurate and consistent.

These were typical results:

-The first is a 1 second Full Range Sweep showing 10hz to 1kHz.

-The second is a low 5 second MLS test showing 20 Hz to 200 Hz.

Looks very similar to the graphs I got with the Radio Shack mic. There wasn't as much randomness in the lows however with the Audix.

Again it shows a big dip in the 92 Hz area. The peaks and dips coincide with the Axis Modes. Could you please give suggestions for concentration of treatment based on these results?


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