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Old 29th August 2007, 02:39 PM   #1
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Default Notch Filter Design

Background: Most listening rooms show catastrophic resonance due to their parallel walls. Unfortunately normal room size result in 3 main resonances which are all in the range of normal music bass.

I was already struggling since long time with a 60Hz resonance, which was caused by the 3m distance between ceiling and floor. After quite some experiments I found a resonable placement for the subs....
But there are still some booming resonances. Today I got angry enough to tackle this with some more efforts.
I love controlled strong bass, but no annoying booming.

I found:
....yes, still some peak at 59Hz...
and "uahuuuu" quite some peak at 24.9 Hz.
Furtheron 34.1 Hz and about 50Hz.
The 24.9Hz is perfectly fitting to the two horizontal main resonances of the room (7.2m x 7.3m).
The 50Hz is the 2nd mode of the 24.9Hz resonance.

Here the measurement at my normal listening position.
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Old 29th August 2007, 02:43 PM   #2
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...if you measure in the middle of the room, all main modes should cancel out...
YES! Wow, practice and theory are fitting together
Even more the 2nd modes will be max in this point and again YES, at least the double of the 24.9Hz resonances become very dominant and cause an ugly peak at 48-49Hz.
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Old 29th August 2007, 03:10 PM   #3
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Up to now I could not figure out the reason for the 34.1Hz resonance. It does not fit to any obvious geometric property of my room and also not to the resonance of the subs.

Anyway... I think I will put my class D amp short time on hold and will settle my bass system at least somehow reasonable. Means I will fit a proper LT ... AND now the topic of the thread: SOME NOTCH FILTERS !

There are two common circuitries which seem to be well suited: The WienRobinsonNotchFilter and the GyratorNotchFilter. ( BTW: JensRasmussen and ACD have a very nice Tutorial about active filters on their WebPage).
The WienRobinson would theoretically offer very nice performance allowing any Q, any f, any bandwidth... hey even a -80db notch with 3Hz bandwith. Well, unfortunately only if you have perfect components.
Some capacitor matching is easy, but the Wien Robinson is demanding two pots, which are perfectly identically.
Unfortunately even good real life tandem potis do easily show 10% mismatch (0.xx db). And with this the performance of the WienRobinson Filter is becoming poor.
On the other hand the GyratorNotchFilter is ending up in quite unpleasant values for the caps. I.e something like 10uF...33uF film caps, if you tackle 20Hz. Also the bandwith and Q will change if you only adjust the gyrator to different L values...

Means both solutions seem to have their limitations.
In the moment I am tending to go for the gyrator solution and settle two pots, one for the frequency and one for Q.

What are your real life experiences with DIY notch filters?
Your infos would be valuable for me to avoid at least the traditional pitfalls.....
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Old 29th August 2007, 03:20 PM   #4
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...here the WienRobinson...
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Old 29th August 2007, 03:26 PM   #5
zobsky is offline zobsky  India
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Unrelated to your topic, .. but you can directly copy ARTA graphs and paste them into a graphics program without having to resort to a screen capture. There should be an option for "Copy" on the program window or menu. This option also allows you to add text to your graphs.
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Old 29th August 2007, 03:27 PM   #6
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...and the gyrator notch filter. Please note, at the output I would of course place a buffer. And the pot would have an additional series resistor in order to limit the min L.
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Old 29th August 2007, 03:35 PM   #7
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Thanks Zob !
...I will go on to play around with Arta. Few hours back I was still a "ARTA-virgin"
But it seems to be a nice tool.
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Old 31st August 2007, 04:17 PM   #8
mikee55 is offline mikee55  United Kingdom
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Default I'm curious too...

Quote:
Means I will fit a proper LT
Are you referring to a Linkwitz Transform?

I'm using a Linkwitz Transform sub and would like to figure out how it behaves and calculate what its doing in my lounge. Where is the most accurate explanation on room nodes etc, that I can read on the net? Oh, I'd need to be able to understand it, without being blown away by numbers.

Cheers

Mike
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Old 31st August 2007, 06:23 PM   #9
Baldin is offline Baldin  Denmark
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Hi ChocoHolic

Why do you need a notch filter ... does it have to be that narrow?

I'm using a single band parametric equalizer. That way you can easily change frequency and bandwidth trying out what actually sound the best.

Cheers Baldin
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Old 1st September 2007, 02:03 AM   #10
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Hi Mike,
yes with LT I am referring to the linkwitz transform.
Of course LT does not solve room resonances, but in the moment I do not have a proper LT for my sub. I am still using some 2nd order mostly integrating low frequency increasing filter, which I designed seven years back for another woofer. It is not matched to my cuurent woofers at all, because I planned to realize the proper LT when putting my class D amp to duty. But that class D amp project I cannot realize within short time.
Now I decided to settle proper bass response before that amp. Room resonances seem to bee the more headache part, but without a proper LT it would be just half of the job for proper bass response. So I am doing both in one step now, the LT should really be an easy side job which I will get with just few additional effort. (..hopefully, you never know before it works...)

Hi Baldin,
it would be interesting to know the schematic and possible filter characteristics of your parametric EQ.
I could imagine that it is something like a Wien-Robinson.
And may be it is adjustable to quite narrow BW.
If you look room resonances they are typically steep.
In my room they seem to have BWs of some single HZ and make 40db...50db/Oct.
Literature is stating tyipcal Q of room resonances around 5-15. From the graphs of my room I getting typical Qs above 10.
If you want to get frustrated about your living room you should do the two same things which I did:
1: Use a sine wave generator and sweep trough the bass frequencies, while you are standing close to the wall.
2: Download ARTA for free and start playing.

Both exercises do unfortunately show quite impressive what most of us know since physics lessons in the age of 15 years, but we usually ignore it.
Your parametric EQ is probably already a big step forward.
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