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Old 9th October 2012, 09:31 PM   #21
Pano is offline Pano  United States
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Martin J King and John K do as well. Important reading for the OB fan.
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Old 10th October 2012, 02:16 AM   #22
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Default Edge response

I ran Edge but I don't know how to post results (graph)
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File Type: txt mid bass dipole response edge.txt (5.6 KB, 10 views)
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Old 10th October 2012, 03:12 AM   #23
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Hi I'm a bit late to the party but here is a post that I wrote before on calculating notch filters.

Quote:
Originally Posted by wintermute View Post
Hi I don't know the speaker you are mentioning, but if you want to learn about notch filters I found this wikipedia page invaluable. It takes the guess work out of it!

RLC circuit - Wikipedia, the free encyclopedia

go to the parallel RLC section (but it helps to read the preceding stuff as well.

If you are like me and shy away from maths, it looks intimidating but you really only need the Q formula Click the image to open in full size. and the Wo formula Click the image to open in full size. to be able to work out a filter suitable for your desired frequency. Note that Wo is in radians, so you need to divide by 2pi to get Hz. (edit: the original formula I linked to was incorrect I've now fixed it, sorry for any confusion!)

Swapping the values for C and L will change the q (without changing the centre frequency) and the resistor determines the amount of cut.

Choose a value for C or L and the frequency and then work out what the value for the unknown component (L if you chose C) is. adjust until you get usable values and the desired Q. The higher the Q value the narrower the notch will be.

I found this to be a much better starting point for simulation (very close to the desired frequency) than the methods I'd found in my speaker books.

Tony.
The centre frequency is easy and accurate using the formula in my experience, getting the Q right is another matter!! Also don't be surprised if the notch has an affect on the lower frequencies outside the range of the notch!

I only recently became aware of the series RLC in parallel with the driver as a form of notch filter. It wasn't until I thought about it that it dawned on me how it could be useful. It is basically a bandpass filter shunting to earth via a resistor (which allows you to control the amount of cut). What it allows you to do (and I'm only thinking about this from an academic standpoint as I've not implemented one) is set the lower and upper frequencies (with cap and coil respectively) and everything in between those frequencies will be attenuated roughly evenly (ignoring any non lineararity of resistance with frequency), thus allowing a sort of shelving circuit only over a given frequency band.

Tony.
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Old 10th October 2012, 03:20 AM   #24
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Quote:
Originally Posted by Pano View Post
Edge is very simple and easy to use, PCD has a different usage. If you have good frequency and impedance measurements you can use PCD to figure your notch filter. Knowing the cause of the peaks is important, there may be a better way to fix it than the notch filter.
As Jay points out, 1150Hz is going to be tough. You'll hear every little thing you do there, so you might want to make the notch broader and shallower than the FR indicates.

Listen, try, listen.
Hi Pano, I found that PCD gave me less than accurate results (maybe because initially I was using relatively high DCR coils (around 0.28 ohms) Speaker workshop allows you to specify the DCR of the coils and gives VERY accurate simulations.

I'm curious about the comment on impedance, I didn't think it mattered at all for paralell RLC notches....

I just saw your latest post, my comments above are from my experiences with BOX speakers so it may be completely different with OB which I've never tried

Tony.
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Old 10th October 2012, 12:04 PM   #25
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Default Dipole peak

Wintermute, i like math. no problem. I figured how to post the graphs that I did in Edge. I'll post them tonight. And yes, I will add the Wiki article to my library! Thanks much!~
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Old 10th October 2012, 03:50 PM   #26
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Tony,
If you are doing a shunt notch filter, you're right, the speaker impedance shouldn't matter for the RC part. How deep it is will matter with the R part.
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Old 10th October 2012, 05:32 PM   #27
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Quote:
Originally Posted by jimbones View Post
I ran Edge but I don't know how to post results (graph)
Here are your results:
Click the image to open in full size.


What you want to notch there?
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Old 10th October 2012, 09:15 PM   #28
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A notch is a notch is a notch. It does not know what kind of box you have. All of these filters, be they they series or shunt are effected by both input and output impedance. As you are trying to deal with a hump, you will be doing a "parallel filter" which is in series with the driver. Parallel refers to the components of the network in parallel, not the position of the network in the circuit. To understand how it works, you might want to simulate it in LTSpice. ( D'Apolitto will even give you a model for a speaker) No matter the simulation, you still need to test. Simulation gets you close. It tells you that you are on the right track.
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Old 10th October 2012, 09:20 PM   #29
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The graphs above show all the symptoms of baffle step hump at about 650. A notch is not the best way to deal with it. Lower the first pole. It also shows pretty bad breakup at about 2200. You may need to put a notch on that puppy if you are not using a very steep crossover.
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Old 10th October 2012, 09:23 PM   #30
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Quote:
Originally Posted by Pano View Post
Tony,
If you are doing a shunt notch filter, you're right, the speaker impedance shouldn't matter for the RC part. How deep it is will matter with the R part.
I was thinking it didn't matter for the parallel series connected but I was wrong! perhaps it doesn't matter so much for the centre frequency but it certainly matters for the Q or the amount of cut....

I just did a check in speaker workshop by doubling the impedance of my MTM measurement and then comparing with the same notch filters.
results shown below.

green is the non filtered response (on the baffle)
blue is with the actual impedance of the speakers
black is with the impedance doubled.
red I just added in, it is a second order bessel target at 2.8Khz (my crossover frequency, the final crossover is actually 4th order bessel at 2.8khz with some added components.

Tony.
Attached Images
File Type: png notch_compare.png (35.3 KB, 90 views)
File Type: png notch_filter.png (6.7 KB, 91 views)
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