# calculating Q for parametric eq formula?



## CobraVin (Jul 31, 2007)

i have read find -3db left and right of peak

say i have a peak at 640, -3 to the left is 500, - 3db to the right is 750

what exactly do i calculate now, and will this tell me how much to attenuate?, thanks


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## ErinH (Feb 14, 2007)

While we're at it, is there a program that will allow you to view x-over points and further EQ adjustments? I'd really like to see what I'm doing in some sort of graphical display

*sorry to you, CobraVin, for a possible post dump. Just made more sense to post here than start a new thread at this point*


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## CobraVin (Jul 31, 2007)

no i appreciate the post bump, and am interested in the answer as well


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## St. Dark (Mar 19, 2008)

I've always just kinda "seat of the pants"'ed it, and used an RTA to watch what happens, and narrow it down (or widen it up! *L*) from there.

Did a quick look and found:
http://www.astralsound.com/parametric_eq.htm  (there are formulae on this page) and
"A filter with a Q of 1.41 has a bandwidth of one octave." from this link in the original link 

and one final note, from JL Audio 
http://mobile.jlaudio.com/tutorials/Parametric_EQ/Parametric_EQ.pdf

Based on my own guesses, and what I've seen above, start out around "2" and see (errr, hear) what that does.


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## npdang (Jul 29, 2005)

It's like...

Q = center frequency / -3db point to the right, minus -3db to the left 

So example, if your center frequency is 1000hz, with the -3db point 500hz to each side (really wide Q), you get 1000 / 1500 - 500 = Q of 1.

If you did the same example with a -3db point of only 100hz to each side, you get 1000 / 1100 - 900 = Q of 10.


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## ~thematt~ (Sep 14, 2007)

Its the reciprocal of the number of octaves that you want to target.

A Q of 1, is 1/1 octaves wide
A Q of 2, is 1/2 octave wide
A Q of 3, is 1/3 octaves wide
A Q of 0.25, is 1/0.25 (or 4) octaves wide.....


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## Fixtion (Aug 25, 2006)

*suggestion*

*winisd* has the ability to see/adjust these slopes, typologies, q-factor, -/+ adjusments for a modeled driver up to high range drivers. 

*-fixtion*


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## ErinH (Feb 14, 2007)

I understand what these do, I just would like to see the effect when you eq outisde of an x-over. I could do the math and get an idea, but it would be nice if I could find something that shows what eq cut or boost is doing to the total system; not just one area.


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## CobraVin (Jul 31, 2007)

Fixtion said:


> *suggestion*
> 
> *winisd* has the ability to see/adjust these slopes, typologies, q-factor, -/+ adjusments for a modeled driver up to high range drivers.
> 
> *-fixtion*


wish i had it, an on line calculator would speed things up a bunch as well, i just want to make sure i know how to figure it on my own



npdang said:


> It's like...
> 
> Q = center frequency / -3db point to the right, minus -3db to the left
> 
> ...


is that the correct formula? subtract -3db freq of the left from the right and divide by center freq

if so wouldnt your last example be a q of 5 (900 from 1100=200, 1000/200=5 

also do you just cut or boost by the amount from peak or is there a guideline that people use, thanks


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## Whiterabbit (May 26, 2006)

honestly, I dont think you can use a calculated guideline.

If the -3dB point is caused by cancellation and you are boosting both drivers, your response will go nowhere.

If the -3dB point is caused by cancellation and you are boosting only one driver, you might fix the problem or you might screw your tonality.

The best policy honestly is to boost or cut then remeasure to determine if you are actually making a change. You might find even heavy EQ doesnt really affect the response much. In which case alternate tuning methods must be evaluated.


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## dewi1219 (Nov 16, 2006)

npdang said:


> It's like...
> 
> Q = center frequency / -3db point to the right, minus -3db to the left
> 
> ...


This isn't exactly correct, because the frequency scale is logarithmic, not linear. So, 1000Hz is not centered logarithmically between 500Hz and 1500Hz. If 1000Hz is your center frequency and you have Q=1, then the lower F3 point will be 618Hz and the upper F3 point will be 1590Hz. The effect is more pronounced at lower Q values. You can see similar examples on the chart from the JL audio link above. 

I have created a very simple Excel file that will perform two calculations:
1) If you have the upper and lower F3 points, it will calculate the center frequency, bandwidth, and Q.
2) If you have the center frequency and Q, it will calculate the upper and lower F3 points and bandwidth.
If anyone wants a copy of the file PM me with your email addy and I'll send it to you. 

Also, there is no formula for boost/cut - the amount depends on the levels just above and below the frequency range that you are adjusting. Some kind of RTA is very helpful for making these calculations and adjustments.


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## WuNgUn (Feb 9, 2008)

Ummm....
Huh?
Is there a thread somewhere on here that will tell me what you guys are talking about??
Thanks


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## CobraVin (Jul 31, 2007)

thanks dewi pm sent


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## dewi1219 (Nov 16, 2006)

WuNgUn said:


> Ummm....
> Huh?
> Is there a thread somewhere on here that will tell me what you guys are talking about??
> Thanks


Yes, as St. Dark said, this is a good place to start:


St. Dark said:


> and one final note, from JL Audio
> http://mobile.jlaudio.com/tutorials/Parametric_EQ/Parametric_EQ.pdf


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## CobraVin (Jul 31, 2007)

thanks again dewi, its very helpful


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## dewi1219 (Nov 16, 2006)

No problem. Does this mean I get to drive the Cobra?


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## CobraVin (Jul 31, 2007)

well youd have to ask the new owner, i sold my last cobra at the Barrett Jackson auction in west palm beach last april, god i miss that one, i did everything on it supercharged fuel injected engine, all bodywork and paint, interior,every nut and bolt and wiring connection.....sniff sniff


















heres some build pics if your interested
http://s150.photobucket.com/albums/s94/cobravin/FFR Cobra builds/


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## dewi1219 (Nov 16, 2006)

Beautiful! Too bad you couldn't get any more power into it


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## CobraVin (Jul 31, 2007)

400 hp and only 2200 lbs, like a slot car, trust me she was a rocket, i could have put a big block in it but it kills the handling


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## dewi1219 (Nov 16, 2006)

That was just a little sarcastic joke by me - a blown 5.0 is the perfect engine for that car.


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## Pulse-R (Jan 14, 2007)

I made an excel spreadsheet a while ago for another project which does a graph even for the 20-20k with up to 10 parametric EQ's.

If someone want it to make into a javascript calc, then let me know - PM or email or something.


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## CobraVin (Jul 31, 2007)

pm sent, ill take a look, thanks


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## CobraVin (Jul 31, 2007)

Whiterabbit said:


> honestly, I dont think you can use a calculated guideline.
> 
> If the -3dB point is caused by cancellation and you are boosting both drivers, your response will go nowhere.
> 
> ...


thanks for the replies

i have noticed that some eq changes dont have much of an effect, can you elaborate on the "alternative tuning methods" , do you mean leaving gaps between crossover points around say the 100hz from cabin gain, instead of trying to fight against it? or just generally everything from install to aiming


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## ziggy1971 (Jun 14, 2015)

I know this is an old post, but I was wondering if the Excel spreadsheet Pulse-R created is still available. I couldn't PM because I didn't have any posts, so this is my first.


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## 14642 (May 19, 2008)

Q = Frequency / [email protected]


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## ErinH (Feb 14, 2007)

Q factor vs bandwidth in octaves band filter -3 dB pass calculator calculation formula quality factor Q to bandwidth BW width octave convert filter BW octave vibration mastering slope dB/oct steepness EQ filter equalizer cutoff freqiency - sengpielau


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## ziggy1971 (Jun 14, 2015)

Thanx for the info


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