# Question about 'Q' and various equalizers



## terry j (Jul 31, 2006)

howdy all

tonight I started mucking about with something I've been meaning to do for a long time...the exact details are not important to this question.

I've had a Behringer dcx 2496 for a while now, and a little while ago finally downloaded the upgrade and the computer interface thingy and mucked about with the interface between the computer and the unit. That was all good.

Previously I have worked out which equalizer to use (drop down menu) with both the deq 2496 and the deqx. (have never used one of the BFD's yet).

Tonight I used TMREQ with the DCX, as it puts out the filters with cut, frequency and Q (it is the one that works with the deqx, don't recall right now exactly which one is suitable for the DEQ).

The predicted results vs the measured results were WAY off, the first time I've ever had that happen (I always find the predicted results spot on).

I then went back out of curiosity trying different equalizers from the menu, and noticed that the figures to enter into the filters varied greatly (on the few that output frequency, gain and Q). In other words, the suggested values to use for the filters change from equalizer chosen to equalizer chosen, even though they are ALL in the same format of boost, frequency and Q.

Why would it be so??? Surely for example the frequency is a constant, as well as the cut/boost. Indeed, doesn't Q have an actual definition and so should not vary??

If that is so, I don't understand why different equalizers would have different values for those (pre) defined concepts.

Can anyone shed light on this?

The long and short of it is to experiment with which equalizer best suits the one you use (if it is not one of the supported ones of course!)


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## brucek (Apr 11, 2006)

> Indeed, doesn't Q have an actual definition and so should not vary??


It has to do with the differences in how the devices define their bandwidths.

For the DSP1124P, it defines bandwidth as:

Bandwidth (Hz) = centre frequency*(BW/60)*sqrt(2)

So, the Q formula becomes:

Q = 60/[(BW/60)*sqrt(2)]

For the FBQ2496 the bandwidth control adjusts in 1/60 of an octave steps from 1/60 to 5/60 of an octave, and so the formula becomes:

Q = sqrt(2)/BW

and so on..................

SMS
The filter bandwidth in Hz between the half gain points is given by:

Bandwidth = centre frequency/Q

TMREQ
The filter bandwidth in Hz between the half gain points is given by:

Bandwidth = centre frequency/Q

R-DES
The filter bandwidth in Hz between the half gain points is given by:

Bandwidth = 1.766*centre frequency/Q

Generic
The filter bandwidth in Hz between the half gain points is given by:

Bandwidth = centre frequency/Q

Yeah, you're right. If your equalizer isn't in the list, check the manual on how it defines bandwidth and the relationship to Q and pick the correct choice in the list to match....

brucek


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## terry j (Jul 31, 2006)

thanks Bruce, you never cease to amaze me! Seems like you have an answer to everything at your fingertips heh heh heh.

ME?? I don't bother with a manual (who reads them anyway?), I just plug and see what happens.

Funny how 'Q' can be defined in different ways, first I came across it was the TMREQ paper I read somewhere written by a good friend of all of us I imagine!! which if I recall correctly was all to do with minus 3 db points and center frequencies and all the rest of it, indeed as you defined it above. I didn't realise then that it could be anything different. I took the time to remember it and work it out for myself, until I found that the predicted results of REW gave such good corrections I gave up after that......until today of course!

still learning, does it ever end??

thanks


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## Glyptoron (Apr 20, 2006)

Hello,

You can find here a quick calculator and a chart for Q factor :
http://www.sengpielaudio.com/calculator-bandwidth.htm


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