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I'm using the Web Audio API to make a graphic EQ with four BiQuad filters; a highpass, two bandpass and a low pass:

enter image description here

You can see each node representing each filter's frequency. It's working nicely but I'd like to draw the shape of the filter's roll-off you'd usually find in other graphic EQ's.

I have the Q-factor of each filter and the central frequency. What I would like to do is get a formula that allows me to get the frequency of the roll-off at a specific db value. For example what is the frequency of the roll-off at -200db?

It's been a while since I've done any heavy mathematical stuff and so any help would be greatly appreciated.

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The Q factor and center frequency won't be enough to plot the full frequency response of a filter. You'll need to know its topology (e.g. Butterworth, Chebyshev, others) and order. – Jason R Feb 23 '13 at 16:59
These are BiQuad filters. Do I need to be more specific than that? – RobotEyes Feb 23 '13 at 17:05
Yes. You can calculate the frequency response from the coefficients directly, or if you're using a well-known topology, there may be cookbook equations that you can use instead. – Jason R Feb 23 '13 at 21:36
ok thanks. So I don't currently have enough information on these filters to do a calculation - correct? The filter's code is hidden behind an API so I'm not sure if I can get more info on them but I'll take a look about... – RobotEyes Feb 24 '13 at 0:56
I think I found the appropriate cookbook here – RobotEyes Feb 24 '13 at 1:12

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