I studied physics over 20 years ago, I really have little practical experience with these things. What I had been taught about Q was that it was the center frequency divided by the bandwidth (-3db points in a notch or band pass). This is irregardless of the shape of the curve. If its asymmetric, I think most people use a geometric means f0 = sqrt(f1*f2). I am not familiar really with the concept of Q in low and high pass filters. What I was taught was the following:

Band pass or Band Reject has:

1) notch depth

2) center frequency

3) high and low cutoff frequencies.

of course the curves can look different, but again the most basic descriptors.

Notches might look different, but their all going to have a center and cutoff frequencies (so long as they notch below 3db)

2) low or high pass filters have a

a) cutoff frequency (-3db point or breakpoint)

b) slope or db per octave or decade

of course there are nonlinear type curves and everything else you can make especially digitally, but these are the most basic descriptors used to describe run of the mill filters.

From Wikipedia talks about initial Q, but then more rolloff:

A

**second-order filter** attenuates high frequencies more steeply. The Bode plot for this type of filter resembles that of a first-order filter, except that it falls off more quickly. For example, a second-order

Butterworth filter reduces the signal amplitude to one fourth its original level every time the frequency doubles (so power decreases by 12 dB per octave, or 40 dB per decade). Other all-pole second-order filters may roll off at different rates initially depending on their

Q factor, but approach the same final rate of 12 dB per octave; as with the first-order filters, zeroes in the transfer function can change the high-frequency asymptote. See

RLC circuit.

I don't have any experience with this initial Q in low and high pass filters, are they talking about how sharp the knee is near the cutoff frequency? Is there a simple definition for Q for low and high pass filters like there are for notches?