first order filter 6dB down

[Marcelinho]

If a standard butterwort first order filter is 3 dB down at say 800Hz, How do i calculate at which freq. it to be 6dB down?

Or more simply said: how do i calculate the -6dB point of a first order filter.

[godfrey]

A low pass filter is -6dB at 1.73 times the corner frequency. (1.73 = the square root of 3)
For a high pass filter, it's the inverse i.e. -6dB at 0.577 times the corner frequency.

btw, one octave beyond the corner frequency, response is -7dB, one octave before the corner frequency, response is -1dB

[Marcelinho]

hmm, an octave is a dubbling of the frequency
The corner frequency is where the response is -3dB (is it?)
Then an octave beyond the corner freq. must be -9dB (6dB/octave)

[tinitus]

Originally Posted by Marcelinho

Or more simply said: how do i calculate the -6dB point of a first order filter.

Linkwitz or Bessel
well, they are not 1. order, but can be made to work like it
someone have said its found somewhere in between 1. order and 2. order

but its not just the -3db or -6db that are different, the slopes are different curves too
not impossible like finding a needle in a haystack, but probably the best way to describe it

the only thing that can said with absolute certainty is that the stricktly calculated values are the ones that wont work

[godfrey]

Originally Posted by Marcelinho

hmm, an octave is a dubbling of the frequency

Yes

Originally Posted by Marcelinho

The corner frequency is where the response is -3dB (is it?)

Yes

Originally Posted by Marcelinho

Then an octave beyond the corner freq. must be -9dB (6dB/octave)

No, because the corner is rounded. Here's what it looks like with a corner frequency of 1 KHz.