Butterworth question

[Soulwax]

If a 1st order Butterworth filter has a minus 3db point at 140Hz and a minus 9db point one octave lower at 70Hz how does one calculate the minus 6db point?

Presumably this will be around 100Hz? Is there a formula?


Forever mathematically challenged but trying..

 

[Soulwax]

105Hz?

doh.. I’ll get me coat on the way out!

 

[jan.didden]

A(dB) = 10 log[1+(wx/wc)^2n]

A =attenuation at wx;

wx/wc is ratio of freq where att is wanted to 3dB filter cutoff freq wc;

n = filter order

Jan Didden