# Guess the frequency response of a filter with little information

I have a processing unit that does a nice EQ filtering on soundfiles. The only thing I can do is : I give the original .wav file, and the processing unit gives me back the filtered .wav. I have no other information than this.

I'd like to know the frequency response graph of this processing unit. How to do it ?

• 1) feed the unit with a "flat spectrum white noise", and see the spectrum of the filtered file? It works but it is not very accurate

• 2) feed the unit with sinewaves soundfile with frequency = 20, 30, 40, 50, etc... 20 000 Hz. Then I examine the response for each file... and I can draw a frequency response graph after 10 hours of work :)

• 3) feed the unit with a frequency sweep ? and then try to see what's the unit's reponse to this sweep ? How to decode the response into a nice frequency response graph ?

• 4) I imagine that there is another better solution?

Thank you.

• All of the methods that you proposed are viable options. The easiest is probably to feed the algorithm with white noise. Why do you feel like it isn't accurate? Oct 22 '13 at 15:50
• Method 1 with white noise is okay for having the general shape of the frequency response curve, but it is not accurate (I know it because I tried this method with some filters for which I knew frequency response, and the dB given by this empirical method is not accurate)
– Basj
Oct 22 '13 at 16:55
• The white noise approach becomes more accurate as you do it longer and average the results. In any given stretch of time the frequency of white noise will not be perfectly flat. It only flattens in long periods of time due to the "regression to the mean" effect. Oct 22 '13 at 17:10
• Yes that's true, I have noticed that the longer the white noise is, the best the shape of frequency response is. However, even with a very long time (e.g. 60 seconds), the frequency response graph made with method #1 (white noise) only gives poor result... the dB attenuations shown by the empirical graph are not the real dB attenuation of the "ideal frequency response graph"
– Basj
Oct 22 '13 at 18:43