I recorded a 10-piece jazz band last weekend indoors at a little place. Had a bunch (mebbe a dozen) LOUD squeaky feedback episodes. Took a chance and found I could hunt them down in Audacity by zeroing in on each one with the Analyze>Plot Spectrum tool.......it lets you see your frequency spectrum for the short moment you have highlighted. It snaps to each peak, so you can make a note of the EXACT feedback frequency by floating your cursor across the frequency graph it makes (feedback was occuring usually in the 6000-10000 Hz range, sometimes there were harmonics in there also.) The guy squeaking was moving around the small stage area a slight bit, so I was noticing that each feedback event had a slightly different center frequency.
I needed a notch filter that would let me specify what resonant frequency each event was happening at........and let me control how narrow a frequency range I wanted to subtract from the waveform for that little amount of time I used the filter 
So, I went looking aroung the interwebs, and found a sweet little Nyquist plug-in called "notch.ny" from the plug-ins area of the Audacity site. I stuffed it into the ...\Audacity\Plug-Ins folder, fired up Audacity again, and then went to work erasing every single one of them, hunting them bastiges down like I was looking for ticks on a dog!! Man, did it ever fix the recording.......big lifesaver.
You just highlight the offending event, plot the spectrum and note the peak frequency (or frequencies), pull up the notch filter, specify the frequency and shape (a "Q" of 4.5 worked best for me), apply the filter, and then re-plot the spectrum to see and then listen that the feedback is erased!
VIVA AUDACITY!!!!
Topic: Filtering feedback? (Read 1406 times)