Hi, and thanks for your response. It seems to me that the highpass
filter I'm using in Audcaity IS actually affecting frequencies above
the threshold I gave it. The amplitudes are being DEcreased ABOVE the
1kHz threshold! This is not good for my purposes! I don't understand
why this is happening, but it is. When I play a file without
filtering, I can hear sounds at about 5kHz, but when I use the
highpass filter, not only do the low frequencies get reduced (good),
but the ones above the threshold are getting dimmed and I can't hear
the 5kHz sounds anymore! Shouldn't the sounds above my threshold be
amplified, if nothing at all??!!
I think I might just 'cut off' the data at 1000hz and below in a
Matlab matrix so I can avoid this problem.
--- In derek holzer <> wrote:
>
> Hi Natalie,
>
> nbioacoustics wrote:
>
> > Does anyone know how to cut off all data below a certain frequency
> > threshold? I don't want to use the HighPass Filter, I just want to get
> > rid of all the sound below 1kHz...
>
> Technically, these are one and the same. A highpass filter will
> attenuate frequencies below its cutoff point. The steeper the slope of
> the filter, the more the frequencies below the cutoff are reduced.
>
> > Also, if I run the the highpass filter, it doesn't affect things above
> > the threshold I give it at all, right?
>
> Filters usually have a "bump" right at the cutoff frequency.
Frequencies
> just above the cutoff frequency will be slightly amplified. The steeper
> the slope of the filter, and the higher the resonance, the more of a
> "bump" you get.
>
> If you wanted an approach which isn't based on the traditional highpass
> filter model, you could investigate an FFT-based model. This is what
> gets used for most software noise-reduction filters (although I don't
> think Audacity has such a noise reduction utility). In this case,
> however, the sound gets reduced to a specific number of frequency
> channels in a process much like MP3 compression. Then you can simply
> "erase" the unwanted frequency channels and reconstruct the sound from
> what's left. I consider FFT-based filtering to be both destructive and
> lossy, especially when you are working with very frequency-sensitive
> material like binaural recordings and bioacoustics.
>
> In short, both methods will change your sound in some way you probably
> don't intend, so you have to decide which method is least destructive
> for your purposes.
>
> best,
> d.
>
>
> --
> derek holzer ::: http://www.umatic.nl
> ---Oblique Strategy # 196:
> "What else is this like?"
>
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