Question: For my speech recognition project, I'd like to completely cut off frequencies above 5000Hz on my wav files using ffmpeg.
I understand there is a lowpass filter for reducing frequencies above a certain value:

```
ffmpeg -i original.wav -af "lowpass=f=5000" lowpass.wav

```

However when I check the spectogram, it seems like frequencies above 5000 have been reduced, not completely cut off

[Voice sample with 5000Hz filter](https://i.stack.imgur.com/Twvoz.png)

How can I completely cut off frequencies with ffmpeg? Any other software is also fine.

Answer: 
Use the acrossover filter with a somewhat lower value than the actual cutoff.

`ffmpeg -i original.wav -af "acrossover=4500:order=20th[k][r];[r]anullsink;[k]anull" lowpass.wav`
I think I found the correct command:

```
sox input.wav output.wav sinc 7.99999k-5k

```