Hi, I’m using Audacity 2.0 on Windows Vista 32-bit. I’m new to using Audacity for more than the simplest tasks. The attached audio sample was originally recorded on a cassette recorder in the 1980’s. There is an annoying ringing in the background. We created an mp3 and used it for copyright registration, but need to improve the quality before issuing the whole song on a CD. I cannot use Noise Removal because the sound cannot be isolated by itself. Is there any way to remove the ringing sound without destroying the guitar and drums? Thanks!
Try using the Equalization effect http://manual.audacityteam.org/manual/help/manual/man/equalization.html
Select the “Graphic Eq” view, and pull down the 3.15 kHz slider to the bottom (all other sliders set “flat” at 0 dB).
Thanks for the suggestion. I tried doing the equalization as you described but it didn’t remove the unwanted sound. Any other ideas?
Deeper notches in the equalization are possible if you use the “draw curves” option rather than using the “graphic EQ” sliders which are limited to -15dB …
A deeper notch removes more sound …
[ BTW it sounds like you’ could have got some sort of feedback problem with the guitar which has a resonant frequency of 3KHz, this could be an electrical or acoustic feedback loop : somehow feeding some of the output back into the input ]
Same thing here, This was a very low sound recording through 2 walls. The high pitched ringing is louder than the conversation.
How can I remove the noise? 
Rather than trying to remove the noise, the better solution would be to place the microphone in the same room as what you are trying to record after ensuring that you are legally entitled to do so.
If the noise level is greater than the signal level then there is probably not much chance of successful noise removal, but if the high pitch ringing is at specific frequencies then you may be able to “notch them out” as described by Trebor. See here for how to identify the frequencies that may need to be filtered out: Missing features - Audacity Support
[moderator note: link removed]
next-door-are-fighting-again
I’m not a legal expert, but it is probably a breach of privacy laws in many countries to record your neighbours and publish the recording on the Internet without consent.
Also the recording quality is so poor that it would not be possible to recover anything other than a bit of “distant noise” from that recording.