# Why can't filter out beeping sound?

While recording an interview I picked up some beeping sound (focus lock sound from a camera). I find this beeping sound to have base frequency of 1500Hz, so I put the sound file in audacity and apply a notch filter at 1500Hz. This remove the middle portion of the beeps but left behind ringing at the start and the end of the beep.

I thought maybe the beeping sound has different frequency at the start and end, so I created a test file with a mathmatically generated burst of 1500Hz pure sine wave.

I repeated the notch filter and sure enough, it left behind some ringing at the start and the end of the burst.

The higher the Q of the notch filter, the more ringing.
The recorded file does not have as abrupt start and stop beeping, but abrupt enough to also have the ringing after filtered.

Is there a way to perfectly remove the 1500Hz wave with no ringing without making the Q very low, which could remove wanted speech?

Is there a way to perfectly remove the 1500Hz wave with no ringing without making the Q very low, which could remove wanted speech?

No. You’re fighting basic electronics. Pure silence is well-behaved and understood and pure sine waves are, too. However, starting a sine wave isn’t. Bad Graphic: sine wave starting.

Note the area under the red bar is not a pure sine. It’s massively distorted. Massive distortion creates multiple foreign tones, overtones and harmonics. It’s even worse if you manage to get a sine to start cold into a full wave from a dead stop. You can get award-winning damage by starting a sine wave somewhere other than the zero point.

That’s the trash you’re trying to get rid of. It’s a performer in your show and I don’t think you can stop it. The desperation method, other than re-doing the interview is cut and paste clean words into the hole made when you deleted the damage.

The Hollywood desperation method is get somebody less important to revoice the whole interview in your quiet studio.

Koz

If you have a lot of time on your hands, you might try notches at odd and even harmonics of the base tone. That’s where most of the trash is going to live.

You might also try copy-pasting several trashes together into a longer sample and Analyze > Plot Spectrum. Use a very high analysis number and stretch the display sideways.

If you cursor over the spikes, the display will tell you the frequency.

Another wacky possibility if you have clean beeps with no voice, is string a bunch of those together, select them for Noise Reduction > Profile. Try Effect > Noise Reduction just on the tone intervals.

https://manual.audacityteam.org/man/noise_reduction.html

Koz

It’s known as the “Gibbs phenomenon”.
Additional information here: Ringing artifacts - Wikipedia

Usually there’s no “perfect” way, only “compromise”.

One exception to that is if you are able to obtain a sample accurate copy of the sound to be removed, which you can then “subtract” from the track (invert the copy and mix with the track to “cancel out” the tone). In most real world cases this is not a practical option.