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I'm trying to record an instrument using a piezo pickup, and while it's not capturing a lot of audio noise, it's capturing a lot of high frequency noise that my instrument shouldn't be making. The highest note my instrument can make is an A5 (880 Hz), yet it's capturing frequencies in the 2000-8000 Hz range.

How do I use Audacity to filter out all frequencies above a specific pitch?

I'm not talking about removing noise in general. When I'm not playing a note on the instrument, there's already virtually no background noise, so the "Noise Reduction" tool can't do anything.

This old question on the Audacity forum talks about it a little, but the filters they mention, like the Equalizer, have long since been removed in the current 2.4.2 version.

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    "it's capturing a lot of high frequency noise that my instrument shouldn't be making." BZZZT! Wrong. See Laurence's answer. Those frequencies are the essence of your instrument's sound. If you filter them away, nobody will be able to tell what instrument it is. Is there something wrong with the sound you're recording? Or is the prolem the way the spectrum looks? If it looks like the spectrum of a single sine wave, then it will sound like a single sine wave. – piiperi Reinstate Monica Sep 22 '20 at 7:41
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You can use the Spectral edit multi tool.

But be aware that the nominal frequency of a note only defines its pitch. All musical sounds contain overtones, at higher frequencies than the basic pitch, and it's these which largely determine the timbre of the sound. Filter out everything above 880Hz and it's not going to sound like that instrument any more!

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You need an equaliser. Not any specific equaliser plugin, but any equaliser available in whatever application you choose to use. It may be a plugin, it may be built into the application.

If you genuinely only want to keep frequencies below some cutoff, then you want a low-pass filter. You'll find them in the list of EQ options, or you might find a separate plugin for it.

From reading your question though, this is not what you want to do. As piiperi's comment says, these higher frequencies are what make your guitar sound like a guitar. With just the fundamental frequency it would sound like a pure sine wave, because that's what it'd be. (A tuning fork is the closest physical equivalent of this.)

It may be that you've got stuff on there which you don't want though. If you've used a piezo pickup, they have a midrange "quack" to their sound which changes the tone. If you've used a microphone and you've not done a good job with positioning and/or playing, you may be picking up fretting noise, finger squeaks, poor picking/fingerpicking technique, clothing rustling, etc.. These are things you mostly don't want - although sometimes you do, because on most acoustic guitar playing you'll hear some of these things, so be wary about aiming for a completely clean, sterile result. But assuming you do want to remove them...

It's time for more EQ. You can use a multiband graphic EQ, traditionally with 31 faders for bands of frequency, or you can use a parametric EQ where you set a frequency yourself. Since you're a beginner, I'd suggest a graphic EQ is going to be easier for you.

You want to start with everything zeroed (at midpoint). Work out roughly where your troublesome frequencies are. Then one fader at a time, push that frequency up (at least halfway, to maximum if you can stand it) and run your recording. The result will sound strange, but you're not listening for sound quality, you're listening to see if this is a frequency band which is adding a ton of crap. If it is, then boosting that band will make the noise really jump out at you. If it is, push that band down to maybe -6dB, and then move on to the next one.

The idea is to have as few frequencies removed as possible. That leaves the tone of your guitar substantially intact, whilst doing a precision strike on the bits you don't want. Initially you're going to be far too heavy-handed, and the result with loads of bands removed is going to sound dull and muffled. No matter - go back and try again.

Doing good EQ is a skill, and it takes practise. A good live sound engineer can dial this in by ear on the mixing desk within seconds, because they know what they're listening for. It could take you half an hour to get something you like. No worries, you'll get there.

But any engineer can also tell you that the best way to get a good sound is to start with a good sound! Your guitar technique is part of that, of course - but after that, you need to record it well. This SoundOnSound article is a very good place to start for techniques on recording guitars. There are many ways to skin that particular cat, and it would certainly benefit you to do some experimentation with that. (And one thing to avoid - never, absolutely never, point the mic straight at the soundhole!)

In general, I would thoroughly recommend you going to the SoundOnSound site and looking through their archives. They've got a lot of articles about how to do recording and how to "massage" recordings with EQ and other FX. You'll learn a lot by going through their articles and then trying out some of those ideas for yourself.

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    It has been 15+ years since I've done live sound, and I was never great at it, but I loved getting a space set up - reading this really takes me back to that "there it is!" feeling, thank you for a great answer. – brnlmrry Sep 22 '20 at 16:24
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    @BriansaysReinstateMonica Thanks! It's been a few years for me too - spending every other weekend with my son after a divorce is not compatible with gigging. Back in the day though, I was running sound for local gigs where I'd have 3 or 4 bands on, and not a whole lot of time to soundcheck. Or for small festivals, where at best you get to line-check. I didn't do any huge events because this was never my day job, but it was a lot of fun and I was reasonably competent back then. – Graham Sep 23 '20 at 9:07
  • I disagree that a graphic EQ is a better choice for beginners. As you've explained, optimal use of EQ requires first finding the critical frequencies, and that's much easier by sweeping through a single band-boost. Especially software graphic EQs are a huge pain to set up since you can just tweak multiple faders with one grab. Beginners using graphic EQs tend to set up ridiculously nonlinear curves that don't really accomplish anything useful. Only for seasoned engineers can (physical-fader) graphic EQs be quicker to set up, because they already know pretty much what curve they want. – leftaroundabout Sep 23 '20 at 9:47
  • @leftaroundabout I don't disagree that a parametric is easier for people who know what they're doing. I don't think it's easier for a beginner though. And the point of this is that it isn't a curve - you're trying to notch out the nasties, not put a gentle curve across it. Depending on how accurate the OP's information is, of course. Without hearing it ourselves, we can't judge what's needed. Hence my point about recording - often the best answer is to re-record and start with something that doesn't have these problems. – Graham Sep 23 '20 at 11:40
  • @leftaroundabout ... But you're right that it's an option. The point really is for the OP to start learning how to use EQ effectively, and that's a wider problem than just "I've got noise on my recording". – Graham Sep 23 '20 at 11:45
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It sounds like what you want is spectral selection and editing.

Spectral Selection is used to make selections that include a frequency range as well as a time range on tracks in Spectrogram view. Spectral Selection is used with special spectral editing effects to make changes to the frequency content of the selected audio. Among other purposes, spectral selection and editing can be used for cleaning up unwanted sound, enhancing certain resonances, changing the quality of a voice or removing mouth sounds from voice work.

There above-linked reference page includes a detailed example of removing a specific pitch.

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Turns out my Audacity installation was screwed up. The low pass filter is what I wanted by it wasn't showing up, probably because I carried over an older configuration file from a previous version.

So I did sudo apt purge audacity*; sudo apt install audacity and then all the regular filters were back.

With low pass removing all the junk high frequencies, it sounds much better. Thanks everyone.

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