noise removal - audacity wiki

Noise Removal

From Audacity Wiki

Audacity's Noise Removal effect can be used to attenuate constant background noise such as tapehiss, electric fans or hums. It will not work very well for removing talking or music in thebackground. There are two steps:

Show Audacity a "noise profile" - a short section of audio containing only thenoise to be removedReturn to the effect to apply the noise profile to all the audio.

Related article(s):

Click Removal (http://manual.audacityteam.org/o/man/click_removal.html) for removal ofdiscrete clicks and pops from audioTransferring tapes and records (http://manual.audacityteam.org/o/man/tutorial_copying_tapes_lps_or_minidiscs_to_cd.html) and other media to computer or CDImproving Recording Quality by system tweaks and hardware upgradesOther Tutorials

Contents

1 How does Audacity's Noise Removal work?2 Noise Removal steps

2.1 Get Noise Profile2.2 The Noise Removal2.3 Noise Removal controls2.4 Special cases

2.4.1 Noise Removal with Cassettes2.4.2 No silent noise sample

3 Other Techniques3.1 Notch Filter3.2 Noise Gate

3.2.1 Nyquist Noise Gate Plug-in

How does Audacity's Noise Removal work?

Audacity's Noise Removal effect is in essence a multi-band digital noise gate, automatically shaped by the NoiseProfile you supply.

Noise Removal - Audacity Wiki http://wiki.audacityteam.org/wiki/Noise_Removal

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In other words it looks at the noise sample, works out what the noise floor is in each of the frequency bands, anduses this as the threshold for a bank of noise gates.

When the audio is processed, it is processed into the same frequency bands, and passed through the noise gates,so if that frequency band has only noise in it, the noise will be blocked. If there is signal in that band (which willalso mask the noise from being heard) then the noise gate opens and the noise is let through.

As with all noise filters, Noise Removal may not always remove all the noise without affecting the quality of therest of the audio. If Audacity's Noise Removal effect does not work as well as you would like, or if you have nosample of audio that is only noise, go to Other Techniques below.

A more technical explanation of Noise Removal can be found here.

Noise Removal steps

Removing noise is a two-step process: noise profiling followed by noise removal/reduction.

Get Noise Profile

In the first step, you select a portion of your sound which contains all noise and no signal, in other words, selectthe part that's silent except for the noise. Usually this will be at the start or end of the track, but if the track hasno lead-in or lead-out, try zooming in to hit a precise spot between a note or word. You select the part of thetrack that is just noise by clicking in the track and dragging a selection out with your mouse or with the arrowkeys on your keyboard. Then choose Effect > Noise Removal and click "Get Profile". Audacity learns fromthis selection what the noise sounds like, so it knows what to filter out later.

The Noise Removal

Then, select the audio from which you want the noise removed, which you do by clicking in the track anddragging a selection area with your mouse. Note however that you may not necessarily want to apply NoiseRemoval to the whole track. This is because most Noise Removal takes some of the music away with the noise,and adds or exposes artefacts in the recording. If you only have a few spots in the track that need a smallamount of Noise Removal, you may well want to select only those areas to remove noise from. But if the spotsneed a greater amount of Noise Removal, they may once noise is removed have a rather different "feel" ortimbre than the rest of the track, and betray that it has been treated at those points. In this case it may be betterto remove noise from the whole track so that it sounds consistent. To apply Noise Removal to the entire track,select all of it by clicking in the Track Control Panel where the Mute/Solo buttons are, or if you only have onetrack on the screen you can use Edit > Select > All or the hotkey CTRL + A (or (CMND + A on a Mac).

Having selected the length of audio from which you want to remove noise, choose Effect > Noise Removalagain, but this time, click the "Remove Noise" button. It may take a few seconds or longer depending on howmuch track you selected.

If you want to apply Noise Removal at the current settings, you can use "Repeat Last Effect"CTRL + R (or COMMAND + R on Mac) to run Noise Removal after grabbing the Noise Profile,rather than reopen the effect dialog.

If not enough noise was removed, or too much of the recording was removed along with the noise, you can useEdit > Undo and try Noise Removal again with a different noise removal level on the slider, which adjusts thenoise threshold. You don't have to get a new Noise Profile again if you think the first one was fine.


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However if the problem is that too much of the signal (music, voice, etc.) has been removed along with thenoise, you can also try going back to the selection chosen for the Noise Profile and reducing its amplification(Effect > Amplify). Then use this as a new Noise Profile. Sometimes running the filter a second time using aNoise Profile that is a de-amplified section of track can give a good result. Another possibility is to select andEdit > Duplicate the track you want to remove noise from before applying Noise Removal, and adjust therelative volume of the two tracks (using the -....+ gain slider on the Track Control Panel) so you get the best mixof the original and noise removed tracks.

Note that if the nature of the background noise changes significantly during the course of the audio, it will bebest to take an individual sample of the changed noise and apply that noise sample to the section of audioaffected by that particular noise.

Noise Removal controls

There are three additional sliders to give more control over the removal algorithm - one controlling thesensitivity of what is considered noise, one controlling the smoothness in the frequency domain, and the othercontrolling the smoothness in the time domain via an attack/delay control.

So to begin with, try setting the two "smoothness" sliders at about the halfway point and adjust them leftwards ifnecessary to make the noise removal more "aggressive", and rightwards if you need to make the the result soundmore "natural" or more like the original music or speech was before Noise Removal.

The main slider controlling the amount of noise reduction works differently now: instead of adjusting thethreshold, it adjusts the gain applied to the noisy part of the signal - i.e. the amount by which the noise should bereduced in volume. If you have a very small amount of noise, you can often reduce it by a large number of dBwith no penalty in added distortion. If you have a lot of noise, you will often be able to reduce it much lessbefore you start to add distortion, so in effect you will be putting it more in the background rather than reducingits volume so much that it is no longer audible.

The sensitivity slider controls what is considered noise. It applies a gain to the noise thresholds that wereprofiled in step 1. Setting this slider to higher values means that more sound will be considered noise and thusfiltered out by the noise removal effect. Setting it to lower values means that less sound will be considered noiseand bypass the filter. Both sensitivity and reduction sliders control how much audio will be filtered, but indifferent ways: the reduction slider controls "how much" will be filtered, while the sensitivity slider controls"what" will be filtered.

The best setting for the noise filter is one in which most of the noise is filtered with minimum hurt to the audio,that means with reduction and sensitivity sliders as close to 0 dB as possible. If there is still noise, try increasingthe value of the sensitivity or reduction slider. Generally, increasing the value of one of these sliders filters morenoise, allowing the other one to be decreased. Try that to get both to the minimum possible value. If the soundgets distorted try to increase smoothness with the other two sliders. Of course, increasing the smoothness sliderswill allow more noise to bypass the filter or will muffle the sound. If noise is bypassing the filter, increasesensitivity or reduction levels or decrease attack/decay time. If the sound is getting muffled reduce sensitivity orreduction levels or decrease the frequency smoothness value.

It may be impossible to get a satisfactory removal when the noise is very loud, when the noise is variable, whenthe music or speech is not much louder than the noise or when the noise frequencies are very similar to those ofthe music or speech.

Special cases


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Noise Removal with Cassettes

Cassettes are slightly different from other sources in that its not uncommon for the noise profile to changethrough the course of the tape, due to accumulation of dirt on the tape heads, either during play or duringrecord, or both.

To optimise the performance of Audacity noise removal it is best to use a noise sample near the beginning of thetape rather than the end. Using a sample late in the tape will sometimes cause poorer discrimination betweennoise and signal, as some of the higher frequency noise will be missing or reduced in amplitude.

No silent noise sample

Audacity's Noise Removal filter relies on taking a noise sample so that it knows what to remove. But sometimesa recording has no noise sample available because there is no passage which is silent except for the backgroundnoise. Examples of this could include tape recordings of music radio, or an audio file recorded from acontinuous LP track where the lead-in and lead-out have been edited away.

There is an option that often works, although it's far from guaranteed, and sometimes fails. Consequently it'swise to back up the original file first if you are planning to export the result from Audacity so as to overwrite theoriginal file.

The plan is to use a recording made in identical conditions that does contain silence, and use a noise samplefrom that for removing noise from the recording with no silence.

This method works very well if the noise in the second recording is identical to the noise in the first. For acassette source, that means using the same type of tape, same source, same deck and same recording amplitude.In the example of a recording made from an LP, you could try recording a band between tracks (or lead-in orlead-out) on any LP that sounds as if it has the same general level and type of background noise. This will beless likely to give a good result than recreating cassette noise, as the noise source is not identical, but in the caseof a noisy recording, it will probably be better than doing nothing.

If the substitute noise sample is not identical, it will still remove some noise, but will not discriminate betweennoise and signal well, so noise removal will be partial.

Other Techniques

Notch Filter

Current Audacity (http://audacity.sourceforge.net/download/) now includes a Nyquist Notch Filterunderneath the divider in the Effect menu.

Suppose you've loaded your audio into Audacity and listened to it, only to be disappointed to find it'scontaminated with a continuous, fairly pure tone of some kind:


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Let's check first to see if the technique that this tip offers will apply to your case. To do this, select the entiretrack with CTRL + A (or CMND + A on a Mac), then click Analyze > Plot Spectrum.

You will need to see a spectrum with the following kind of pattern for this tip to work successfully:

Notice that the spectrum is fairly smooth except for a few evenly spaced peaks. I placed my mouse over the firstone of these peaks on the left (ie, the one at the lowest frequency, just above 3 kHz) and Audacity identified thepeak more accurately for me at 4255 Hz. I make a note of this number for later use because this is the first ofthe troublesome frequencies, and perhaps the only frequency in the whistling, that I need to eliminate.

To eliminate one narrow band of frequencies one uses a so-called "notch" filter (named for the fact that it makesa notch in the frequency spectrum). To do this with Audacity, click on Effect > Nyquist Prompt and enter thefollowing code:

(notch2 s 4255 50)

The line above tells Nyquist to process the notch filter on both channels.

Substitute "4255" with the frequency that you identified in your own spectrum. That other number, 50 in thisexample, is the Q factor of the filter. You might find that you need to experiment with it. Varying this valueinfluences the width of the notch. A lower value makes for a wider notch and vice-versa - just experiment a bit.


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Alternatively to using the built-in Nyquist prompt, you can download a Notch Filter(http://audacity.sourceforge.net/nyquist/notch.zip) plug-in to save a lot of the typing if you have a stereo track.To install this plug-in, place the unzipped notch.ny file in the Plug-Ins folder inside Audacity's installation folderand restart Audacity. On Windows computers, your installation folder is usually under Program Files while on

Mac OS X, it is usually under Applications. After restarting Audacity you will find Notch Filter in Audacity's

Effect menu underneath the divider. Simply enter the notch frequency and notch Q value you want - as before,the smaller the Q value, the wider the notch.

If you now play your recording you might find that you have eliminated the continuous sound. If not, you couldtry recalculating the spectrum as described above to get the next frequency peak for elimination and so on. Inthe case of my recording here is how the spectrum appears after I notched it:

As you see, that first peak has been eliminated. In fact you can see the dip in the spectrum and perhaps I couldconsider experimenting with that value that was set to 50. However, as it happens, the recording is good enoughfor my purposes.

Noise Gate

A Noise Gate (http://en.wikipedia.org/wiki/Noise_gate) is a type of "audio gate" that is "open" and allowssounds to pass unaltered when the level is above a "threshold" level. When the audio signal is below thethreshold level, the gate "closes" and stops, or reduces the signal making it substantially quieter. A Noise Gatedoes not remove noise from a signal but reduces the noise level during the quiet periods between sounds. Thereare many situations where this can be useful.

When there is very low level noise that is effectively masked by the recorded material, a Noise Gate canlower the noise level during silent parts of the recording where the low level noise would otherwise beapparent.

Example: Noise Gate can remove dither noise that may be evident in silent passages in 16-bitaudio files. Be sure in Quality Preferences to enable 32-bit Default Sample Format and todisable "High-quality" dither - more explanation here (http://forum.audacityteam.org/viewtopic.php?f=28&t=38756) .


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When noise removal by other methods causes unacceptable degradation of the sound quality, a NoiseGate can reduce the noise level to some extent between sounds without affecting the actual recordedsounds.Where there is low level intermittent noise of a similar type to the actual recorded sound (for example, ifsound from a distant television or radio is audible during pauses in a speech recording) a Noise Gate canmake the pauses more silent.A Noise Gate may be used after the Audacity Noise Removal effect to further reduce the noise levelduring periods that should be silent.

Nyquist Noise Gate Plug-in

The Nyquist Noise Gate plug-in has a number of features and settings that allow it to be both effective andunobtrusive. One of the most critical settings when using a Noise Gate is to set the Threshold carefully. Thisplug-in provides an analysis tool to assist with setting the threshold level correctly:

Make a selection that is "noise only" and open the Noise Gate effect.1. Select Function > Analyse Noise Level and make a note of the suggested threshold level.2. Select a section of the audio track that includes both sound and silence.3. Apply the Noise Gate with the following settings: Select Function > Gate Level reduction > -100.Listen carefully to the result to check that the noise is being cut and the audio is still present. This effectwill probably sound unnatural as the gate is fully closing and producing absolute silence when closed, butit will allow you to easily hear if the threshold level is set correctly. Click CTRL + Z to undo and ifnecessary adjust the threshold level and retest.

4.

When you are happy that the threshold level is correct, raise the "Level reduction" setting to the highestsetting that produces an acceptable amount of gating (the default level of -12 dB usually works well).

5.

When you are happy with the settings, apply to the entire song.6.

Advanced Tip: By default the Noise Gate will take 1/4 second or 250 milliseconds (ms) to fully open and fullyclose. In some cases it is desirable to make the gate open faster or more slowly. At the minimum (10 ms) thegate will fully open and close almost instantly as the audio level crosses the threshold. This could cause the gate


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to 'flutter' or 'snap'. At the maximum (1000 ms) the gate will begin to slowly open (fade-in) 1 second before thesound level exceeds the Threshold, and will gradually close (fade-out) after the sound level drops below theThreshold for a period of 1 second. Longer gate times (up to 10 seconds) may be achieved by typing in the textbox instead of using the slider.

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noise removal - audacity wiki

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