Reducing noise

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Noise reduction is the constant challenge for the recording engineer, as perhaps the most significant aspect of improving recording quality. Following the advent of realistically priced recording hardware, and free software like Audacity, anyone now can be their own recording engineer and grapple with the same problems.
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Contents

Quick summary

  • Avoid noise in the first place instead of trying to remove it afterwards
  • Spend as much money as you can - use a sound card with balanced inputs, and use shielded cable, sufficiently long so that you can move the microphone right away from the computer
  • If money is at a premium, an inexpensive external USB sound card should (other things being equal) be much quieter than the motherboard sound device that came with the computer
  • Place the microphone on a floor stand and ground it separately from the computer, or use a ceiling-suspended microphone
  • Keep all microphone cables away from mains electricity cables

Prevent noise

To prevent noise entering your microphone recording:

  • Set the correct input level of your sound sources. Set it as high as possible to increase the dynamic distance of sound and noise, but as low as necessary to prevent clipping.
  • If you have it at hand use a hardware limiter as long as Audacity cannot process sound in real-time. Personally, when recording a band I use a simple foot pedal limiter on the vocals, because these seem to be the most dynamic sound source. The input level of instruments can be adjusted quite easily.
  • When you have found the optimal levels, decrease them about a dB or two when recording music just to be sure. When actually performing, musicians tend to be a little bit louder than in rehearsal mode.
  • Shut every non-used sound channel and sound source. Mute non-used channels on your mixing board, switch off non-used amps, keyboards, and don't forget shut the door and the window.
  • Especially in a home-recording environment, avoid switching lights or electric machines on or off during recording, because a spark can cause a knack on the track.
  • Avoid fluorescent lighting (but don't use dimmer switches with incandescent light bulbs because a mic can easily pick up the noise acoustically or create induced noise as a result). See below for more about noise from lighting.
  • Keep cell-phones a good distance away from any equipment. If you are close to a cell-phone tower, try to record in a room that is farthest away from it.

Noise tends to stay on the same low level and cannot be controlled in general (you don't have a fader for it, do you?), but the record source is often dynamic and may change and can be controlled. So you may get aware of your potentials in noise control here. Initially non-audible noise often comes to attention when a low signal is amplified and/or normalized because normalizing and amplifying increases both the wanted signal and the unwanted noise. Therefore, the measures and proceedings I described prevent some of the typical noise problems later.

Environmental Noise

Room Noise

  • Use a directional microphone. Cheap PC mics, besides having rotten sound quality, are nearly omnidirectional (i.e. they pick up from almost all directions equally). Many professional mics (Shure SM57 and SM58 for example) are highly directional. If the business end is not pointing at the noise source, it won't pick up the noise. It may still pick up ambient noise, including any sound originating elsewhere in the room and bouncing off the walls. That will be reduced if you put the microphone right on top of the sound source. When recording vocals, the performer's lips should almost be touching the mic, and sing straight into it (not across the top). Ambient noise will be blocked by the singer's head.
  • Use a noise-blocking stand and a long enough cable to distance the mic from the computer. Often times the vibrations from a computer's fans and drives will vibrate the computer desk and the surrounding area. If the microphone and its stand are on the computer's desk, the microphone often will pick up the vibrations and produce a noise on the audio track (often referred to as a "warble" sound: a soft, repeating hum). To help prevent this, use a ceiling-suspended microphone stand or a full-size floor stand that can have its height adjusted. If these (pricey) options are not available, an alternative is to support a desk stand using a sound-insulating lift, such as a flimsy cardboard shoe box or a number of newspapers. These things insulate the noise rather well, making it difficult for any vibration noises to flow through to the microphone. Almost any lift made of non-rigid, flexible material will do.
  • Direct connection. If recording instruments like keyboards and electric guitars, feed their signal directly into your sound card's line input, or to a sound board and then into your PC. Guitars will need preamps. If you're recording acoustic instruments, use directional microphones placed close to the instrument, or use a pickup with preamp and connect direct.
  • Get the desired signal as loud as possible (without clipping) into the microphone. This allows you to reduce the gain, which will also reduce the low-level noise. The further a microphone is away from the source, the more you have to amplify the mic's input signal to get to a usable level. But, boosting the gain amplifies everything, including background sounds and even the internal electrical noise of the amplifier. Ideally, the microphone should be right on top of the source, with the gain no higher than necessary to get peaks around -3dB. If you are doing multitrack recording, record each individual track as loudly as possible. Set the final volume of each track during post-production mixdown.
    Note: placing the microphone "right on top of the sound source" might not be ideal when recording certain sources (such as bowed instruments like violins and cellos). Instead of placing the mic right on top of your sound source without regard to factors other than noise, you should experiment with different kinds of microphone placement until you find one that provides the best sound. If the "optimal" placement is too noisy you can look for other ways to reduce the noise. In the end, nothing beats an ideal recording environment.
  • Post-recording noise reduction. Noise can often be reduced during post-production, by use of various plugins. Typically, they are fed a sample of the noise alone and then subtract that noise from the rest of the recording. To facilitate this, be sure to record a second or two of "silence" before you start the actual performance. This gives you a clean sample of the noise. This works extremely well with low-level background sound like air conditioning.
  • Don't forget the possibilities of non-technical noise reduction:
    • Turn off your refrigerator and furnace / air conditioner during the recording session.
    • Watch out for telephones, cell phones, pagers, ticking clocks, lawnmowers, and the like.
    • Avoid locating the recording session near airports, train tracks, and fire stations.
    • Hang blankets on the walls, to dampen a live room. Or record in a room with wall-to-wall carpeting.
    • If you can, record in a basement, to help isolate your session from outside noise. You'll probably need to use the blanket-on-the-wall trick here, since concrete walls make good sound reflectors.
    • Record late at night to reduce traffic noise leaking in from outside. (It's up to you to work this out with the neighbors.)

PC noise

  • Do a Google search on Quiet PC or Silent PC for all kinds of sites with tips and techniques for reducing the noise coming out of your PC.
  • If using a laptop, don't use its built-in mic. Besides being cheap and omnidirectional, it will pick up a lot of the mechanical noise of the laptop itself.
  • Directional microphones. (see above)

Electrical noise

60/50 Hz hum and/or crackle

  • A common problem. Make sure all your recording equipment is connected to the same ground. This is easiest to accomplish by plugging everything into the same power strip. Then ground the computer separately from the recording equipment.
  • Keep microphone cables well away from mains electricity cables (including those behind walls)
  • Try to use incandescent light bulbs (including halogen lamps) but avoid using dimmer switches because of the noise risk. Avoid using fluorescent lamps near a signal path (cables and equipment), especially for low-power signal lines such as microphone cables.  Fluorescent lamps often generate a significant amount of high-frequency RF noise, which may then be captured by the cable or the equipment.  Incandescent lamps on the ceiling do not usually induce buzzes (because they are far away), but if used in a group of 4 or more, they may introduce buzzes into the power line, which may affect other equipment on the same power circuit.  Power conditioners may be used to alleviate this problem.
  • If all else fails, get rid of the hum during post-production by using a de-noise plugin or an extremely narrow notch filter. Crackle will be much more difficult to remove.

Remove noise

On virtually any recording you can find noise. It is not always necessary to get rid of it completely. First-of-all, it is often audible only in very low-volume passages. Second, the average not-too-picky listener will accommodate to the noise level of your recording. In this regard it is comparable to the odor of a room: When you enter it you become aware of it, but once you stay in there for a period of time you'll probably cannot smell it anymore. Third, he/she may listen to your programme in the car or while washing dishes so he/she may be not in the position at all to hear the noise.

Sometimes, a completely silent passage, e.g. between sequential parts of a programme, can irritate a listener more than a constant low-level noise throughout the mix. This is so because complete silence may disrupt the ambience of the material. There are situations where you actually want to add noise (e.g. in film production between a cuts of the same scene).

So, you may want to change your attitude towards noise here: It's not just dirt that needs to be removed, but it's a natural part of all listening experiences that has to be dealt with appropriately. In general, we need to accomplish two things: The noise just has to stay on a roughly similar level throughout the material and it should not be too obvious to let it be ignored.

Hi-Band Noise

Removing hi-band noise appears to be an option you can choose to take or to leave out. If you apply it, do it on the most basic level you can reach, on unprocessed tracks (for example, before adding reverb), on single tracks or even on single passages.

On multiple tracks:

  • Before doing anything else, increase the tracks to a working level with the Audacity Normalize function. Leave a little bit of headroom when amplifying.
  • Mute all passages where there is nothing to be used in the mix. You may use the envelope tool to make silent passages of a track really silent.
  • Fade-ins and fade-outs are much better than sharp edges cut-and-pasted that very likely will cause clicks.

On a single noisy track, you may want to use the Noise Removal feature of Audacity. You accomplish this task in two steps:

1) You pick a "noise-only" part of the track's signal. This part should not be longer than approx. half a second. This is a sample of the noise that will be used to compute the neccessary changes to the track to remove just the noise (though this idea is always merely theoretical).

You have to be careful in selecting your sample. If you pick a sample that contains not only noise but also a slight part of - let's say - a reverb tail, you'll remove that, too. To give you another example, the sound of breathing can be quite similar to noise but it provides a lot of the vitality of a vocal track.

Now, select a small portion of noise, call "Noise Removal" and Select "Get Noise Profile". If you are unhappy with your selection, you can repeat this step. Every time the last sample will be overwritten with the new one.

2) Now, select the portion of the track that needs noise reduction (in most cases this will be the complete track). Call the "Noise Removal" function a 2nd time. You can click on "Preview" to listen to the first seconds of your selection or on "Remove Noise" to execute the noise filtering. The less/more slider is quite obvious, I think, and can be changed for testing and applying the effect.

If you're unhappy with the result you can Undo all changes on your track and try again.

Noise Removal often helps to reduce hi-band noise such as hissing and (to a certain extent) crackling.

Subsonic Noise

While removing hi-band noise might be considered optional, removing subsonic noise seems mandatory to the writer. In contrast to hi-band noise you can apply subsonic reduction as a step in the mastering process, on the mixed and processed material, before a final Normalize.

Subsonic or low-band noise can enter your recorded material in many ways, such as through physical vibrations during the recording or noise from the tape machine (if you still use one of those).

Everything below 20 Hz is called "subsonic" because the human ear is unable to perceive it as recognisable sound. You can recognize subsonic noise by eye when the zoomed in waveform in Audacity is not symmetrical along the time axis. If you have already applied normalization in Audacity as recommended above, this should have removed any DC offset in the recording. The reasons for removing subsonics are the same as with DC offset - they will reduce the headroom available on the recording by taking up dynamic range, and can introduce clicks when editing.

To remove subsonics from your track you may filter it with:

  • Audacity's built in Equalizer under the Effect menu
  • Audacity's built-in High Pass Filter under the same menu - set the cutoff frequency to around 25 Hz. You can repeat this same effect a couple of times if a sharper cutoff slope is desired.
  • This Nyquist plug-in subsonic filter. Right-click or control-click over this link, and save it as "subsonic.ny". Then drop it into the "Plug-Ins" folder in your Audacity installation folder and restart Audacity.


After removing subsonic noise you can generally re-normalize your track, and it will appear louder yet much more defined in the bass.

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