There are many situations where noise reduction can be useful:

A perfect take being ruined by street noise, an air conditioner turning on, or other noise common to home recording settings.

Touching up demo recordings before release, or before pitching to labels.

Restoring audio of any kind to make it more listenable.  This could be anything from old master tapes, recordings with historical or archival value, low quality cell phone recordings, and more.

In film audio, reducing audible differences between location and studio quality audio.

Reducing amp noise, ground loop noise, or 50-60 Hz hum from individual audio tracks prior to mixing.

Fixing digital clipping resulting from incorrect input volume.

Reducing opamp hiss caused by lower quality preamps being driven hard.

Removing pops, clicks, ‘plosives, sibilance, and other distracting items for a more polished and professional sound.

This depends on the type of project in question, your goals, release plans, and budget. 

Noise reduction can be added onto any mastering project on a per song basis.

A noise reduction/repair project can also be standalone; this works well for film audio, during the mixing process, or for audio restoration projects.  Particularly when working with DIY home recordings, doing a noise reduction pass at the beginning of mixing can result in a much more polished sounding end product.

Because every project has unique needs, I quote each project individually. The first step is to contact me; we’ll discuss the details of your project, and go from there.

One of several common tools used for professional noise reduction is called a spectrograph.

Skillful use of a spectrograph is often compared to using a program like Photoshop to edit photos: this tool allows highly surgical and precise adjustments to be made to audio.

If you’re thinking of getting into this yourself, make sure you practice on test audio! This tool is like using a scalpel; it can be either a lifesaver, or a lethal weapon, depending on the skill of the operator.