Noise Reduction is a DirectX plug-in, it actually contains four discrete plug-ins: Noise Reduction, Click and Crackle Removal, Audio Restoration, and Clipped Peak Restoration. You can use these plug-ins to analyze and remove background noise from your audio recordings.

The Sony Noise Reduction plug-in is designed to analyze and remove background noise such as tape hiss, electrical hum, and machinery rumble from sound recordings. Unlike a normal filter, it can do so without removing part of the source material. This is accomplished by separating the audio into its frequency components and using a "noiseprint" to distinguish unwanted noise from the desired signal.

A noiseprint is created by analyzing a portion of the recording where there is no source material, such as during a silent lead-in. The Noise Reduction plug-in stores frequency and amplitude information about the noise in the recording. Once a noiseprint has been generated, you can modify it to determine what parts of the frequency spectrum you want to remove. However, this type of manual modification is not usually necessary.

The noise reduction algorithm works well to remove constant background noise. Rapidly changing or "erratic" noise such as traffic sounds or vinyl pops are not well suited for this algorithm). However, different types of noise can be removed by generating a separate noiseprint for each type.

Also note that in cases where the noise is of equal or greater volume level than the source material, it can be difficult, if not impossible, to clean up without some resulting audible side effects.

Which Noise Reduction mode should I use?
In general, Mode 0 will sound the cleanest, but can sound unnatural and electronic when the noise level is high. Mode 3 will not completely remove the noise, but will generally sound the most natural for very noisy recordings.

Modes 1 and 2 compromise between Mode 0 and Mode 3. You may switch between the modes while previewing in real time and decide which one works best for your needs.

When trying the different modes, the Noise bias control should remain near 0 dB when using Mode 0, but it should be raised or lowered for the other modes to find the right balance. When the noise level is low, the modes will sound very similar and Mode 0 usually performs best.

The license of this software is Free Trial Software, the price is $279.95, you can free download and get a free trial.