Sound processing software has made substantial leaps in recent years. In addition to merely sampling the background noise floor and minimizing it, it is now possible to target individual elements in an audio recording, either by dedicated algorithms or, when the audio is viewed in spectrograph form (see home-page banner), by surgically "painting out" obtrusive sounds manually. This enables us to further suppress background noises and/or effectively remove specific unwanted sounds.*
The next step in this process is enhancement. First, frequencies above and below the voice spectrum are filtered out. Next, if applicable, we suppress echoes and reverberation, which tend to obscure clear speech. Finally, equalization targeting the voice spectrum is applied in conjunction with compression to boost low-level or quiet passages.
This enhancement process is not a 'paint-by-numbers' affair. The incorrect or excessive application of processing at any step can actually worsen the fidelity of the voice portion of the recording. It takes the discerning ear of a trained audio professional to carefully apply the various processes in the right order and in the proper manner to coax the recording along toward maximum intelligibility.
* The effectiveness of the audio cleanup process is limited by the amount of detail actually present in the digital recording. While dramatic results can be achieved working with uncompressed WAV files recorded at standard sampling rates, in the case of files recorded via compression codecs such as MP3 or recorded at low sampling rates, the potential of uncovering very low-level dialogue is compromised.