I am often given digital audio recordings by litigators to enhance. They want to hear the voices in the recording better than what they currently hear. In litigation, it should be termed audio clarification enhancement. This has a negative connotation, implying that the audio has been in some way altered. Clarification is a better word and you don’t open yourself up to having your investigation questioned, or an accusation that your enhancement somehow altered the recording.
Some of the time, when I receive the files the first thing I discover is that the file is not an original recording and there are filters that have been previously applied to the file.
The other day I received a file from a litigator from another country and I agreed to do a pro bono listen, as I often do when I’m learning what their expectations are versus what reality will allow. (Television shows like CSI have given a false perception of what is and is not possible when it comes to audio enhancement).
The file I received from an international pro bono job had a very heavy noise reduction filter applied to it. I can tell just because I can recognize the sound of noise reduction on an audio recording. It would have been a complete waste of time to try to work with that file so I asked if they had an original version of the file or a copy of the file that had not had any processing on it. The next morning there was a link to the complete file without the noise reduction.
My message here is you can try things on your own but always save the original unprocessed audio file before filtering. Then, apply one filter at a time to the maximum capability of the filter to help your clarification.
As an audio forensic expert, I sometimes apply several filters through the course of an investigation for a clarification project. However, amateurs tend to select a lot of different filters and not use them to their optimum potential before applying another filter.
This is why it’s important to seek advice and retain an audio forensic expert to handle your evidence, have a chain of custody that can be reported at the end of the investigation, and properly apply audio engineering best practices and techniques, forensically. Following a protocol that is accepted in the scientific community, allowing you to maintain integrity on your audio evidence will bring you the best results for your clarification process.
If your budget doesn’t allow for an audio forensic expert keep in mind a secondary message in this blog post: apply one filter at a time to its maximum potential when restoring the low quality in an audio recording.