MediaSlap blog

Forensic Audio Enhancement

by Danny Reeves and Dwight Cook

Sometimes it seems impossible to discern the details on a recording.  There is too much background noise, or the volume is too low. The solution is a process called Forensic Audio Enhancement; the attempt to correct defects in recorded audio.  Defects might include clicks, hum, buzz, pops, excessive background noise and poor signal strength, any of which can lose a case.

But now technology has progressed to such a point that these defects are not altogether impossible to overcome. There are two basic types of enhancement that can clear up your recording; listenability and/or intelligibility of the sound source and both are non-destructive.

Easy on the ears

Listenability enhancement is a relatively simple procedure applying equalization (EQ), compression, amplitude increase or limiters to an audio recording which is already intelligible, but needs to be made more listenable.

Easy on the brain

Intelligibility enhancement is a more complex and demanding engineering task.  Enhancement procedures involve complex scientific filtering, digital compression, digital EQ and amplitude increase and limiting.  Sometimes there is the need to target small sections of the recording with “region specific” processing.

For the forensic audio engineer, this requires knowledge of the physics of sound, human auditory perception, phonetics, critical bands, as well as knowledge of electronics and professional audio recording techniques, and of course experience. An engineer’s past experience is key to knowing what to do and can save you time and money in the discovery process. The results can be impressive, with previously unintelligible dialogue becoming intelligible and coherent. In some cases, previously unknown audio events are uncovered!

What is the typical process for enhancing audio?  There are four basic steps in the forensic audio enhancement process.

1. TRANSFER the source recording to a digital format.  The source recording can be any of the following: micro-cassette, regular cassette, any tape based recording format; digital hand held recorders, police videos and voicemail.
2. EVALUATE the material and then develop an enhancement plan.
3. EXECUTE the enhancement plan until the desired outcome is reached.
4. CREATE a final digital master of the original recording along with the final enhanced version.

Double Check

Make sure that your audio enhancement studio is able to offer a certified statement or is available to be deposed for certification. Also make sure that their process has previously been accepted in federal, state, and local courts for credibility.

Danny Reeves is an audio engineer, Dwight Cook an owner of Sound Works, Houston who offers forensic audio services including enhancement.

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