Blindspot Episode 10: How to Authenticate Digital Audio Evidence

The authentication of digital audio evidence verifies the integrity of an audio recording for use in court. Establishing a solid chain of custody is the first step in the authentication process. However, it does not authenticate a piece of evidence on its own. We have seen unreliable audio evidence stored in the original recorder that supposedly created it. Let’s take a closer look at how to authenticate digital audio evidence and why the process is so important.

How to Authenticate a Digital Audio Recording

WHY IS AUDIO AUTHENTICATION IMPORTANT?

The authentication process determines whether or not the audio recording in question has been tampered with. In the age of digital audio, edits can be made and covered up very easily. Editing software now has the ability to alter original events within digital audio recordings. Often times, analysis will detect if the audio recording was edited after downloading to a computer and before authoring a CD.

HOW TO AUTHENTICATE DIGITAL AUDIO EVIDENCE IN 5 STEPS

There are five steps that one must complete to properly authenticate digital audio evidence.

1. ESTABLISH A CHAIN OF CUSTODY

If the expert is able to retrieve the evidence from the original source, in most cases that will automatically create and establish a chain of custody. If it’s not possible for the forensic expert to retrieve the recording, then the forensic expert must carefully go through all of the documents and reports that arrived with the evidence. Without an established chain of custody, the forensic examiner must rely on other techniques as well as their own expertise to determine the authenticity of the evidence.

2. CRITICALLY LISTEN TO THE AUDIO RECORDING

During this process, the expert should note unusual sounding sections in the recording, referred to as anomalies. They should place markers near any anomalies they hear for later reference when compiling a forensic report. Inconsistencies in sound quality, noise floor, and level of the recording are all important to pay attention to.  The forensic expert should use both studio monitors and headphones with flat frequency responses to best hear everything that is going on in the recording.

3. ELECTRONICALLY MEASURE ASPECTS OF THE RECORDING

The forensic expert should use the audio forensic software they have to note the frequency ranges, levels and other aspects of the recording. Marking what frequency ranges voices or other sounds are in comparison to the noise floor can also help the expert better detect sudden changes and other anomalies in the recording that may indicate tampering.

4. VISUALLY INSPECT THE AUDIO RECORDING

This step goes hand in hand with electronic measurement. The forensic expert should analyze the waveform characteristics and look for any anomalies present. Additionally, the expert can use spectrum analysis and spectrograms to better see the behavior of the frequencies and detect breaks or changes in the signal or noise floor.

5. ANALYZE THE METADATA

The forensic expert will also need to inspect the recording’s digital information. This includes hex information, sampling rate, bit depth, and file format. Further comparison with an exemplar recording will detect any additional anomalies. Digital footprints are almost always left on recordings when they are created and when they are edited using other software.

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For a forensic examiner to authenticate a piece of audio evidence, they must prove beyond any doubt that the recording is in its original form and has not undergone any tampering. If a piece of evidence is not authentic, it should not be used in court. This piece of evidence may be incomplete or altered to purport events that did not occur.

 

Listen to this week’s Blindspot episode to further explore how to authenticate digital audio evidence. And subscribe to receive the latest episodes downloaded to your computer or portable device.  

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