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Forensic Audio Evidence Handling

Acquisition, analysis, and evaluation of audio recordings required as evidence in court is an area which audio forensic specialists regularly deal with. Generally, this kind of audio evidence is submitted as a part of a law enforcement investigation or official inquiry into a civil incident such as an accident.

In the past I have received enquiries for such audio documents and have felt the need to direct customers to a law professional so that their evidence complies with legal requirements. In short, just because you have a recording in your possession does not necessarily mean that it is admissible as evidence in court. Certain legal processes and procedures need to be followed in order for the court to accept an audio recording as evidence. You need to be aware of the applicable laws and jurisdiction in your country, state, before submitting the evidence.

If you are in possession of a digital recording that may be submitted as evidence, it is important to maintain the integrity of the recording, as this will be the first consideration legal professionals make to determine its admissibility. Most courts apply the “fair and accurate approach” in order to qualify the audio evidence submitted. With this in mind, the following three aspects are highly recommended.

  • The chain of custody must be absolutely clear as this will determine the decision of authenticity and admissibility – eg. how did the recording come into your possession, on what type of device was the recording made, have others made edits to a recording prior to coming into your possession?
  • Avoid any kind of editing or processing of the audio file – this includes, for example, submitting a two minute segment of interest from an one hour recording, editing as well as processing with tools that increase/decrease gain or noise, or any kind of destructive audio processing.
  • Do not attempt to alter any of the audio file’s metadata.

In many cases audio recordings will be made in less than ideal circumstances, such as when someone has placed an audio recording device in a pocket or is wearing a body wire. Forensic audio specialists have the tools and knowledge to enhance recordings, deliver better speech intelligibility and recover details that aren’t noticeable when listening to the unprocessed recordings.

Typically audio recordings that a forensic audio specialist deals with suffer from noise disturbances that may interfere with the speech present in the recording. If this recording is presented as evidence in court, the court may need to be convinced that during the enhancement process no changes  took place resulting in alteration of the dialogue, be it with or without intention.

The task of determining the presence of alterations or tampering of a digital recording is more complicated than with an analog recording. As an example of this is Spectral Noise Reduction commonly used to attenuate or eliminate the noise present in an audio recording but may also interfere with fricative and plosive components that are of outmost importance to speech intelligibility.

Analog and digital media have their own strengths and weaknesses exhibiting advantages and disadvantages in the workflow and processes that the forensic audio specialist has to implement to tackle the issues he is faced with.

Digital is more complicated than analog because tampering is easier to do and disguise. However, the digital tools are very sophisticated, and with a thorough approach we can determine any added or altered content. Audibly and spectrally examining the content of an audio file can reveal discrepancies or other characteristics present in the recording. Enhancement of the audio is limited by the resolution of the original audio recording.

As digital evidence is rapidly increasing in importance, both courts and examiners must adapt to changes and develop new methods and capabilities to create more robust methods in handling and storing digital evidence in the future.

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