FORENSIC AUDIO ANALYSIS

Acquisition, analysis, and evaluation of sound recordings

Forensic enhancement is to better hear dialogue by removing unwanted noise OR ENHANCING BACKGROUND NOISE OF INTEREST

The primary aspects of audio forensics are establishing the authenticity of audio evidence, performing enhancement of audio recordings to improve speech intelligibility and the audibility of low-level sounds, and interpreting and documenting sonic evidence, such as identifying talkers, transcribing dialog, and reconstructing crime or accident scenes and timelines.

Forensic audio enhancement services are the scientific process of clarifying audio recordings using non-destructive techniques to preserve speech quality. This is crucial for the trier of fact can make determinations about the events within the recorded evidence. Above all, the goal of forensic enhancement is to better hear dialogue by removing unwanted noise. The noise reduction process uses a noise profile to systematically reduce unwanted frequencies without affecting the dialogue or human voice signals. This scientific process of improving the quality of dialogue is also known as speech enhancement or voice enhancement.

ASSESSMENT

Audio forensic examiners should assess digital evidence thoroughly with respect to the scope of the case to determine the course of action to take.

ACQUISTION

Digital evidence, by its very nature, is fragile and can be altered, damaged, or destroyed. This is mostly due to manipulation or improper handling or examination.

EXAMINATION

The purpose of the examination process is to extract and analyze digital evidence. Extraction refers to the recovery of data from its media.

DOCUMENTING

The expert documents actions and observations throughout the forensic processing of evidence. This will conclude with the preparation of a written report of the findings.

Audio transcription

Creation for the trier of fact.

Forensic audio transcription is the scientific observation of words under controlled conditions derived from an enhanced audio recording. Audio enhancement is the process of reducing unwanted sounds and enhancing the wanted sounds. For example, dialogue is frequently enhanced within an audio evidence recording. If the dialogue is still difficult to interpret, then the audio transcription services and forensic transcript are tools used by an expert as to what was said. 

ACQUISITION

The acquisition of digital audio recordings MUST be done in accordance with the acceptable protocols within the scientific community. These protocols have been established by an organization called SWGDE. The acquisition of digital audio recordings is broken up into three categories; establish/examine the chain of custody, request the original, retrieve the recordings using acceptable methods.

First, examine the chain of custody. What type of equipment was used to create the evidence? How was the evidence handled from the time of its creation to the delivery to the courtroom, as well as investigators and experts? Are there authentic chain of custody documents and reports that outline the chain of custody? Sometimes a chain of custody log from law enforcement will be included, which will strengthen the authenticity of the audio evidence. If the investigation of the chain of custody reveals inconsistencies, more often than not that recording is determined to lack authenticity and integrity.

If the expert is able to retrieve the evidence from the original source, in most cases that will automatically create and establish an authentic chain of custody, IF DONE PROPERLY. This retrieval process must be documented through video recording or images in order to provide an accurate record of what the expert did during the retrieval process. If it isn’t possible to retrieve the recording(s), then the forensic expert must carefully go through all of the documents and reports that arrived with the evidence. But if the chain of custody cannot be established, the forensic expert must rely on other techniques as well as their own expertise to determine the authenticity of the chain of custody, such as request of the original.

As a general rule, a forensic audio laboratory should request the original recording or the earliest generation available. An original recording is the first manifestation of sound in a recoverable stored format. If the original recording is on analog media, playback and duplication rely on physical processes that introduce noise and degrade the signal, even if slightly. A copy of an analog recording can never be an exact duplicate. An original digital recording is a bit stream from which the acoustic audio signal can be generated. Exact copies of that bit stream can be made. With digital evidence, each stage of copying can be exact with no loss of quality between generations. The exactness can be tested and confirmed through the use of a hash function. Therefore, a bit stream duplicate of a recorded file is equivalent to the original.

“Means of securing the recorded evidence must be evaluated based on their effect on the recorded signal, and the available method of transfer preserving the evidence in a condition as close to the original as possible should be chosen. Use multiple means of collection if it is not apparent which available means will produce the highest quality.” (SWGDE Bast Practices for Forensic Audio PG 10) Appropriate retrieval methods are as follows:

– The original recorder/recording system
– A forensic image of the original storage medium
– A forensic image of the original file(s)
– A file transfer of the data from the original storage medium
– Other; analog transfer, digital signal transfer, copy, etc.

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