A well-written expert opinion should be comprehensible not only to the expert who authored it and other specialists in the field but also to the requesting authority. The summary of the opinion should be clear and concise, aiding the requester in understanding its implications. This principle guided us when we developed three methodological papers in the field of audio forensics. Our goal was to present our methodology, and especially the conclusions drawn from our examinations, in a manner that would be understandable to non-experts. The principles we employed can undoubtedly be applied in other fields where the integrity of an object submitted to an expert needs to be assessed.
I will illustrate the importance of clear communication with a simple example, but first, let's briefly introduce the field of audio forensics.
Audio forensic experts primarily analyze audio recordings. The most common tasks include verifying the authenticity of an audio recording, improving the quality of an audio material, enhancing the intelligibility of speech, and identifying the speakers in a recording.
Today, everyone carries a device capable of recording audio and video. Expert experience shows that the use of audio recordings as evidence is becoming increasingly common year by year. The examined audio materials are not only privately recorded but also include recordings of telephone conversations made by service providers, financial institutions, and authorities.
When an audio recording is used as evidence, there is often a question, not only from the opposing party but also from the authority, as to whether the recording accurately reflects what was actually said. The authenticity of a recording can be examined by an expert, and the results are documented in an expert opinion. During the analysis, the audio forensic expert examines whether the recording has been subsequently modified, whether the content has been altered, or whether the recorded material has been manipulated even during the recording process. These examinations are complex, and it is essential that the expert expresses their opinion precisely. Our methodology outlines the potential examination results and creates a categorization system.
First, it is important to clarify that any modification to a recording is considered an intervention. For example, starting and stopping a recording, adjusting the volume, renaming, converting, or copying an audio file are all actions that require human intervention, but these actions are not considered manipulation until it is proven that they were done with the intent to deceive.
Let's consider a simple example to illustrate why it is important to formulate an opinion precisely and clearly. During a business meeting, we record a conversation using the recording function of our mobile phone. During this conversation, our counterpart makes a statement that we would like to use as evidence in a later dispute. To do this, we copy the recording from our phone to a CD and attach it to the case file. Before burning the CD, we convert the audio file from the phone's unique format to MP3 format to ensure that it can be played in court. After playing the recording in court, the other party claims that they never said what was recorded and suggests that the recording was certainly manipulated. The court orders an expert examination. For the examination, the expert receives the recording on a CD, on which no signs of cutting are detected. However, the expert finds that the audio file has been modified compared to the original recording because the original format has been converted, meaning that the recording has been intentionally altered. The expert concludes their opinion with this statement. Since the expert (correctly) states that the recording has been altered, the court may no longer consider the audio material to be credible and may exclude it from the evidence. This is because the expert did not explain to the requesting authority in an understandable way that although the entire recording was technically altered, the detected intervention did not change its content. The content of the recording is identical to the original recording.
To avoid such misunderstandings, our methodological description of audio authenticity examination defines categories and their explanations that allow decision-makers who are not familiar with the field to understand the conclusions drawn from the examinations. Briefly, these are:
- If no signs of a compromise to the integrity of the recording are found during the expert examination, the expert opinion will state that no signs of intervention, manipulation, or audio technical errors were detected. In terms of intervention in the integrity of the recording, we distinguish between interventions that modify the content and those that do not.
- Intervention that modifies the content: As a result of the intervention, some of the audio events that actually occurred are inaudible in the recording, or new content that did not actually occur is created by changing the content of the speech or modifying audio events.
- Intervention that does not modify the content: In digital audio recordings, human intervention can occur that does not change the content or intelligibility of the speech or audio events. Such interventions include renaming the file, copying the file, converting the audio file, and changing the volume level. If it can be proven by audio forensic methods that a non-content-modifying intervention has occurred, the expert opinion will state the fact and nature of the intervention, adding that the intervention did not result in a change of content.
The latter definition should be applied in the above example. Thus, the expert correctly states that the court was not presented with the original recording, but rather a converted copy of it, the content of which is identical to that of the original recording. Following this, the court can base its judgment on the audio recording as evidence.
We believe it is important to document in the audio forensic methodology which interventions in the examined recording change its content and which do not, thereby assisting the requester in understanding the expert's findings.
Gabor Balla
Forensic Audio Expert