Audio Authentication and Visual Inspection
Sound waves can tell us a lot about a recording. Like metadata, the visual elements of a sound wave can expose the characteristics of an audio recording without even having to listen to it. These characteristics can be important, especially when it comes to detecting edits within audio evidence. The process of observing these characteristics is called visual inspection. This is a part of the audio authentication process.
Visual inspection (a general term that comprises a variety of forensic tests like narrowband spectrum analysis) is a crucial part of an audio forensic expert’s job. To understand how crucial visual inspection really is, it’s important to understand the concept and value of the noise floor.
The noise floor (usually unwanted sound) of a recording is the present background noise and overall ambiance of a recording. For example, if you’re recording yourself speaking on the street in New York City, and you’re speaking into a microphone while standing in one place, the sound of the cars going by, the conversations happening around you, and the overall city noise (unwanted sound) will contribute to the noise floor.
If you’re standing in one spot recording that audio, the noise floor will never change, because the environment your audio device is picking up will stay consistent the entire time. The second that noise floor is altered, you know you have an edit.
There are many ways to examine this. One of the most reliable ways to observe this noise floor is what’s known as a spectrogram. The spectrogram is meant to read the spectrum of an audio recording. To put it simply, a spectrogram takes the contents of an audio recording and conforms the characteristics to blends of color that represent the spectrum of an audio recording in Hz. You can see that below.
Now, because the noise floor of a recording never changes, you can tell when you have an edit when the spectrogram shows a change in, or absence of, color. The noise floor will always stay consistent, so when there’s a short drastic change such as the one pictured below, you know you have an edit. This makes the recording inauthentic.
Surely there are other ways to visually detect edits. Even the sound wave itself can expose an edit.
All sound waves should be smooth and continuous. Even if someone were to loudly clap during an audio recording, the sound wave will still remain smooth and continuous. When you see gaps or a wave that is not smooth and continuous with another piece of the audio file, you know you have an edit.
Though a critical ear is generally considered the most important part of audio forensics, a good eye for edits in the visual inspection can teach you a substantial amount about the evidence you’re working with before even taking the time to listen to it. Visual inspection really comes in handy when trying to determine the authenticity of a piece of audio evidence and to make sure a proper chain of custody was kept throughout the distribution of audio evidence.