One of the most common audio issues with audio enhancement is noise. Noise and other extraneous ‘unwanted’ sounds make up the noise floor in a recording. Unwanted sounds interfere with the wanted sounds. Removing the unwanted sounds and raising the intelligibility of the wanted sounds is audio enhancement.
A noise floor is usually consistent throughout the recording and can be removed to varying degrees by a trained audio expert using software.
Non Continuous Sounds
The most complicated issues are the unwanted sounds that are not continuous. These sounds could include anything from a plane flying overhead to someone whistling while people talk. It is harder to pinpoint these sounds. A spectrogram is used so the audio expert can see and hear the unwanted sound allowing the expert more options for removal.
Use of a Spectrogram
A spectrogram shows both the frequency content of a recording and the level of those frequencies over time. It may be the most helpful tool to an Audio Expert because it presents all sound throughout the recording in one view. The spectrogram allows the expert to identify and address individual unwanted sound in the recording. With the right software, these individual sounds can be selected and removed without affecting any other part of the recording. It is important to remember that there is a right and a wrong way to do this, which is why only a trained Audio Forensic Expert should be hired to complete an enhancement for use in court.
The Role of Artifacts
When processing audio, it can be easy to introduce artifacts to the recording. Artifacts are unwanted noise that is produced from various processing and compression techniques. Considering the goal of an audio enhancement is to eliminate extraneous noise, introducing artifacts is the exact opposite of what you want when working with a recording. Many things can introduce artifacts, but the simplest way to describe the cause is over processing. By over processing, I mean using extreme settings within individual audio tools.
For example, I often work with audio evidence that is extremely quiet. This often requires a gain increase of portions where only voice content exists. If the gain is increased too much, it can cause clipping of the audio output. When this occurs, the edges of the waveform are essentially clipped off, producing a distorted and noisy audio signal. The end result is a less intelligible voice than the original, essentially defeating the purpose of the whole process.
When adjusting individual ranges of frequencies on the spectrogram, it is very important to be aware of artifacts. Being able to recognize artifacts and know the limitations of what processing can be done is what makes an Audio Forensic Expert necessary. When isolated portions are processed with a trained ear and the right knowledge, noise can be eliminated and voices can be brought out without introducing any artifacts.
I recently worked on an audio recording that had a siren present during a portion of talking. Because it was so loud, it made the underlying dialogue difficult to hear. Luckily, the siren could be isolated in the recording. By selecting only the siren and then decreasing the gain a moderate amount, the voices became more audible while still avoiding any artifacts.
Audio Forensic Experts have a plethora of tools at their disposal, which is making audio enhancements more and more effective. There are some things to be cautious of when enhancing audio. This is determined on a case by case basis. Any technique that is taught in training should be used as long as the technique is accepted in the scientific community.
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