As an Audio Forensic Expert, knowing what tools are available to me and how they work is extremely important. One type of signal processor that I frequently use is a compressor. While this is often thought of as a tool for music production, it serves many functions in the Audio Forensic lab. Like with most audio signal processors, it takes training and experience to operate compressors properly and effectively when performing forensic audio enhancement. This experience also helps me determine whether or not the compressor is needed for enhancement.
A compressor is a device that automatically attenuates the gain of an audio signal. This means that when the audio reaches a certain level, the compressor will lower the gain of the audio signal. When the audio drops below this certain level, the compressor will stop attenuating. It is similar to a person manually adjusting the volume on a stereo as a song is playing. A benefit of a compressor is that it also has a ‘make up gain’ control. This allows the operator to raise the overall level of the audio after it has been attenuated. Through this process, the recording can be made louder without clipping or distorting the signal.
I will typically use a compressor when certain sounds in a recording are much louder than the rest of the audio and I need to balance the overall volume. An example would be a dog barking occasionally throughout a recording that is peaking much louder than the people talking. Using a compressor, I can attenuate the level of the barking without affecting the level of the people talking. Once the louder signal has been attenuated, I can use make up gain to increase the overall level of the recording. This becomes extremely helpful when the sound source that needs to be heard is quieter than other sounds. I will often receive recordings where the conversation that needs to be heard is buried or behind another sound source, like a television or even other people in the room. By adding a compressor, I can decrease the difference in level between the two signals.
Compression is not always the best approach for an audio enhancement and in some cases, I avoid using it completely. One of the biggest issues in recordings is a loud noise floor. The noise floor is the sum of all of the extraneous and unwanted noises in the recording. As I mentioned before, sometimes this noise floor is louder than the desired sound and therefore compression helps make the desired signal louder with respect to the noise. In some audio, the noise is already quieter than the desired signal. In these cases, using too much compression can actually increase the level of the noise relative to the desired signal. This can actually make the desired signal more difficult to hear and hurt the overall enhancement.
This is why it takes training and experience to properly use a compressor. With the knowledge that I have gained from my 30 plus years as an Audio Forensic Expert, I know when to use and when not to use a compressor on audio. I also know how to properly use it so that I improve the quality of the audio instead of making it more difficult to hear.