All recordings–both digital and analogue–have a noise floor. The term originated when manufacturers of analogue audio recorders referred to the extraneous noise that their machine created in addition to the desired recorded audio signal.
Often a background noise constitutes most of the audio recording and covers a portion of speech that needs to be audible in order to determine a series of events pertinent to the case. These noises can often be removed by the audio forensic expert to help determine facts about the series of recorded events.
Background noise and noise floor extraneous sound can consist of a heating or air conditioning fan running, refrigerator motor, window fan, clock, fluorescent lighting, wind, rain, car running and even radio or television. All these sounds contribute to the background noise and noise floor of a recording and aid the forensic examiner in authenticating a recording. However, this background noise can interfere with the forensic examination. Clarification is part of the forensic examiner’s job. It is appropriate for the forensic examiner to remove these background sounds in order to authenticate or clarify an exhibit of audio recorded evidence.
Some of the recordings experts are asked to authenticate are confession recordings created by law enforcement agencies. Defendants exclaim, “That is not what I said, they edited it” or “There is more I said that has been edited out of the recording.” Due process entitles both parties in litigation to examine any evidence presented in their case. However, original recordings are not always available for examination. How do you as a law enforcement official feel about the absence of original recordings?
I have worked on cases where missing “original evidence” was considered spoliation of evidence. Personally I believe that circumstances of each case should be considered by the forensic examiner before any decision has been made by either party.
If the forensic examiner observes characteristics that are noticeably questionable, then the expert must notify the officials in charge of their findings during the preliminary examination phase of the forensic investigation. Original recordings are required, and if not produced, a motion to suppress the evidence should be filed.
photo credit: zoom h1 recorder for use as studio backup & ontheground audio via photopin (license)