There are many other ways audio evidence can be spoiled. Consider the following scenario: An officer is working undercover to crack a major drug ring in the city. For months he has worked his way through a number of small-time sellers and has finally gained enough credibility to be presented to the top guys for a major buy. His plan is simply to make the buy and record the event. He has studied enough to know that a digital recording cassette machine will give him much better sound than an old-style micro-cassette recording device. His department has acquired the best digital device on the market specifically for this score. He has studied the digital recorder's manual and knows that while it can be set to record for 2, 4 or 8 hours - the longer the recording time the lower the audio quality. Due to compression, the longer the recording time, the more audio information that must be squeezed (compressed) onto the tape. He sets his machine for 2 hours since he is sure the meeting will not last more than 10 to 15 minutes.
He is now ready to make the buy and obtain the vital evidence. However, even with all of his preparation, he has made a basic mistake. The digital recorder is inside the lining of his sports coat. He has his cell phone turned on and clipped to his belt near the recorder. The buy goes down exactly as planned, and as the officer makes his way back to the station, he feels pretty good that his efforts are paying off.
As the audio technician begins to analyze the tape, it becomes evident there is a great deal of background noise occurring at regular intervals. The impulse signals produced from the apparently silent cell phone come through the recording at a much greater level than the audio and sound like small arms fire on the tape. Cell phones and other digital devices must always be kept far away from digital recording devices. This tape, despite the officer's careful preparation, will be difficult, if not impossible, for a jury to hear and clearly understand.
Years ago, this would be the end of the story. But with today's forensic audio analysis software, such evidence can be salvaged. The technician simply applies a set of adaptive filters to the tape. These so-called "smart filters" can effectively remove a variety of background noises and are particularly good at clearing out impulse signals from cell phones. The end result is an audio tape that clearly identifies what is happening and who is involved. The cleaned-up tape can be easily heard and understood by a jury. Even so, problems like this can be avoided if an audio technician is consulted as officers develop operational plans for a case like this.
Interpreting the data
A problem also arises with advanced methods of audio analysis in the interpretation of the analyzed data. When any evidence is presented in court, prosecutors expect it to be challenged by the defense. A major challenge is always to the credibility of the laboratory and the technician who performed the analysis.
In all areas of forensic analysis, laboratory and technician credibility is important. This is particularly true in the audio area where improper manipulation of analytical software can easily lead to a wrong conclusion. Any size department can buy the software and recording equipment to set up an audio lab. However, this lab will only be as good as the quality of the technician running it. Unlike other software, where users can read books or pick up the basics through trial and error, running the proper software and filters is just a small part of the audio analysis equation. More important is data interpretation. And this only comes from hands-on experience analyzing and interpreting audio data. A well-trained technician is essential, especially in the area of speech recognition. Identifying a specific voice, isolated from a tape with several voices, and then comparing it to tapes of suspect voices, requires a great deal of experience. Only a certified speech recognition expert should evaluate this type of data.
Many forensic audio experts say audio analysis is part science, part art and a large part gut feeling that only comes from experience. A feeling for the right filter or parameter to apply to a tape to bring out a needed layer of sound comes with time. This experience can greatly decrease the time needed to obtain necessary evidence from a tape. Consider the following:
A call comes in to a 911 operator from a distraught, frantic woman. She is screaming and hard to understand. As the operator calms her down, she realizes the woman is being threatened with violence and may already have been beaten. The operator immediately dispatches a patrol car to the address. When officers arrive, they find the caller alone, very agitated, and insisting the call was "a mistake" and nothing was wrong. On the way to the scene, the officers learned there was a restraining order against the woman's ex-husband. They pay a visit to the ex-husband on the job. He claims to have been there all night, and his supervisor backs up his story. Officers then ask a forensic lab manager to listen to the 911 tape, since they believe it might provide a clue as to what actually happened at the woman's apartment.