With the advent of surveillance cameras and in many cases microphones there is now a need for advanced forensic audio analysis and experienced technician who can accurately analyze audio evidence. A young girl is abducted from a parking garage. In addition to her scream for help is the sound of voices. One male and one female, the presumed abductors are captured by a surveillance microphone. Outside a department store in a major mall two men plan the final stages for a robbery of the store. Their plans captured by security camera with microphone but the details buried under the background noise of shoppers hurrying by them.
Sound Analysis - The Beginning
Forensic audio analysis was originally developed during World War II when an instrument called the sound spectrograph was used to try and identify enemy voices during radio broadcasts. Unfortunately the sound spectrograph and similar early sound analyzing equipment were limited in that they only allowed the investigator to change the amplitude and frequency of voices or other sounds. The development of digital audio has given forensic investigators a whole new spectrum of ways in which to analyze sound recordings. A number of sophisticated software products are on the market which allows the forensic audio expert to take apart the various component of a recording and analyze them separately.
Enter The Digital Age
Background noise reduction, eliminating hiss and pops that can occur in a recording, enhancing a voice or voices to determine what is the nature of their conversation. Isolating certain background sound like a church bell ringing, a train passing by, school children playing at recess etc, all are now capable thanks to digital audio analysis. These background sounds can often be important piece of supporting evidence in identifying a location where a crime is committed. In kidnapping and ransom cases these can be especially important in identifying where the victim might be being held captive.
Background sound enhancements can also potentially identify engine noise, airplanes passing overhead to an airport and in some cases gunshot sounds in the distance. Gunshot sounds can be particularly important because by enhancing and breaking down the sound components it might be possible to identify that two guns are fired at essentially the same time resulting in two shooters and different gun types. A door slamming in the background and its echo pattern might identify a location as a large open warehouse, or a building with particularly high ceilings. All of this might be essential in determine where a recording was made.
In Ransom cases the perpetrators often disguise their voice during the ransom call. Most often they use a cheap sound modulator like those sold by local electronic stores. These devises are limited in their ability to disguise the voice. The audio forensic expert can adjust the amplitude and frequency of these recordings to reveal the true voice of the abductor. Periodic clicks or other electronic noise can often be used to identity the type of phone or device that the call was place on.
One problem with tape recordings whether of meetings or from surveillance devices is that they can be tampered with. Portions of the tape can be deleted, items can be added, and sometimes music or other sounds can be added to obscure underlying events. In the past when forensic investigators were limited only to sound spectrographic information these events would often go unnoticed. The digital age has changed all that. New digital analysis software can detect discontinuities in the sound recording; discontinuity events that are not noticed by the human ear. These may be extremely small gaps, bumps or dips in amplitude or frequency that can signal something has been changed on the original recording. Literally in seconds this software can scan a recording and flag these types of discontinuities or other unusual events.
However, identifying these events and determining whether they are tampering or simply glitches in the recording device itself can be a long and tedious process. The evidence does not usually jump out on the first try as it does on those popular CSI TV shows. It also takes a well trained examiner to utilize the audio analysis softwares to their full potential.
In this digital age with the internet and music storage and playing devices it has become possible for individuals to hide message in music files and then send them undetected to another party or group. The types of people that potentially might use this type of technique to cover their real activity are varied. Some examples are pedophiles, terrorist cells, criminals or black market traders. Pedophiles might use it to make contacts with children, other pedophiles, or to share pictures. Terrorist groups could use it as a way to send message while planning an attack. Drug dealers, white slavery and prostitution rings, corporate espionage and money laundering activities all have the potential for using this type of digital cover.
The forensic digital audio investigator has a number of software tools available to analyze these messages and look for hidden message in an internet transferred file. In the simplest case these message might simply be added to the end of the song. However, the more sophisticated individual would bury the message under a certain part of the recording. It might be in some type of cipher or character that is only known to the recipient or the message might even be recorded backwards so on the surface it does not appear to make sense.
Steganogrpahy, a type of audio analysis, is the study of writing in ciphers, or in the forensic world it translates into covered writing or hidden messages. It is also possible to hide data as well as message in these digital files. Corporate espionage or drug dealers can hide data like type, time, and size of a drug delivery. The analysis of these files and music files allow investigators to detect this hidden message and decipher their meaning. In many cases such analysis may allow law enforcement to intervene and stop an illegal activity or provide critical backup evidence to ensure a conviction when an actual arrest is made.
As more and more digital advance are made the criminal element will continue to utilize these techniques to their advantage. It is important the forensic laboratory continues to utilize and improve these software methods for uncovering the hidden messages in audio recording and files.