Explaining Forensic Audio - Part 1

The concept of forensic audio investigation may have become popular in recent years along with the forensic sciences in general but it’s been in practice since World War II.  With audio use in full swing for radio transmissions across long distances, scientists were trying to identify the voices of their enemies among the many radio broadcasts that took place on open frequencies.  The work done in forensic audio investigation today is based on the pioneer work of those scientists.

Specifically, forensic audio has to do with any type of audio of an evidentiary nature.  In modern cases, law enforcement or other legal professionals (e.g., defense attorneys and prosecutors) will turn to a forensic audio examiner to perform any one of a number of services where audio is concerned.

In many cases these specialists are hired on a contract basis.

While the most common Hollywood portrayal is audio enhancement with a few twists of a knob (if only it were so easy) there is a great deal of work that can be done:

• Audio Enhancement - the most common and offered by just about every forensic audio company you can find.
• Audio Authentication - geared toward tape and digital formats
• Forensic Transcription
• Speaker Identification

There are also a number of other services or procedures that can be classified in their own right as a specialty process or “miscellaneous” in terms of categorization.  This article will focus specifically on Audio Enhancement and Authentication.

Audio enhancement is the most common and well known service where forensic audio is concerned. 

It’s not likely that one can take a garbled and mangled inaudible conversation and “tweak it” to produce clear speech that is intelligible. While audio enhancement isn’t specifically focused on speech, that is usually the intent behind cleaning up or enhancing audio for legal purposes.  It’s a means of reducing or filtering out unwanted noise from a poor recording in order to clear up the speech that’s covered with noise or is a victim of poor recording methods.

A forensic examiner is not a miracle worker however, and while modern software and equipment has provided a variety of tools to improve the quality of speech in a recording, there are still limitations.  When it comes to enhancing speech, you can’t fix something that’s just not there.  If the recording doesn’t contain the robust elements of someone’s speech, and the equipment only picked up bits of a word or phrase, there is no magic software to fill in the blanks. Enhancement techniques can sometimes have fantastic results with intelligibility, but more often would be a disappointment to the layperson.

In terms of speech enhancement, an examiner can offer critical listening in combination with forensic transcription and speech decoding methods to help identify and discern what is being said.  Again, technology can only do so much so an examiner with linguistics and phonetics experience is usually best for the job. However, if the speech waveform isn’t picked up by the recording device or is masked by noises of the same frequency range, he won’t be able to decipher the speech.

Forensic audio enhancement sometimes simply involves increasing the volume of a whisper from a suspect where traditional playback - even at high volume - can’t help an individual understand the utterance. Audio enhancement offers the most benefit in situations where noise can be eliminated or at least reduced so as not to distract the listener from the speech. Enhancement techniques are quite good at getting rid of electronic buzzing or hum and other noises such as tape hiss, the crackles and pops of a phonograph record or the beeping of a backing truck, open car door, low battery warning of a fire detector, etc. Enhancement used in this way is often called improving the listenability of the recording. Unintelligible speech isn’t made intelligible but the recording is easier on the ears.

Authenticity is an important part of legal matters where evidence is concerned.  This is especially true when it comes to audio that is or will be entered into evidence in a criminal or civil matter.  As technology advances, it becomes progressively easier for an individual to tamper with a recording.

Audio authentication - either involving tape authentication or digital file authentication - is a way to ensure that the audio being utilized as evidence has not been tampered with in some way.

Not only will a forensic audio examiner use software to examine the actual recording, but authentication involves examining the physical tape itself and its casing.

This includes:

• Checking the tape for splices
• Examining the tape’s plastic shell to check for prying or disassembly
• A process of examining the tape call magnetic development

Magnetic development involves using a magnetic liquid called ferrofluid in the examination of audio or video tape. The ferrofluid allows the forensic examiner to see the magnetic patterns on a tape. The types of things the audio engineer would look for include magnetic signatures left by stopping, pausing and starting the recording process. It can also be possible to tell if the tape has been recorded over or simply erased and left blank.

Newer software provides an edge when it comes to detecting authenticity in audio as it looks into variances that could lead to falsification of a recording - particular in tape recordings:

• Equipment noise - hums, pops, varying pitch in inconsistent forms over the whole of the recording
• Fading - Any gradual or sudden decline in volume that effects the interpretation of noise or dialogue.  If the sound cuts out completely that becomes a gap in the recording
• Gaps - Any segment where there is a change (often unexplained) in the content or context of an audio recording.  A perfect example of this is the infamous Watergate recording where there is an 18 and ½ minute gap in the recorded audio that was later discovered to be an overlaid recording of electrical interference.
• Transients - These are clicks or pops and other “attack” sounds within a recording and may signify that there has been a splice or some other alteration to the recording

There are a number of methods that are used to authenticate a recording, checking it specifically for originality. An obvious and important tool that a forensic audio examiner has is the ear.  Critical listening affords a great deal of benefit and it takes training and experience to know what to listen for while playing a track over and over.

In some cases, an examiner may have to run through a section of audio many dozens of times. They examine bits of a conversation or sections of a recording to fully understand the sounds - including both foreground and background sounds.  If something stands out the examiner will further investigate that specific segment using their trained ear to discern if the recorded event is authentic or not.

Beyond listening, the examiner utilizes physical inspection as well as spectrum and waveform analysis to visually inspect the quality and construct of a specific piece of audio.  All of these methods come together allowing the examiner to certify the authenticity of a tape recording.

Things become more difficult when a recording is made using a digital recorder that creates audio files such as .wav or .mp3. While many of the above methods may be useful with a recording that originated as a digital file, it is somewhat easy for a knowledgeable person to edit the file without leaving any known sign of doing so.

One promising method of digital authentication is the use of ENF (electric network frequency) data. Whenever someone is recording while plugged into an electrical outlet or using a battery operated device within an ENF magnetic field, their recording will include a 50Hz or 60Hz (depending on country) waveform signature. This signature can be compared to an ENF database to possibly determine where and when the recording was made, whether it’s an original or a copy, or whether it was edited or altered in anyway. Digital audio authenticity examination, and more specifically the use of ENF, is in its infancy and has a way to go before forensic audio examiners reach the level of sophistication that has been achieved with authentication of analog tape recordings.

The use of software to determine authenticity and enhance audio is just the beginning when it comes to forensic audio.  Depending on the situation, additional specialty services or skills may need to be applied such as speaker identification and the analysis of gunshot sounds to determine if more than one weapon was used, which type of firearm was used, or the sequence of the shots fired.  Forensic audio is a robust trade that encompasses a number of specialists who work together to bring clarity through the study and examination of audio.