Equalization settings can be adjusted to help the forensic expert recover lost or poorly recorded audio. This blog post covers basic audio clarification.
The sound spectrum is measured in frequencies. All audio sounds like a car horn, gunshot and spoken words have frequencies associated with them. Music has the broadest frequency range and covers 80HZ to 12,000 kHz in most cases. In an attempt to keep this blog post easy to understand, I am going to focus on spoken word audio.
Forensic spoken word recordings range in frequencies from 1Khz to 3Khz. Most sound spectrum frequencies above 4Khz and below 800 Hz do not contribute to the spectrum of spoken word recordings. The forensic expert should remove frequencies above and below 1Khz to 3Khz when background noise is present to increase the clarity of spoken word recordings.
Beautiful spoken word fidelity can include frequencies from 400 Hz to 5,000 kHz when recorded in an audio recording studio using state-of-the-art digital audio recording instruments like mixing council, preamplifiers, microphones and digital computer recording software. However, when it comes to forensic audio, most recordings that need authentication, clarification and identification have been made on digital pocket recorders and wireless interception devices.
These recordings are subject to extreme background noise because they are recorded outside the professional studio environment, next to noisy furnaces and air conditioners and in public environments like restaurants and other places where unwanted chatter and background noise can spoil a recording. Equalization helps the forensic expert begin the clarification and restoration process and is a very important element of restoring the audio recording.
The best way to determine what frequencies need to be adjusted is to first listen to the audio recording. The trained critical listening skills of the audio expert can identify both wanted and unwanted frequencies during the initial listening pass.
Next, the expert will remove frequencies where unwanted sounds exist. Background hums in room ambience, air conditioners and furnace blower motors exist in low range frequency spectrums like 250 Hz all the way down to as low as 60Hz.
Radio frequency interference is probably the most difficult distortion to remove from the audio recording. When radio frequencies have static, it is best to first try noise reduction before setting equalization’s settings to clarify the spoken word recordings. The best methods are those that are tried and tried again until the desired outcome is achieved.
It is best for the expert to save the original audio file before applying any equalization treatments just in case it becomes necessary to go back and start over. Also, every step of equalization should be noted in the expert’s work notes so they can recreate the process for the court should it become necessary.