Forensics Audio – Intelligibility and Stealth Recording

Photo: Spy Gadgets

Audio Enhancement and Intelligibility

Last time I talked about the importance of speaker recognition and voice ID in the field of forensics audio. Another key job for a forensics audio expert is to improve the intelligibility of voices, which entails audio enhancement of the raw recording. The goal of audio enhancement is not necessarily to make a pretty and polished sounding audio file as is the case in most entertainment mediums, but to make the words that are in the recording intelligible and useful for a lawyer to use as evidence in court. Audio enhancement of seemingly incomprehensible dialog can make or break a case. For example, listen to this – compare the first raw recording with what it sounds like afterwards with the audio file enhanced. This is pretty much a recorded confession to a murder. If the lawyer didn’t have access to a forensics audio expert who could enhance the audio, the conversation here would be a lot less intelligible and a lot less convincing when presenting their case to a jury. Notice how it is common practice to provide audio transcripts with the recording. Forensics audio experts do their best to make each word in the dialog intelligible, but if there are certain words that aren’t comprehensible, the protocol is to write the word “[inaudible]” where the incomprehensible word is in the transcript.  The extent of the audio enhancement needed depends on the state of the recording. Often times, the material is recorded with stealth recorders in prison cells or in transport where there is a lot of noise and unwanted sounds that make discerning words and speech difficult.

Screen Shot 2016-09-10 at 4.32.20 PM.pngPhoto:

Forensics audio expert Kent Gibson talks about an experiment done in a Pasadena holding cell, where they introduced a low frequency air conditioner rumble-like noise to lure the suspects into thinking that it was noisy and that they could speak freely. What the suspects didn’t know is that software made by  Cedar Forensics, which ranges in price from $3-$60k depending on how fancy of a configuration you want, can easily isolate the background noise from the speech, making what they’re saying completely intelligible.

Audio enhancement and Noise Reduction software

Cedar made DNS (Dynamic Noise Suppression) which is commonly used also on dubbing stages and is capable of reducing noise across the whole spectrum, including noise from stealth recordings with poor mic placements. Cedar also made Cambridge, which has lots of adaptive filters and dialogue noise suppressors.  Adaptive filters are an interesting and important feature for forensics audio because the software can sense background noise and then automatically and dynamically attenuate the noise over time. Another high end tool used by experts is Cardinal, made by DAC, which is around $25k; it also provides adaptive and noise canceling filters as well as other noise reduction and EQ tools. Something that’s a little more cost friendly is Izotope Rx, a common tool amongst sound designers, which retails for $350 and Rx Advanced is $1200.

Stealth recordings’ Admissibility

It is important to note that recordings made in stealth are not always legal or admissible in court. The federal government law states that creating an audio recording only requires one person’s consent, but some states require everyone who is in the recording to give their consent.  In the state of Washington, you need consent by all parties in the recording as stated here in order for it to be admissible in court. However, things are not always clear cut when it comes to what is legal or illegal for stealth recordings, especially in the area of audio surveillance. An interesting case that happened recently in July 2016 in regards to the FBI placing audio surveillance mics without a warrant around the Courthouse steps in California counties led to an interesting ruling by Judge Hamilton.

Renda writes,

the FBI placed recording devices in a light fixture along the steps of Alameda County Courthouse and just outside the Contra Costa County Courthouse, among other locations in proximity to both courthouses. The government then used the conversations they gathered from the devices during the grand jury proceedings. The defense argued that the recordings were gathered in violation of the Fourth Amendment provision that guards against illegal search and seizure, and asked for the recordings and other evidence tainted by the recordings to be suppressed. In her ruling, Hamilton agreed with prosecutors who argued that the men “did not have a reasonable expectation of privacy in their public oral communications outside the county courthouses.”

This is an important case that may very well set a precedence for audio surveillances procedures and protocols in the future.

Stealth Recorders – Investigator’s Perspective

Photos:, Maigret Sets a A Trap

So what tools are commonly used by those in this field? In these two photos, the microphone placements and recording environments are very different. In one you have a professionally treated recording room with a high end condenser mic primed for recording and in the other, a small hand-held stealth recorder, recording everything from behind the rustles of the detective’s pocket. One of the things I find the most fascinating about stealth recording is that you can’t control the recording environment that you’re in. Anything and everything can happen. So if the quality of the recording dictates how much enhancement is needed to be done to the audio file, then what type of recorders are good for stealth recording? In general, the human voice’s fundamental frequencies are between 100-500 Hz with some overtones and harmonic content in 1-4kHz, according to Audio Forensics expert Ed Primeau. So a recorder that can record up to just 8kHz can be good enough.

It is interesting looking at stealth recorders from the perspective of a law enforcement agent or investigator versus from that of an audio professional. As audio professionals, we care a lot about the quality of the recording, but the detective or investigator often focuses more on the practicality and stealth potential of the recording tool, as their life or someone else’s life may very well be dependent on the ability to capture this information in a conspicuous manner. You can always get an audio forensics expert to clean up and enhance the audio later anyway!  Here are some interesting audio surveillance recording tools that are used in the field:

Stealth Tek 2010
: up to 365 hours of recording time. It comes with a universal ear bud mic that allows you to hook up to your cell phone or home telephone line, automatic telephone recording, or cell phone recording.

MicroDot Audio TinyTek 2010
– just slightly bigger than a quarter, it’s the world’s smallest digital audio recorder and was manufactured under a microscope. Features include voice activation, time and date stamping, programmable timer and will record up to 37 hours.
And of course, you can’t go undercover without your James Bond stealth pen or key chain recorders! 😛



Stealth Recorders – Audio Professional Perspective: Binaural Recording

So what would be recommended for stealth recordings from an audio perspective?  We can learn a lot from movies. For instance, when recording the voice of an actor on set for a film, what is one of the considerations? We don’t want the audience to see the mic on the actor because that would take away or distract us from the magic and “realism” of the story that they’re trying to tell. Lavalier mics are often taped underneath the actor’s shirt in order to capture their speech in a conspicuous way; those of you who have worked in post production audio also know how important audio enhancement or noise reduction is when it comes to using material from a lav.

Photo: Wikipedia,
Binaural recording is a similar idea to lavs, but you use two little mics and place them just above where your ears are, which is cool because you’re basically recording what your ears are hearing. Binaural recordings are best experienced through headphones because you really get that immersive sound, making it feel like you’re in that actual environment. Pearl Jam actually released an album called “Binaural” that uses binaural recording. A higher quality lavalier that makes for great binaural recordings is Countryman B6 Omnidirectional lavs. They have a 20Hz to 20kHz frequency response, come in different skin tones, and can be hidden in your hair, behind a button, in the side of your glasses, or any other location you can think of!

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Photo: B6 lavalier datasheet.pdf

Hopefully this overview of the importance of intelligibility in forensics audio and the ability to improve it with audio enhancement as well as stealth recording equipment provides a better understanding into the interesting and rewarding challenges in this field. Stay tuned as we dive into our last Forensics Audio topic next week!



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