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Posts Tagged ‘Noise Reduction’

Forensic Audio Enhancement: Removing a Single Sound

Monday, August 24th, 2015

sound board 1416985 - Forensic Audio Enhancement: Removing a Single SoundOne of the most common audio issues that I address during an enhancement is noise and other extraneous ‘unwanted’ sounds. The noise floor is usually consistent throughout the recording and can be removed to varying degrees by using noise reduction software. The most complicated issues are the extraneous sounds that are not continuous. These sounds could include anything from a plane flying overhead to someone whistling while people talk. These sounds are difficult to pinpoint with standard tools like noise reduction and equalization, but they can be identified using a spectrogram.

A spectrogram shows both the frequency content of a recording and the level of those frequencies over time. It may be the most helpful tool to an Audio Forensic Expert because it visually presents everything that is happening throughout the audio in one window. Using this, the expert can both identify and address individual harmful noises in the recording. With the right software, these individual sounds can be selected and removed without affecting any other part of the recording. It is important to remember that there is a right and a wrong way to do this, which is why only a
trained Audio Forensic Expert should be hired to complete an enhancement for use in court.

When processing audio, it can be easy to introduce artifacts to the recording. Artifacts are unwanted noise that is produced from various processing and compression techniques. Considering the goal of an audio enhancement is to eliminate extraneous noise, introducing artifacts is the exact opposite of what you want when working with a recording. Many things can introduce artifacts, but the simplest way to describe the cause is over processing. By over processing, I mean using extreme settings within individual audio tools.

For example, I often work with audio evidence that is extremely quiet. This often requires a gain increase of portions where only voice content exists. If the gain is increased too much, it can cause clipping of the audio output. When this occurs, the edges of the waveform are essentially clipped off, producing a distorted and noisy audio signal. The end result is a less intelligible voice than the original, essentially defeating the purpose of the whole process.

When adjusting individual ranges of frequencies on the spectrogram, it is very important to be aware of artifacts. Being able to recognize artifacts and know the limitations of what processing can be done is what makes an Audio Forensic Expert necessary. When isolated portions are processed with a trained ear and the right knowledge, noise can be eliminated and voices can be brought out without introducing any artifacts.

I recently worked on an audio recording that had a siren present during a portion of talking. Because it was so loud, it made the underlying dialogue difficult to hear. Luckily, the siren could be isolated in the recording. By selecting only the siren and then decreasing the gain a moderate amount, the voices became more audible while still avoiding any artifacts.

Audio Forensic Experts have a plethora of tools at their disposal, which is making audio enhancements more and more effective. There are some things to be cautious of when enhancing audio, but any technique that helps should be used as long as the science is sound.

 

Audio Clarification 101: The Science of What the Ear Perceives

Thursday, April 26th, 2012

15935870235 bc57227892 b - Audio Clarification 101: The Science of What the Ear PerceivesEver since I began my career as an audio engineer back in the 70’s, noise and sound quality have always been a focus mine as an audio expert. I remember when my interest first started back in the day when records—those twelve inch round black things—would play at parties and the background  noise was never a point of focus. But today as vinyl is making a comeback, audio philes all over the world are starting to collect vinyl again. Last week, in fact, The Who released a 10 inch vinyl record to celebrate National Record Store day, but I digress…

There are several things that I do with almost every audio recording that needs to be clarified. Step one is to listen to a good portion of–if not the entire–audio recording and make observations about the noises that I feel can be removed. The second step is to determine what tools I have available that I can use to remove those unwanted sounds.  Lastly, I begin the process of noise reduction in the steps of audio clarification.
Almost every software program that’s available has a noise reduction program in it. Once you learn how to use that noise reduction program, you first select the audio file you are working with and begin by removing unwanted noise. You’re only going to be able to remove one type of that unwanted noise, so my advice is to try different settings within the noise reduction program. Then, the next step is to use equalization to first remove additional unwanted sounds and then use a second equalization process to increase the desired or wanted sounds. When that’s complete, the last step of the restoration and clarification process is compression. Compression basically helps to make all of the remaining sound even so that if what you’ve done so far has, for instance, decreased the volume of the digital file to such an extent that the volume is very low, the output of the compressor plug in will help you raise that volume to a more desirable level.

Also, if it is an audio recording that you are trying to hear a conversation of, the playback of what you have done with all of these tools is almost as equally important as the entire clarification process is. I have found that listening to a very hard to hear audio recording on different devices helps my ability to create a transcript or to hear all of the sounds more clearly. For example, if I am restoring an audio that has bad or hard to hear conversation on it even after I have finished the restoration process, I will then burn that file onto a CD and listen to it, for instance, on a boom box or on a clock radio. Because both devices will play that recording back differently, my physical sound perception will pick up those sounds differently and aid in the transcription process.

This whole process is all about the science of what the ear perceives. Because the perception of sound is different from person to person, it is important to listen back to the hard to hear conversations, music in the background, any other sound that you’re trying to identify (gunshots, screams, etc.) on more than one playback device. The main reason is because headphones and the various types of playback devices that have external speakers will reproduce that sound differently based on each device that you’re listening to the sound on.

So in conclusion, follow the steps that I’ve outlined above as you are using your software program and do the best you can with the tools that you have to work with when restoring your audio file. It is also almost as equally as important to listen to it played back on multiple devices, which will help your clarification process out.

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Noise Reduction for Sound Clarification

Friday, October 8th, 2010

2372161745 a1e69fcea4 b - Noise Reduction for Sound ClarificationThe first step when restoring audio is to remove background noise. More often than not, a recording in need of sound clarification or restoration has background noise covering the sounds that are desired to be heard. Noise reduction is the process of reducing and often eliminating that unwanted background sound. Sound like wind, motors, lawnmowers, electronic hums and buzzes and other sounds that may be louder than the spoken word and cover that speech so it is not audible.

Once the background noise is removed the desired spoken words can be heard. There are many levels of clarification acceptance. Some of the time the recording needs to be clarified so a jury can hear it. Other times the recording may only need to be audible to a transcriptionist or court reporter. Then, once the transcription is created, the audio forensic expert can go back, listen to the clarified recording comparing to the transcript and correcting any discrepancies using the expert’s critical listening skills.

The recording can then be certified by the forensic expert and an affidavit created as to the genuineness of the transcript for the legal proceeding. That way, if the audio recorded evidence is difficult to hear in a court room, the audio expert not only certifies the audio recorded evidence but can also testify on its accuracy.

This is a good example of a strategy between lawyers and expert witnesses. It is the expert witness’s job to suggest strategies to lawyers, public defenders, police and other government agencies that retain the expert witness. The lawyer will have difficulty and not represent the client to the best of their ability if they do not understand the audio forensic process and retain an audio forensic expert. The audio forensic expert of course cannot develop a strategy without a lawyer who is licensed to present the law case in the court room.

Before beginning your case that includes audio evidence, consult with a qualified audio forensic expert.

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