Forensic Audio Enhancement: What It Can and Cannot Do

Forensic audio enhancement is one of the most requested and most misunderstood services in digital evidence work. Attorneys and investigators often arrive with a body camera clip, a jail call, or a phone recording made in a noisy room and a reasonable hope that the right software will make every word clear. Sometimes it does. Often it helps a great deal. But the Scientific Working Group on Digital Evidence (SWGDE), whose best practices guide much of the forensic audio field, states plainly in its guidance on forensic audio enhancement that the goal of increasing intelligibility or signal quality "may not be achievable" for some recordings. Knowing where that line falls before a case depends on it saves time, money, and credibility.

What Forensic Audio Enhancement Actually Is

SWGDE defines audio enhancement as the processing and filtering of audio recordings to improve the signal quality and intelligibility of the signals of interest, such as speech, by attenuating noise or otherwise increasing the signal-to-noise ratio. In its Best Practices for Enhancement of Digital Audio (document 20-A-001, version 2.0, approved in 2025), SWGDE separates three ideas that people tend to lump together:

• Intelligibility. How much of what a speaker said a listener can readily understand.
• Listenability. Whether the audio can be listened to without discomfort, which matters for long recordings a jury or reviewer has to sit through.
• Signal quality. How accurately the recording represents the acoustic events that actually happened, often described as a signal-to-noise ratio (SNR).

The distinction matters because forensic audio enhancement can make a recording far easier to listen to without making a single disputed word any clearer. A good examiner is explicit about which of those goals a given enhancement achieved.

Just as important is what forensic audio enhancement is not. It reduces or removes what is masking the voice; it does not add information the microphone never captured. Every legitimate technique works by attenuating noise, correcting distortion, or rebalancing what is already in the recording.

What Forensic Audio Enhancement Can Do

SWGDE's guidance includes a table of common problems and the mitigation strategies examiners use for each. Many of the problems that make a recording hard to follow respond well to careful processing:

• Steady background noise. Air conditioning, engine noise, and the hiss of a cheap recorder are broadband noise that dedicated or adaptive noise filters can attenuate substantially.
• Hum and constant tones. Electrical hum and other stable tones can be targeted with notch and comb filters, often with dramatic improvement.
• Talkers at very different levels. A phone call where one party is much louder than the other can be leveled with dynamics processing so both sides are audible at a normal listening volume.
• Muffled or low-level audio. A microphone buried in a pocket or a recorder set too low can be improved with gain correction and equalization.
• Clicks, pops, and cell phone interference. Short impulses and the repeating buzz from a nearby transmitter can be reduced with interpolation and specialized tools.
• Wind, rustle, and mixed sources. SWGDE's 2025 revision specifically added deep learning filters (trained neural networks) as one of the recognized strategies for wind noise, clothing rustle, broadband noise, and separating overlapping sound sources.

SWGDE also describes a general order of operations drawn from published research: address distortions first, then separate sources, then attenuate continuous noise, then varying noise, then reverberation, then balance the voice, and finally correct gain. The order is described as neither exhaustive nor compulsory, but it reflects a core point about forensic audio enhancement: each step can create new problems for the next one if it is applied carelessly.

What Forensic Audio Enhancement Cannot Do

The limits are where expectations and reality most often collide. Based on SWGDE's guidance and the physics of recorded sound, forensic audio enhancement cannot:

1. Recover audio that was never recorded. SWGDE lists drop-outs, meaning missing samples or stretches of silence where the recording system lost the signal, with a mitigation strategy of "None, but document its location and duration." A gap is a gap.
2. Separate a voice from a sound that completely covers it. When a siren, a slammed door, or another talker fully masks the target voice in both time and frequency, there may be nothing underneath to recover.
3. Undo heavy processing that happened before the file arrived. SWGDE notes that processing applied to the audio before it reaches the examiner may limit the effectiveness of enhancement. Aggressive compression, a phone app's built-in noise suppression, or a screen recording of a playback all discard information that later work cannot restore.
4. Guarantee a particular word. SWGDE observes that perceived intelligibility is subjective and can be evaluated differently by different listeners. An enhancement can make a passage clearer; it cannot settle by itself what a disputed word was.
5. Work without an original or best-available copy. The quality of the enhanced audio depends on the quality of the submitted recording, and SWGDE directs examiners to ask for an earlier generation recording whenever one may exist.

That last point is often the most practical one for attorneys. Before asking for forensic audio enhancement, ask where the recording came from and whether a closer-to-original copy exists: the native file from the device or recording system is almost always a better starting point than a copy that was emailed, texted, or captured by holding a second phone up to a speaker.

AI Tools and the Risk of Adding What Was Not There

Machine learning has a legitimate place in forensic audio enhancement, and SWGDE's 2025 revision acknowledges trained neural network filters for several noise problems. The same guidance adds a caution that applies to every tool: software may implement proprietary or unpublished algorithms, and their effectiveness should be tested before use.

The risk with some consumer AI tools is different in kind. A tool that reduces noise is removing something. A tool that regenerates or "restores" speech may produce clean-sounding words that the original recording does not actually support. Courts have already confronted that problem on the video side. In State v. Puloka, a Washington state superior court in 2024 excluded AI-enhanced video after testimony that the enhancement "added information that was not in the original files," finding that the method relied on opaque processing that was not peer reviewed or reproducible by the forensic video community. Audio produced by a generative tool invites the same challenge. Forensic audio enhancement that is going to be offered in court should rely on methods an examiner can explain, test, and reproduce.

How Courts Evaluate Enhanced Audio

An enhanced recording still has to be authenticated like any other exhibit. Federal Rule of Evidence 901(a) requires evidence "sufficient to support a finding that the item is what the proponent claims it is," and Rule 901(b)(9) recognizes authentication through "evidence describing a process or system and showing that it produces an accurate result." For forensic audio enhancement, the process is the evidence of accuracy, which is why documentation carries so much weight. SWGDE's guidance lines up with that expectation:

• Repeatable results. SWGDE states that enhancement results must be repeatable and reproducible to the extent that they are perceptually consistent.
• Detailed records. Examiners should document the processes, settings, software and versions, and time segments processed in enough detail to repeat or reproduce the final result; project files, history logs, and screenshots can serve as that record.
• Comparison to the original. The final review compares the enhanced result to the unprocessed original, and SWGDE suggests delivering a copy of the submitted audio alongside the enhanced version.
• Checking what was removed. Examiners are directed to review the filter residue (the part of the signal being removed) so that pieces of the target voice are not stripped out along with the noise, and to avoid over-processing that introduces new artifacts.

Handled this way, forensic audio enhancement produces an exhibit that can be defended on cross-examination: the original is preserved, the steps are written down, and another qualified examiner could follow them and hear the same result. Our post on when to call a forensic audio and video expert covers the point in a case where bringing in an examiner makes sense.

Getting the Most Out of a Forensic Audio Enhancement Request

Attorneys, investigators, and agencies can improve the odds of a useful result before an examiner ever opens the file:

1. Send the original or native file. Export directly from the recording device, body camera system, or call platform whenever possible, and avoid re-recording, converting, or compressing it first.
2. Do not try to fix it yourself first. Free noise reduction apps can permanently discard information and leave nothing for a later examiner to work with. If someone already processed it, say so and send the unprocessed version too.
3. Identify the region of interest. Give timestamps for the passages that matter. SWGDE's workflow begins by locating the region of interest, and different segments of one recording often need different processing.
4. Describe the recording conditions. Where the device was, what it was, and what else was happening in the room can help an examiner choose the right approach, though SWGDE also cautions examiners to guard against bias from case information they receive.
5. Ask for the right deliverable. Sometimes multiple enhanced versions are more useful than one, for example when two overlapping voices each need different processing. Plan to play the result in court over proper equipment, such as over-ear headphones, rather than laptop speakers.
6. Keep enhancement and transcription separate. An enhanced recording can support a transcript, but the transcript is an interpretation. The enhanced audio and the original should both be available to anyone checking it.

For a sense of what typical cleanup work involves in practice, see our overview of audio cleanup services. Agencies preparing recordings for public release should also review our guide to 911 call audio redaction, since enhancement and redaction often happen to the same files.

Conclusion

Forensic audio enhancement can turn a frustrating recording into usable evidence by reducing noise, correcting distortion, and balancing voices so the words that were captured can be heard. It cannot restore sound that was never recorded, reach a voice that was completely masked, or reverse damage done before the file arrived, and no ethical examiner will promise a specific word. The recordings that get the best results start as original files, go to an examiner early, and come back with a documented, repeatable process that holds up when opposing counsel asks how the exhibit was made.

Focal Forensics performs forensic audio enhancement following SWGDE best practices, with full documentation of every processing step and the original recording preserved alongside the result. Learn more about our forensic audio services or reach out to discuss a recording before you decide whether enhancement is worth pursuing.

Phone: 303-900-3585 · Email: info@focalforensics.com

Sources

• Scientific Working Group on Digital Evidence: Best Practices for Enhancement of Digital Audio, SWGDE 20-A-001-2.0 (approved June 27, 2025)
• SWGDE: Best Practices for Enhancement of Digital Audio, document listing
• Cornell Law School Legal Information Institute: Federal Rule of Evidence 901, Authenticating or Identifying Evidence
• Greenberg Traurig: Washington Court Rejects Novel Use of AI-Enhanced Video in Trial (State v. Puloka, 2024)