Crack and Bang: Butler’s Audio Investigation

On July 13, 2024, during an outdoor rally in Butler, Pennsylvania, gunshots rang out. In seconds the scene turned chaotic: panic in the crowd, people injured, one spectator killed, and the speaker was struck before the shooter was ultimately neutralized. It’s a heavy political and human event. But that’s not what matters here. What matters is what an investigation can extract from sound when images don’t settle everything, when eyewitness accounts conflict, and when rumors outrun facts.

In Robert C. Maher’s study, a low-key object becomes central: the lectern microphone. A mic built for speech, not ballistics. And yet it captured an acoustic signature that stressed listeners often blur together: a sharp crack, followed by a rounder bang. Two sounds, two speeds, and a lead for investigators.

the crack and bang duo: a physical signature

When a bullet travels faster than sound, it produces a shock wave, a kind of moving mini-boom that rides with the projectile. If that bullet passes near a microphone, the first thing recorded is often that crack: the shock wave slicing through the air. The bang comes later: the muzzle blast, the gun’s pressure wave traveling through the air at the speed of sound. In a recording, that sequence can seem like a detail. In audio forensics, it becomes a rule of the game.

measuring a fraction of a second to estimate distance

Between crack and bang there’s a tiny delay, but it’s measurable. By pinpointing it in the waveform, you can estimate the distance between the shooter and the microphone, combining two pieces of information: how sound propagates through air and the (supersonic) bullet speed. In the Butler case, analyzing those intervals on the early shots leads to an estimate of about 130 meters between the shooter’s position and the lectern microphone. Put differently: roughly a sports field, measured by ear… and by timing.

What stands out is the nature of the work. You’re not looking for a generic “gunshot sound.” You’re hunting transients, avoiding echoes, accepting the messiness of an outdoor venue with crowd noise, screams, audio compression, and possible clipping. It’s millisecond-level investigation in a world that looks nothing like a lab.

from the lectern mic to smartphones: piecing the puzzle together

Another hallmark of our era is the sheer number of recordings: smartphone videos from different angles, with tiny mics and aggressive automatic processing. Each one is flawed on its own. Together, they can be powerful. By comparing timing patterns across multiple sources, the analysis identifies ten audible gunshots: the first eight consistent with the same origin, then two more attributed to law enforcement gunfire from other positions.

Audio forensics doesn’t replace field investigation, but it can sharpen it quickly: helping sort what’s plausible from what’s imagined, stabilizing a timeline, and shrinking the space left for interpretation.

what sound can do, and what it can’t

Sound doesn’t tell you everything. It comes with uncertainty: atmospheric conditions, the projectile’s true speed, reflections from structures, file quality. But it has a rare strength: it leaves a measurable trace. And when that trace is cross-checked, synchronized, and handled transparently, it becomes a verification tool, almost an antidote to social noise.

What do you think: should audio forensics be as routine as video in public investigations whenever crowd recordings exist?