The similarity of shock waves with water waves-here caused by a family of ducks fleeing an intruder (b)-is no coincidence. The first photograph of oblique shock waves from a supersonic bullet was taken by Ernst Mach and Peter Salcher in the 1880s (a). Although the technology has changed significantly, particularly for capturing large fields of view, that name for this method persists today.įigure 2. Toepler named his optical method the Schlieren method (Schlieren means 'streaks' in German). He saw spherical waves in the air from loud spark discharges and thought he was observing sound, but actually he was the first to see shock waves. But Hooke was ahead of his time, so this observation principle lay unused until the mid-19th century, when the German scientist August Toepler rediscovered it and used it to observe electric sparks. In this way an antique glass windowpane warps one's view of the world outside. Hooke also discovered another visible trait of transparent phenomena: They can distort the features of a background pattern that is viewed through them. What Hooke described is now called the 'shadowgraph' method, and it's a simple approach that works extremely well for visualizing shock waves. Above the flame he saw a plume of hot air that was not directly visible but cast a shadow because the heat changes the density of the air, which refracts light rays. Robert Hooke discovered this effect more than three centuries ago while observing the shadow of a burning candle cast by sunlight. Certain aquatic predators find their transparent prey by the shadows that the Sun casts on the ocean floor. For shock waves these signs can include moisture condensation, dust disturbance, whitecaps on water, optical distortion and shadows. This allows us to capture the development and progress of these wave fronts on a scale that has not been possible in the past.Įven a transparent phenomenon sometimes leaves telltale signs. Now, however, our research group has taken modern high-speed videography equipment and combined it with some classical visualization methods to image shock waves from explosions and gunshots in more realistic environments. Usually they can only be seen clearly by special instruments under controlled conditions in the laboratory. Like sound waves, shock waves are as transparent as the air through which they travel. Their almost-total invisibility has given shock waves a mystique that has been exploited by Hollywood in countless scenes where explosions send heroes diving for cover. These waves can be powerfully damaging in their own right, but in addition, studying them can help to quantify their originating explosions and can provide insight into how buildings and airplanes can be hardened to resist damage resulting from such blasts. Recent attacks by terrorists using improvised explosive devices have reinforced the importance of understanding blasts, explosions and the resulting shock waves. Historically, the study of shock waves has taught us much about the properties of gases and material responses to a sudden energy input, and has contributed to the development of gas lasers and the field of plasma dynamics. The first three audible shots are “beanbag” rounds from a law enforcement shotgun, followed by seven shots from a law enforcement pistol.Shock waves play important roles in modern physics and engineering, military operations, materials processing and medicine. (Courtesy Robert Maher) A waveform of an audio recording captured by a surveillance system. More Photos Gunshot acoustical testing at a closed firing range for the National Institute of Justice.

Here & Now's Robin Young talks with Montana State University professor Robert Maher about why the sound of a gunshot has become an important forensics tool. Analysts can examine sound waves and determine the direction of gunshots, what type of gun they came from and a time frame of the shooting, which can be used as evidence in the courtroom.

Brown/AFP/Getty Images) This article is more than 4 years old.Ĭellphone recordings and police body cameras have opened up a new field of forensic science. Pictured: A gun enthusiast fires a pistol at the LAX Firing Range in Inglewood, Calif. The sound of a gunshot has become an important forensics tool.

How to tell the sound of a gunshot