Whenever a bridge collapses, sensor engineers propose the use of the latest and various technologies to prevent future catastrophes and the loss of human life. For example, microelectromechanical systems (MEMS) technology was proposed for bridge monitoring after the 2007 collapse of the I-35W bridge over the Mississippi River in Minneapolis. With more than 617,000 bridges across the United State, 42% of all bridges are at least 50 years old. Of these older bridges 46,154 bridges or 7.5% are considered structurally deficient (in poor condition).
One approach proposed recently by researchers for structural health monitoring (SHM) involves the use of acoustic sensing methods to provide a non-destructive evaluation (NDE) solution. The acoustic sensors would detect internal cracks and determine current material damage properties through a change in the properties of sound waves as they pass through a structure to provide external acoustic monitoring of internal cracks. Acoustic emission methods have previously been used to determine crack modes and locations. In fact, piezoelectric devices implanted in various structural components have been used to both produce and monitor acoustic emissions. When used in this manner, they provide an integrated NDE.
Over a decade ago researchers demonstrated that a device called the SpeechJammer could be used to disrupt people’s speech. In the protype, the speech of the human target is received and then sent back to them with a delay of a few hundred milliseconds. While involving no physical discomfort, it immediately causes the target to stop speaking and does not involve anyone but the speaker. This phenomenon (called Delayed Auditory Feedback (DAF)) was implemented in two prototype versions by combining a direction-sensitive microphone and a direction-sensitive speaker. It proved effective in disturbing the speech of a specific person. With the increasing number of inconsiderate speakers, this could provide an interesting solution.
During World War II, the Nazis experimented with using sound as a weapon in what was called a sound cannon. Their researchers found that human targets (prisoners) exposed to low frequency sound (infrasound) panicked, felt dizziness, and experienced pain in their internal organs. After the war, American solders recovered a prototype of this acoustic weapon. When American researchers evaluated it, they concluded, “sample acoustic cannon emits a loud sound that makes people closer than 50 meters from the source to lose consciousness, and at a closer distance could be fatal.” However, it was never produced in Germany or the U.S.
More recently, highly directional and amplified sound waves are produced by sonic cannons or long wave acoustic device (LRAD) systems. The LRAD is optimized to clearly deliver recorded or live voice messages over crowd, engine, and background noises. One high-end version boasts broadcasts will be clearly heard and understood over distances up to 5,500 meters (18,044 feet) in any type of terrain and environment. Even the base model provides sound levels 20 to 30 decibels louder than typical bullhorns and vehicle-based P.A. systems.
Since the LRAD acts as an amplifier, projecting a human voice or recording across thousands of meters, it also has a second mode to emit a “deterrent tone” for police to use in riot control situations. By concentrating sound waves into a narrow cone extending about 15° in every direction from the axis, a large amount of kinetic energy is packed into a tight space to disrupt and disperse the crowd.
Overview of Bridges, https://infrastructurereportcard.org/cat-item/bridges-infrastructure/
Nazi Secret Weapons: Schallkanone -The Sound Gun, https://pictureshistory.blogspotcom/2009/10/nazi-secret-weapons-schallkanone.html?m=1