2017-10-31
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Electronic Engineering
possible before reflecting it away. Their special shape reflects energy into the apex of the wedge, dissipating it as vibrations in the material rather than the air. Anechoic chambers are frequently used for testing microphones, measuring the precise acoustic properties of various instruments, determining exactly how much energy is transferred in electro-acoustic devices, and performing delicate psychoacoustic experiments.
The world's first wedge-based anechoic chamber was built in 1940 at Bell Labs in New Jersey. It is encased in more than a meter of concrete to shield it from external noise. Its creators have boasted that the chamber absorbs over 99.995% of the incident acoustic energy above 200 Hz. The wedge-shaped panels are poor at absorbing lower frequencies, but these frequencies carry little energy and are inaudible to human ears. This chamber received the Guinness Book of World Records' award for being the world's quietest room.
http://www.wisegeek.com C. Long Range Acoustic Device (LRAD)
The LRAD is used as a combatant deterrent or crowd-control weapon by emitting sounds painful to the ears. It is capable of emitting sound within a 15 to 30 degree beam. Its range is 300 to 500 meters. At maximum volume it can emit sound 50 times greater than the human threshold for pain. The LRAD uses a set of piezoelectric transducers capable of converting electrical energy into sound. They are permanently polarized, so any distortion of their shape creates an electrical impulse. By using a power source to supply this electrical impulse, piezoelectric transducers can rapidly change their shape creating sound waves in the process. The transducers are also arranged so that they are in phase with each other. The resulting sounds they emit can combine to make the projected sounds louder. The sound produced by the LRAD can be directed so there is less-than-normal dispersion. This results in a 20-dB drop in the volume of sound 15 degrees outside the beam. This directional sound propagation stems from the fact that the LRAD employs outer and inner transducers in creating sound waves not completely in phase with each other. This enables other sound waves to cancel out those in the outermost portion of the beam. The resulting wave front of the sound is also flatter than usual, preventing the sound from being dispersed as it propagates. As the LRAD-produced sound waves interact with the air, they create additional frequencies within the wave, thus amplifying the sound and pitch.
http://www.tech-faq.com
From infrasonic to ultrasonic. Scientists use two general types of sonar – active and passive. Active sonar transducers emit an acoustic signal or pulse of sound into the water. If an object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the sonar transducer. If the transducer is equipped with the ability to receive signals, it measures the strength of the signal. By determining the time between the emission of the sound pulse and its reception, the transducer can determine the range and orientation of the object.
Passive sonar systems are used primarily to detect noise from marine objects, such as submarines, ships, and marine animals like whales. Unlike active sonar, passive sonar does not emit its own signal. Rather, it only detects sound waves coming towards it. Thus, passive sonar cannot measure the range of an object unless it is used in conjunction with other passive listening devices.
http://oceanexplorer.noaa.gov D. Sound reinforcement system
A sound reinforcement system that makes live or pre-recorded sounds louder and distributes them to a larger or more distant audience involves a signal path starting with an instrument pickup or a microphone plugged into a multicore cable ("snake"). The snake routes the signals of all of the inputs on two mixing consoles. Once the signal is at a channel on the console, this signal can be equalized, compressed, or panned before being routed to an output bus. A group of signals may be routed through an additional bus before being sent to the main bus to allow the engineer to control the levels of several related signals at once.
The signal is then typically routed with everything else to the stereo masters on a console. Mixing consoles also have additional sends (auxes) on each input channel so that a different mix can be created and sent elsewhere.
Then, in smaller systems, the main outputs are often sent to an additional equalizer, or directly to a power amplifier, with one or more loudspeakers (typically two) connected to that amplifier. In large-format systems, the signal is typically first routed through an equalizer then to a crossover which splits the signal into multiple frequency bands with each band being sent to separate amplifiers and speaker enclosures for low, middle, and high-frequency signals. Low-frequency sounds are sent to subwoofers, and middle and high-frequency sounds are typically sent to full-range speaker cabinets.
http://en.wikipedia.org/
