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Originally posted by bobs_uruncle
Originally posted by Jameela
would having an Mp3 player headphones in and listening to your Mp3 player be a protection against this?
Or is the only protection going to be expandable foam ear plugs?
No, that wouldn't work and dependent on the frequency of the LRAD, even expanding foam plugs wouldn't work. If the sound pressure transfers through the skin and bones in the head, you've got a serious problem. You know the old saying, "fight fire with fire?" That stands as truth in this case, except you fight an audio wave with an equal but inverted audio wave. I don't know if this is feasible however as it is sound pressure, your inverted source would have to be far enough away from your ears that it didn't damage your hearing.
But here's a thought... If you made a helmet that had at least one layer of vacuum with a baffled breathing system and the space between the inner part and outer part of the helmet was suspended by elements that were either magnetic (not touching) or were resonant at a much higher frequency than the LRAD signal, the vacuum would stop the sound. If it isn't patented, anyone can do it now, I just made this method using a vacuum and magnetic suspensors public knowledge ;-)
Cheers - Dave
Frequency is the pitch of a sound, and is determined by the space between sound waves. Short distances between waves give a higher pitch and longer waves give a lower pitch. Pitch or frequency is measured in Hertz (Hz). Named for Heinrich Hertz (1857-1894) the measurement simply means cycles per second. In this case a cycle is one wave. Since the speed of the waves is fixed at (speed of sound) the number of waves occurring in a second determine the sound's frequency.
Amplitude is the volume of a sound wave. The larger the compression and decompression of the air from the normal atmospheric pressure the louder the sound is. Amplitude is measured in decibels (dB). Unlike the Hertz the decibel has no fixed unit, but rather is a measurement of relationships between sounds. However, there is a convention as to sound pressure level . The threshold of hearing is measured at .0002 microbar. This measurement is one of atmospheric pressure, it means that the faintest sound perceivable to young human ears is 1/20000th the pressure of the average atmospheric pressure measured at sea level. The "threshold of hearing" is listed as 0db on a conventional sound level chart. Another common reference is the " threshold of pain" which is listed as 140db on a conventional sound level chart.
LRAD provides the acoustic performance, flexibility and reliability needed for operation at distances from 10 to 3,000 meters. LRAD’s beam width, frequency range and maximum continuous output ensures 100% intelligible communication over distance and loud background noise. Each LRAD model presents the loudest and most intelligible product in its size and weight category. LRAD’s performance record coupled with military construction and survivability standards has made each model the communication tool of choice for police departments and military units around the world. Additionally LRADs provide a portable capability in a rugged lightweight waterproof casing, as well as the option to operate the device remotely across an IP-accessible network. LRAD can provide communication solutions to meet any operational requirement.
The LRAD uses the phase of the sound waves, the size of the device and the properties of air to create more directional sound:
The outer transducers are not completely in phase with the inner transducers. The sound waves interact with one another, canceling out some of the outermost waves and making the sound less audible outside of the "beam."
The device's diameter is larger than most of the wavelengths it produces. This allows the device to create a wave front that's more flat than rounded, keeping the sound from dispersing.
Air interferes with sound waves as they pass through it. As the LRAD's sound waves interact with the air, they create additional frequencies within the wave. Such waves are referred to as parametrically generated, and many speakers try to prevent them. The LRAD uses them to create a greater range of pitches and to add volume
Basic specs for the LRAD 2000x:
Range: 8,900 meters (speech)
Beam width: +/-15° @ 1.0 kHz/-3dB
Maximum output: 162dB SPL at 1 meter