This week, CR4 interviewed Dr. F. Joseph Pompei, inventor of the Audio Spotlight directional-sound system and founder of Holosonics Research Laboratory. Next week, the Engineer's Place for News and Discussion will meet Elwood "Woody" Norris, Joe Pompei's chief rival. Although these inventors have been profiled by the likes of ABC News, CNBC, Forbes and Newsweek, there's still a story to tell. Based on your feedback, CR4ers want to know more about how directional-sound systems work. Today, we'll turn up the volume and explode what one magazine calls "audio dynamite".
Life Before Directional Sound
Let's start with the basics. Traditional stereo speakers produce sound by moving an electromagnet attached to a flexible cone. Changes in the electromagnet's polar orientation determine the direction of repulsion and attraction. As the voice coil moves back and forth like a piston, the electrical audio signal determines the frequency and amplitude of the sound wave produced by the diaphragm. Typically, a passive crossover breaks the audio signal into different pieces. Drivers such as woofers, tweeters, and midrange devices are used to produce low, high, and mid-spectrum sounds. The speaker's audible sound waves then spread out in all directions like a pebble cast into a pond.
Ultrasound: It's Not Just for Babies
Unlike traditional stereo speakers, directional-sound systems convert an audio signal into an ultrasonic frequency that is focused like a beam of light at a distant target. This narrow beam is inaudible because its high frequency (above 20,000 Hz) is beyond the range of human hearing. Like audible sound waves, however, ultrasound waves get distorted as they travel through the air. But this is not a problem for a good directional-sound system. As Joe Pompei and Woody Norris have learned, there is a way to use this distortion to break ultrasound waves into lower-frequency, audible sound waves. Moreover, because these ultrasound waves can travel along a narrow path, people to the left and right of a target remain in silence.
Got a Minute?
Come on inside Conference Room 4 (CR4). If you point your new directional-sound system at a person standing 100 yards away (hey, it's a big conference room), only that person can hear the song you're playing. Point the emitter at the walls, however, and an unsuspecting listener may think that your violin concerto is being piped in from a hidden speaker near the light switch. Now hang from the ceiling and "shine" the directional-sound system on an unsuspecting listener below. If you get this right, the poor soul may start complaining that he can't get violins out of his head. Tell him not to worry. He doesn't need a psychiatrist. He just needs a directional-sound system like yours in his car and home entertainment center.
Resources:
http://electronics.howstuffworks.com/speaker12.htm
http://www.gotjuice.org/thesoundwar.pdf
http://www.woodynorris.com/Articles/Newsweek3.htm
Steve Melito - The Y Files
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