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Almost everyone knows there are kosher hot dogs and pickles, but you have to be a real aficionado of kosher to know that there also is such a thing as a kosher sound system. Seeing a niche market, a small Washington, D.C.area research company last year received a patent for just such a system. And in what must be a textbook example of how a new technology can be transformed into other products, the company is developing a listening device based on the same principles that would allow farmers to hear insect larvae munching away inside grain elevators.

Both applications evolved out of the same core problem: How do you make a sound system without electricity? For Orthodox Jews, the question arises in connection with the Biblical commandment not to do work on the Sabbath or other holy days. Traditionally, one definition of forbidden work was lighting a fire. In the modern age, orthodox rabbis have taken the modern incarnation of fire to be electricity, and therefore any effort on their congregants’ behalf that initiates the flow of electricity is forbidden. In practice, this means that on the Sabbath, Orthodox Jews are not allowed to turn on a light or push a floor button in an elevator. The prohibition also rules out the use of electrically driven microphones.

This observance exacts a price. The farther back that members of a congregation are seated in a synagogue, the harder it is for them to hear. Moreover, rabbis and cantors must regularly strain their voices in their efforts to be heard by all worshippers. This problem is exacerbated when they are praying or singing facing the altar with their backs to the congregation and on the High Holy Days of Rosh Hashanah and Yom Kippur when the services are so crowded that people often must be seated outside the main sanctuary. “Our cantor says that he would rather sing four operas than one Yom Kippur service,” says Marc Schneier, a rabbi with the Hampton Synagogue in Westhampton Beach, Long Island.

It was into this quandary that Defense Research Technologies (DRT) of Rockville, Md., has stepped with a nonelectrical sound-amplification system. The technology was first developed by DRT president Tadeusz Drzewiecki and others at the U.S. Army’s Harry Diamond Laboratories in the late 1960s and early 1970s, when they, too, faced a problem with sound and electricity. Crew members on the decks of noisy aircraft carriers wanted a way of talking to each other, but they were afraid that sparks from traditional electric microphones would ignite the jet-fuel fumes that waft across the deck surfaces.

In response, the Army researchers came up with the principles for a system that Drzewiecki has patented as an “acousto-fluidic” technology. The system works on the principle that sound can travel farther if it is wind-born. In fact, some rabbis have dubbed the new technology a “wind microphone.”

Drzewiecki says the system works by “taking advantage of the mechanical gain obtained by the deflection of a high-energy jet of air.” As a person speaks into an input horn, the pressure waves generated by the voice hit a smooth jet of air released from a canister of compressed air and cause it to vibrate.

The wavering jet stream is then split by a thin piece of laminated plastic into two channels, one carrying the upper range of the sound waves and the other carrying the lower range. The split sound waves are then routed through separate tubes so they will bang from opposite sides into another, faster-moving jet of compressed air. As waves from one channel push on the jet stream with positive pressure, waves from the other channel, which are completely out of phase, pull on it from the other side with negative pressure, increasing the amplification effect.

The split jets of air, which travel with much more force than a voice, cause the smooth jet they come in contact with to vibrate at the same frequency but with higher amplitude. This phenomenon can add enough kinetic energy to the pressure wave to raise its volume tenfold. The amplified sound, imprinted with the original voice harmonic, travels along plastic piping and emerges from speaker horns at the ends of the tubes.

The army engineers were able to demonstrate that the principle worked-but were not able to get the sound loud enough to be heard on flight decks where the cacophony frequently reached 120 decibels. Part of the problem was that they did not realize they could theoretically repeat the steps of the process indefinitely to achieve the desired volume. Static in the line-an issue that Drzewiecki later resolved by making the flow of pressurized air less turbulent-also stalled development.

Instead, Drzewiecki built a model of the technology, which he implemented as a temporary intercom system in his office building. Then in the late 1970s, a rabbi walking down the halls saw the system with its air bottles and lack of electrical connections and realized that it could be used in a synagogue.

After discussing the idea with the rabbi, Drzewiecki sought kosher status for the acousto-fluidic technology by petitioning the Rabbinical Council of America, a governing body for North America’s 1,000 Orthodox Jewish congregations. Fifteen years later, the system was formally declared kosher because the amplification process, according to the rabbis’ interpretation of the Halacha (Jewish law), was deemed not to be “work.” Moreover, the use of the valve that released the jet of air from a tank of compressed air was not considered a violation because the valve could be automatically programmed to be turned on and off before the Sabbath and holy days. The decision was confirmed by the Institute of Science and the Halacha, an organization in Jerusalem devoted to finding solutions to difficulties created by orthodox observance. While 15 years might seem a long time if one were seeking, say, a patent, Drzewiecki was informed by the rabbis that it was unusual for deliberations on such a matter to be concluded in so short a period.

In 1994, Drzewiecki mounted a demonstration at Rabbi Schneier’s synagogue during crowded Rosh Hashanah and Yom Kippur services. He found that the volume 50 feet away was roughly the same as when the sound came out of the speaker’s mouth-and people 100 feet away could also easily hear what was being said or sung. Schneier described the technology as “revolutionary.”

The kosher sound system is currently too costly for many synagogues. Drzewiecki estimates that a system for a large 1,600-member congregation, for example, would cost about $20,000 because it would have to be custom designed and hand built. But he believes that if the units could be produced in quantity, the price could easily be halved.

There is also the aesthetic question of what a synagogue would look like with two long pieces of plastic piping stretching along its walls and onto the altar. The best solution, suggests Drzewiecki, would be for systems to be installed in newly constructed synagogues.

Listening for Larvae

Along with its attempts to break into the synagogue market, DRT is turning its attention and its technology to a completely different sound-amplification problem-bugs in the granary. Specifically, how does one detect the presence of weevil larvae in grains such as maize and rice before they hatch and destroy an entire stored crop? A traditional sound-amplification system might be able to register the larval eating sounds, which are generated at a level 10 times below that of human hearing. But electrical microphones could generate sparks that in turn could trigger an explosion in the grain dust, which is highly combustible.

Drzewiecki has begun work with the U.S. Department of Agriculture to see if a multi-stage acousto-fluidic system could detect the faint munching sounds. Grain samples would be put into tubes with cloth-covered holes in the sides. Small horns placed over the holes would feed the sounds into an air-driven amplifier, much like the one used in the synagogue sound system, except that it would amplify sound through a series of three interactions with smooth air jets. This solution not only reduces the risk of fire, Drzewiecki adds, but because Jewish law also prohibits the eating of insects, it also helps ensure that the grain is kosher.

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