And once again Friday is here, bringing with it the weekly "A Taste of Physics" column — number 40.
This week: sound waves, constructive interference, and a surprising connection to the scandal in Serbia.
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The video shows one of the recent protests in Serbia against government corruption.
An estimated half a million people took part in this demonstration.
At a certain moment, with no apparent cause, the crowd suddenly breaks into a panicked run — as if struck by some invisible force.
What actually happened there?
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The intensity of sound waves is measured in decibels.
Decibels are used to measure a fixed ratio between two numbers, and they are widely used in other fields as well — for example, measuring antenna gain.
For sound waves, decibels are base-10 logarithms, so 20 decibels is ten times 10 decibels, 30 decibels is ten times 20, and so on.
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A sound level of 125 decibels causes us considerable discomfort.
At 140 decibels it becomes outright pain, and the brain no longer interprets it as sound — only as pain.
According to estimates, a device capable of generating sound waves at 160 decibels or more was used against the protesters in Serbia — comparable to the noise of a rocket launch at point-blank range.
At that intensity, anyone standing near the source of the sound may lose their hearing permanently.
Those slightly farther away experience acute distress, sharp pain, loss of balance, and disorientation.
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Generating a sound wave of such intensity is no simple feat, and beyond that, the wave must be directed in a single direction in order to maintain its strength and prevent it from dispersing in all directions.
To achieve this, an array of many small speakers is used, along with a kind of "lens" that focuses them toward the same direction.
An electronic controller synchronizes the speakers so that the sound waves they produce combine with one another to form a single, more powerful sound wave — a process called **constructive interference**.
A lack of synchronization causes the waves to partially cancel each other out, a phenomenon known as **destructive interference**.
Interference can be observed in water waves.
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This technology also has more benign applications — for example, deterring agricultural pests or keeping pirates away from ships.
Shabbat Shalom 😊
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