IYPT 1991, Задачи на турнира — Задача 4. Self excitation

Problems for the 4th International Young Physicists’ Tournament

Critical edition

Olympiets Youth Center, Moscow, Soviet Union; July 22‒28, 1991 [1]

Critical edition: translated, restored, and commented text [2]

FINAL DRAFT. — Please do not re-publish. Suggestions and criticism welcome

Translated, edited and commented by Ilya Martchenko. The original edition’s historical notes, sources and acknowledgments are retained with problem 1.

Епиграф

Is this black currant?
No, it is red currant.
But why is it white?
Because it is still green. [3]

Условие

A strong hum [11] sometimes happens on the concerts of newbie rock bands, when the microphone appears close to the speaker that reproduces the signals amplified from this very microphone. How do the frequency and the amplitude of the produced sound oscillations depend on the distance between the microphone and the speaker, and on their mutual orientation?

Note [10]

“Feedback” in the Slovak and the 2006 English texts. “Auto-excitation or Larsen effect” in the French text

Note [11]

“Buzzing” in the 1991 English text. “Whistle” in the Slovak text. “Strong noise” in the 2006 English text. The original Russian «гудение» is an open-ended term and does not necessarily mean a low-frequency tone

Решение

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Оригинал в Архива: 1991.pdf