Why does feedback between a microphone and speaker generate a high-pitched noise, and not a low one?
鈥� Sounds can be viewed as combinations of sine waves of different frequencies. The states that any frequencies that perfectly fit in the system鈥檚 鈥渄istance鈥� (from microphone to speaker and back) and are amplified along the way will be sustained and amplified further.
This distance is hard to pin down because it depends on factors like delays in the electronics, the room鈥檚 acoustic properties, the positions of the microphones and the resonant frequencies of the instruments and speakers. But higher frequencies are more likely to enter a feedback loop because the waves are shorter. The odds are higher that you can perfectly fit many short waves into a certain distance than many long waves. So although you do get throbbing low-frequency feedback, high-frequency feedback is more likely.
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Moving a microphone near a speaker causes feedback because live speakers are never truly silent: close up, you can hear a multi-frequency hum. And as you move the microphone, you鈥檙e changing the distance and sweeping the range of frequencies that cause feedback, resulting in a high-pitched whine.
Ron Dippold, San Diego, California, US
鈥� As a musician, I have been a victim of feedback at all points on the frequency spectrum. There are two main reasons why higher frequencies resonate more often. First, most microphones, amplifiers and loudspeakers are designed to be less responsive to lower frequencies than the higher ones of voices and guitars.
Second, lower frequencies need more space to become 鈥渋n phase鈥� and for their waveforms to flow back and forth. In smaller venues, there is not enough distance between the microphone and speakers for this to happen. Once a stage is large enough, it is more common for bass frequencies to resonate (and for band members to complain about them).
Connor Fitzgerald, Berlin, Germany
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