To grasp the mechanism, a brief look at acoustics is helpful. Sound propagates through air as vibrational patterns that ultimately vibrate your eardrums. Wave frequency, measured in oscillations per second, determines the perceived pitch. Lower frequencies, such as aircraft engine hum, are heard as deep tones, while higher frequencies, like emergency sirens or barking, are perceived as sharp, high-pitched sounds.
Coway Airmega 300 —— 308.99美元（原价443美元，省134.01美元）
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Любовь Ширижик (Старший редактор отдела «Силовые структуры»)
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