a)Đơn vị đo tần số là gì?Đơn vị đo độ to của âm là gì? b)Hạ âm là gì?Siêu âm là gì? c)Tiếng sét và tia chớp được tạo ra gần như cùng một lúc, nhưng ta thường thấy chớp trước khi nghe thấy tiếng sét. Hãy giải thích vì sao?

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