Realactivex has answered most of your questions. So I will not elaborate on that unless you have some more questions.
I just wanted to correct some terminology uasge. As you said, the frequency of sound is measured by the unit of measurement - Hertz - usually written as Hz. Hertz is a unit of frequency and can be used to measure any periodic event. If something is moving at 30 times a second, it is said to be moving at 30Hz or 30 cycles per second.
Most commonly, Hz is used to measure radio and audio frequency. In audio, analogue signals are sinusoidal. In other words, they change once every second. This is 1 Hz or one cycle per second.
Most amplifiers and speakers generally refer to their range between 20Hz and 20KHz or 20 Kilo Hertz which is 20,000Hz. When you say 30Mz, you are referring to 30 Mega or 30 Million Hertz. This is never used in audio signals as 30Mhz is well beyond the hearing capability of most living beings. Mega Hertz is used in radio signals used for wireless transmission of data.
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