The frequency of radio bands?

Question

I don't understand frequency all that well.. but I do some. But correct me if I'm wrong. Frequency (Hz) can encompass different things. Like, the sounds we can hear (vibrations) are around 20Hz to 20kHz. And most electrical power is generated at 50 or 60 Hz. That's not sound (vibration) Hz though, It's a current Hz, which is why we can't hear it.

But then we have radio bands. Which go way higher in Hz. Like Ham radio goes up to 10 meters (28000-29700 kHz) and FM radio that has 87.5-108.0 MHz (87500000-108000000Hz). Obviously wayyy higher then the Hz we hear, so it can't be that version of Hz (which confused me at first, since you'd think radio = sound. so it's be the Hz we hear). So what Hz do radio bands run on? Can anyone explain? I'm confused.

Answer 1

I think that alot of the problem with using these terms in different arenas cause confusion.  However Hz is a proper term to replace the older measurement called cycles per second or cps or just c.  It refers to the amount of times the wave cycles in one second.  Perhaps given the frequency of alternating current in the US it is about 60Hz or 60 cycles.  The current of US electricty completes a entire cycle (starting at zero and going positive then returning back to the zero crossing and going negative and back to zero again) 60 times in one second.

Most humans can hear between 20 cycles to 20kc, this is true.  But we can hear them because of the vibrations in the air.  Our ears convert the vibrations into electrical engery that our brains can translate.  The reason you can't hear these audible signals is because they are in transport with wires or other conductors.  Just like in your sound system, the wires carry the sound, but you can't hear it because your ears can only pick up vibrations.  Once you connect the speakers, the electrical energy is converted into acoustic energy that you can hear.  Summary, your ears can only hear acoustic energy, not the electrical energy.

In the digital world, it's completely different because they use the words frequency and Hertz differently to measure how many times a second something happens.  Like in digital audio streams, Red Book compact discs have a sampling rate of 44.1kHz, but the actually frequency repsonse of the resulting samplerate is only 22050 kc.  The samplerate is the amount of samples in one second, not the actual audio range maximum.

I think what's confusing you is that even though we have frequencies that run much higher than that of our hearing range, you can still hear stuff on them.  The frequency that you receive that station with is definetely outside our hearing range.  These frequencies are modulated with information within our hearing range.  When it's picked up by your receiver, the different stages of the receiver pick out the modulated signal and convert it into electrical energy and transfer to the speakers.  The speakers vibrate the air near you and enables you to hear the sound.  I think you're missing the technique of modulation.

Modulation is the process of modifying the waveform to carry messages within the waveform's modifications.  They are many different types of modulation...more than can be covered here.  Check out the wikipedia article about modulation at http://en.wikipedia.org/wiki/Modulation