Why do we use 44,1 kHz frequency on CDs while we’re only being able to hear up to 20 kHz?
There is a thing, called “Nyquist frequency”.
Basically, with 44.1kHz sample-rate, you can sample/store sounds up to 22.5kHz, accurately.
Imagine a sine-wave .. you need multiple points (samples) on that sine-wave, to be able to store and reproduce it, accurately.
The closer the tone gets to the sample-rate, the worse the audio-quality becomes.
So, 44.1kHz is basically just good enough to get the frequency-spectrum of the human hearing covered.
This is called the “nyquist frequency.” Basically, in order to accurate record (and later replay) sounds at a given frequency (in this case around 20 kHz like you mentioned) you needs to “sample” it twice as often as that frequence. Sampling means taking a snap shot of the value at a given point in time.
This is because sound is a wave and you need to require both the peak and the valley to accurately recreate it. And a freuncy of 20kHz means there’s 20k peaks and 20k valleys every second. So that’s 20+20 =40.
As for why we go up to 44 instead of staying at 40 (a lot of people can’t even hear about 17 kHz so even lower might work) I’m not entirely sure.
Hopefully that makes sense, this stuff is a lot easier to explain/understand with pictures.
If the sampling frequency is too close to the acoustic frequency, higher frequency sound waves won’t map neatly into the rigid digital containers that record their values. This causes distortion.
By sampling sound at a frequency that is at least twice the highest expected acoustic frequency, we minimize (but do not eliminate) these acoustic artifacts.
Ideally, sampling would be at least 10 times (an order of magnitude) higher than the highest acoustic frequency. Of course, higher sample rates produce correspondingly more data. So acoustic recording becomes a balance between quality and data size.
Though audio CDs sound quite good, experienced listeners can discern recording inaccuracies, especially in music with cymbals or other very high frequency sounds.
CD’s sample at that frequency, 44,100 times per second.
If they sampled at 20,000, then it would sample once every sound wave, and every sample it got would see the same part of the sound wave. This system wouldn’t detect any sound at all in 20khz, because it sees the same part of the wave every time (and that wouldn’t look like a wave).