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Since our hearing range is said to be 20Hz to 20,000Hz. So, how can we possibly hear something at 44.1Khz? Or do we hear only half of the sound produced at 44.1Khz by any audio CDs? What is going on?

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2 Answers

up vote 12 down vote

The 44.1kHz it's the sampling frequency, i.e. the frequency at wich the encoder samples the audio data. It has nothing to do to the frequency of the audio data.

You can generate a 1000Hz sine wave, sample it at 44kHz and play it back: what you hear is still the 1000Hz sound. Take a look here.

Actually there is a reason to why CD audio data is sampled at 44kHz, that is slightly more than double the maximum frequency humans can hear.

It's because of the Nyquist-Shannon sampling theorem which states that:

If a function x(t) contains no frequencies higher than B hertz,
it is completely determined by giving its ordinates at a series
of points spaced 1/(2B) seconds apart.

In layman terms, just sample at double the maximum frequency of the audio to get an "accurate" reproduction of the original signal.

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Good answer! You said that 44100 hertz is slightly more than double the maximum frequency humans can hear. Why then do you get so much better quality if you record it at 48 000 hertz? (Yes it is strange if you look theoretically at it, but I think for example the drums sound dull and the brighter "air" sounds dull on 44.1 kHz.) – Friend of Kim Feb 16 '12 at 20:06
@50ndr33 well, professional recordings use sampling rates of 96kHz or more. That's because 2 times the frequency is the theoretical minimun. And when you record music you don't have just pure sine waves, but lots of harmonics/overtones which colour the sound (think a distorded electric guitar). Take a look at en.wikipedia.org/wiki/Oversampling. Also, the quality of the Analog-to-Digital converter has a role. With a professional AD @ 44.1kHz you will record with better quality than with a cheap PC soundcard @ 48kHz or 96kHz. – Mr Shunz Feb 16 '12 at 20:41
Yes, I know this, exept for what you call "not just pure sine waves". Do you mean that we percieve soundwaves above 20 000 Hz and even though we can't really hear it, it gives an impact? – Friend of Kim Feb 16 '12 at 20:54
@50ndr33 well, i really don't know if we "perceive" >20kHz soundwaves but sure they interact with the udible ones contributing to create a "rich" sound. If you play a 440 Hz (that's the A note) sine wave it just sound sterile, and surely not as the same A played on a piano or on a violin, or on a guitar, etc.. – Mr Shunz Feb 17 '12 at 9:42
Yupp, that's an interesting theory. You're probably right! – Friend of Kim Feb 17 '12 at 12:03
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up vote 1 down vote

Good answer Mr Shunz.

The Wikipedia article you included has this passage:

... however in some cases ultrasonic sounds do interact with and modulate the audible part of the frequency spectrum (intermodulation distortion).

For an explanation of Intermodulation Distortion (IMD), see http://en.wikipedia.org/wiki/Intermodulation#Intermodulation_in_audio_applications

In the top-right figure there, the signal at the lowest frequency could fall in the hearable range (< 20 kHz). This peak could be lost in an ADC-process (analogue to digital conversion) if the sampling rate is too low. That is one of the reason hard-core audio-enthusiasts cherish all-analogue audio; from the recording studio, via the LP and to the speakers.

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That's right... but actually the greatest argument against audio CD format is not the 44kHz sampling rate, but the 16bit depth which has "not enough headroom". That's why nowadays most digital recording is done at at least 24bit resolution. – Mr Shunz Feb 16 '12 at 10:11

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