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Why we need super-high frequencies, even if we can't hear them

Even if it's true that only frequencies up to 20 kHz matter, even higher frequencies are important so that the 20 kHz response can be as good as it needs to be...


The ear has a frequency range of around 20 Hz to 20,000 Hz (20 kilohertz). If you could wave your hand backwards and forwards twenty times a second, you could hear it. Likewise, the fastest vibration of 20,000 times per second is audible (to young people with their hearing still in pristine condition). Anything faster than that, our ears just don't have the mechanism to register.

20 kHz might not seem like a very high frequency, when video signals range up to 5.5 Megahertz (5,500,000 Hz). Your FM radio is sensitive to frequencies up to 108 MHz, and satellite TV achieves frequencies in the Gigahertz region.

So why should 20 kHz be anything of a problem?

The answer is that we want it exact. Two common problems in audio are a 'droop' in the high frequency response, and also a smearing in time that often puts high and low frequencies out of step with the mid range. In technical terms we would talk of a poor frequency response and of phase distortion.

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The best solution is to make sure that the high frequency response extends way beyond 20 kHz. If it can reach up to, say, 100 kHz, then 20 kHz is a breeze. There will be hardly any loss of level at 20 kHz, and the phase will be as close to spot on, compared to the mid range, as makes no difference.

But 100 kHz is a challenge.

Even handling 100 kHz as an electrical signal isn't without problems. High frequencies tend to leak everywhere in electronic circuits, so controlling such a high frequency is difficult. Also, as an old hi-fi saying goes, the wider you open the door, the more muck gets thrown in. So if the door is open as wide as 100 kHz, then you shouldn't be surprised if there is some noise and interference, all of which can damage frequencies further down in the audio band. So designers have to be careful.

When it comes to recording such high frequencies then we have another problem. Standard CD-quality digital audio is good up to around 20 kHz only. In theory it should manage half of the sampling rate, which is 44.1 kHz, but in practice this is not possible.

More current 96 kHz sampling rate equipment should fare better. An HF response up to around 40 kHz will allow a gently tapering slope above 20 kHz. (Manufacturers tend to be rather shy of specifics, but it wouldn't hurt to know that they are doing the right thing).

If you can afford a sampling rate of 192 kHz, then in theory you should be able to record frequencies up to around 80 to 90 kHz. If frequencies above 20 kHz are rolled off very gently up to 80 kHz, then everything below 20 kHz should be pretty damn good.

Maybe we can't hear frequencies above 20 kHz, but higher frequencies still need to be handled with great care if we are to achieve perfection within the audio band.

By David Mellor Monday December 11, 2006