[Elecraft] Numbers, Numbers, Numbers

[Jim Brown]

On 12/2/2010 8:55 AM, k6rb at baymoon.com wrote:
> The guy told me the THD was 0.1 % to which I
> replied "but the human ear cannot distinguish a THD much below 1 %." And
> he replied, "of course, you can't hear it, that's why we have the
> oscilloscope." The price for the system was in excess of $10,000.

As an audio professional (and a Fellow of the AES), I can tell you that 
THD is a poor indicator of equipment performance or audibility. It's not 
that the numbers don't matter, but rather that simple-minded descriptors 
like THD are not useful. The human ear DOES hear harmonic distortion at 
levels as low as 0.02%, but it depends on the ORDER of the harmonic 
(that is, which harmonic). The second harmonic can be somewhat pleasing, 
and some studio EFX boxes add second harmonic. The control that adjusts 
the level of the second harmonic is called "warmth."  In general, orders 
of harmonics that relate to western musical scales (octaves) are least 
objectionable, and high orders are most objectionable.  Third order 
harmonic distortion is no pleasant.  It is those higher orders that are 
audible at low levels.

Also, any non-linear process produces both harmonics and intermod.  Some 
intermod products are also quite audible at fairly low levels, while 
others that are common to western musical scales are less audible.

73, Jim Brown K9YC