The Swedish thesis shows that around 500Hz the ear/brain is at its best in
copying wek signals in noise.

73, Dick, PA0SE

For many years I was used to a 800Hz tone for CW (just happened to be the
'sidetone' in my tranceiver). But on LF I found out that I had a better
copy at lower frequencies, also because the IF-shift of my TS440 seemed to
be more effective here. After some experimenting I ended up with best
results at 400Hz.
That is a bit less that the 500Hz tone from the Swedish thesis, but maybe
it is just my brains that are 25% slower than those of the average
(Swedish) ham.

73, Rik  ON7YD