A number of factors are at work, and each person's abilities/sensitivity
vary. Depending on the situation, a lower or higher pitch will be more
appropriate for a particular operator.
1. Pitch Discrimination. The ear can resolve nearby tones more
easily at low frequencies than high. Two tones separate by 10 Hz at
1000 Hz are only 1% apart... but at 200 Hz they are separated by 5%.
There are many conflicting factors at play here. A
just-noticeable-difference (jnd) in pitch depends on frequency, volume,
duration, and the speed at which frequency changes (for a changing
tone). As a result, there is a band of frequencies for which we have
maximum pitch discrimination.
If you want to measure this for yourself, take the test at:
http://www.ucihs.uci.edu/hesp
(You must use Internet Explorer for the test.) Click on "Hearing Test"
at the left and pick "frequency discrimination". The other tests are
fun, too.
My results were:
1000 Hz: average 6 Hz, std deviation 2.1 Hz
500 Hz: average 1.9 Hz, std dev 1.9 Hz
250 Hz: average 2.9 Hz, std dev 1.2 Hz
This confirms my personal preference to listen around 440 Hz (musical
A) rather than much lower.
2. Masking. The louder of two pure tones, close together, will mask
the weaker. The range of frequencies masked increases as the intensity
difference between the tones increases. For pure tones, a tone at
higher frequency masks a wider range of frequencies than a tone at lower
frequencies. Thus, listening at lower pitch means adjacent, louder
signals are less likely to mask the nearby weak signal.
Most masking occurs to frequencies above the masking signal. If a
lower-pitched station is interfering with one you wish to copy, flip the
BFO around on CW to reduce/eliminate the masking effect.
3. Pitch Detection. It takes just a few cycles for the brain to
identify a pitch for low frequency signals (50-100 Hz)... and about 12
cycles at 1 kHz. That means 40 ms or longer at very low frequencies
compared to 12 ms at 1 kHz. If the tone begins softly (soft keying rise
time), the assignment of pitch can be as short as 3 ms. This perception
means the operator will prefer slightly higher frequencies. It also
means soft CW keying makes it easier for other operators to pick out
your signal!
Using each of the above three factors might give you some more tools
to tease apart a pileup.
73,
-- Eric
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