N6KB:
> Low frequency audio into the ear canal is a bit like VHF RF into a
microwave waveguide. Energy transfer from outside to the ear drum is no
doubt much more efficient at higher audio frequencies.
Indeed this is correct. The ear's actual peak
response is around 3 kHz as shown in Fletcher-Munson
curves:
http://www.silcom.com/~aludwig/images/Fletcher-Munson.jpg
However this is very different than the ear/brain's
response to signals in noise. This gets into an
area called psychoacoustics which is fascinating to
research. Here's an interesting summary:
How "wide" is your brain?
You may be reading some of this and say to yourself, "Self, this
doesn't make sense - I can hear a weak signal as well with the CW
filter as I can with the SSB filter. What's the deal here?"
We are getting into a field referred to as Psychoacoustics, or "How
the brain perceives sounds." As it turns out, the "trained ear" can
fairly easily resolve a bandwidth of less than 30 Hz - assuming the
presence of random noise in the background. This means that if you
have a single CW signal amongst 2.4 KHz of white noise - or even
other CW signals that are at roughly the same strength but at a
different pitch, then your brain/ear is perfectly capable of picking
it out, being able to "ignore" the 2.4 KHz of noise and the other
"dissimilar" signals: Under these conditions, narrower filters won't
always improve the "copy" for a skilled operator.
Put this same CW signal in amongst other similar signals - some of
which are much stronger and very close to the same frequency- and
even the trained ear is hard-pressed to make out the "buried"
signal. Under these conditions, the brain isn't able to reject those
"other" signals and a our hypothetical 30 Hz filter may make a world
of difference.
A simple detection/indication circuit - like an audio level meter -
does not have any means to be able to pick out that 30 Hz of signal
among the 2.4 KHz of noise, so if you were to look at the meter
alone, you would never be able to see the meter deflect in sync with
the keying of the signal. Take the output of a 30 Hz bandpass filter
- one that passes only that range of frequencies occupied by the CW
signal- and feed it into the same meter and then you will likely see
the meter deflect with the CW signal. Why? Instead of 80 parts
noise and 1 part CW signal bandwidth, we are now feeding it only that
part of the bandwidth containing the CW signal.
http://www.ka7oei.com/qrss1.html (from box at right)
73, Bill W4ZV
|
