Eric
Absolutely fascinating and confirms many years of my own experience and
suspicions.
Am I right that the ear having shut down for a loud noise, takes a few ms to
recover? So, a static crash doesn't just cover the signal for the period of
the crash but also for a short period thereafter due to our hearing AGC.
I suspect that, ideally, our headsets should be preceded with a peak limiter
calibrated to the headset sensitivity to limit peak sound pressure into our
ears to prevent *any* natural AGC.
David
G3UNA
snip
Yes, signal range could be from S1 (or less) to S9+40 dB, so more than
90 dB. However, several other aspects of human hearing come into play:
1. The most sensitive part of audio spectrum for typical hearing is 2
to 5 kHz. If we take a K3 with very wide filters, and no antenna, in an
extremely quiet listening environment, and just gradually advance the
audio gain until we can just begin to hear the receiver noise floor, we
will be listening to a higher-pitch hiss in this range of 2 to 5 kHz.
White noise at lower frequencies won't be perceptible yet until the
receiver gain is advanced another 10 dB (at which point frequencies down
to 500 Hz are audible) or 20 dB (good for frequencies down to 250 Hz).
If we narrow the receiver bandwidth so we are only listening to 100-700
Hz, for example, the receiver noise floor will appear about 10 dB louder
(relative to the minimum threshold of hearing) at the higher end.
2. Another frequency-sensitive aspect of human hearing is the
attenuation reflex. This reflex tightens two muscles in the ear, one of
which tightens the ear drum slightly and the other moves the three bones
of the middle ear to reduce the transmission to the cochlea (inner ear).
This is our own, human protective AGC.
The attenuation reflex begins to act at 65-70 dB above the threshold of
hearing at 200 Hz... but 80 dB above the threshold of hearing at 700 Hz.
The "slope" of the attenuation reflex is about -0.6; i.e., a signal that
is 18 dB above the attenuation reflex threshold will be reduced to just 6
dB above that threshold (i.e., 12 dB attenuation added) by the time it
reaches the inner ear.
Now let's look at an operator listening to a K3 in a perfectly quiet
listening environment (no other local sounds). If he adjusts the receiver
so that antenna/band noise is 5 to 10 dB above his threshold of hearing at
a pitch of 400 Hz, and then tunes across a CW signal that is +95 dB above
the band/antenna noise floor, that CW signal will be about 100 to 105 dB
above the threshold of hearing.
That signal will also be about 30 dB above the threshold for triggering
the attenuation reflex. At a slope of -0.6, the attenuation reflex will
cut that signal down by 20 db... so that it is now 80-90 dB above the
threshold of hearing. -20 dB of attenuation is about the maximum the
attenuation reflex can deliver -- but that is only in children and
teenagers. For adults, the maximum attenuation level declines with age,
so I (at age 55) can no longer get -20 dB of protective attenuation.
Maybe I get 10-15 dB of attenuation, leaving the CW signal at something
like 95 dB above the threshold of hearing. Of course, once this
attenuation reflex is activated, that very weak CW signal down near the
noise level will be attenuated below the threshold of hearing, so no more
copy.
Even worse, long exposure to signals above the attenuation reflex
threshold results in incremental and permanent hearing damage. So that CW
signal, at 95 dB above the threshold of hearing... and 20 dB above the
attenuation reflex threshold... represents an important hazard. The USA
National Institute for Occupational Safety and Health has set a limit of
about 1 hour per day at this level... and that limit declines quickly at
higher levels.
3. Fortunately, by setting the receiver gain at these low levels, that
loud CW signal is below the threshold of pain (about 110 dB above the
threshold of hearing at 400 Hz). The threshold of pain is where the
operator rips off the headphones and says "ouch"! We want our receivers
to limit signals (or static crashes) before they reach this level!
So, we can't use a receiver that is perfectly linear over a 130 dB
range -- it would destroy our hearing! But we need to listen to signals
in a very quiet listening environment, as quiet as we can get... and set
the gain levels appropriately... and use some form of signal limiting to
keep signals well below the pain threshold.
And we should recognize that hearing varies from person to person. As a
result, one person with poor hearing range, listening in a noisier
environment and having his attenuation reflex triggered often, will have
receiver AGC and his own attenuation reflex interacting to reduce signal
strengths... eliminating weaker signals... while another operator
listening to the same radio with good hearing (big dynamic range between
his threshold of hearing and attenuation reflex trigger point), with
minimal receiver AGC, will find a rich range of signals in the pileup.
The psycho-acoustic phenomenon of "masking" further complicates the
management of a pileup. But that's a subject for another time...
-- Eric K3NA
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