Hi David --
Thanks for your interest in this arcane subject.
When a signal just a little bit above the trigger point for the
attenuation reflex appears, it takes about 150 ms before attenuation
develops. For a very strong signal, the reflex is faster (25-35 ms) but
there is still a period when the full power of the signal slams all the
way through to the inner ear.
73,
-- Eric K3NA
on 08 Mar 14 Fri 06:07 David Cutter said the following:
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
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
