[Elecraft] VP6DX -- the true story ?
Eric Scace K3NA
eric at k3na.org
Fri Mar 14 07:03:24 EST 2008
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
>>
>
More information about the Elecraft
mailing list