[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
>>
>


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