[Elecraft] VP6DX -- the true story ?

[David Cutter]

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
>