[ARC5] dB Power Apples and dB Voltage Oranges, was Re: Selectivity Ratings...

[Fuqua, Bill L]

    Noise floor is meaningless when one is listening to coherent signals. Noise floor is dependent on bandwidth. Assuming white noise
the noise floor is increased by 3 db if the bandwidth is doubled. However, the human ear is made up of 
hundreds of individual resonators each connecting to a nerve. Think of it as a real time spectrum analyzer.
When listening to your stereo you can hear annoying coherent signal such as a harmonic of 60Hz while it is too small of a signal to move the needle of 
a voltmeter a fraction of a dB above the noise floor. That is also why a bell can be heard for long distances. It is because all the
energy from the bell entering the ear vibrates only a few of those resonators. Bells were used as warning alarms for just that reason. 
   In the old days a well trained operator could easily hear a CW signal in a broad receiver as  even though the signal would not
even register on a AC voltmeter. The real benefit of narrow filters was when there were other signals that distracted the operator
not just random noise.  Today, most of us would not get by with out narrow filters.

73
Bill wa4lav
 
   
    
________________________________________
From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of mac [w7qho at aol.com]
Sent: Tuesday, October 29, 2013 9:33 PM
To: hwhall at compuserve.com
Cc: arc5 at mailman.qth.net
Subject: Re: [ARC5] dB Power Apples and dB Voltage Oranges,     was Re:  Selectivity Ratings...

1db above the ambient noise floor
On Oct 29, 2013, at 6:25 PM, hwhall at compuserve.com wrote:

>
> >
> Good CW operators
> can hear as little as 1.5dB change and oft times even under 1 dB
> will bring
> a very marginal signal out of the noise.
> >
>
> I seem to recall that the original 1 dB definition was based on a
> consensus of the smallest power change in an audio circuit that
> could be discerned by the human ear. I've always wondered how they
> conducted the tests to arrive at that, though.
>
> Wayne
> WB4OGM
>
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