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

[Mike Feher]

You are mostly correct Bill. However, the noise floor remains the same for
"white" noise regardless of the BW. The total noise power is what doubles if
the BW is doubled. So, the S/N assuming S>>N, decreases by 3 dB if the BW is
doubled. That is why as you say, we use narrow filters to increase the
effective S/N.  73 - Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 


-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Fuqua, Bill L
Sent: Wednesday, October 30, 2013 12:17 AM
To: mac; hwhall at compuserve.com
Cc: arc5 at mailman.qth.net
Subject: Re: [ARC5] dB Power Apples and dB Voltage Oranges, was Re:
Selectivity Ratings...

    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