[Elecraft] dB Changes, Loudness, and Signal to Noise Ratio

[Jim Brown]

On Tue, 15 Dec 2009 06:32:42 -0800 (PST), Bill W4ZV wrote:


>http://www.audioholics.com/education/acoustics-principles/human-hearing-
amplitude-sensitivity-part-1

>In Table 1 we see a collection of studies spanning 60 years. It should be
>kept in mind that in each case the results were obtained under
>laboratory-ideal conditions. Even so, we see a range of values from .25 dB
>to 3 dB. The resultant range is owing to varying methodologies used by the
>researchers and, of course, the response of the human hearing apparatus to
>the applied acoustic signals.

I posted the following before, but somehow it appears to have been missed.

The quotations above, and ALL of the discussions about what's perceptible 
and what isn't are talking about the LOUDNESS of a sound. Note the title in 
the link -- "amplitude sensitivity."  That is VERY DIFFERENT from radio 
communications, where we are mostly concerned with SIGNAL TO NOISE RATIOS 
and INTELLIGIBILITY. There's nothing WRONG about the tutorial that Bill has 
posted, but it applies to a different problem. 

The first time I had this point driven home to me was around 1974, when I 
was balancing levels for an acoustic paging and background music system in a 
multi-story federal office building in downtown Chicago. In rooms where the 
background music was held just above the noise level, paging was 
significantly hotter. Changes in level of only a dB were quite obvious on 
the background music, and either "right" or "wrong"  BECAUSE they were in 
reference to the background noise level!  But that same dB change was hardly 
noticable on the paging, because the paging was at a higher level relative 
to the noise. 

When we make our transmit signal a few dB stronger with an amplifier or a 
better antenna, or when we add audio processing (compression, peak 
limiting), or when we run those multiple Beverages that Bill has talked 
about, or when we narrow up the RX bandwidth to pull out a weak signal, we 
are working on the signal to noise ratio, NOT LOUDNESS. It's apples and 
oranges!  The only thing in common is that we're using a log ratio to 
describe it. 

73,

Jim Brown K9YC