[TowerTalk] dB Differences

[Jim Brown]

On Thu, 11 Feb 2010 21:43:04 -0800, jimlux wrote:

>At threshold, 3dB is important.

>At S9 or 60 over, 3dB is irrelevant.

>Now, if you start talking 1dB... 1dB is very close to a "just noticeable 
>difference" in audio levels.. that is, if you have two tones that are 
>0.5dB apart in level, you'll have a hard time telling the difference 
>between them.  (see, e.g. Weber-Fechner law, etc.)

Sort of.  As you noted, 1 dB is NOT DEFINED as the change in sound level 
that is barely noticeable. Rather, experimental work by people who study 
how people hear (this field is called "phsychoacoustics") shows that for 
UNTRAINED listeners, 1 dB is the smallest change in OVERALL LOUDNESS of 
everything hitting their ears that they can hear as DIFFERENT. Trained 
listeners (for example, recording engineers) can resolve smaller 
differences. In the perception of overall LOUDNESS, it takes a change of 10 
dB to be perceived as "twice as loud" (or "half as loud").  Our receivers 
have AGC (automatic gain control), so changes in signal strength are 
removed. We hear no change in loudness, because a stronger signal causes 
the gain to be reduced by the increased signal strength. 

The dB is a RATIO of two signals, and is used to measure many things. One 
of them is the ratio between a DESIRED SIGNAL and NOISE or INTERFERENCE. 
Human perception of that DIFFERENCE is considerably greater for signal and 
noise (or two signals) that are very close in level. For example, one of 
the things I've done professionally is mix the outputs of many mics that 
pick up a musical performance, for sound reinforcement (PA), recordings, 
and broadcast. When we do that, we are BALANCING the loudness of sounds 
picked up by as many as 30 or 40 microphones to achieve the MUSICAL balance 
that the performers are trying to create. Performers often change their 
loudness in that mix by moving around or by moving the mic, or by wandering 
around the stage. Sometimes that change is intentional, but often it is 
not. When I have a band properly balanced so that they sound "right" and 
then something changes to make it "wrong," the changes I make to correct it 
are often only 1-2 dB. 

In a previous post, I noted that changes in MY SIGNAL STRENGTH of only 2dB 
were often the difference between missing and making a QSO with a weak 
station (or with a station who has a lot of RX noise). 

The dB is also used to measure frequency response. OFTEN, a system with a 
"bump" or "dip" in frequency response of only 1dB can make a voice or an 
instrument sound "wrong" -- that is, un-natural, harsh, dull, bassy, boomy, 
etc. 

As to "What's a dB worth?  You hit it when you noted that if you're trying 
to improve your signal, you run into certain limits, (1,500 watts, the size 
of your city lot or rural property, the building codes, and the size of 
your pocketbook, and that each element of improvement has both a cost and a 
limit. 

Last year, I put a 3-el SteppIR on a 120 ft tower. Even with all used tower 
sections, used feedline, and a lot of volunteer labor, that project cost me 
$11K (antenna, tower, rotor, tower hardware, guys, money to pay climber). I 
already had two dipoles at 120 ft, so I paid $11K for 6dB. I was already 
using a legal limit amp to drive it. My design "guideline" is to not lose 
more than 1dB in feedline. It's 380 ft from my rig to the antenna 
terminals, so I needed hard line or heliax to achieve that on 10M. 

What will 3dB MORE cost me?  Another 3-el SteppIR, a bigger mast section, 
and lot more labor. IF the tower would hold it. I can't put up a LONGER 
antenna -- no room between the trees to turn it.  

What will 3dB cost me on 160M?  I already have an 86 ft vertical with top-
loading and 70 radials, and I'm running legal power. To pick up 3dB in some 
selected directions, I've got to add at least one more equivalent to that 
(vertical plus 70 radials) plus phasing lines/network to feed them. And 
I've got to figure out where to put that second element. 

What's 1 dB worth in a contest?  Bill, N6ZFO, did some statistical analysis 
suggesting that 1 dB is good for a 2.6% improvement in our score for the 
ARRL Sweepstakes. Three 1 dB improvements were good for 8%. Obviously, 
using radios with better receivers, and  practicing to become a better 
operator also helps. :)

73,

Jim K9YC