# Decibels usage?

Discussion in 'Electronic Basics' started by terryS, Feb 3, 2008.

1. ### terrySGuest

Based on one tenth of the Bel named in memory of Alexander Graham of
telephone fame. Have always understood that Decibels are the
logarithmic ratio (base 10) of 'two' or more, power levels.

So am familar that doubling (or halving) a power ratio is approx +3
(minus 3) decibels. etc.

But in day to day parlance one sees (and often hears) that something
is loud, at say 90 dBs, very loud at say 105 dBs. or liable to damage
ones hearing at say 125 dBs.

Or that 'normal office noise is 76 dBs or something.

But in relation to what?

On other hand have seen auto magazine articles reviewing vehicles
that, very carefully, will say, for example, "Interior noise at 'x'
mph. is so many dBA".

What is that 'A' ? Ambient or something?????

Also recall doing 'Noise measurements' on communication circuits using
dBa (small 'a'). Meaning IIRC 'adjusted'? Those were using a 'Typical
weighted circuit response' with zero dBa appearing to be somewhere
around minus 90 dBm?

Various 'trades' also seem use dBm, i.e relative to dB Zero being
being one milliwatt; dBw, i.e. relative to one watt and others.

And then there are loud speaker measurements; something to do with
Sound Pressure Levels at a certain distance from the speaker?

But is there any one single power level or standard that is generally
accepted as being a 'Normal' reference. Hence people tending to drop
the reference in everyday usage?

2. ### Tom BiasiGuest

There is a power point presentation here;
http://homepage.ntu.edu.tw/~karchung/Phonetics II page twelve.htm

Tom

4. ### EeyoreGuest

In relation to the threshold of hearing.

Also they are sound PRESSURE levels - not Power.

A weighting. The LF and HF is filtered.

I'm sure Wikipedia explains it very well.

Graham

5. ### terrySGuest

Thanks guys. TerryS

6. ### Tony BurchGuest

The other guys have given some good info and links, but I thought I'd just
also give my short summary...

As you said Terry, decibels always relate to ratios of power levels. Power
is measured in Watts.

dB = 10log(P1 / P2).

When we are talking about "How many Decibels" we always need to know what
the "reference power" is first. This is the number P2, in the equation
above.

For sound, the proper notation to use is "dB (SPL)" or "dBSPL". People often
leave off the "SPL" part of the notation.

The equation is:

dB (SPL) = 20log(SPL1 / SPL2)

Note that it is 20log, instead of 10log because SPL1 and SPL2 are measured
in micropascals instead of Watts.

For sound, it was decided that the reference SPL2 is 20 micropascals. 20
micropascals is a very quiet sound. A 20 micropascal sound is 0dB (SPL).

So if we have a sound meter that measures SPL1 in micropascals, the meter
gives us the reading dBsound = 20log(SPL1 / 20 micropascals)

Also, here's a Wikipedia reference
http://en.wikipedia.org/wiki/Sound_pressure

Anthony Burch http://www.SuperSolderingSecrets.com