# DB level?

1. Aug 27, 2007

### keetat

can anyone describe wat is dB level.
can dB level be showed through receiver or field strength meter?
thx for helping

2. Aug 27, 2007

### mgb_phys

dB is decibel is a measure of the relative power or amplitude between two signals.
It is an exponential unit because signal strength covers such a wide range of values, basically all you need to know is that every 3dB change is a doubling of power and every 6dB is a doubling of amplitude.

Last edited: Aug 27, 2007
3. Aug 27, 2007

### Manchot

When referring to the ratio between two power levels, it is defined as:
$$dB = 10 \log_{10}{\frac{P_1}{P_2}}$$
Be careful, though: when you're dealing with field quantities (e.g. voltage), the 10 out front becomes a 20 since the power is proportional to the field strength squared.

4. Aug 27, 2007

### rbj

i think it's more common to call it a "logarithmic unit".

there are more intrinsic reasons. it's because, if viewed linearly, no particular value of signal strength (or relative change in signal strength) deserves to be defined as the unit for which all other signal strengths to be measured against.

for example, regarding the price of stocks: http://www.fool.com/foolfaq/foolfaqcharts.htm

$$\%$$ ( dummy - the first % symbol is not rendered correctly)

if we redefined the meaning of "percent change" from the existing:

$$\% \mathrm{ change} = \frac{ V_{after} - V_{before} } { V_{before} } \ \times \ 100 \%$$

to

$$\% \mathrm{change} = \log _e \left(\frac{V_{after}}{V_{before}} \right) \ \times \ 100 \% = \left( \log _e (V_{after}) - \log _e(V_{before}) \right) \ \times \ 100 \%$$

then we can confidently say that if the stock rose in price exactly 5%, then later fell precisely 6%, then later rose another exact 1%, with the latter definition we could say that the final value of the stock is exactly what we started with. not so with the conventional definition of % change.

dB is similar to that but a dB would be more like 11.5%, the difference being just one of convention. we can say that a signal that increases exactly 5 dB, then later fell precisely 6 dB, then later rose another exact 1 dB, has the exact same amplitude at the end as it started with.

it would be good to know more than that.

Last edited: Aug 27, 2007
5. Aug 27, 2007

### rbj

i think so, if there is a reference value for what 0 dB means.

in acoustics, they sometimes assign "0 dB" to the threshold of hearing which, in SI units, is 0.0000204 N/m2 pressure variation or 10-12 watts/m2 intensity at a frequency of 1000 Hz.

Last edited: Aug 27, 2007
6. Aug 27, 2007

### chroot

Staff Emeritus
The decibel is a way of expressing ratio -- nothing more, nothing less.

A ratio of -10 dB is 1:10 (10^-1.0)
A ratio of -6 dB is 1:4 (10^-0.6)
A ratio of -3 dB is 1:2 (10^-0.3)
A ratio of 0 dB is 1:1 (10^0)
A ratio of 3 dB is 2:1 (10^0.3)
A ratio of 6 dB is 4:1 (10^0.6)
A ratio of 10 dB is 10:1 (10^1.0)

etc. The conventions regarding 20 and 10 are just that: conventions. They're not always followed, so consider the context carefully when comparing two numbers expressed in dB from two different sources.

- Warren

Last edited: Aug 27, 2007
7. Aug 28, 2007

### rbj

i think there is a teeny bit more than that. although it comes from convention, the convention is informed a little by psychoacoustic data. for a person of normal good hearing, a dB roughly corresponded to a Just Noticeable Difference in loudness.

http://en.wikipedia.org/wiki/Just_noticeable_difference
http://en.wikipedia.org/wiki/Weber–Fechner_law
http://en.wikipedia.org/wiki/Decibel

that's where the 10 (for power ratios) or 20 (for voltage ratios) came from. it may have been fudged slightly (because they liked powers of 10 and nice even numbers), but the size of a dB (as opposed to some other log unit) was chosen in such a way that you might be able to just barely hear a 1 dB change, if you have good hearing and are in an otherwize quiet environment.

Last edited: Aug 29, 2007