Explain dB formula ?

1) explain dB formula(p2/p1), I do not understand it. 
2) What is meter band,BBc world service uses 16 meter band,19meter band etc. for its broadcasting frequency specturm is between 15 to 18MHz. Who we devide it into scale in this typical exmaple scale is between 8 to 10

dB = 10 x Log10 (P2/P1) formula is used to quantify Attenuation in fiber optics.
dB (decibel) measures the relative strength of two signals or a signal at two different points, Log10 is base 10 Logarithm and P1 and P2 are two powers to compare.

As we know that Attenuation means any reduction in the strength of a signal. So, imagine a signal travels through a transmission medium and its power is reduced to half. Suppose its original strength or power (P1) is 1 which reduces to its half 0.5 , represented by P2. By putting these values in the formula we can calculate or measure Attenuation.

dB= 10 x Log10 (P2/P1)
dB= 10 x Log10 (0.5 / 1)
dB=10 x Log10  (0.5)

In telecommunication, a band sometimes called a frequency band is a specific range of frequencies in the radio frequency (RF) spectrum, which is divided among ranges from very low frequencies (vlf) to extremely high frequencies (ehf). Each band has a defined upper and lower frequency limit.

Regulatory organizations assign each transmission source a band of operation, a transmitter radiation pattern, and a maximum transmitter power. Same is the case with BBC.

For more detail about this topic visit the following link and its related pages:

http://en.wikipedia.org/wiki/Band_%28radio%29

http://en.wikipedia.org/wiki/4-meter_band