Please help me know

what are you wanting to know?

this db unit can add like a ordinary arithmetic.

but dbm is another.

at first, you have to understand what is db, what is dbm?

if the number is very large, or huge and multiply or divid, you can take a logrithm algorithm. so that it s changed into add and subtration.

some time people specialize some unit like mV or mW they use dbm, so you have to understand what your dbm represent? mV or mW will get diffenent result.

for example, 1mV [=] 0dbm, 10mV[=]20dbm, 100mV[=] 40dbm

but 1mW [=] 0dbm, 10mW [=] 10dm; 100mW [=] 20dbm.

none will write that like 3dbm + 4db.

corresponding, if you know the unit, and neednt point out a reference, yoiu can simple write as db.

enough,

Thank you for your reply soon. The dbm is in mW, db is power factor. So, i want to know the basic principle of this function. Best regard!

dB is 10 * (log of power ratio). dBm takes the reference power as 1-mW. So 5mW is about 7-dBm. Adding dB has meaning if you are looking at the total gain of two succeeding amplifiers (or loss of attenuators) - then the dB numbers add.

Adding dBm has meaning if you are adding signals. But you need to know a bit more than that. If the signals are uncorrelated, then you can simply add the powers [NOT in dBm, but converted to mW, etc. e.g. A + B (both in dBm) = 10*log(10^(A/10)+10^(B/10)]. When there is correlation between the signals, you need to understand the whole business.

Be warned - in order to use dB and dBm safely, you need to take the time and trouble to understand both signal theory and the dB convention properly.

db (DeciBel) is a kind of a logarithmic scale, used to measure sound and sound products.

In acoustic terms (SPL for Sound Pressure Level), the softest, absolute quiet is 0 db and up to, say +120 db, the human threshold of pain from noise.

In Recording Sound terms the top, loudest possible level recorded (also called "clipping level") is 0 (zero) db, and the softer it gets, the higher the numbers in minus sign.

Compared to the examples above, the softest, absolute quiet is -120 db, and up to, say -3 db, the machine threshold of distortion from clipping the complex signal. The under-clipping top level is usually -6 db for recording.

Here is an example of this, from a PC recording system:

Here is Wiki

Thank you for the technical explanations.

Greatly appreciated!

Now I know why they would never mike me on the drums! Sound PRESSURE level!