I also have this DB unit in my Audio Receiver. Suppose it shows -30[DB]. Does it mean that the output to my system speakers has the same voltage and current as of the voltage and current to my PC speakers if it was played in Audacity at -30[DB] ?
Assuming your receiver doesn’t have a dB meter, it’s a gain/attenuation indication. That is, if it reads -30dB when the music is loud or the music is silent, then of course, it doesn’t indicate the signal level. If it does have a meter, it will usually show Watts and dB.
There is a direct correlation between the dB level of the digital file and the wattage or loudness, but it’s unknown (unless you have a calibrated home theater systems and you don’t touch the volume control and throw it out of calibration).
But even though the exact calibration/relationship is unknown, the RELATIVE dB changes are valid. For example if your receiver says -20dB, changing the receiver’s volume to -26 is the same as using the Amplify effect at -6db. Or, if you are listening at 90dB SPL* and you reduce the level by 6dB (either digitally or with the volume control) the loudness will go down to 84dB SPL.
Some handy numbers to remember:
3dB is a power factor of two. If your amplifier is putting-out 100W, then +3dB is 200W and -3dB is 50W.
6dB is an amplitude factor of 2. That is, +6dB is twice the numeric value in a digital file or twice the voltage. And, 6dB is a power factor of 4. (If you double the voltage to your speaker, the current also doubles and that’s 4 times the power.)
10dB is a power factor of 10 and 20dB is a voltage/amplitude factor of 10.
***** SPL is the loudness of the sound in the air. 0dB SPL is the threshold of human hearing, the quietest sound that can be heard by a human with normal hearing.
The SPL level depends on the sensitivity/efficiency of your speakers, the wattage, room size and room acoustics, and how close you are to the speaker.