It is a relative unit of measure - a bit like the apparent and absolute magnitude of a star. The metre is a defined unit of measure because 1m will always be the same length. It will always be the same.
A relative unit of measure simply compares a reading with a standardized unit of measure. Take proton nmr as an example, where chemical shifts are compared to the chemical shift of tetramethylsilane. A more real world example would be saying that a pillar is twice my height. I am comparing the height of the pillar to my height which gives you a relative idea of how tall the pillar is. However this measurement is defined by how tall I am - if I compared the height of he pillar to the length of my arm, the answer would be different.
Now dBA is used is compare how loud something is by using sound pressures.
Since by definition, dB = 10 log(I/I0) where I is the measured intensity and I0 is the standard. Now in acoustics, the intensity of a wave is directly proportional to the square of amplitude, dBA = 10 log(A^2/A0^2) = 20 log(A/A0).
A is the max sound pressure that is measured and A0 is 2 x 10^-5 Pa.
Now some rules of thumb. A 6 dBA increase means the sound source will sound twice as loud as it was before. A 12 dBA increase will mean that the sound is 4 times as loud (2^2 = 4) and so on and so forth.
It is similar in reverse: a 6 dBA decrease means the sound will halve in volume, 12 dBA will mean it will decrease by a factor of 4.
A relative unit of measure simply compares a reading with a standardized unit of measure. Take proton nmr as an example, where chemical shifts are compared to the chemical shift of tetramethylsilane. A more real world example would be saying that a pillar is twice my height. I am comparing the height of the pillar to my height which gives you a relative idea of how tall the pillar is. However this measurement is defined by how tall I am - if I compared the height of he pillar to the length of my arm, the answer would be different.
Now dBA is used is compare how loud something is by using sound pressures.
Since by definition, dB = 10 log(I/I0) where I is the measured intensity and I0 is the standard. Now in acoustics, the intensity of a wave is directly proportional to the square of amplitude, dBA = 10 log(A^2/A0^2) = 20 log(A/A0).
A is the max sound pressure that is measured and A0 is 2 x 10^-5 Pa.
Now some rules of thumb. A 6 dBA increase means the sound source will sound twice as loud as it was before. A 12 dBA increase will mean that the sound is 4 times as loud (2^2 = 4) and so on and so forth.
It is similar in reverse: a 6 dBA decrease means the sound will halve in volume, 12 dBA will mean it will decrease by a factor of 4.
