magnitude observed in Audacity is also proportionnal to the acceleration unit ‘g’ and not to dB. But since Audacity only has a dB scale i don’t know what i am seeing exactly in Audacity. Could anyone shed some light on this?
If you have an independent way of measuring g-force you can make a calibration and do the calculations on a spreadsheet, but there are couple of tricky things, especially if you’re not familiar with decibels -
If you look to the left of the waveform you’ll see a scale from zero to +/- 1. 1.0 (or 100%) is the digital 0dB reference. Normal soundwaves waves (and natural vibration) is symmetrical with approximately equal positive & negative deviations. So, you can ignore the negative values, or take the absolute values, or average the absolute values, or calculate the RMS, etc. (With digital audio we are usually interested in the absolute-peak values.)
There is no default calibration. For example if you play a 0dB audio file, the actual loudness depends on the particular digital-to-analog converter (or soundcard), the volume control setting, the gain/power of the amplifier, the efficiency of the speakers, how close you are to the speakers, and any sound reflections in the room. The same is true on the recording side. And, the same will be true for g-forces.
Note that the reference for dB SPL (sound pressure level or acoustic loudness) is approximately the threshold of hearing (the quietest sound that a person with normal hearing can hear). So digital dB levels are usually negative numbers and dB SPL is a positive number.
However, there is a direct correlation. If you reduce the digital level by 3dB, the acoustic level will also drop by 3dB. (That’s assuming everything is linear so you’re not driving the amplifier into distortion or something like that.)
Decibels are always relative, and since they are logarithmic subtraction is a ratio.
It might be better if you look up the formulas because the forum doesn’t seem to allow subscripts or superscripts. Note that the amplitude formulas use a factor of 20 and the power/energy formulas use a factor of 10, so make sure you are using the amplitude formulas…
A = Amplitude (g in your case)
A = Aref x 10 to the power of (dB/20)
dB = 20log x A/Aref
…Everybody that works with audio knows that 6dB is an amplitude factor (or ratio) of 2. (+6dB is double, and -6dB is half.)
So here’s an easy example of a 6dB increase -
Let’s say we have a calibration/reference of 2g = -10dB. Then, let’s say we now have -4dB on the digital scale. That’s a +6dB difference so we plug +6db into the formula:
g = 2g x 10 to the power of (6/20) = 4g
…That’s all very easy once you have a calibration reference and a formula in your spreadsheet. Note that your recording & playback calibration references will be different.