How to Measure Sound with a Handheld Meter - Workbench Wednesdays 

that's right. Ideally these kind of measurment should be made in a deaf room. BUT, if you want a before/after comparison. and not accurate value, it makes sense. I would use a wooden box lined with wooden fiber or any kind of absorbent instead of a plastic tub though. That's for 2 reasons: 1_ few mm of plastic will not absorb much from the outside, I think the sheer distance between devices is makes a bigger difference. 2_ absorbent in the inside would limit most reverberation (reflections). at least in the high frequency range, which is the usefull range in this example. About 4-5 cm of wooden or glass fiber would be enough. Don't by so called "acoustic foam", it's cheap but pretty useless. Also, all acoustic measurement are supposed to be at least one meter away from source. The reason for that is sound pressure decreases by 6 dB every time you double the distance, so the dB reading you'll get will increase dramatically the closer you get to the source and will not reflect actual perceived sound level. One meter away allows you to quickly calculate the SPL level at any distance given the formula: Ld= Lref- 20log(1/d) where Lref = sound pressure measured at 1 meter, d = distance from source, L= Sound pressure measured at distance d from the source. This is only true in free field environment, but can be usefull in most cases as long as your room is not too reverberent.

How would you feel if I had lined the plastic box with some sound absorbers? While I was going for a comparative measurement, I did wonder what would have been a more appropriate measurement. Because you're right, we got an artificially high measurement.

That would be better, but still not the best. If i had to make this kind of measurement, i would try to lower the background noise by getting away of other appliances. Try this: place the DUT as far away as possible to any other source of noise or walls and turn it off. Place the microphone one meter away from it, set it for dBA in slow mode. Now, turn off every thing you can in the room: PCs, scopes, ventilation, AC... and measure the background noise. Then, turn the DUT on to measure its noise. If you can get at least a difference of 6 dB between the two measurement, you can assume that the biggest contributor to noise is the DUT. The bigger the difference (the lower the background noise), the more accurate the measure. This would still not be perfect, but at least it should give you a more accurate value. And would not cost a penny.

Not quite, A simulates the ears freq. resp. at the threshold of hearing i.e. at very low noise levels. C simulates the f.resp. at more normal soundlevels. See here for explanations of the averaging times also: www.nti-audio.com/en/support/know-how/frequency-weightings-for-sound-level-measurements

wrong. Both are weighted, but different curves. A curve is indeed weighted to reflect human ears response, C curve is supposed to be the human ear response at higher SPL levels. dBspl or Z weighting are "flat response". Z weighting is equal to spl but defines limits for the bandwidth.