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[BrianSoddingBoru4]

According to my sound meter, my shop vac/dust extractor runs at 80dB, my table saw at 105dB. When I run both together, I get a reading of 105dB.

Am I looking at this wrong?

Boru

Not really

To be precise: db = 20 log (a1/a2)
+20db = x10
+3db =   x1.4 (sqrt(2) to be precise)
-20db = x 0.1 (1/10)
-3db =   x0.7 (1/1.4, or 1/sqrt(2)to be precise)

Thus something +3db bigger than something else is 40% bigger!
Thus something -3db smaller than something else is 30% smaller!
Adding two EQUAL things is doubling them. 20x log(2) = +6db, or x sqrt(2)^2

The log is the reason why you cant just add dbs. Adding dbs would be multiplying values. You have to convert to "normal" numbers, then add, then convert back to dbs. Dbs arent good for adding numbers, but for MULTIPLYNG.  adb + bdb = a x b (old fashioned slide rules used this, before we had electronic calculators).
Dbs are good for showing any multiplicative effects
Dbs are good for estimating dominant and negligible effects (because of the multiplicative nature)

Now, as i previously said, your table saw is actually x10 times louder than the dust extractor. Or, the dust extractor ads just 10% (1/10) of the table saw to the total sound. The total sound is 1,1 the table saw. The big question is: How many db is x1.1 (see above)?
db is 20 x log (a1/a2), a1/a2= 1,1, thus both devices combined are  +0.8db louder than the table saw. Be careful when taking measurements. Air pressure decreases with the square of the distance. So make sure to

#1 measure both devices from the same distance individually
#2 put them next to each other (or place your meter exaclt yhalfway between both devices) and measure with same distance FROM BOTH

If you do it properly, and if your meter is sensitive enough (10% should be easily possible), you should see a total just short of +1db more than your table saw.

Your homework for today: Check it out and report back to class, ASAP :-P

Then I wasn’t actually wrong when I said, ‘Decibels are not additive’?

I reject the homework assignment. I’ll take the fail.

Boru