In article ,
LRod wrote:
On 1 Sep 2005 12:03:45 -0700, "Never Enough Money"
wrote:

Fein Turbo II is rated at 58 decibles. The Shop Vac "Quiet Deluxe 6.5
HP" is rated at 82 decibels.

The rule of thumb I know is that 3 decibles corresponds to a doubling.
Thus the Shop Vac Quiet Deluxe 6.5 HP is 24 decibels louder, or 3 db +
3 db + ... + 3 db (8 times), or 2x2x2x2x2x2x2x2 = 2 to the 8'th power =
256 times louder.

Is that correct? If so, holy moly!

As someone else pointed out, it's logarithmic, so your calculation
isn't quite. An easier way to understand it is that 10 dB is actually
10x, and the next 10 is a power of 10 (10^2=100). So what you have is
20dB is x100, the next 3dB doubles that to 200, and the 1dB left over
is a fraction of x2, so around 220 or so is the decimal equivalent of
24dB.

24dB is a factor of 251.2+ according to my engineering calculator.

It's those stray dB between 9 and 10 that messes your calculation up.
9 dB is 8x, but 10 dB is 10x.

His calculation was closer than yours. grin

when you've got a number that is a multiple of 3dB, "doubling by 3s"
will get you very close to a 'correct' answer.

consider a factor of 30dB. 'doubling', 10 times gives you a factor of
1024x, where the 'accurate' value is 100x. 'error' is less than 2.5%.

At 60dB, the 'doubling' error is less thant 5%. on a factor of 'one million'.

"Good enough for -most- practical purposes." grin