As I understand it, when we measure sound in decibels, it must be relative to a certain distance from the object producing the sound. i.e. a rock concert produces 120 dB if you’re sitting in the front row, but only 110 dB if you’re 15 rows back.
Why is it so rare to express the distance associated with a dB level? i.e. why do people say “a jet engine produces 140dB” without specifying a distance? Do they mean with your ear to the engine? At 10 meters? 100 meters?
Is there some standard distance that I am unaware of?
In the real world, sound pressure level vs. distance is not that predictable. Rock concerts are not usually held in free space. The wikipedia article has a table of SPL measurements that often specify distance from the source.
When measuring SPL, there is not a standard distance that is used to calculate the value. While whoever took the measurement should provide a reference distance, its not usually provided, especially in instances like you’re talking about.
Decibels SPL is inherently a pressure measurement, and is calculated using the following formula
dB=20 Log(Measured Pressure value / 20 µPa)
The only reason distance is specified is because as a general rule, the SPL follows the inverse square law, which when translated to sound, means that for every doubling of distance, the SPL goes down by 6 dB SPL. This is true only for conventional speakers. Your typical concert sound system will only drop 3 dB for every doubling of distance.
Here is a Wikipedia link for sound pressure level. One other thing to keep in mind is that you will often here SPL’s quoted with weighting such as A or C. This refers to a couple of different curves that are represent the way that the human ear actually reacts to different frequencies and so modify the SPL a little bit.
I think the OP has a good point. I’ve never seen figures for distance.
I presume (perhaps incorrectly?) that dB figures for vacuuming or attending a rock concert are for normal users at closest normal distance. Vacuuming would be standing at the handle, not crouching to pick up that stray bobby pin. Rock concert would be at the front row, not either in the middle nor on stage with the amps.
For airplanes, I’m going to guess they mean for the normal ground attendant requiring ear protection, not the person in their yard as the plane passes overhead.