I got into figuring out decibels, and loudness measurements, and found the number 194dB as the boundary at which a sound starts distoring itself, namely creating vacuums which supposedly turn into shockwaves.

I know shockwaves are created around bodies that travel faster than the speed of propagation of sound in a chosen medium (let's keep it to air in this one).

What is the relationship between the aforementioned loudness measured in decibels, and the shockwaves created by supersonic movement? I cannot seem to bridge the two concepts.


Here is the phenomenae I'm trying to connect to this concept, and also to each other:

Famous video of a volcanic eruption (YouTube)
Research video of a trombone producing a shockwave (YouTube)

Note that my interest in this phenomena is from a Sound Design standpoint, not merely physics.

1 Answer 1


Your research is heading in the right direction, but you should firstly understand that as an absolute measurement metric, the "decibel" is meaningless. It's like asking someone "how long is that piece of string" and getting the answer "half". "Half of what?" is usually where that goes...

Decibels are useful as relative measurements with reference to a known constant reference point. The reason this needs to be clarified is that there are many different types of reference that can be aligned with "decibels" as an absolute measurement metric.

"Decibel" on its own can be used as a relative measurement along a known measurement axis. For example - increasing a signal level by 6dB is valid.

The 194 number is actually 194 dBSPL (Sound Pressure Level) and relates to the maximum possible sound pressure level at standard temperature and pressure at mean sea level (101.325kPa) which is 1 atmosphere at 0 degrees C.

Supersonic shockwaves are distorted sound waves and can (theoretically) produce sound pressure levels greater than 194dBSPL.

  • Hi :) I did mean SPL, so I specified in air. What I still do not get is how come 101.325kPa (= 1 bar, right?) is the maximum SP? Say for example there's a sound source that produces 195dB (SPL)... does it have to emanating from a supersonic source, and given that MSL is just MSL, why is that amount of pressure chosen as a reference for shockwaves? Commented Nov 22, 2018 at 11:06
  • please also refer to added examples in the question Commented Nov 22, 2018 at 11:16
  • also feel free to edit my question to fit your answer, I'm a bit of a noob to Stack Exchange. Commented Nov 22, 2018 at 11:27
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    I found this paper that might help get us closer to an answer - what do you think? Particularly the equations at the end are interesting. homepage.physics.uiowa.edu/~fskiff/Physics_044/…
    – Mark
    Commented Nov 23, 2018 at 11:50
  • 1
    2992 is the reference pressure used for aviation at Mean Sea Level - follows that it is a standard reference for all applications that are pressure related.
    – Mark
    Commented Jul 17, 2019 at 0:04

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