1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Supersonic wave and distance calculation

  1. May 20, 2015 #1
    To calculate the distance from a stationary visible object using the speed of sound when dealing with a supersonic blast wave from that object, how would one proceed?
    Is the standard 340m/s good enough reference?
    How is the velocity reduction over time and distance?

    Does all the variables (temperature, elevation and such) have a notable impact on velocity reduction?
    For these examples lets say sea level at 20C.

    Example 1:
    Looking at a large amount of high exploive, TNT, going off from far away, say 5km. How to calculate distance?

    Example 2:
    This is what triggerd me into reserching this.
    A volcano eruption caused a shock wave that traveles for 13sec from point of origin to the viewer.
    At the viewer it still sound like the wave was supersonic (through a video).
  2. jcsd
  3. May 20, 2015 #2

    I'm trying to understand what exactly you are trying to do. Are you:
    A) calculating a distance by determining the difference between when you see something and when you hear it?
    B)calculating a distance by determining the difference between when you see something and a super sonic blast wave hits you?

    If it is the former, this should be an easy calculation, since nothing is moving - neither you, the listener, or the thing creating the sound - the source object. Yes, you can use the speed of sound vs the speed of light [if that really matters at all - you can probably assume it being simultaneous] to get your calculation.
    We do this all the time when figuring out how far away a storm is - when you see lightning you can count the seconds until you hear the thunder. 3 seconds is roughly 1 km.

    If it is the latter, I do not think you would be able to easily determine how fast something is moving if it is supersonic - unless someone else can say otherwise?
  4. May 20, 2015 #3


    User Avatar

    Staff: Mentor

    This document from FEMA implies that the supersonic blast wave only lasts for a few milliseconds as it expands outward from the high explosive (HE) or other source:


    So it's not supersonic for very long, and if you are several seconds away from it, it sounds like using the standard speed of sound (adjusted for temperature) would be fairly accurate. :smile:
  5. May 21, 2015 #4
    I was thinking of the two as the same thing, as in looking at the thing go off, then "hearing" the supersonic wave.

    Yes ive done this myself with thunderstorms. But when dealing with the lightning you get that distinct loud crackle(blast wave) that i would say is supersonic when struck close by.
    But at a distance(not really far away) its clearly just dark rumbling, which i would guess is subsonic.
    From this i get that thunder is only supersonic for a short duration.
    Am i correct at assuming this?

    That is where i am running into problems.
    Or is there a wave traveling behind the supersonic blast wave at normal 340m/s that we can use for calculating?

    Well to me it sounds like maybe they are talking about the duration "the listener" is in the wave as it passes by..

    If it is supersonic for such a short time, then why do we still hear what sounds like a sonic boom at a great distance from the object??

    What kills everything for me with the volcano thing is that by timing the wave as it decends down the mountain, it looks to do about 223m(volcano height according to wiki) in about a sec or maybe more... But it still sounds so much like its the supersonic boom at a distance of 13sec. Explenation?

    Video in question:
  6. May 21, 2015 #5

    I watched the video, but i am not convinced that this is a sonic boom, or rather a shock wave hitting the boat. First off, what I seem to see going on in the video, is a large explosion - very very loud one - where the sound reaching the people in about 13 seconds. This is august 2014 in Papua New Guinea 0 which at that month had a mean temperature of 28 degrees C. - which tells us that the sounds was traveling around 348.3 m/s and therefore the blast took place roughly 4.53 km away - this seems reasonable by what I see in the video. A shock wave generally - as stated earlier in this thread - dissipates quickly so it is unlikely it would be a shock wave 4 and a half km after it started.

    Ok, then this is fine. I think really you are simply looking in the wrong places for sonic booms. They are generally made by moving objects. Which makes the whole business of finding out where they are a moot point. You are interested in stationary objects making shock waves that do not reduce in energy over distance and i am afraid this is unlikely to be happening.

    I believe - now correct me if i'm wrong - that this is what i did just now when looking at your video. I find it likely that you are looking at nothing more than the sound of the initial blast having traveled towards the camera at the speed of sound.

    I think what you are thinking is to be a sonic boom is really just an incredibly loud explosion. It may be very loud, but it is not necessarily a sonic boom.

    I hope this helps.
  7. May 22, 2015 #6
    So basicly most of my thoughts are invalid because the wave is only supersonic for a very short duration, if at all.
  8. May 23, 2015 #7

    Perhaps, but you may also put an entirely positive spin on this and consider that this makes your calculations much much easier!
  9. May 30, 2015 #8


    User Avatar

    Another way to look at this is by using real numbers. Atmospheric pressure is roughly 100000 Pa. 100 Pa (1 part in a thousand) of over pressure corresponds to 133db (130db corresponds to a jet engine at 100 ft). So your calculation is essentially a far field calculation. Shock velocity is needed when you need to make near field corrections. For example a 1kton explosion at 1km has an average sound speed of 423 m/s with a Mach number of 1.034 while at 5 km 357 m/s with a Mach number of 1.004 (for air with a sound speed of 340 m/s)
  10. May 30, 2015 #9


    User Avatar

    Oops, the blast numbers came from Kinney and Graham Explosive Shocks in Air
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook