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!

How noisy was it at source?

  1. May 28, 2012 #1
    If I am using a brush cutter creating 98db at ear (manufacturers claim) and I can still hear music from 30m away what would the db be at the source of the music.

    I was wearing ear defenders but presume that they would work equally on both sources of noise.

    I am lead to believe that the source (i.e. speakers) were pointing away from me. Does the direction of the speakers have any impact?

    What are your thoughts and is there a db range as an answer?
  2. jcsd
  3. May 28, 2012 #2


    User Avatar
    2017 Award

    Staff: Mentor

    To find any number, you would have to find out which db difference you could hear, which depends on the music and the noise and probably your ear and brain as well.
    As a second problem, "db at the source" is not well-defined - the sound level depends on the position where you measure it, point-like sources do not emit sound.
    And as third issue, speakers usually don't emit the same power in all directions. The orientation of the speakers does matter, and it is complicated to evaluate how.
    In addition, the environment can reflect sound in some non-trivial way.
  4. May 29, 2012 #3
    Thank you for those comments.

    I agree that there are numerous factors that can affect the calculation for instance the vertical mudstone escarpment 12 meters in front of the speakers.

    Most equipment these days seems to quote a db value for either a meter or for the user.

    What I would like to achieve is something very simplistic.

    I would like to calculate from the known facts a graph that would show what the sound would be at 10 meters and 20 meters ignoring all factors like orientation, atmosphere and topography etc.

    Is there a simple formula to do this? If so it would be very helpful for the discussion I am having.
  5. May 29, 2012 #4


    User Avatar
    2017 Award

    Staff: Mentor

    Without any reflection, absorption and other issues, the intensity drops with the inverse distance squared, if the distance is large compared to the size of the source.
    This can be transferred to dB - a factor of 10 is 10dB, a factor of 4 (10m->20m) is something like ~6dB.
  6. May 29, 2012 #5


    User Avatar
    Gold Member

    Your question reminds me of the difficulty sonar operators have when trying to isolate a submarine target by finding the "signal" when it's immersed in "noise". They use sopiisticated digital signal processing, including dFFTs to try to do this.

    Your cutting machine generates the background noise, yet you can hear the music even with those ear protection "muffs" on. On an aircraft carrier flight deck with many jet engines running one can hear verbal communications from nearby neighbors. Seems our brains can filter out the "signal" from the "noise" without too much effort.

    Here’s an excellent overview that contains several answers to your questions: http://www.physicsclassroom.com/class/sound/u11l2b.cfm

    As for ear protection from noise from machinery and jet engines see what’s done at airports: http://www.noisebuster.net/nw-aviation.html
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook