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Sound Power, Sound Pressure

Discussion in 'Electronic Basics' started by Winston, Jun 17, 2011.

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  1. Winston

    Winston Guest

    I'm a noob trying to get a visceral grasp of the science.


    For a perfect free-field site, sound power and sound pressure
    are said to be related:

    Lw = Lp + 10 * log10(2*pi*r^2)

    Lp = Sound pressure in dB. 0 dB = 20 micro Pascals
    Lw = Sound power in dB. 0 dB = 1 pico watt
    r = Distance from device to microphone in meters

    Does this mean that if I could place my microphone at a
    distance of 398.94 mm from the device under test that
    my sound pressure reading will be within 0.0001 dB of
    my sound power reading even though 'sound pressure' dBs
    are different from 'sound power' dBs?


  2. Phil Allison

    Phil Allison Guest


    ** Looks that way to me - at least in theory, with a perfect point source.

    All it tells you is that a point source with a sound power of 1 pW produces
    a SPL of 0dB at 399 mm.

    ..... Phil
  3. Winston

    Winston Guest

    By extension, doesn't it also say that a point source with a sound power
    of say 30 dB would measure 30 dB sound pressure, all else being equal?


  4. Phil Allison

    Phil Allison Guest


    ** Yep.

    SPL and Sound Power match numerically at that magic distance.

    Dunno what good it is to you ....

    ..... Phil
  5. Winston

    Winston Guest

    Phil Allison wrote:

    It's easier for me to understand the arithmetic on a
    'gut level' if I can draw some conclusions from the
    equations. I'd like to get to a point where I can tell
    if a reading is within a reasonable range or if it is
    'probably wrong'.


  6. Phil Allison

    Phil Allison Guest

    ** Same here - you have to get a feel for the quantities involved.

    ** That is usually called a " sanity check ".

    .... Phil
  7. Winston

    Winston Guest

    I appreciate your help.

  8. Winston

    Winston Guest

    I went to Rochester, Minnesota on a service mission once.
    Those folks *know* about cold. Man!

  9. Rich Grise

    Rich Grise Guest


    60 degrees F: Southern Californians shiver uncontrollably. People in
    Minnesota sunbathe.

    50 F: New Yorkers try to turn on the heat. People in Minnesota plant

    40 F: Italian & English cars won't start. People in Minnesota drive with the
    windows down.

    32 F: Distilled water freezes. The water at Lake Bemidji in Minnesota starts
    getting cooler.

    20 F: Floridians don coats, thermal underwear, gloves, wool hats. People in
    Minnesota throw on a flannel shirt, buttons open.

    15 F: New York City landlords finally turn up the heat. People in
    Minnesota have the last cookout before it gets cold.

    0 F: All the people in Miami die. Minnesotans close the windows.

    10 below zero: Californians escape en masse to Mexico. Girl Scouts in
    Minnesota sell cookies door to door.(In fact, they're selling them right

    25 below zero: Las Vegas disintegrates. People in Minnesota rummage
    around the attic to find some winter coats.

    40 below zero: Washington DC runs out of hot air. People in Minnesota let
    the dogs sleep indoors.

    100 below zero: Santa Claus abandons the North Pole. Some Minnesotans are
    frustrated when they can't start their cars.

    460 below zero (absolute zero on the Kelvin Scale): All atomic motion stops.
    People in Minnesota start saying . . . "Cold 'nuff for ya?"

    500 below zero: Hell freezes over. In Minnesota, VIKINGS win the Super
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