# Sound Power, Sound Pressure

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

1. ### WinstonGuest

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

Background:

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

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

www.me.mtu.edu/courses/meem4704/lab/lab4homework.pdf

Where:
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

Question:
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?

Thanks!

--Winston

2. ### Phil AllisonGuest

"Winston"

** 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. ### WinstonGuest

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?

Thanks

--Winston

4. ### Phil AllisonGuest

"Winston"

** Yep.

SPL and Sound Power match numerically at that magic distance.

Dunno what good it is to you ....

..... Phil

5. ### WinstonGuest

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'.

Thanks!

--Winston

6. ### Phil AllisonGuest

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

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

.... Phil

--Winston

8. ### WinstonGuest

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

--Winston

9. ### Rich GriseGuest

MINNESOTA TEMPERATURE CONVERSION CHART

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
gardens.

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
now!!)

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
Bowl!!