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Re: Estimating Sound propagation  Rob Fletcher
 Jun 15, 2005 13:57 PDT 

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Just a quick note to point out a few items regarding sound attenuation. It is correct that the air attenuates the sound...but there is a lot more going on that needs to be considered. This only accounts for the actual dampening of the sound waves by the air, but does not account for any geometric effects of the sound source itself. One must consider if the sound is emanating from a point source or a line source. A point source could be a single wind turbine, an example of a line source could be a continuously used highway with heavy traffic, or perhaps a line of wind turbines in a row as observed from a distance. As a result the dB attenuation, though small due to air...as indicated, can indeed actually be significant. Note that a point source with a power level of 100 dB will have a sound pressure level of 52 dB at 100 meters given typical atmospheric temperature and pressure levels. The required equation is SPL = PWL + DI - 20*log(r) - 11dB, where:

SPL = sound pressure level at the distance from the source
PWL = power level at the source
DI = directional index (for spherical = 0, for hemispherical = 3, and for reflecting plane single wall = 6 and for three corner walls = 9)
log = base 10 logarithms

So for example if we have a single wind turbine (which we will call a point source) operating at 140 dB what would the sound be at 400 meters away?

SPL = 140 + 3 - 20*log(400) - 11
SPL = 140 + 3 - 52.04 - 11
SPL = 79.96 dB

So almost half! This does take into account the attenuation of the air.

For a line source the simplified equation is

SPL = PWL + 10*log(alpha/r) - 8dB (note that there is actually a little more required that indicated here to deliver high accuracy, but what is illustrated here keeps it simple), where

alpha = aspect angle in radians of the observer to the line source
r = distance from the source to the observer

So calculating for a line source again for 140 dB and assuming an aspect angle of about 45 degrees and 400 meters

SPL = 140 + 10*log((pi/4)/400)) - 8
SPL = 140 - 27.07 - 8
SPL = 104.9 dB

For a string of wind turbines with each spaced at least 5 rotor diameters apart perpendicular to the wind (as would be required for a minimum to limit rotor turbulence) this dB number will be lower.

You can go to the following web site to calculate sound attenuation from a wind turbine over a give distance...

http://www.windpower.org/en/tour/env/sound.htm#minor

Rob Fletcher

-----------------------------------------------------------------
Robert W. Fletcher, Ph.D.
Associate Professor
Director, Alternative Energy Program
Mechanical Engineering Department
Lawrence Technological University
21000 West Ten Mile Road
Southfield, Michigan 48075-1058
USA
Telephone: 248-204-2525
Fax: 248-204-2576
e-mail: rflet-@LTU.edu
-----------------------------------------------------------------
----- Original Message -----
From: "Daniel Alberts" <galactic-@nethere.com>
To: "Michigan Wind Working Group" <mw-@topica.com>; <amsn-@mtu.edu>; <gboke-@fsec.ucf.edu>; <ghar-@garfield-twp.com>; <hsgo-@mtu.edu>; <hutz-@mtu.edu>; <jessess-@yahoo.com>; <jlri-@mtu.edu>; <jsh-@pasty.com>; <jsil-@nmu.edu>; <mjha-@hotmail.com>; <thes-@hotmail.com>; <pcjo-@chartermi.net>
Cc: "Darren Brown" <djbr-@kandse.com>
Sent: Wednesday, June 15, 2005 2:45 PM
Subject: [Delphi] Estimating Sound propagation


 Everyone,

This web site provides a rough first estimate of outdoor sound attenuation
based on ISO 9613.


http://www.measure.demon.co.uk/Acoustics_Software/iso9613.html


For example, at 20 C, and 77% humidity, a Concert A (440 Hz) will reduce in
volume 2.3 dB per kilometer. A low frequency sound (200 Hz) will reduce only
.8 dB per km.

Of course, this doesn't take into account sound absorbed by trees or
reflected by buildings. But it gives a good 'first estimate'.

-Daniel

Daniel Alberts
Lawrence Tech's Wind Energy Delphi
www.ltu.edu/engineering/mechanical/delphi_wind.asp
dja1-@nethere.com
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<DIV>
<DIV><FONT face=Arial size=2>Just a quick note to point out a few items
regarding sound attenuation.  It is correct that the air attenuates
the sound...but there is a lot more going on that needs to be considered. 
This only accounts for the actual dampening of the sound waves by the
air, but does not account for any geometric effects of the sound source
itself.  One must consider if the sound is emanating from a point source or
a line source.  A point source could be a single wind turbine, an example
of a line source could be a continuously used highway with heavy traffic,
or perhaps a line of wind turbines in a row as observed from a distance. 
As a result the dB attenuation, though small due to air...as indicated, can
indeed actually be significant.  Note that a point source with a power
level of 100 dB will have a sound pressure level of 52 dB at 100 meters given
typical atmospheric temperature and pressure levels.  The required equation
is SPL = PWL + DI - 20*log(r) - 11dB, where:</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>SPL = sound pressure level at the distance
from the source</FONT></DIV>
<DIV><FONT face=Arial size=2>PWL = power level at the source</FONT></DIV>
<DIV><FONT face=Arial size=2>DI  = directional index (for spherical =
0, for hemispherical = 3, and for reflecting plane single wall = 6 and for
three corner walls = 9)</FONT></DIV>
<DIV><FONT face=Arial size=2>log = base 10 logarithms</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>So for example if we have a single wind turbine
(which we will call a point source) operating at 140 dB what would the sound be
at 400 meters away?</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>SPL = 140 + 3 - 20*log(400) - 11</FONT></DIV>
<DIV><FONT face=Arial size=2>SPL = 140 + 3 - 52.04 - 11</FONT></DIV>
<DIV><FONT face=Arial size=2>SPL = 79.96 dB</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>So almost half!  This does take into account
the attenuation of the air.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>For a line source the simplified equation
is</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>SPL = PWL + 10*log(alpha/r) - 8dB (note that
there is actually a little more required that indicated here to deliver high
accuracy, but what is illustrated here keeps it simple), where
</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>alpha = aspect angle in radians of the observer to
the line source</FONT></DIV>
<DIV><FONT face=Arial size=2>r = distance from the source to the
observer</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>So calculating for a line source again for 140 dB
and assuming an aspect angle of about 45 degrees and 400 meters</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>SPL = 140 + 10*log((pi/4)/400)) - 8</FONT></DIV>
<DIV><FONT face=Arial size=2>SPL = 140 - 27.07 - 8</FONT></DIV>
<DIV><FONT face=Arial size=2>SPL = 104.9 dB</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>For a string of wind turbines with each spaced at
least 5 rotor diameters apart perpendicular to the wind (as would be
required for a minimum to limit rotor turbulence) this dB number will be
lower.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>You can go to the following web site to calculate
sound attenuation from a wind turbine over a give distance...</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><A href="http://www.windpower.org/en/tour/env/sound.htm#minor"><FONT
face=Arial
size=2>http://www.windpower.org/en/tour/env/sound.htm#minor</FONT></A></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Rob Fletcher</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial
size=2>----------------------------------------------------------------- 
<BR>Robert W. Fletcher, Ph.D.<BR>Associate Professor<BR>Director, Alternative
Energy Program<BR>Mechanical Engineering Department<BR>Lawrence Technological
University<BR>21000 West Ten Mile Road<BR>Southfield, Michigan
48075-1058<BR>USA<BR>Telephone: 248-204-2525<BR>Fax: 248-204-2576<BR>e-mail:
</FONT><A href="mailto:rflet-@LTU.edu"><FONT face=Arial
size=2>rflet-@LTU.edu</FONT></A><BR><FONT face=Arial
size=2>-----------------------------------------------------------------</FONT></DIV></DIV>
<DIV><FONT face=Arial size=2>----- Original Message ----- </FONT>
<DIV><FONT face=Arial size=2>From: "Daniel Alberts" <</FONT><A
href="mailto:galactic-@nethere.com"><FONT face=Arial
size=2>galactic-@nethere.com</FONT></A><FONT face=Arial
size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>To: "Michigan Wind Working Group" <</FONT><A
href="mailto:mw-@topica.com"><FONT face=Arial
size=2>mw-@topica.com</FONT></A><FONT face=Arial size=2>>; <</FONT><A
href="mailto:amsn-@mtu.edu"><FONT face=Arial
size=2>amsn-@mtu.edu</FONT></A><FONT face=Arial size=2>>; <</FONT><A
href="mailto:gboke-@fsec.ucf.edu"><FONT face=Arial
size=2>gboke-@fsec.ucf.edu</FONT></A><FONT face=Arial size=2>>;
<</FONT><A href="mailto:ghar-@garfield-twp.com"><FONT face=Arial
size=2>ghar-@garfield-twp.com</FONT></A><FONT face=Arial size=2>>;
<</FONT><A href="mailto:hsgo-@mtu.edu"><FONT face=Arial
size=2>hsgo-@mtu.edu</FONT></A><FONT face=Arial size=2>>; <</FONT><A
href="mailto:hutz-@mtu.edu"><FONT face=Arial
size=2>hutz-@mtu.edu</FONT></A><FONT face=Arial size=2>>; <</FONT><A
href="mailto:jessess-@yahoo.com"><FONT face=Arial
size=2>jessess-@yahoo.com</FONT></A><FONT face=Arial size=2>>;
<</FONT><A href="mailto:jlri-@mtu.edu"><FONT face=Arial
size=2>jlri-@mtu.edu</FONT></A><FONT face=Arial size=2>>; <</FONT><A
href="mailto:jsh-@pasty.com"><FONT face=Arial
size=2>jsh-@pasty.com</FONT></A><FONT face=Arial size=2>>; <</FONT><A
href="mailto:jsil-@nmu.edu"><FONT face=Arial
size=2>jsil-@nmu.edu</FONT></A><FONT face=Arial size=2>>; <</FONT><A
href="mailto:mjha-@hotmail.com"><FONT face=Arial
size=2>mjha-@hotmail.com</FONT></A><FONT face=Arial size=2>>;
<</FONT><A href="mailto:thes-@hotmail.com"><FONT face=Arial
size=2>thes-@hotmail.com</FONT></A><FONT face=Arial size=2>>;
<</FONT><A href="mailto:pcjo-@chartermi.net"><FONT face=Arial
size=2>pcjo-@chartermi.net</FONT></A><FONT face=Arial size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>Cc: "Darren Brown" <</FONT><A
href="mailto:djbr-@kandse.com"><FONT face=Arial
size=2>djbr-@kandse.com</FONT></A><FONT face=Arial size=2>></FONT></DIV>
<DIV><FONT face=Arial size=2>Sent: Wednesday, June 15, 2005 2:45 PM</FONT></DIV>
<DIV><FONT face=Arial size=2>Subject: [Delphi] Estimating Sound
propagation</FONT></DIV></DIV>
<DIV><FONT face=Arial><BR><FONT size=2></FONT></FONT></DIV><FONT face=Arial
size=2>> Everyone, <BR>> <BR>> This web site provides a rough first
estimate of outdoor sound attenuation<BR>> based on ISO 9613.<BR>>
<BR>> <BR>> </FONT><A
href="http://www.measure.demon.co.uk/Acoustics_Software/iso9613.html"><FONT
face=Arial
size=2>http://www.measure.demon.co.uk/Acoustics_Software/iso9613.html</FONT></A><BR><FONT
face=Arial size=2>> <BR>> <BR>> For example, at 20 C, and 77% humidity,
a Concert A (440 Hz) will reduce in<BR>> volume 2.3 dB per kilometer. A low
frequency sound (200 Hz) will reduce only<BR>> .8 dB per km.<BR>> <BR>>
Of course, this doesn't take into account sound absorbed by trees or<BR>>
reflected by buildings. But it gives a good 'first estimate'.<BR>> <BR>>
-Daniel<BR>> <BR>> Daniel Alberts<BR>> Lawrence Tech's Wind Energy
Delphi<BR>> </FONT><A
href="http://www.ltu.edu/engineering/mechanical/delphi_wind.asp"><FONT
face=Arial
size=2>www.ltu.edu/engineering/mechanical/delphi_wind.asp</FONT></A><BR><FONT
face=Arial size=2>> </FONT><A href="mailto:dja1-@nethere.com"><FONT
face=Arial size=2>dja1-@nethere.com</FONT></A><BR><FONT face=Arial size=2>>
<BR>>
<BR>> <BR>> <BR>> <BR>> <BR>> <BR>> </FONT></BODY></HTML>

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