Not an HVAC guy, but FWIW,
at best the new ducts will reduce airspeed 30%. Maybe the perceived noise level will drop more than this.
Re: 4 feet, adding internal duct vanes may reduce noise due to air turbulence.
This
http://www.faqs.org/patents/app/20090036047
claims to tune the system for reduced noise, I guess much like an auto muffler is tuned to cancel out noise at some freqs.
This one
http://www.faqs.org/patents/app/20090098818
is hard to read but seems to claim it can "straighten" air flow with vanes.
From the Web, looks like dampers increase turbulence, but maybe I should have said, "Sounds like dampers increase turbulence."
I guess an auto A/C system almost always equals a poor ductwork design/install because the auto marketing people define the overall auto internal geometry and leave it to others to reduce the auto A/C noise level, so I'd look into auto A/C designs for solutions to problems like this.
Here's another option
http://www.acousticalsurfaces.com/hvac/duct_silen.htm?d=26
at best the new ducts will reduce airspeed 30%. Maybe the perceived noise level will drop more than this.
Re: 4 feet, adding internal duct vanes may reduce noise due to air turbulence.
This
http://www.faqs.org/patents/app/20090036047
claims to tune the system for reduced noise, I guess much like an auto muffler is tuned to cancel out noise at some freqs.
This one
http://www.faqs.org/patents/app/20090098818
is hard to read but seems to claim it can "straighten" air flow with vanes.
From the Web, looks like dampers increase turbulence, but maybe I should have said, "Sounds like dampers increase turbulence."
I guess an auto A/C system almost always equals a poor ductwork design/install because the auto marketing people define the overall auto internal geometry and leave it to others to reduce the auto A/C noise level, so I'd look into auto A/C designs for solutions to problems like this.
Here's another option
http://www.acousticalsurfaces.com/hvac/duct_silen.htm?d=26
