Noise reduction calculation 1

[StephenT]

I have an air handling unit in a mechanical room (surrounded by office space) whose walls are heavily insulated and has a baffle wall return air plenum to reduce noise transmittance. I am not concerned about the noise transmittance through the walls or baffle return. However, I am concerned about the noise transmittance through the supply ductwork.

At the fan discharge, the sound pressure is:
93 dB @ 63 Hz
95 dB @ 125 Hz
95 dB @ 250 Hz
89 dB @ 500 Hz
87 dB @ 1,000 Hz
78 dB @ 2,000 Hz
72 dB @ 4,000 Hz
67 dB @ 8,000 Hz
Because this is a low temperature (45 F) fan powered VAV system, we require additional insulation and have decided to use 1" sound lining (2 lbs/ft^3), 16 gage sheet metal (up to the first elbow outside of the mechanical room), and 1" external insulation (1.5 lbs/ft^3) to prevent condensation and reduce noise level. There is (1) 90 degree mitred elbow before the ductwork exits the mechanical room and the office space has a suspended acoustical ceiling.

My question is... Does anyone know where I can find USEFUL information to help in calculating the noise level I can expect in the office space based on the reduction from the elbow, sound lining, sheet metal, insulation, and SAC?

I am trying to stay away from duct silencers but still need to maintain an NC level less than 30 in the space. I would appreciate any information or suggestions you may have.

Thank you,
Steve

 

[MintJulep]

http://www.seam.com/consulting.htm

 

[ChasBean1]

There might be too many variables here to plug in numbers and get an accurate answer. Missing are duct velocities and installation of vibration isolators between fan systems and duct work. If velocities are kept low, details such as ceiling grid suspension wiring in contact with duct work is looked at and vibration isolators are used, the majority of the noise in the space will probably be from the fans in the FPTs. I'm not sure if there are any pre-made calcs as they would have to be very rough estimates without knowing specific installation details. Not very helpful, but some thoughts on the matter...

 

[StephenT]

Additional info:
The velocity in the ductwork is 1,550 fpm.
We use a flexible (canvas) connection to attach to the fan.
No vibration isolators to suspend the ductwork at this time. Has anybody else used/recommend vibration isolators to suspend the ductwork in a situation such as this?

 

[lilliput1]

For NC 30 I think you should plan on using sound attenuator or locate the 1st tap far enough from the fan and route it back to the room served. See IAC (Industrial Acoustics) "Snap" calculation form. Use minimum 3 db safety factor at each octave frequency.

Make sure you specify the proper pressure class duct construction and require beading/reinforcing of ductwork to prevent drumming or vibration of the ductwork.

Check also noise in the return ductwork.

Per ASHRAE, vibration isolation duct hangers are recommended on ducts suspended above or near a noise sensitive area. These hangers are recommended for ducts with velocity above 1500 FPM and for all duct sizes when duct pressure is 2" wg and over.
Verify design of walls with architect to minimize noise transmission. The mech room side wall panel should be isolated (not connected) to the other side wall panel & acoustic insulation should be in between.