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Acoustic insulation in supply & Return Duct up to 3 Mtrs

Acoustic insulation in supply & Return Duct up to 3 Mtrs

(OP)
Dear member,
It is general practice to provide 3 mtrs Acoustic insulation in supply and return duct from AHU. Irrespective of AHU capacity. i.e CFM. However, low capacity AHU/FCU/Split Ducted unit may not required upto 3mtrs acoustic insulation. Hence, on what basis acoustic insulation shall be provided upto 3mtrs or less?

RE: Acoustic insulation in supply & Return Duct up to 3 Mtrs

For me leaving out the acoustical insulation is a function of:
Is the unit located near a sound sensitive space
How many elbows is in the ductwork
Does it serve noise sensitive spaces.

Good questions to ask an acoustical consultant, the next time you work with one on a project.

knowledge is power

RE: Acoustic insulation in supply & Return Duct up to 3 Mtrs

Both Vibro Acoustics and IAC publish sound analysis software that applied correctly will answer your question based on the noise generated by your unit. Trane has/had a good program for purchase. Many local representatives will apply the programs to generate attenuation selections for lazy engineers. The calculations involve logarithmic addition and subtraction across octave bands and is not difficult. I feel design for noise control in HVAC systems is part of a complete deliverable similar to sizing a fan for static pressure.

3 mtrs is a lazy someone's rule of thumb that can get you in trouble in demanding applications.

RE: Acoustic insulation in supply & Return Duct up to 3 Mtrs

Usually acoustic lining do not provide enough attenuation. You may need silencers. See ASHRAE or SMACNA for how to calculate sound pressure loss through ductwork and fittings and recommended acceptable sound level for room functions. Get fan sound data from fan manufacturer. Select silencer type, size and length that would provide adequate attenuation and acceptable pressure drop.

RE: Acoustic insulation in supply & Return Duct up to 3 Mtrs

Get your hands on insertion loss data for internally lined duct to determine what length of treatment you need. I agree that a mech engineer should be able to handle 90% of HVAC acoustic design situations.

RE: Acoustic insulation in supply & Return Duct up to 3 Mtrs

Lining of ductwork is now not preferred and is absolute no no in hospitals. They retain dust and subject to growing mold if the air stream is wet.

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