a. Is it structure borne i.e. through the unit into the building and shaking it to bits
or
b. is it radiated from the condenser fans or compressors to the surroundings.
If (a) then either check with the manufacturer to see if the spring isolators are ok or can be modified. Have the retaining bolts been removed that hold the springs in place.
You could also mount the unit on spring isolators. An inertia base would also help..
speak to IAC Industrial Acoustics Ltd.. They can advise you.
b.Radiated sound to the surroundings can be taken out by fitting a parapet wall to intercept the sound. If the sound path is interupted it greatly reduces the noise.
A 'wall' or fence/barrier just as high as the unit or slightly higher will have a big effect.
Some units can also have attenuators fitted to each individual fan and an acoustic 'jacket' fitted to the compressor.
Consult the manufacturer (and IAC Ltd as alreday mentioned )
What would the load profile of the unit look like. If there is extra condenser most of the time consider installing VFD's on the condenser fan motors and running more of them at slower speeds.
While this won't make your unit any quieter on a design day, it might help quiet it down enough 7 days out of 10. You would also need to install a pressure sensor and setpoint controller to handle the VFD modifications.
Here's a bit more info on the problem. It appears that most of the noise is low rumbling from the supply duct. The problem is further compounded because there is no shaft (open to office areas below). Any suggestions on how to dampen the rumble?
If the supply duct near the discharge from the unit is accessable, consider adding a sound trap to the duct.
But first check the unit to see if there are vibration isolators on the supply fan and have the shipping brackets been removed? Are the drives and belts adjusted properly, in good condition and aligned properly? Is the fan wheel clear, free of any debris (Like paper, insulation, roofing material the roofers discarded in the return duct or maybe the installation / operating manual left inside the unit on start-up?)A fan out of balance can make a lot of noise and vibration.
Firstly, put in some good quality Vibration Isolators.
For the Supply Duct borne noise, you must use a Sound attenuator. This must be selected to provide the right Insetion Loss.
If costs permit also look at Floating Floors.
Low frequency noise 'in duct' is relativel;y easy to get rid of. The attenuation needed will ahve to be 50mm thick or more. Line the bends if you can as this will have the greatest effect.
Self adhesive 'pyrosorb' high density foam is good as it is also class 'O' fire rated.
Low frequency noise can be tackled by attenuating the larger ducts rather than the smaller ones. Treating smaller ducts tends to affect the higher frequencies.
It would help to actually measure the problem with a sound meter to see how much noise you need to take out. You can hire meters pretty cheaply. Use a meter with frequency filters instead of a simple dBA read out.
On the supply air side, the air handling system should be designed for very low velocity to minimize turbulence, which is what creates noise in ductwork also using long-radius elbows and lining return ducts to attenuate any residual noise.”
Sound traps, which are special sections of ductwork that have very thick internal sound attenuating baffles provide a lot of attenuation in a very short distance of duct.
If you design the sound traps, specially shaped and custom built, so that they could be conveniently installed in the existing structure that would expedite the problem.
If the air handler fan motors are equipped with variable frequency drives such that only enough air is distributed to meet the cooling load then this feature not only contributes to low noise, but also saves considerable energy.
