Silencer Installation

The attentuator should be mounted as close to the Air Handling Unit as possible. That is its purpose, to attenuate the noise from the machine. If the equipment room is too small to install the attentuator on the outlet of the air handler, I would suggest you transition from the discharge of the air handler to the attentuator, sizing the duct for the cfm and velocity you intend to carry in the duct. If the attentuator is larger than the duct, I would make the transition to fit the attentuator, then on the outlet of the attentuator, transition the duct to your required size.

The manufacturers of such equipment should be able to advise as to the size and location of silencers as that is their business and stock in trade.

yes it is alright but there are some other things to consider. what is the NC level you are trying to achieve? i assume that you are attenuating air-borne noise from the machine but if you concerned of the noise besides the machine room consider sound proofing the walls instead. sometimes the equipment sound just passes through the walls and on to the adjacent rooms without feeling the effect of the duct mountd attenuator. do the basics of sound transmission and determine what are those that need to be provided with attenuators. using internally lined ducts instead is sometimes more economical than using sound attentuators.