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Sound Attenuators

Bill Dunn

Happy Thanksgiving!

(Just in case you’ve visited Home Depot or Walmart during the past 6 weeks and forgot there is another big holiday before Christmas)



This month, let’s talk about a minor aspect of the HVAC system, which can have HUGE impact on room comfort: Sound Attenuators.

Sound attenuators, often called duct silencers, reduce noise in the HVAC system, so the occupied spaces are not uncomfortably noisy. (The responsibility for making the occupied spaces uncomfortably noisy is left in the capable hands of LuEllen, who likes to have loud and angry phone calls with her latest boyfriend; and Zeke, who emits some stunning sounds – and smells – every time he has lunch at Taco Bell. Note: the names of the exuberant coworkers in your office may be different.)


Sound attenuators are typically fabricated with the same dimensions as the ductwork they are serving. They become a section of the duct system. Internally, however, sound attenuators have special baffles that are engineered to reduce noise energy, while keep pressure drop as low as possible.


There are three primary types of attenuators:


Dissipative – the internal baffles are made of perforated metal with sound absorbing fiberglass packed behind it. These models have the best sound reduction performance. The Vibro-Acoustics models begin with “RD” (straight) or RED” (elbow).


Film-Lined – with this type, plastic sheeting encapsulates the fiberglass behind the perforated metal to make sure no fibers break free and get into the airstream. They are often used in medical facilities.The sound reduction is slightly less effective compared to Dissipative attenuators. The Vibro-Acoustics models begin with “RFL” (straight) and “REFL”(elbow).


No-Media – the no-media attenuators have a series of chambers, similar to a car muffler, but no fiberglass media. They are often used in laboratory applications. Acoustical energy is captured internally, but the effectiveness is noticeably less than the first two types. The Vibro-Acoustics model numbers begin with “RNM” (straight) and “RENM” (elbow).



Since these three types of attenuators deliver different sound performances, please make sure the scheduled model numbers and specifications match. If they don’t, please do not guess what is needed. Check with the design team. Here are some other aspects of sound attenuators to be aware of:


1. High Transmission Loss (HTL) casings. In certain situations, “break-out noise” can be a problem. This is noise that radiates right through the duct before ever reaching the supply diffuser. In these cases, the attenuator is fabricated with thicker gauge sheet metal to keep that noise from getting out.


For contractors: make sure you check the schedule notes and specs to see if HTL is required, as it is an expensive up-charge.


For consulting engineers: make sure you consider during the design stage of a project whether “break-out noise” may be a problem. It’s extremely hard to solve after everything is installed. Feel free to contact us for assistance.



2. System Effect Pressure Drop - Because the internal sound-absorbing baffles reduce the free area, the air velocity through an attenuator is much faster than the rest of the ductwork. The high velocity air coming off the discharge of the attenuator does not enjoy an immediate change of direction because of an elbow, damper, transition, coil, etc. A great deal of turbulence will occur, which can make the pressure drop listed on the data sheet double or triple. This is called System Effect, which can cause a reduction in airflow throughout the entire HVAC system. Quite often, the solution to potential System Effect is to use an elbow attenuator rather than trying to squeeze a straight model in a tight spot. Again, feel free to call us for assistance.




3. Low Frequency Noise – Low-pitch rumbling noise is the most difficult to attenuate. For example, the sound wavelength in the first octave band, 63 Hertz, is approx. 18 feet long. And guess what air handlers produce in abundance? That’s right, low frequency noise. The most effective way to attenuate these long wavelengths is with a long attenuator. This means if the length of an attenuator is reduced, the sound performance also will be reduced – sometimes drastically.


For contractors: Please do not arbitrarily reduce the length of attenuators because they will not fit in the duct layout. Check with the design team first. The reduction in sound performance could be disastrous.


For consulting engineers: If you make a last-minute change to the length dimension of attenuators on your schedule, PLEASE also adjust the sound performance numbers (Dynamic Insertion Loss) accordingly. If the model numbers and dimensions do not match the sound performance, it can be a big problem.


If you’ve ever worked with acoustical consultants, then you know making last-minute changes, especially to the length dimensions of attenuators, will make them very cranky. When this happens, those acousticians are anything but quiet.

Preventing HVAC noise problems can be a challenge.When the owner and occupants of the building start complaining, it is not a fun situation. At Melia Associates LLC, we have decades of experience in this field. Plus, we have some of the finest application engineers in North America on our team: the staff at Vibro-Acoustics in Toronto.


As I mentioned at the beginning, sound attenuators area a minor aspect of the HVAC system, but they can have a HUGE impact on room comfort. So, please don’t cross your fingers and hope everything works out. Give us a call early on and, if needed, we’ll get the experts involved who can do an acoustical analysis – free of charge!


Happy Thanksgiving!


Bill Dunn, Engineering Sales, Melia Associates LLC 860-290-6969, bdunn@meliaassociates.com

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Address

43 Thomas Street, East Hartford CT

06108

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