The HVAC Factor: Sound Attenuation For Exposed Ductwork

By Nick Kaufmann, LEED AP
From the July/August 2016 Issue

HVAC ductwork
At Mi-T-M, fabric sound attenuators abate noise from 14 AHUs ranging from 2,625 to 4,400-cfms. (Photo: DuctSox Corp.)

Noisy spaces are difficult to work in, and productivity can decrease amid an excessive ambient hum of employees in cubicles and at desks, students in classrooms, or people in sound-sensitive spaces such as libraries, recording studios, and laboratories. However, there are solutions when a significant portion of that ambient noise rings out from exposed HVAC ductwork systems—whether these are round spiral metal or fabric-based air dispersion systems in open architectural ceilings.

There are three main reasons noise propagates through HVAC ductwork in an occupied space:

  • HVAC mechanical equipment fan noise reverberating its operating noise through the duct;
  • Internally generated duct noise due to air turbulence from ductwork, duct fittings or mechanical equipment, especially if it’s operating higher than the recommended 1,000 to 1,200-feet per minute (fpm) of airflow; and
  • noise caused by air outlets, such as vents, registers, or diffusers, that disperse air into the space.

And there are three main types of mechanical equipment noise: equipment vibration; equipment noise through walls, floors, or ceilings; and reverberated noise down the duct line.

Mechanical equipment vibrating a roof, floor, or wall that transcends into the space can be solved with isolation springs or pads. Meanwhile, equipment noise that passes through walls, floors, and ceilings is limited by the choice of building material components.

The third type—equipment noise reverberated down the duct—is more common and can be corrected. Excessive reverberation sound through ducts could occur in two possible scenarios: a poorly installed system; or an area not part of the original HVAC design that is converted to occupied space in close proximity to mechanical equipment.

Typically, open architectural ceilings—where ductwork is in view along with support trusses and the underside of the roof—will often feature either spiral round metal or round fabric ductwork.

Spiral round metal and, more recently, the fabric duct industry have introduced sound attenuating accessories that can be added to a duct run to absorb noise. Fabric sound attenuators are able to quiet air handling unit (AHU) and variable air volume (VAV) box operational and airflow noise in the 500, 1,000, and to 2,000-Hz octave bands by 28 to 35-dB. These sound attenuators, or duct silencers, are typically two duct diameters long and are slightly larger than the duct diameter to accommodate sound absorption materials, such as rock wool.

Probably the first action toward quieting an HVAC system is hiring an HVAC engineer or contractor to check the mechanical equipment for proper duct layout, airflow adjustments, balancing, and other refinements. The subsequent action is modifying the duct system. One or more metal/register duct runs can be modified with metal sound attenuators. Or metal duct runs can be replaced with quieter fabric duct and sound attenuators.

If the air distribution system is already fabric duct, then in most cases, a fabric sound attenuator can be zippered in, which was the approach employed by the staff at Mi-T-M, an industrial pressure washer and air compressor manufacturer in Peosta, IA.

The company was experiencing mechanical equipment noise reverberation from 14 AHUs in its new two story, freestanding 68,000 square foot office building. The facility’s 20 duct runs totaling 1,388 linear feet were specified as fabric duct. The noise was surprising, given that fabric duct in general has a soft texture that absorbs noise.

Mi-T-M’s plant maintenance staff unsuccessfully attempted attenuating mechanical noise by reducing AHU airflow by 60% with the AHUs’ built-in variable frequency drives (VFD).

Success was achieved when a contractor retrofitted the duct runs by adding sound attenuators designed for fabric duct. Most fabric duct lengths and fittings zipper together; thus, the sound attenuators were easy to install by zippering them inline and connecting them to the support cable. These attenuators reduced the sound by 11-dB, an impressive number considering that dB ratings are exponential.

Kaufmann is director of manufacturing and engineering for DuctSox Corp., a Peosta, IA-based manufacturer of fabric ductwork/accessories. He is a 22 year veteran of the HVAC/R industry.

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