Acoustics and HVAC Systems
Low Frequency Noise in Ducts:
A SOUND Argument for the Benefits of Foam Insulation
Insulation materials traditionally have been applied to the interior or exterior of ductwork for a combination of purposes, including thermal efficiency, condensation control and noise control. Of these, effective noise control is by far the most challenging. It is important to remember that a conventional duct liner, and to an even lesser extent, duct wrap, will only resolve a portion of the noise issues in any building. An effective noise-attenuation strategy begins at the design stage and utilizes a combination of methods including mechanical layout, vibration isolation and duct insulation.
Types of Noise Associated with Ducts
The standard definition of noise, which is any unwanted sound, is a deceptively simple summation of a complex issue that can have a tremendous impact on our work and learning environments. According to the National Institute for Occupational Safety and Health, ambient noise also affects people’s health by increasing general stress levels and aggravating such stress-related conditions as high blood pressure, coronary disease, peptic ulcers and migraine headaches.1 Productivity and learning are affected as well.
Acousticians like to talk about sound in terms of two categories: structure-borne sound and airborne sound. The latter, airborne sound, is the noise we actually hear. It can and will travel anywhere there is air. Structure-borne sound results from a physical vibration of materials caused by some impact event or other form of mechanical excitation that causes vibration.
In most cases, structure-borne sound travels through the building structure via construction materials, frame and interior elements. It eventually becomes airborne sound that can be heard at some distance from its source, perhaps several floors away. Airborne sound also can become structure-borne, causing surrounding surfaces to vibrate. Consider a sheet-metal duct, a virtual wind tunnel through which any noise can travel. It provides a structural and an airborne path for noise generated by mechanical equipment such as fans and chillers. It also can transmit noise from virtually any other source in the building, including people, speakers and machines. This airborne-to-structure-borne conversion can repeat multiple times until the sound source is switched off.
This is why good acoustic engineering in a building requires an integrated noise control strategy.
Structure-borne noises are most effectively controlled using vibration-isolation and/or other structural-isolation techniques. Acoustic duct lining is mainly efficient for controlling air-borne noise and can, to an extent, minimize the panel vibration.
Significance of Frequency
Figure 1, taken from the ASHRAE Chapter on “Sound and Vibration Control,” breaks down the likely sources of sound-related complaints in an HVAC system. All of these noises can travel through the air in the duct in the form of sound waves or through the duct panels in the form of mechanical vibration. As the graph illustrates, sources for sound-related complaints in a mechanical system are typically broadband and range from the low-frequency “THROB” of turbulent airflow and fan instability to the high-frequency “HISS AND WHISTLE” of grills and water valves.
Problematic noises in the lowest frequency ranges (below 125 Hz) and the highest (above 1000 Hz) are not likely to be alleviated by duct liner as they occur either due to poor equipment isolation or stability or emit downstream from the duct. It is generally well known and accepted by acoustic engineers that the 125-1000 Hz range is typically the most problematic in an HVAC system. This is the range that can and should be addressed with duct liner.
It is worth noting that the American Standard ANSI S3.1 – 1977 identifies 500Hz, 1000Hz, and 2000Hz as the key frequencies for speech intelligibility. This means that noise in those frequencies is arguably the most disruptive to the human ear when it comes to conversation and should be minimized, particularly in such spaces as classrooms and lecture halls.
Comparative Performance of Elastomeric Foam at Critical Frequencies
Commonly used fiber-based duct liners absorb sound primarily through viscous and frictional losses of the air oscillating inside the material. Noise is reduced because of frictional effects inside the material. These materials, in typical 1-inch thicknesses, are effective over a relatively broad range of frequencies, but their sound absorption performances diminish significantly at 500 Hz and below. Thus, when low-frequency noise is anticipated as a problem, greater thickness (and length) of fibrous material is required or else the designed sound attenuation will not be achieved.
Elastomeric foam products, such as AP ArmaFlex and AP CoilFlex, react to sound quite differently. Unlike fiberglass, which allows sound to freely enter into the spaces between the fibers, closed-cell elastomeric foam is too highly resistive to enable the viscous friction of air inside to develop an effective absorption mechanism. However, instead of simply reflecting the sound like many other types of rigid foam materials, the physical properties of elastomeric foams are such that their structure actually absorbs sound. This response can be particularly pronounced at critical low frequencies below 500 Hz. In the case of AP ArmaFlex and AP CoilFlex, a significant proportion of the incident-sound energy is converted into movement of the foam and eventually into heat.
1. From the January 2010 Scientific American Mind, “How does background noise affect our concentration?”