CONTROLLING HVAC NOISE
Controlling HVAC noise is vital. Continuous or intermittent noise from an HVAC system can be distracting as well as disruptive. While mechanical engineers focus on a system delivering proper airflow, acoustic performance is often ignored. When determining if an HVAC system requires additional attenuation, two noise paths must be considered; noise that travels through both the supply and return duct system paths, and “breakout” noise that travels through the walls of the ductwork into neighboring spaces. Airborne noise in an HVAC system is attenuated with the addition of duct silencers, and breakout noise is attenuated with the addition of a dense duct lagging material wrapped around the outside of the ductwork to increase the STC of the duct walls.
There are two main categories of duct silencers; active duct silencers and passive duct silencers. Active duct silencers operate electronically by means of microprocessors, loudspeakers and microphones. They cancel sound by feeding back an additional noise source that is 180 degrees out of phase with the original noise. In theory, noise reduction is effective for low frequencies, usually below 300, Hz. Initial research to develop a commercial product was particularly strong in Great Britain, but today research still goes on throughout the world. Passive duct silencers are so called because they do not require mechanical or electrical means to function. They do their job very simply by providing a trouble free static means for the dissipation of sound energy by converting it into very minute quantities of heat. HVAC noise control requirements are rarely confined to a narrow low frequency range, therefore the broad band low and high frequency attenuation capabilities of typical passive attenuators are practically always required and are the standard in the industry. There are many factors involved in choosing the correct duct silencers, so they should always be properly specified by an expert mechanical engineer or acoustic consultant.
Since fans generate the primary noise source in HVAC systems, acoustic analysis should start with the fan power levels per octave band, and ends with the resultant absorption or reflective effects in the receiving space. Laboratory test results can be obtained from fan manufacturers showing total power sound levels from all of the fan components that may be contributing to radiating noise. Fans should always be selected to operate near their maximum efficiency. Fans are noisier when they are either oversized and operating under design speed, or undersized and operating above design speed. While individual duct components have properties that can provide natural attenuation, they also can generate noise. The smaller the face area of a duct, the faster air will travel through that duct, the more noise it will generate, and the higher the pressure drop will be, so it is important to size the ducts based on the CFM, (cubic feet per minute), of airflow.
Once everything has been done to naturally attenuate the noise generated by an HVAC system and it is determined that additional attenuation is needed, then the addition of HVAC silencers is required. HVAC silencers attenuate sound when they are installed directly in the ducted air path. Standard rectangular HVAC silencers are fabricated with a series of perforated metal baffles filled with a sound absorbing material such as fiberglass, and encased in a solid metal shell. Circular or “conical” silencers are fabricated with an acoustic bullet in the center also filled with a sound absorbing material. For applications requiring a high degree of cleanliness and hygiene, silencers are available with a lined acoustical fill. For industrial applications where corrosive contaminates may be present, packless silencers are available.
In addition to attenuating noise generated by the HVAC equipment itself, another common use of duct silencers is for attenuating sound being transmitted between spaces via the return and exhaust registers. This could involve unwanted airborne noise being transmitted through the ductwork in and out of a home theater, or to secure conversations in conference rooms or Sensitive Compartmented Information Facilities.
Acoustic performance of duct silencers is generally described in terms of “insertion loss” which is the measure of noise level reduction determined by comparing the noise level with and without a silencer in place. Since the silencer itself can generate noise by disturbing the airflow, its self-generated noise has to be added to the attenuated sound level. Baffle and bullet-type silencers also block a portion of the air stream and will cause additional pressure drop (PD) in the system which needs to be accounted for. Silencer manufacturers will supply these values in their acoustical performance data. Over the years, manufacturers have designed and manufactured duct attenuators in response to specific requirements from acoustical consultants, consulting engineers, owners and contractors, so there is a wide diversity of types and models available for any noise control problem encountered in HVAC engineering.
Acoustic Louvers are used as part of the intake and exhaust air system of buildings or equipment to help reduce noise generated by the system by allowing airflow while acting as a barrier. Acoustic louvers reduce noise pollution from plant cooling towers, boiler rooms, and air handling systems in all types of buildings, including power stations, chemical treatment plants and offshore applications. Acoustic louvers can play an essential role in helping to combat environmental noise problems that affect mixed commercial and residential areas.
Acoustic louvers are available as louvered screens, freestanding barriers and enclosures, and are available in a wide range of colors and finishes to match the overall scale and aesthetics of any building. As with duct silencers, acoustic louvers need to be properly specified by an expert mechanical engineer or acoustic consultant. In addition to engineering details such as airflow and pressure drop, environmental issues such as rain and snow infiltration, weather resistance, and wildlife deterrents have to be taken into consideration.