Roof Top Units

Packaged roof top units (RTUs) provide a common method for comfort heating and cooling in small to medium sized buildings. To save energy (both thermal and flow-related) and to save ductwork, designers have progressively placed RTUs closer to occupied spaces. In many cases, roof top units are installed directly over an office or meeting space. The challenge is how to reduce both the airborne noise and structure borne noise from multiple noise sources that are located so close to the receiver.

Aerodynamics

Problem	Solution
click image to view larger Aerodynamics Fan type, fan orientation and turbulence can lead to generated noise by the silencer. This can lead to more than just noise in the occupied space: air distribution and energy consumption become major problems if the silencer isn’t selected and, more importantly, applied properly.	click image to view larger Fit-the-system silencers are built to the required shape and size to provide sufficient insertion loss while meeting space restrictions. These silencers also include flow-shaping internals to help reduce pressure drop and provide uniform air distribution. EX casing silencers are designed with the acoustic media out of the airstream, resulting in noise reduction with minimal added pressure drop (compared to ductwork).

Problem

Solution

click image to view larger

Aerodynamics

Fan type, fan orientation and turbulence can lead to generated noise by the silencer. This can lead to more than just noise in the occupied space: air distribution and energy consumption become major problems if the silencer isn’t selected and, more importantly, applied properly.

click image to view larger

Fit-the-system silencers are built to the required shape and size to provide sufficient insertion loss while meeting space restrictions. These silencers also include flow-shaping internals to help reduce pressure drop and provide uniform air distribution.

EX casing silencers are designed with the acoustic media out of the airstream, resulting in noise reduction with minimal added pressure drop (compared to ductwork).

Environmental Noise

Problem	Solution
click image to view larger Environmental Noise Environmental noise from the RTU’s condenser fans, fresh air intakes, exhaust fans and compressors is a major concern. Its impact on neighboring property and spaces can instigate lawsuits, tarnishing the reputation of the owner and design engineer.	click image to view larger Typical condenser fans have little static pressure to spare for silencing noise radiated to property lines. Intake and discharge silencers are designed to minimize pressure drop and resist corrosion due to the environment. Condenser noise control solutions also include access panels for the condenser fans for maintenance purposes. Acoustic vertical barriers are another option. These barriers do not add static to the system. The position of barrier has to be carefully engineered to factor in air intake requirements of the fan. However, these barriers do not provide as much noise reduction as intake and discharge silencers.

Problem

Solution

click image to view larger

Environmental Noise

Environmental noise from the RTU’s condenser fans, fresh air intakes, exhaust fans and compressors is a major concern. Its impact on neighboring property and spaces can instigate lawsuits, tarnishing the reputation of the owner and design engineer.

click image to view larger

Typical condenser fans have little static pressure to spare for silencing noise radiated to property lines.

Intake and discharge silencers are designed to minimize pressure drop and resist corrosion due to the environment. Condenser noise control solutions also include access panels for the condenser fans for maintenance purposes.

Acoustic vertical barriers are another option. These barriers do not add static to the system. The position of barrier has to be carefully engineered to factor in air intake requirements of the fan. However, these barriers do not provide as much noise reduction as intake and discharge silencers.

Radiated Noise

Problem	Solution
click image to view larger Radiated Noise Radiated noise is often overlooked. It originates from internal fans and compressors, penetrating the casing and floor of the unit to enter the occupied space. Lightweight building construction, packaging of components and faster/louder fans are some reasons why radiated noise is one of the most common noise problems in the HVAC today.	click image to view larger An engineered barrier, built into the noise control curb solution, will attenuate low-frequency noise before it passes through the ceiling and into the occupied space.

Breakout Noise

Problem	Solution
click image to view larger Breakout Noise Breakout noise travels down the return or supply duct and breaks through the duct walls and into the occupied space. Lightweight building construction, packaging of components and faster/louder fans are some reasons why breakout noise is such a common noise problem in HVAC today.	click image to view larger High transmission loss (HTL) casing should be used for silencers to attenuate breakout noise. This is a better alternative to field applied duct lagging because of single-source responsibility.

Seismic & Wind Loading

Problem	Solution
click image to view larger Seismic & Wind Loading Wind and seismic forces are factors that require a design engineer’s close attention. Depending on the region, RTUs may be exposed to winds of 90 mph (145 kph) to in excess of 150 mph (241 kph) and/or extreme seismic forces.	click image to view larger For code compliance, Vibro-Acoustics performs all required anchorage calculations and provides connection details for the curb. In addition, the design and calculations are stamped by a professional engineer.

Vibration Path and Structure-borne Noise

Problem	Solution
click image to view larger Vibration Path and Structure-borne Noise Structure borne noise travels through the structure rather than air. Vibration from the fans and compressors in the RTU is transferred to the structure and creates noise problems throughout the building. This problem is very difficult to diagnose post-design.	click image to view larger Vibration isolation, designed into the noise control curb solution, can isolate the entire RTU system externally. Vibro-Acoustics takes into consideration the location of the equipment in relation to neighboring occupied spaces, roof deflection, as well as sound criteria.

Flanking Radiated External Noise

Problem	Solution
click image to view larger Flanking Radiated External Noise Flanking problems are most common in sound sensitive areas. This occurs when noise discharges from the air intake or discharge opening. Through the roof deck and into the adjacent space. * Common - Where the roof construction has metal decking and does not include concrete.	click image to view larger Intake and discharge silencers are designed to minimize pressure drop and resist corrosion due to the environment. Condenser noise control solutions also include access panels for the condenser fans for maintenance purposes. Please feel free to contact our team of experts to discuss your noise control, vibration isolation and restraining needs for roof top units.

Duct-Borne Noise

Problem	Solution
click image to view larger Duct-Borne Noise This is the most common path addressed by consulting engineers. However, incorrect selection and application of the silencer can lead to unnecessary noise and pressure drop problems. Often a silencer is picked from a catalog with little consideration of the required performance or the inlet and discharge conditions	click image to view larger The first step is to complete an acoustic and system analysis to determine the required noise reduction as well as the system constraints. Fit-the-system silencers with flow-shaping internals should then be integrated into the curb. To gain the benefit of single source responsibility, the curb and silencers should be scheduled and specified as one noise control system.