Commissioning for sound and vibration
One of the most overlooked areas in building systems commissioning is the proper control of sound and vibration. This is a shame, because most of the sound or vibration issues that need to be corrected at the end of the project usually could have been avoided through proper commissioning at the beginning of the project. Commissioning for sound and vibration control is not as complicated as most providers might think. A basic understanding and a little common sense can go a long way. If you do not have a basic understanding of sound and vibration fundamentals, consider taking classes through a certifying organization. You can also hire a consultant to assist you when necessary.
Commissioning for sound and vibration is the same as commissioning for any other discipline—it must start at the beginning of the project to provide maximum benefit. This means the expected or acceptable levels must be clearly defined in the owner’s project requirements (OPR) and properly addressed in the basis of design (BOD). If you do not see these requirements in the OPR, you need to stop and ask both the owner and the design team why. Remember, there is no way for you prove that the systems are installed and operating as designed if you don’t have a target.
Sound and vibration levels are not unlike lighting levels, in that owners frequently do not know how to describe or quantify their expectations. Never assume that because you don’t see any expectations listed, the owner doesn’t have any. The owner does have expectations, and you do not want to be the one explaining to him at the end of the project why his new ceiling-mounted water source heat pumps are so much louder than the single-duct variable air volume (VAV) system he had in his previous facility.
A word of warning: Don’t be surprised if you initially get a lot of resistance when you raise these topics. Many design professionals do not believe in putting this type of information in writing. As one architect recently stated, “You’re asking me to give the owner the rope he’s going to hang me with.” That is a poor business attitude, and nothing could be further from the truth. In reality, you are doing the design team a big favor by putting the expectations in writing. Many projects are designed and built under tight budget constraints. This often leads to higher sound levels in the finished spaces due to less expensive HVAC equipment, and less expensive construction materials and methods. Documented sound and vibration expectations are your insurance that the owner receives what he is paying for, especially when the owner isn’t paying for a lot.
Commissioning review of design documents
Once the target sound and vibration levels have been established (along with the owner’s other requirements), it is time for the design and construction documents to be created. As the sound and vibration commissioning provider, it is critical that your contract includes at least one review of the design documents before they go out for bid. It is better to have at least two reviews because sound and vibration issues are more expensive to correct the later in the design process they are discovered. This is a great opportunity to sit with the owner to explain the pros and cons of the systems that have been selected. It is hard for an owner to relate to noise criterion (NC) or room criterion (RC) ratings. In some cases, it may be necessary to take the owner to a facility that has systems similar to what have been proposed (or build a mock-up) for the owner to get a feel for the differences in sound and vibration levels between the types of systems.
During your reviews, you need to ensure that the sound and vibration testing requirements are included in the project specifications. Make sure that the testing specifications require that the testing firm is certified in sound and vibration measurement. A sample sound and vibration spec can be downloaded from the National Environmental Balancing Bureau website. During your review of the plans, make sure that the system as designed can achieve the sound and vibration levels established in the OPR. For example, if the owner desires all private offices to have sound levels of NC35 or less, you want to make sure there is no equipment above the office ceilings that has fans or compressors. Classrooms designed for NC30 should not have open ceilings when there is ceiling hung equipment (it happens more than you think).
Typical items to consider during your commissioning review include:
- Does the building have a lightweight structure, specifically a lightweight roof structure? If so, have isolation requirements been addressed for all rooftop mechanical equipment?
- Has inertia mass, i.e., housekeeping pads, of normal weight concrete been shown beneath the pumps/air handling units?
- How are the supply and return chases constructed? For plenum returns, mechanical equipment rooms should be provided with return air baffle walls.
- Are duct airflow velocities excessive? For rules of thumb, see Table 1.
- Do the plans have installation details incorporating the proper vibration isolation methods?
- Is the supply ductwork lined where required? Some building owners prohibit internal fibrous liner. There are nonfibrous liners on the market that are gaining acceptance by owners.
- Are inlet ducts to VAV terminals hard-ducted instead of being connected with flexible duct?
- Pay close attention to plenum ceilings. The return paths used can cause the ceiling assemblies to essentially be acoustically transparent. If light fixtures are being used to return air to the plenum, do not install equipment above the lights. Pay particular attention to return grill placement.
- Pay attention to damper locations. Dampers must be located as far from the diffusers as possible. Avoid face dampers in the diffusers if at all possible. If face dampers must be used, verify that there are air straightening vanes prior to the damper.
- Pay attention to diffuser locations. There is an additive effect from multiple diffusers; for each doubling of the number of diffusers, the sound pressure levels increase 3 db.
- Have sound attenuators been used properly? Ask to see the equipment selections with and without the manufacturer’s attenuators. Do not assume that the attenuator will always make the equipment quieter, because in many cases the attenuators will increase the sound levels in some octave bands while reducing levels in others.
- Are duct silencers properly applied where required? Remember that duct silencers require laminar flow to be effective. When straight ducts with laminar flow cannot be provided, the fan speed changes required to overcome the increased pressure drop can actually create higher sound levels than if the silencer wasn’t used at all. Duct silencers with pressure drops in excess of 0.25 in. wg should be avoided if at all possible.
- Are sound sensitive areas located away from areas with the highest sound levels? Conference or training rooms are often placed in interior areas since these areas offer easy access to restrooms and elevators. Often these rooms end up right next to the mechanical equipment room.
- Are mechanical equipment room partitions of proper construction (i.e., alternating stud partition, double-wall, double-layered drywall)? Many designs show only stud-to-stud partitions for the mechanical equipment rooms.
- Is mechanical equipment oriented to reduce sound pressure levels in the normally occupied spaces? If the mechanical room is situated adjacent to a stairwell, it is usually possible to orient the unit such that the noisiest end of the unit is directed toward the stairwell in lieu of the occupied areas.
Your contract should include periodic site inspections during construction of the project, and it is wise to perform one of those inspections early in the project, especially when the equipment is being installed. Missing or improper isolators are easy to correct before duct or other systems are installed. Isolators that do not match the approved submittals are often found on projects, so be diligent in your inspections.
Also remember that new equipment can mask future vibration problems. Over time, belts will wear, sheaves get out of round, etc. A system that does not exhibit a vibration issue during startup or the warranty period may still develop issues later in life, especially if the original installation was not performed per industry standards and correct practices. The equipment manufacturer’s specific sound and vibration requirements and recommendations should be incorporated into the prefunctional checklists for the project.
During your field inspections, be on the lookout for the items that were identified during your design phase review. Information is not always conveyed to the personnel who are installing the equipment. Also ensure the following:
- All ductwork is properly isolated from walls and lights (no physical contact).
- There are no poor (highly turbulent) duct fittings.
- All duct connections are properly sealed. (Air leaks can generate noise.)
- All penetrations of partitions are properly sealed.
- There are no improperly adjusted isolators.
- There are no improperly adjusted duct connectors.
- All shipping blocks have been removed.
- There are no flanking paths of transmission.
- There is no excessive dirt on blower wheels.
- There is no misalignment of couplings, sheaves, and other drive components.
- Flexible conduit has been used between rigid electrical conduit and reciprocating or rotating equipment.
- Fan discharge and inlet fittings are correctly installed and fan orientations are correct. (This not only reduces system effect but also reduces sound levels.)
Once all equipment and systems have been properly cleaned, started, and balanced, it is time for the acceptance phase to begin. All specified systems must be properly tested in accordance with the project specifications and other design documents. It is extremely important to perform actual testing or witness verification in proper order. First and foremost, all test, adjust, and balance (TAB) work must be complete before sound or vibration testing for acceptance takes place. If pumps are cavitating or fans are in any kind of surge, the readings of sound and/or vibration are falsely elevated.
If applicable, vibration testing and verification should be performed and completed before moving to the sound portion of acceptance. All sound and vibration measurements should be reported on the specified test forms, and included in either the final TAB report or the commissioning report, whichever is specified in the contract documents. Sound measurements should be given in terms of NC or RC, or, in some cases, both criterion are specified.
For vibration reporting, final verification of “isolation in place” should be performed by the commissioning provider, but some specifications allow this to be done as paperwork verification by the installing contractor. This should also be done for proper “housekeeping” of the equipment (pump pads cleared of debris, isolators properly set, bolt down of fans removed, etc.).
The next procedure should include verification of vibration testing being actually performed. It is not always realistic to be able to witness every piece of equipment being tested, and if not actually seen, proper forms with the equipment readings, as set out in the commissioning specifications, must be properly and fully filled out. These forms should also note any piece of equipment that does not meet proper vibration specification. These deficiencies should then be routed to the proper people for remediation with follow-up testing. If the commissioning provider is not performing the sound or vibration testing, it is imperative that sound and vibration verification be performed with the sound and vibration testing contractor. The verification process is the same as for the TAB process, with random measurement locations selected for retesting. The systems must be postured as indicated in the original report.
Incorporating sound and vibration requirements into your commissioning tasks is not incredibly time-consuming. You are already reviewing most of the plans, specifications, contract documents, and systems as part of your other commissioning disciplines. Adding sound and vibration as an additional service brings added value to your customers, the end users, and the facilities. If the buildings could talk, they would thank you.
– Gaghan is owner of his own mechanical contracting business, Gaghan Mechanical. He obtained most of his knowledge in the sound and vibration area while with Trane, where he participated in the company’s national vibration standardization. McGregor is a full partner and senior acoustical engineer at Engineering Dynamics. He has experience analyzing, designing, and measuring noise and vibration from building mechanical systems. Huber is the president of Complete Commissioning, where he performs building system commissioning for new and existing facilities and systems. All three are on the sound and vibration committee of the National Environmental Balancing Bureau (NEBB).