A practical example
Figure 1 shows a hypothetical installation of a pair of classrooms using numerous good practices for meeting ANSI S12.60. The unit is over a corridor, which reduces breakout sound directly into the room and allows attenuation before entering the room. Both the supply and return are ducted, allowing a chance for attenuation. Linear slot diffusers are used, and they are often quieter or can be installed with a desirable length of straight duct before the diffuser.
Placing the diffuser as the noise-generating device closest to the room is critical in meeting the standard. If there are turns or elbows close to the diffuser, it can easily increase the generated sound beyond the rated sound from the device. Further, the use of sound ducts would need to be evaluated on a case-by-case basis, because they can reduce the breakout noise but may also reduce attenuation of sound traveling within the duct. Finally, there are acoustic plenums below the unit, which allow for a significant sound reduction between the unit and the ductwork.
Helpful resources available
There are numerous resources that can be of help when considering HVAC equipment. These resources include acoustical analysis software that may accurately predict and compare sound levels of various HVAC systems and construction plans. The software takes into account different sound sources and accurately predicts a classroom’s sound level, which helps in selecting the right solution during planning and design. Inputs include specific building requirements along with the type and placement of HVAC equipment, duct configuration, and wall and ceiling type. Equipment manufacturers offer this type of acoustical analysis and software. The American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) also has an HVAC Applications Handbook that provides guidance on conducting an acoustical analysis.
Bringing in an acoustical consultant familiar with the requirements early in the process can also help prevent issues and save money down the road. The National Council of Acoustical Consultants (NCAC) is an organization that can provide guidance and help in selecting a consultant for a specific project.
Sound levels that promote learning
HVAC noise can be a common culprit in noisy classrooms. Excessive noise and reverberation interfere with speech intelligibility, resulting in reduced understanding and learning. Engineers and architects need to consider all the sound sources impacting a classroom space.
School acoustic standards provide guidance on maximum background noise levels and reverberation times for optimal learning. It is important to be aware of these standards—and know there are feasible and affordable options available to help meet them. When the system is properly designed, excessive mechanical noise can be substantially reduced at little or no extra cost.
In the end, a good listening and learning environment is achievable when classroom acoustics are considered at the outset of the design process, and with early collaboration among school planners, architects, contractors, and suppliers.
Stephen Lind is an acoustics engineer in sound and vibration testing at Trane, a supplier of indoor comfort systems and solutions, and a brand of Ingersoll Rand. His work focuses primarily on measuring sound for air-conditioning systems—he is active in the Air-conditioning, Heating, and Refrigeration Institute’s (AHRI’s) Technical Committee on Sound and on the Acoustical Society of America (ASA) Standards Committees S1 and S12. Lind received a degree in physics from the University of Northern Iowa and a master’s in engineering (acoustics) at the University of Texas at Austin. He can be reached via e-mail at slind@trane.com.
