Thursday, October 4, 2012

School Acoustics

I was recently asked about the issues with meeting classroom acoustics requirements, both for the upcoming LEED V4 and many local code requirements.

The Acoustical Society Standard ANSI S12.60 is the base requirement. For most classrooms, it requires a dBA no greater than 35, which is about 26 NC, depending, of course, on the shape of the sound spectrum. This is a pretty tight requirement, especially because it doesn’t state from where the sound comes. Measurements in many schools exceed 35dBA from outside noise alone.

LEED for Schools presently has a 45 dBA limit for getting a point, again not specifying from where the sound comes.

The upcoming LEED V4, however, has a prerequisite of 40 dBA, but specifies that it is for predicted sound from the HVAC system alone; AHRI 885 is referenced as a calculation method. This is a major improvement, I believe, as one no longer needs to hire an Acoustician for this value.

Reverberation time is also specified, but in V4, it is covered by room construction details.

Now, what to do about the HVAC system? One thing is clear, if the 40 dBA requirement is to be met, there will be no mechanical equipment in the classroom and likely none above the ceiling if it contains a fan. 40 dBA is about NC 31. Putting equipment in the corridor with lined duct will be the likely solution. Fire dampers are often required at the wall penetration. There are also space issues that need to be worked out.

Displacement Ventilation diffusers are the quiet solution for air outlets, but there are challenges:

--- The air should not be any colder than 65F to avoid discomfort, but the dew point has to be much lower. The mix of ventilation air and humidity control will be a challenge. Some codes now require that the ventilation be shut off in unoccupied classrooms. This implies a separate ventilation supply, which likely has to be pressure independent VAV that allows easy demand controlled ventilation.

---  The “near zone” is likely about 4 ft at 250 cfm. This means a clear space around the diffusers, which is no problem for the ones often placed on each side of the white board, but it may be a challenge in the rear of the ever more crowded classrooms.

---  Displacement diffusers do not heat as well as other methods. Higher velocity air at the floor is likely required, or at least a baseboard or some other alternate heating solution.

---  The thermostat location in a stratified environment is problematic. Some trial and error is likely, as the ADA height requirement may not be ideal; some offset will be inevitable.

---  Finally, with many classrooms in perimeter zones, the internal heat gain in the winter will offset the heating demand. Many times, discharge delta-t’s are very low or neutral once the classroom is in use. This adds to the problem of controlling humidity without reheat.

This means that the engineer will have to look at a number of solutions. It seems that a dual duct system is a pretty good one, with ventilation air in one duct and recycled air in the other. The dual duct unit may require a heating coil in some areas, but it could be located above the classroom and still be able to meet acoustical requirements. Locate the two supply ducts one above the other, as they will be supplying air from each side.

The DOAS fan unit may be another solution, given its ability to vary the ventilation rate while separately managing heating and cooling. The unit of course, will likely have to be in the corridor.

The chilled beam has also been considered, due to its quiet nature, but openable windows offer a challenge in controlling condensation.

In any case, classroom HVAC design won’t ever be the same.

Authored by: Dan Int-Hout, Chief Engineer Krueger