With spring right around the corner, April showers are on the way. In most buildings there is only one thing between the occupants and spring rains – the roof. Some may find the sound of rainfall soothing but in noise-critical spaces like classrooms, performance halls and recording studios, this natural sound source can be particularly disruptive. Without flooding you with information on the size of rain drops, wind speed, time intervals between drops, and other droplet factors, we offer this crash course in the acoustics of roofs.
The first key ingredient in reducing sound transmission is mass – the more, the better. The general trend in roof construction is lighter weight materials, typically consisting of a corrugated metal deck topped with rigid insulation and a membrane layer for water protection. While this makes great sense in terms of dollars and cents as well as thermal insulation, it does not stop much sound from either airborne noise sources or impact from rain or sleet. This same deck topped with 2″ of concrete or a few layers of roofing board or sheathing reduces significantly more rainfall sound. This is due to the fact that the rain drops have a much more difficult time energizing the heavier weight construction, while the lightweight structure is much more easily induced into motion. This motion translates to sound, unless it is dealt with in some other manner.
The second key ingredient is the introduction of a resilient layer into the assembly. This resilient layer will absorb the motion of the layers above it, transforming that kinetic energy into heat as opposed to acoustic energy. Options in this category include compressed mineral wool, entangled nylon strands, and good old-fashioned tarpaper. By laying these materials under roof sheathing, they act as damped springs, absorbing the energy that would otherwise manifest itself as rainfall sound. Additional benefits of these types of resilient layers are thermal; the material between the upper and lower members of this resilient “sandwich” provides a insulating layer that helps to lower heating and cooling costs.
River rock ballast provides both mass and resiliency. Because ballast consists of loose, round stones that are not fixed in place, when a raindrop hits a stone, a portion of the impact energy is absorbed. Additionally, ballast provides the rooftop equivalent of diffusion – as a raindrop hits a stone, it breaks up into several smaller drops each of which has a fraction of the energy of the original. This distributes the impact energy over a greater area and translates to less intruding sound below.
One final thought is the choice of roofing material. Standing seam metal roofs, while visually appealing, are conduits for the transmission of impact sound from rain. Interior sound levels in rooms with exposed decks could increase by 20 decibels during heavy rain storms. Conversely, green roofs are good choices as they have both mass and sound damping characteristics.
The best roofs are those that keep building occupants dry and building interiors quiet. With Metropolitan Acoustics on your side, the only problems you’ll have with April showers are those pesky May flowers!