Another common sound absorption rating system used in Europe is the weighted sound absorption coefficient, αw, measured in accordance with EN ISO 11654. Absorption values vary between 0.00 and 1.00, but are only expressed in increments of 0.05. This system uses fixed reference curves from 125 to 4,000Hz and is similar to NRC except that it assigns “classes” to each absorption value range as listed in the following table.

The advantages of single number absorption rating systems is that materials can be quickly classified and compared with each other. The shortcoming is that these systems only account for a limited frequency range and do not allow for frequency-specific comparisons. As such, care should be taken when using the above absorption ratings and acoustical consultants should always be involved on more noise-sensitive projects.

General rules of thumb for achieving acceptable absorption can be dangerous, since each space is different both in usage and geometry. Because room absorption is directly impacted by surface area, an absorptive ceiling (NRC ≥0.80+) can provide much needed sound absorption and reduce reverberant noise levels by 10 dB. Walls are another good surface to treat, though modern construction often results in one or more being glass. Assuming it is thick enough, carpet can also provide sound absorption. Besides the thickness issue, another limitation of carpet is its distance and orientation from the occupants. Unless occupants are hiding under the table, much of the sound absorption provided by carpet is limited by the typical large conference room table, which is usually a hard, reflective surface. Increasing a room's size results in higher reverberation time and can increase speech intelligibility issues. This is why large classrooms, for example, must employ multiple absorptive surfaces to ensure that speech is not garbled by elevated reverberation levels. Utilizing acoustic ceiling tile, thicker carpet, and acoustic panels on at least two non-parallel wall surfaces are all methods for achieving a suitable reverberation time. Open office sound absorption is very straightforward: treat the entire ceiling with acoustic ceiling tile having a minimum NRC of 0.80. Carpet and absorptive cubical walls help, but remember the importance of maximizing absorptive surface area and occupant orientation. For example, speech levels at the front of a speaking person are around 10 dB louder than at their rear.

Acoustics and LEED

The specifics seem to constantly change, but currently both school and healthcare projects have several acoustical requirements and/or credits. LEED 2009 for Schools requires Minimum Acoustical Performance (IEQp3) based on background noise levels (≤45 dBA) and surface absorption in classrooms and core learning spaces depending on room size (20,000 cubic feet is the cutoff). LEED documentation recommends using acoustical ceiling tile to achieve required sound absorption performance (NRC ≥0.70). Not surprisingly, classrooms have many of the same issues as open offices discussed earlier. A German study² found that 65 dBA is the average noise level in a classroom—a threshold level for possible noise-induced heart-attacks. It can be argued that this level of stress is totally unnecessary particularly when reducing reverberation time in a classroom is a relatively simple and inexpensive fix. Enhanced Acoustical Performance (IEQc9) is worth 1 point and requires both lower background noise levels (≤40 dBA) and minimum sound transmission performance (≥STC 35). For reference, these background noise and sound transmission requirements are less stringent than what most acoustical consultants and industry guidelines recommend.

LEED 2009 for Healthcare provides credits in the way of Acoustic Environment (IEQc2) that is worth 1-2 points and encompasses sound isolation, background noise, surface finish absorption, and exterior noise levels. Again, many of these acoustic credits can be easily earned by simply achieving industry accepted design criteria and good building design. A knowledgeable acoustical consultant can help ensure the correct criteria are met.

In the past few years, post-occupancy evaluation research from the Center for the Built Environment have identified acoustics as having the lowest occupant satisfaction ratings in both green and non-green buildings. Equally important is the fact that occupant satisfaction with acoustics actually decreased with the move from non-green to green buildings. Other IEQ metrics including lighting, office layout, and office furnishings were higher for green-buildings. In other words, acoustics are the biggest occupant complaint in green buildings. This should come as no surprise, since many of the methods employed by LEED have the added effect of reducing speech privacy, noise isolation, and/or sound absorption. For example, typical open office design often results in complaints of reduced speech privacy between occupants. Using large quantities of glass can increase natural light, but glass is also a very acoustically reflective material and typically provides lower sound isolation from outdoor noise. These are just a few examples of why LEED buildings are often plagued by low occupant satisfaction when it comes to acoustics.

A Holistic Approach to Noise Problems

Solutions to most “noise problems” typically require the holistic evaluation of multiple paths with each requiring individual attention. By properly addressing noise issues, offices are more productive, conference room meetings are more effective, and hospital patients are more comfortable. All of the design criteria and terms described above may not be directly relevant for every job, but each should be considered. Doing so will improve the end result, and provide more opportunity for project team integration throughout the building process. Most importantly, clients will be confident that all of their noise control concerns, not just the “usual suspects,” are addressed prior to construction. It is important for architects to play an active role especially in areas where there is no clear owner. In many cases, this “gray area” occurs at mechanical and air distribution equipment “in” the occupied space, such as terminals and diffusers. While engineers typically select the equipment, architects drive space planning and are ultimately held accountable for proper noise levels (since they often hire the acoustical consultant). Success is achieved when acoustics are prioritized. Clear communication between project team members on the desired acoustic environment and various available options lead to exceeding client expectations.

Price Industries is the leading manufacturer of air distribution products in the North American market, and works to bring about the vision of the design community by collaborating on high-quality, high-performing, and customizable air distribution solutions. www.price-hvac.com