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By Sara Hart

Simple solution

Mitigation of external noise into a building’s envelope is one kind of acoustical challenge; improvement of desired sounds created within the envelope is another. Whereas in the previous projects, defense against noise is invisible, hidden in the wall cavities and window units, occasionally a client wants a solution that is conspicuous, one that serves an architectural purpose as well as an engineering one. The Southern California Institute of Architecture (SCI-Arc) in Los Angeles was one such client willing to grant architects license to experiment. The school commissioned Hodgetts + Fung Design and Architecture to improve the acoustic quality of the school’s main space, seeking a result that would be all things to all activities—lectures, performances, presentations, and ordinary conversations. The Los Angeles–based firm is familiar with the integration of acoustics with architecture, as witnessed in its resurrection of the Hollywood Bowl [Record, January 2005, page 152].

The solution, named XSS—Experimental Sound SurfaceCeiling—by its inventors, bears no resemblance to anything one would expect to be applied to what is basically a straightforward box. Hodgetts + Fung chose a unique architectural solution. The result is an upside-down terrain, some kind of inverted gray topography that alters the spatial experience completely, almost surreally. And yet, the intervention, while ingenious, is a low-tech solution. The acoustical material is industrial wool felt, the kind used to cushion heavy machinery. It can be specified up to 2-inches thick.

Initially, the architects chose a thickness of 3¼8 inch. Although acoustical calculations didn’t drive the process, the architects consulted engineers McKay Conant Brook about the thickness of the felt. The engineers determined that if they increased the thickness to 5¼8 inch, they would achieve the .4 acoustical coefficient that was required.

The 12-by-24-foot bays are supported by a lightweight aluminum frame. The openings in the felt increase sound absorbency. The material was fabricated off-site and then delivered to the school. Once the felt panels were attached to the frame, the material was manipulated by the installers. The contours of the the final configuration further diffuse the sound. The wool felt is somewhat fragile and vulnerable to pull-through in this particular application, so the architects went in search of a special fastening device to secure it to the frame without damage. Sticking to their inexpensive, off-the-shelf materials imperative, the appropriate device appeared in the form of a nylon ratchet fastener designed for use in upholstery. The fasteners secure the felt to a flexible polypropylene flange, which is then free to rotate about a curved ABS spine supported by the aluminum substructure. The aluminum frame was assembled on the floor, raised above shoulder height, where the felt was attached, then the entire structure was hoisted to the concrete ceiling by ropes.

As the built environment gets noisier, so have the protests against noise, fueling litigation and consequently creating intense debate over acoustical standards. At the moment, claims are argued in terms of negligence, breach of contract or warranty, liability, and acoustical nuisance, to name a few, and evaluation of claims varies widely from state to state. Until there is a more uniform acoustical standard, clients may choose to spend money up front in prevention and, hopefully, avoid costly court battles and a public-relations nightmare later.

 

A-weighted Sound Level One’s ability to hear a sound depends greatly on the frequency composition of the sound. People hear sounds most readily when the predominant sound energy occurs at frequencies between 1,000 and 6,000 Hertz (Hz, cycles per second). Sounds at frequencies above 10,000 Hz (such as high-pitched hissing) are much more difficult to hear, as are sounds at frequencies below about 100 Hz (such as a low rumble). To measure sound on a scale that approximates the way it is heard by people, more weight must be given to the frequencies that people hear more easily. A method for weighting the frequency spectrum to mimic the human ear has been sought for years. Many different scales of sound measurement, including A-weighted sound level (and also B-, C-, D-, and E-weighted sound levels) have evolved in this search. A-weighting was recommended by the Environmental Protection Agency (EPA) to describe environmental noise because it is convenient to use, accurate for most purposes, and is used extensively throughout the world. Source: The EPA Library, a collection of related documents from the Noise Pollution Clearinghouse (www.nonoise.org).