Is this ‘acoustical’ product right for my project?
The answer is often difficult to determine. The term ‘acoustical’ is applied to a wide variety of products, materials, and systems that promise to solve problems ranging from privacy to occupant discomfort. But what makes for a good acoustical solution? As with most questions in building design, the answer depends on multiple factors specific to the facility, its users, and the design intent. In this article, we clarify some of these factors and provide a few tips on how to assess the options for your next acoustical conundrum.
First, let’s define ‘acoustical’
Firstly, we must clarify some terminology. Being labeled ‘acoustical’ means only that a product is designed to achieve some goal related to acoustics. Depending on the product and its use, it could be just what your project needs, or just the opposite. For example, acoustical vinyl is a material often used to block sound transmission through ceiling plenums. It is acoustical because it is engineered to block sound. Acoustical fabric, on the other hand, is used in performance spaces because it is considered acoustically-transparent. It is acoustical because it is engineered to not block any sound at all. To really understand a product’s acoustical values, one must look beyond the sales literature and delve into the performance metrics.
Sound Transmission vs Noice Reduction
There are literally hundreds of ways to measure the acoustical performance of building elements. In this discussion we will only look at two – sound transmission and noise reduction. In product literature, these factors are by far the most common measures of acoustical performance.
Sound transmission deals with limiting sound ‘trespass’ from one space to another. Or from outdoors to indoors. The most common metric is Sound Transmission Coefficient (STC). The higher the STC, the better the material is at blocking sound. Ceiling assemblies use a related metric called Ceiling Attenuation Class (CAC). There are many factors that contribute to blocking sound but, in general, materials and assemblies with greater mass have higher STC ratings. The ideal material for blocking sound will be heavy, dense, and solid. We can visualize an ideal sound barrier as a brick.
Noise reduction deals with the propagation of sound within a space. In spaces dominated by smooth, hard surfaces, excessive reverberation creates distracting levels of noise and causes problems with both speech privacy and intelligibility. Noise reduction is achieved by introducing surfaces that effectively absorb sound. The most common metric is Noise Reduction Coefficient (NRC), which measures how much sound reflects from a given surface. The ideal material for noise reduction will be porous, soft, and highly textured. We can visualize this ideal sound absorber as a sponge.
With these visualizations in place, we can start to see why an acoustical product aimed at noise reduction often makes a poor sound barrier. A sponge doesn’t make a very good brick.
Acoustical ceiling panels provide an example where product selection must balance both noise reduction (NRC) and sound transmission (measured using CAC for ceilings). One of the highest CAC values available in a lay-in ceiling panel is essentially a piece of drywall. It makes a great sound barrier, and a terrible sound-absorber. It’s all brick and no sponge. On the other end of the spectrum, the highest NRC panels have only mediocre CAC ratings. In each case, the specific needs at hand must be weighed before making the right choice. In a lively open office space prone to excessive levels of noise, NRC may be the most important factor. In the adjacent office used for focused work, it may be more important to block out unwanted sounds, and CAC may take priority. A drywall panel could be the right answer in a room rarely used by personnel that houses a noisy piece of equipment. The best solution is highly dependent on the context in which it is applied.
The first and most important step in solving your next acoustical problem is – as always – clearly defining the problem. Only then can one determine the right solution. Is it a sound transmission problem? Or a noise reduction problem? Do you need a sponge? Or a brick?