When you combine loud activity, hard surfaces, a mass of water and a raucous crowd in a large space, you get an acoustic nightmare. Such is the case with natatoriums, or indoor pool areas. These high-volume spaces filled with materials that reflect sound create poor acoustics that give rise to excessive noise and make the spoken word difficult to understand, creating an environment that is uncomfortable at best and unsafe at worst.
Many of a natatorium’s inherent structural elements are sound-reflective: Wall, floor and ceiling surfaces that are resistant to water and chlorine damage — such as concrete and tile — also are resistant to sound absorption. These hard surfaces increase reverberation, as does the water within pools. And with the large volume of space, the noise level easily becomes distracting, unpleasant and even intolerable.
Activities such as swimming and diving generate noise. Building services features such as ventilation systems also contribute to the din. Spectators do add noise absorption, but when they cheer, they add to the acoustic problem. Water runoff areas at the pool perimeter, if poorly designed, can sound like a constant waterfall.
As a result, speech intelligibility is significantly compromised, making it difficult to accommodate swimming practice, instruction and competitions, or programs that combine music and aquatics. These conditions not only interfere with enjoyment, but can lead to dangerous situations.
When assessing a room’s acoustics, it is vital to measure reverberation time, or the number of seconds it takes for acoustic energy to dissipate by 60 decibels from its original value. The average reverberation time in a natatorium tops 6 to 8 seconds at 500 Hz (the center-of-speech frequency), and often it’s higher. Generally, the target reverb time at this frequency is 1.5 to 2 seconds. The challenge is how to control the reverberant field to a manageable level and achieve the ideal reverb time.
Making sense of acoustic absorption
- When sound hits a material or surface, three things can happen. It can be: reflected, or bounced off and redirected, by the material or surface
- transmitted through the material or surface
- absorbed by the material or surface
When sound is absorbed by a porous material, air gets trapped between the fibers and converted into a very small amount of heat. Imagine a football stadium filled with screaming fans. If all that sound could be absorbed into one cup of coffee, there might just be enough energy to heat it.
It’s important to note that acoustic absorption is frequency dependent: Two materials may look the same, but one might absorb high frequencies and another mid- or low frequencies. Typically, as the thickness of the material increases, so will the absorption of low frequencies.
Testing and analysis
The first step toward an acoustic solution for a natatorium is to conduct testing and analysis of existing conditions. This can be achieved in two ways: desktop calculation or on-site measurements.
With desktop calculation, acoustic engineers gather architectural specs such as the volume of the space and internal surface absorption coefficients (these can be supplied by manufacturers). They enter all of these data into modeling software that generates an estimate of the average reverberation time within the space.
While desktop calculation produces close estimates (and is a great option for new construction projects), on-site measurements are more accurate and are the preferred method whenever possible.
For on-site measurements, a team of engineers generates a noise field that “excites” the whole natatorium environment. Depending on the volume of the space and the number of pools it contains, the engineers use anything from loud speakers to shotguns that fire blanks — whatever it takes to produce the impact noise required. Whether employing a steady-state noise source (such as a loud speaker generating pink noise) or an impulsive source (such as a starter pistol), a sound level meter is used to calculate the reverberation time (often referred to as T60). In large spaces such as natatoriums, the source noise and measurements are made over multiple locations, including over the pools using mic booms. The average reverb time then is derived from these measurements.
Armed with data, engineers model the installation of acoustic absorption panels and recalculate the estimated average reverb time until they determine the installation scenario that achieves the target level. Other considerations that impact the type of acoustic panels to install include moisture-resiliency, adaptability and aesthetics.
There is an inherent conflict between acoustically absorptive materials and the atmospheric conditions of natatoriums. For obvious health reasons, no facility manager wants acoustic panels that soak in or retain moisture. V-ridged metal acoustic panels allow moisture to hit the metal exterior of the panel and drip off without getting into the sound-absorbing material within. V-ridged panels also allow more flexibility in placement around existing elements such as pipes, speakers, ducts and lighting. And they can be painted any color to complement the aesthetic design.
Ideally, when the project is completed, engineers perform post-installation testing to verify they have achieved the target level. A chart is generated that clearly illustrates the reverb time measurements from pre-installation through testing and modeling to post-installation.
Acoustic treatment of a natatorium can be a sizable and daunting project. Facility managers and administrators should put the job out to bid. This will give them a sense of how different contractors will work with their budget and scheduling restrictions so downtime can be minimized and ROI maximized.