A recent trip to Maine brought an unexpected encounter with thick fog near the tip of a 4,000+ foot breakwater. This rendezvous brought to mind Stephen King’s novel “The Mist”. The physics behind harbor sheltering structures such as breakwaters share some of the same principles we encounter in room acoustics and sound isolation analyses. Whether or not physics is scary is up to the reader, but what could happen to the harbor without careful engineering certainly is!
Breakwaters are barriers used to protect harbors or coastlines, and the ships in or near them, from the force of ocean waves. Sound waves (like those analyzed on a daily basis here at MA) and ocean waves follow similar principles but propagate through different media. Diffusion, scattering, reflection, energy mitigation, and wavelength considerations are all part of both acoustic and breakwater design.
This particular breakwater was constructed by the United States Army Corps of Engineers in 1900 out of approximately 700,000 tons of rock. The visible top of the breakwater is about 43 feet wide formed out of coursed granite blocks, while the underwater base reaches an estimated width of 175 feet.
For this rubble-mound style berm breakwater, every aspect was intentionally engineered. The outermost ocean-side layer consists of loose rock arrangements. The space between the rocks allows them to move, absorbing some of the wave energy in the process. The spaces also interrupt the propagation of an ocean wave, allowing them to break in random fashion instead of crashing in unison unleashing all the energy at once. The random geometry of the breakwater (which extends well below the surface) helps to diffuse and scatter incoming ocean waves instead of creating specular reflections back out to lingering ships. The depth of the breakwater and trapezoidal shape with a steep sea-facing angle is needed to address low frequency ocean waves with long physical wavelengths. The result is a calmer harbor for ships to wait out a storm and a reduction of the erosion and destruction that might be caused if the powerful ocean waves were allowed to enter unencumbered.
All of this is analogous to acoustical design that we encounter on a daily basis: utilizing a resilient element in a ceiling or wall improves transmission loss. Diffusion and scattering are used regularly to help create uniform listening environments and reduce adverse reflections. Diffusive and reflective panels in performance halls are sized to be as large or larger than the wavelengths of soundwaves they are to reflect; otherwise, low frequency soundwaves will diffract or “bend” around the panel.
If not carefully engineered, breakwaters can have unintended consequences, such as salient formations: if too many ocean waves are redirected to the same place, sediment can accumulate over time. Similar phenomena can occur in room acoustics if diffusor and reflector size, type, and placement are not carefully considered, the result could be hot spots or undesirable build-up of spectral content.
Acoustical engineering has the power to prevent frightening destruction. Music has the ability to create moods and induce fear, but that is last year’s October newsletter. Stay safe and have a Happy Halloween!