Vibrations & Excitations

Among our clients, there is often confusion between what is sound and what is vibration. We concede that the two can be difficult to distinguish to the untrained ear. Read on to learn more about the relationship between the two and how vibration can transduce into sound.

Vibrations are all around us. Nearly every motion creates some type of vibration, whether we perceive it or not. But what are vibrations? Vibrations are oscillations that occur about an equilibrium point. If you’ve ever watched someone jump on a trampoline, you’ve got a great visual reference. The jumping surface is taut at rest, which is its equilibrium point. When someone jumps up and down, the surface moves up and down, eventually returning to its equilibrium. It’s very similar to plucking the strings of a musical instrument.

How does this relate to buildings and sound? Buildings are meant to move, whether that is from swaying in the wind or protecting against seismic activity. Buildings also move when people walk on their floors, loud music is played, or rigidly mounted rotating or reciprocating equipment is attached to them. Put your hand on the interior gypsum board while watching your favorite rock band and you can feel the vibrations in the wall. Once vibration is in a building structure, it can travel efficiently to other areas.

For the most part, vibration in buildings is so low that it is outside of human perception. However, when the source is high enough, the energy waves of a surface are transduced into acoustical energy; this is known as structure-borne sound. For example, sound from a nightclub can carry through brick or masonry walls of a building up to apartments several floors above. When it gets to the residential floors, the gypsum board attached to the brick wall will radiate out sound annoying the occupants.

Attempts to reduce noise are often related to issues of vibration. A great example of this is a problem we solved at a condominium building. Residential occupants on at least eight upper floors were disturbed by a thumping noise in the early morning hours. After the building investigated, they realized it was caused by a heavy-stroked swimmer doing laps in a second floor pool before work. The pool shell was tied into two columns on the short end, and there was an open water containment area around the shell. We were hired to investigate the problem, and after many measurements and experimentation, determined that the pool was acting as a cantilever on the two columns – when the waves from the swimmer hit the sides of the pool, the impact moved the pool very slightly which carried the vibration up through the columns and radiated out as sound in the condominium units above. By bracing the pool and providing a resilient interior skin, the problem was solved.

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Vibrations & Excitations