For today’s topic, let’s think of driving our car with the wind rushing by, when all of a sudden our attention is drawn by that annoying squeak coming from the dashboard. The changing of the squeak with time was the topic of a recent blog, but this time we’ll examine its tonal nature.
A tone has the characteristic of pitch, something like a musical note. Contrast this with the broadband nature of the wind and road noise, which contain many different frequencies but don’t have the character of pitch. Despite that tonal dashboard squeak being relatively quiet, we can hear it in the sea of broadband road and wind noise because humans are quite good at selectively hearing tones, even when immersed in broadband noise. In fact, it is possible to hear tones that are quieter than the louder broadband noise that it is immersed in. Because tones demand our attention, they tend to be more annoying than broadband sounds, especially when they are part of unwanted sound.
There are many examples of this phenomenon in building acoustics. Rotating mechanical equipment (fans, pumps, chillers, etc.) as well as electrical equipment (motors, transformers, variable frequency drives, lighting dimmers) all emit tonal noise to varying degrees. Without proper sound and vibration isolation of this equipment, these tones can creep into adjacent spaces and annoy occupants. A tone doesn’t have to be loud to irritate people! Preventing this situation means including heavy enough sound blocking construction along with adequate vibration isolation. This is trickier than it sounds since cost is a factor that increases with better noise isolation. A balanced design approach is needed, which is one contribution that an acoustical engineer brings to the project. Noise fixes after the building has been constructed are almost always more costly than designing it properly initially. Also, some options may not even be possible due to existing constraints. With this in mind, it is easy to see that an ounce of prevention is worth a pound of cure when it comes to preventing annoyance from tones produced by building equipment.
At Acoustics By Design, we offer noise control consulting for projects all along the process, from conceptual design to construction administration. We like to get involved early in the process because it generally saves the client money and produces a better end result.