Page 24 - COMSOL_News_2016
P. 24

NGI, NORWAY

Shake, Rattle,
and Roll

Norwegian researchers are tracking how
low-frequency sound waves travel within
buildings so that they can recommend
design adjustments to alleviate
annoying vibrations.

by JENNIFER HAND

Anyone who has slept near an airport will know the                               We have never achieved
             sensation — an early morning ight wakes you                         this level of agreement
             from sleep, not only because the engine is noisy but                with real-life testing
             also because everything around you seems to be                      before and it is all down
shaking. Likewise, people living near wind turbines, military                    to how we were able
sites, or hospitals with helicopter landing pads often complain                  to model the di erent
that windows rattle and everyday objects buzz when there is                      structural elements in
external noise. More puzzling for them is the fact that even                     COMSOL Multiphysics.
when they can discern no sound, they may still notice                            – FINN LØVHOLT, NGI
irritating vibrations.
                                                                   persists for longer distances. The amount of sound transmitted
   If the response of the sound is 20 vibrations per second        from the outside to the inside of buildings is greater. We are
(20 Hz) or less, it is described as infrasound, meaning that       interested in what happens at the threshold of hearing,”
the original sound is not usually audible to the human ear.        explains Løvholt. “We want to understand how sounds
The effects, however, are very easy to detect. As waves hit        from external sources interact with buildings and generate
windows, spread to the oor, and affect internal walls, they        vibration that is perceived by people. We can then recommend
induce a noticeable indoor vibration. Low-frequency sound          countermeasures to prevent vibration and may be able to
waves are notorious for their potential to create                  propose standard units that recognize the need to account for
annoying disturbances.                                             the ‘annoyance’ factor.”

ð LOW-FREQUENCY SOUND WAVES                                        ð SIMULATING THE SPREAD OF
IN BUILDINGS                                                       SOUND WAVES

Noise is part of modern life and there are formal standards        Løvholt and his colleagues decided to create a computer
that use sound pressure level measurements to recognize            model that would allow them to pick apart the mechanism
high-frequency sound waves at levels of sensitivity, intrusion,    of low-frequency sound waves hitting and penetrating a
and danger for humans. According to Finn Løvholt of the            building. They used the COMSOL Multiphysics® software to
Norwegian Geotechnical Institute (NGI), the generation of          simulate a wooden structure with two rooms separated by
building vibration due to infrasound is an area of research        a wall (see Figure 1, top), closely mimicking the laboratory
that has not been explored extensively. For this reason, NGI,      experiment setup. Within the model, they assigned a
an international center for research and consulting within         loudspeaker to one room, a microphone to the other, and
the geosciences, has been running investigative programs for       placed various probes around the structure in order to monitor
several years on behalf of the Norwegian Defence                   sound pressure levels and vibrations. Every component was
Estate Agency.

   “Low-frequency sound encounters less absorption as it
travels through the air than higher-frequency sound, so it

24 COMSOL NEWS 2016
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