Low Frequency Floor Vibrations

This was a post on the LinkedIn Acoustic Society group page that I thought would have broader interest about low frequency vibrations in a building caused by the heating system air hander unit. David Wright posted this question on the LinkedIn  Acoustical Society of America (ASA)   http://www.linkedin.com/groupItem?view=&srchtype=discussedNews&gid=161832&item=190934921&type=member&trk=eml-anet_dig-b_pd-ttl-hdp&ut=0TQ8n-Ubl2BRw1 Vibration ills. What are the metrics A […]

This was a post on the LinkedIn Acoustic Society group page that I thought would have broader interest about low frequency vibrations in a building caused by the heating system air hander unit. David Wright posted this question on the LinkedIn  Acoustical Society of America (ASA)

  http://www.linkedin.com/groupItem?view=&srchtype=discussedNews&gid=161832&item=190934921&type=member&trk=eml-anet_dig-b_pd-ttl-hdp&ut=0TQ8n-Ubl2BRw1

Vibration ills. What are the metrics

A commercial 4 story “soft” building has occupants complaining of large amounts of vibration which is distracting them from working in their open office areas. Some are saying the floor vibration is making them ill. Sources seem to be walking nearby, but is unknown. Newly constructed poured concrete floors with steel trusses and framework. We are preparing an accelerometer to measure. What metrics would be useful to compare to for human annoyance vibration like this?   David Greenberg • Hi David. What I’m about to say, you know already, but here goes: Let’s say you uncover a vibration annoyance standard. I’ll bet there is one somewhere. Then what? You go about discovering the source(s) and isolating them. You compare the levels before and after with the standard. Even if you succeed in reducing the levels below this standard (from every receiver position of which there may be many), but the laws of subjectivity and issue-driven super-sensitivity, to say nothing of the extreme complexity of vibration energy propagation through structure and sympathetic things sitting on it, you (they) could still be in trouble when some of the occupants say they are still bothered. (The tonal components will likely be the problem.) If our colleagues do come up with a standard for you to use, I suggest you become familiar with it and then shelve it high out of reach. The potential for letting the client know that the standard has been met when they are still being bothered will do nothing for your relationship. Instead, I suggest you use the accelerometer to help locate the offending source(s) (though turning ALL machines off and then back on sequentially would be how I’d do it) and get the building owner to isolate them. But don’t set yourself up for an psycho-acoustic failure. Edward Veale • David, two things: * Here in the UK we have a British Standard that is useful, a summary of which follows – often the problem is making useful measurements sub 20Hz: Structural vibration in buildings can be detected by the occupants and can affect them in many ways; their quality of life can be reduced, as can their working efficiency. BS 6472-1 provides best available information on the application of methods of measuring and evaluating vibration in order to assess the likelihood of adverse comment. BS 6472-1:2008 provides guidance on predicting human response to vibration in buildings over the frequency range 0.5 Hz to 80 Hz. Frequency weighting curves for human beings exposed to whole-body vibration are included, together with advice on measurement methods to be employed. Methods of assessing continuous, intermittent and impulsive vibration are presented. BS 6472-1:2008 describes how to determine the vibration dose value, VDV, from frequency-weighted vibration measurements. The vibration dose value is used to estimate the probability of adverse comment which might be expected from human beings experiencing vibration in buildings. Consideration is given to the time of day and use made of occupied space in buildings, whether residential, office or workshop. BS 6472-1 offers guidance on how people inside buildings respond to building vibration: the judgement criteria are more stringent at higher frequencies than in the superseded standard due to changes in the vertical frequency weighting. * Anti-vibration systems fail if not designed and implemented correctly – as appears to be the case you are describing. You may find the following links useful reading but, as is often the case, many assume the base against which the vibrating mass acts against through the mount has infinite mass and, as in the case you describe, this is not so. As a “rule of thumb” guide, the area of the static mass beneath and covered the mount needs to be at least ten times that of the imposed moving mass at 20Hz and double that for every halving of the frequency. http://www.rostainc.com/pdfs/applications/Rosta_Catalogues/Antivibration%20Mountings.pdfhttp://www.farrat.com/pdf/DTD-AV-Mount-Theory-Selection-10a.pdf


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