One-Third-Octaves for Dummies
One-Third-Octaves for Dummies
(OP)
Hello all. I do a fair amount of work with structural vibrations, but our usual frequency band of interest is fairly narrow, like 0-20 Hz. No background in acoustics or higher frequency mechanical vibrations. Therefore, I've never had to deal with plots using one-third-octaves. I've done some reading online and have an idea of what's going on, but not quite enough to allow me to really deal with these effectively. My current application is checking a structure's ability to support a CT scanner. The manufacturer's tolerance plot is a spectrum with log scale rms acceleration vs One-Third-Octave Center Frequency in Hz. I'm not sure how to turn this into a spectrum with linear x and y axes for comparison with the predicted acceleration spectrum. If someone would be kind enough to point me to a good discussion, I'd appreciate it.
BTW, I searched, but only found the following, which was helpful, but I need more.
http://www.eng-tips.com/viewthread.cfm?qid=150400
BTW, I searched, but only found the following, which was helpful, but I need more.
http://www.eng-tips.com/viewthread.cfm?qid=150400





RE: One-Third-Octaves for Dummies
So the psd is a horizontal line, height (rms_accel_in band)^2/(.22*centre frequency of band in hz).
However you'd be better off turning your linear Hz measurement into third octaves for comparison.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: One-Third-Octaves for Dummies
A really silly question: Why would one want to use a one-third octave spectrum instead of a regular FFT magnitude? I work with relatively low frequency vibrations in the 3 Hz to 20 Hz range 99% of the time. A few of the equipment manufacturers express their vibration limits in terms of one-third octave spectra, hence my original question.
RE: One-Third-Octaves for Dummies
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Eng-tips forums: The best place on the web for engineering discussions.
RE: One-Third-Octaves for Dummies
http://colingordon.com/pdf/Gordon-SPIE99.pdf
As Pete says, it probably evolved from a historical use of available instruments. Third octave filters are really quite narrow in the frequency range below 20 Hz!
Walt
RE: One-Third-Octaves for Dummies
RE: One-Third-Octaves for Dummies
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?