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Natural Science Forum / Physics / Acoustics / May 2005cut off frequency in rubber for vibration isolation
Hi. everyone I want to use rubber for vibration isolation. Someone ask me to calculate cut off frequency. But what does cut off frequency means for rubber ? Is it the range for application? How to calculate? Please teach me... >Hi. everyone > [quoted text clipped - 7 lines] > >Please teach me... It doesn't have one. WHat you must do is form a lowpass filter with an appropriate resonance frequency, which is what I presume they mean. For this you need to things. 1. The mass being suspended by the mounting (kg) 2. The stiffness of the rubber (kg / metre) Now substitute these into the formula f = 1 / (2 pi) * sqrt(stiffnes / mass) to get the cutoff frequency in Hz. At this frequency the amplitude of the vibration will be at its maximum, and you will need to arrange damping to reduce it by a suitable amount. Above this frequency, the amplitude will drop quite rapidly with frequency, giving you the necessary vibration isolation. So the story is that the softer the rubber, the lower the frequency at which isolation starts to operate. d Pearce Consulting http://www.pearce.uk.com Thank you for your rapid response ! Sincerely... > 1. The mass being suspended by the mounting (kg) > 2. The stiffness of the rubber (kg / metre) [quoted text clipped - 4 lines] > > to get the cutoff frequency in Hz. However correct you description of the method is, there is something wrong with the details. The above expression gives f in sqrt(1/meter) wheras it should produce 1/sec. best regards Peter >> 1. The mass being suspended by the mounting (kg) >> 2. The stiffness of the rubber (kg / metre) [quoted text clipped - 11 lines] >best regards >Peter Oops, my error! The stiffness should have been metres per Newton, of course. d Pearce Consulting http://www.pearce.uk.com >>> 1. The mass being suspended by the mounting (kg) >>> 2. The stiffness of the rubber (kg / metre) [quoted text clipped - 14 lines] >Oops, my error! The stiffness should have been metres per Newton, of >course. OK. Third time lucky. Newtons per metre. There - did it! d Pearce Consulting http://www.pearce.uk.com "Don Pearce" <donald@pearce.uk.com> wrote in message news:428aa63d.109192078@news.plus.net... > f = 1 / (2 pi) * sqrt(stiffnes / mass) > [quoted text clipped - 3 lines] > amplitude will drop quite rapidly with frequency, giving you the > necessary vibration isolation. In theory the vibration will be amplified up to a frequency of sqrt(2) times the resonance frequency. At this frequency the isolation will be zero, so useful isolation will happen only at frequencies well above this. Also the usual warning should apply: don't use a formula when you don't understand how it is derived and under what conditions it applies. We don't know the type of vibration isolation that the OP is trying to achieve, and there are many situations where the simple formula doesn't apply. Apart from anything else, practical systems will have a number of different modes of vibration.  SignatureTony W My e-mail address has no hyphen - but please don't use it, reply to the group. >"Don Pearce" <donald@pearce.uk.com> wrote in message >news:428aa63d.109192078@news.plus.net... [quoted text clipped - 10 lines] >the resonance frequency. At this frequency the isolation will be zero, so >useful isolation will happen only at frequencies well above this. This depends on the degree of damping deployed. WIth critical damping there will be no amplification at any frequency, just a smooth rolloff. The penalty is a reduced attenuation in the stop band. >Also the usual warning should apply: don't use a formula when you don't >understand how it is derived and under what conditions it applies. We don't >know the type of vibration isolation that the OP is trying to achieve, and >there are many situations where the simple formula doesn't apply. Apart >from anything else, practical systems will have a number of different modes >of vibration. I presume that he has been told what frequencies are to be suppressed, and by what amount. As an approximation a first order response can be assumed. The formula will allow him to reach a pretty good estimate of the kind of resonance frequency he needs to achieve his isolation. And of course we are dealing with a single mass/spring situation so the simple formulas do apply - at least for small amplitudes. Unfortunately rubber is the classical rising rate suspension system, and if it is a typical cone layout, the resonance frequency will rise considerably at the limits of compression. d Pearce Consulting http://www.pearce.uk.com > And of course we are dealing with a single mass/spring situation so > the simple formulas do apply - at least for small amplitudes. Sorry, maybe my news server has missed a post. All I've seen is "I want to use rubber for vibration isolation." That's why my response was rather wary. SignatureTony W My e-mail address has no hyphen - but please don't use it, reply to the group. http://forum.studiotips.com/viewtopic.php?t=1653 Here, and in the Paul Woodlock studio diary on the same forum you find all about vibration isolation with rubber, neoprene, epdm, etc., with newby explanations AND hardcore math :-) Regards, Bert "Sorbothane"? Any good? Martin SignatureM.A.Poyser Tel.: 07967 110890 Manchester, U.K. http://www.livejournal.com/userinfo.bml?user=fleetie > "Sorbothane"? > Any good? Depends on your intended use as well as on your definition of good. |
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