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Natural Science Forum / Physics / Acoustics / May 2006non contact measurement
Dear All, I've seen vibrations measurement systems that use laser as a pick-up, I guess not as a mic would do, but often to check on resonance frequencies of cones i.e. I'm not talking about the very expensive systems that lets you see breakup modes, but very simple ones, to focus on a cone vibrating for sympathy and gives you a signal from it's vibrations. Anybody can tell me where to get them, and how do they work? Thanks, R. > Dear All, > [quoted text clipped - 10 lines] > Thanks, > R. There are only two relatively inexpensive systems for making non-contact measurement of vibration of which I am aware. Both systems measure displacement and are therefore frequency limited. One is the MTI photonic sensor, for which the vibrating surface needs to be reflective to light, which can be accomplished by bonding a very thin reflective piece of aluminum foil at the measurement location. The other is a capacitive probe, for which the vibrating surface needs to be electrically conductive, which can be accomplished by spraying it with a conductive paint. >> Dear All, >> [quoted text clipped - 19 lines] >probe, for which the vibrating surface needs to be electrically conductive, >which can be accomplished by spraying it with a conductive paint. A simple IR emitter detector mounted together will tend to work without much concern with reflectance. A small microphone capsule held close to the surface has also been used. grge > >> Dear All, > >> [quoted text clipped - 26 lines] > > grge what's expensive? :) it's a very relative term. i suppose you could use a bruel&kjaer mic, model 4193 if i'm correct. infrasound measurement. pro equipment. it's _VERY_ expensive... of course 4 my pocket. don't know about yours :) >>> Dear All, >>> [quoted text clipped - 27 lines] > > grge but lets say I want to pickup the signal of a resonant body like the passive radiator of a subwoofer, then a mic, even if nearby, will mainly sense the "loud" signal of the driver... > >>> Dear All, > >>> [quoted text clipped - 30 lines] > radiator of a subwoofer, then a mic, even if nearby, will mainly sense the > "loud" signal of the driver... i think there are ways to pre-filter the measured frequency. from what i saw you get a cap of some sort that you put on the mic capsule and it shriks the working/recieved freq. range. >> >>> Dear All, >> >>> [quoted text clipped - 38 lines] > i saw you get a cap of some sort that you put on the mic capsule and > it shriks the working/recieved freq. range. You really need to get off the drugs. > >> >>> Dear All, > >> >>> [quoted text clipped - 40 lines] > > You really need to get off the drugs. nah. i like them :) they help me. you don't wanna see what happens when i don't :) <cut> > > You really need to get off the drugs. > > nah. i like them :) they help me. you don't wanna see what happens when i > don't :) *when i don't take them >> >>> Dear All, >> >>> [quoted text clipped - 32 lines] >> radiator of a subwoofer, then a mic, even if nearby, will mainly sense the >> "loud" signal of the driver... On the cover of 6/1988 Speaker Builder magazine is a device hovering over the cone of an 8 inch driver, which is a Panasonic mic element.It has a lever whereby the position can be changed. This was for developing a map of the driver being driven. greg >i think there are ways to pre-filter the measured frequency. from what i saw >you get a cap of some sort that you put on the mic capsule and it shriks the >working/recieved freq. range. >>> >>> Dear All, >>> >>> [quoted text clipped - 51 lines] >>the >>working/recieved freq. range. I am in Malaysia, I don't know how to get that specific release of Speaker Builder. is there a link? if let's say I use infrared, then I shall modulate a high frequency signal (40kHz i.e.), and using the FM coming out from doppler effect detect the signal (does it make sense for a such relatively slow moving part to cause any shift in frequency for doppler effect?). Or I could measure the change in intensity of the reflected beam. Do you think ultrasounds would do? >>>> >>> Dear All, >>>> >>> [quoted text clipped - 60 lines] >any shift in frequency for doppler effect?). Or I could measure the change >in intensity of the reflected beam. Do you think ultrasounds would do? The change in intensity should be fine. A little adjustment of angles and reflectance. greg >>>>> >>> Dear All, >>>>> >>> [quoted text clipped - 63 lines] >The change in intensity should be fine. A little adjustment of angles and >reflectance. I was thinking, there is probably non-linearity since doubling distance is going to produce less than 1/2 the light. greg >>>>>> >>> Dear All, >>>>>> >>> [quoted text clipped - 80 lines] > > greg But if I'm just seeking the period to retrieve the frequency, it should be fine, right? Ric |
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