I've never seen such a thing, but what do people think of the idea of a mechanical impedance meter? I envision a box (or PC) that reads out something like impedance and loss, using a probe similar to a scope probe, but with a tip that vibrates. In theory, one could put it against a speaker cabinet and get the response and resonances, or a driver itself. If it were sensitive enough it could look at turntable bearings, platters and arms. Probably impossible to do cantilevers though.
Is this completely crazy, or could such a thing be built? Would it be worth the effort? Has there ever been such a thing?
Is this completely crazy, or could such a thing be built? Would it be worth the effort? Has there ever been such a thing?
You will hit the usual problems from transducers, and tightness of coupling between the mechanical and electrical domains. Try it. Take an old speaker, remove the cone and carefully attach some sort of thin stiff lightweight probe to the voice coil. Measure the impedance over the audio band. Poke the probe into your speaker cabinet. Measure again. The difference between the two measurements tells you something. I guess at this point the maths get horrible!
Hi Conrad
I guess you are envisioning something like this:
http://yabe.algebra.com/~ichudov/misc/ebay/FtLewis-1/s9r_data.pdf
I have used it and some other equipment which operate in some different mode.
They are good in comparing material samples which have the same geometry at the area of testing.
Due to the small spacing of the probes (a vibrator and a receiver, both piezo transducers), they can not provide any data for complete structures
For structures testing in the field of our hobby, you may use small piezo pick-ups and do the data manipulation with software.
http://www.diyaudio.com/forums/multi-way/125332-water-sound-deadening-medium-enclosures-4.html#post1549021
As DF96 said, a lot depends on the sensor-material coupling. For comparing data (qualitative assessment), this coupling has to be well specified and consistent. This is difficult to achieve.
Regards
George
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After some Google searches it seems that these things do exist, though not in the form I've got in mind. Once you use something like an HP 4815 vector impedance meter for electrical impedance, the idea of a mechanical equivalent is very enticing. If I ever figure out how to do the probe, I'll post it!
Do you really means impedance?
Impedance (mechanical or acoustical) is the ratio of pressure to velocity. It is just another ohms law type relationship.
The most interesting look at impedances would be for system resonances. For a 6 sided enclosure or a long tube (TL) then the acoustical impedance swings high at each resonance (high pressure results in low velocity) and low at each antiresonance (or do I have that backwards?). Usually pressure and velocity are seperately measured and impedance is calculated.
Sounds like you just want an accelerometer. They can be applied to boxes and will measure acceleration or velocity and show all the resonant activity. Light ones can be applied to woofer cones, or if you are concerned about their mass influencing matters you can use a laser probe.
David S.
Impedance (mechanical or acoustical) is the ratio of pressure to velocity. It is just another ohms law type relationship.
The most interesting look at impedances would be for system resonances. For a 6 sided enclosure or a long tube (TL) then the acoustical impedance swings high at each resonance (high pressure results in low velocity) and low at each antiresonance (or do I have that backwards?). Usually pressure and velocity are seperately measured and impedance is calculated.
Sounds like you just want an accelerometer. They can be applied to boxes and will measure acceleration or velocity and show all the resonant activity. Light ones can be applied to woofer cones, or if you are concerned about their mass influencing matters you can use a laser probe.
David S.
I pondered that as well, but I think the impedance concept applies. As you transfer energy into and out of a material (think a block of rubber), there will be losses on each cycle. My guess is, depending on the density of the stuff, there will also be frequency dependent phase shifts. IMO, it looks similar to an electrical component, and it's well known that the electrical an mechanical worlds mirror each other to a large extent. I'd think one could even express mechanical impedance in a variety of forms (consider ohms, Siemans, the old mhos, admittance, susceptance, etc.), just as you can with electrical impedance, though I've no idea what's typically used.
That is all true. The mechanical ohm is a newton per (meter per second), so a force over velocity measurement. Usually I believe you measure the two items (force, velocity) seperately and divide. You can also use a constant velocity generator and just measure force or conversly a constant force generator and measure velocity.
These are analagous to driving an electrical circuit with very high or very low impedances. As an example you can drive a crossover network with high impedance (constant current) and measure the voltage across it, which become representative of the impedance curve. Mechanical and acoustical circuits are just the same.
In the end, the mechanical impedance curve shows the susceptability of the device to a force stimulus. Where impedance is high the force results in not much velocity. Where impedance is low the resultant velocity will be higher for the same force input.
I still suspect you are more interested in the acceleration profile, although, if you know the driving force then they are related as described.
David S.
These are analagous to driving an electrical circuit with very high or very low impedances. As an example you can drive a crossover network with high impedance (constant current) and measure the voltage across it, which become representative of the impedance curve. Mechanical and acoustical circuits are just the same.
In the end, the mechanical impedance curve shows the susceptability of the device to a force stimulus. Where impedance is high the force results in not much velocity. Where impedance is low the resultant velocity will be higher for the same force input.
I still suspect you are more interested in the acceleration profile, although, if you know the driving force then they are related as described.
David S.
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