Mechanical Vector Impedance Meter? - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th July 2011, 12:42 PM   #1
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
Default Mechanical Vector Impedance Meter?

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?
__________________
I may be barking up the wrong tree, but at least I'm barking!
  Reply With Quote
Old 5th July 2011, 01:50 PM   #2
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
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!
  Reply With Quote
Old 5th July 2011, 09:06 PM   #3
gpapag is offline gpapag  Greece
diyAudio Member
 
gpapag's Avatar
 
Join Date: Nov 2002
Location: Athens-Greece
Quote:
Is this completely crazy, or could such a thing be built? Would it be worth the effort? Has there ever been such a thing?
Hi Conrad

I guess you are envisioning something like this:

http://yabe.algebra.com/~ichudov/mis...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.

Water as a sound deadening medium in enclosures???

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
__________________
["Second Law is a bitch." - SY] ["The Road To Heaven:Specify the performance & accept the design. The Road To Hell:Specify the design & accept the performance"-Bruno Putzeys]

Last edited by gpapag; 5th July 2011 at 09:33 PM.
  Reply With Quote
Old 5th July 2011, 10:38 PM   #4
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
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!
__________________
I may be barking up the wrong tree, but at least I'm barking!
  Reply With Quote
Old 8th July 2011, 11:21 AM   #5
diyAudio Member
 
speaker dave's Avatar
 
Join Date: Nov 2009
Location: The Mountain, Framingham
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.
  Reply With Quote
Old 8th July 2011, 12:27 PM   #6
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
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.
__________________
I may be barking up the wrong tree, but at least I'm barking!
  Reply With Quote
Old 8th July 2011, 01:27 PM   #7
diyAudio Member
 
speaker dave's Avatar
 
Join Date: Nov 2009
Location: The Mountain, Framingham
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Toa EPM 104 impedance meter anatech Solid State 12 12th April 2013 12:34 PM
A LED meter, Volt meter, power meter or light indicator... a nice gadget destroyer X Solid State 5 25th February 2011 06:30 PM
Vector meter - or steering indicator AuroraB Equipment & Tools 2 24th July 2010 02:59 AM
QED Vector Reference georgehifi Solid State 0 17th July 2007 01:08 AM
Need Impedance Meter suggestions e.lectronick Multi-Way 7 15th December 2003 09:53 PM


New To Site? Need Help?

All times are GMT. The time now is 04:12 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2