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Old 30th July 2012, 01:53 PM   #1
Pagnol is offline Pagnol  Germany
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Default Measuring excursion in real time to perform corrections

I've been looking for websites and forum topics which deal with techniques to measure the exursion of a driver in real time, but I haven't found any. Probably I don't know the keywords.

In case it's not entirely clear what I want to achieve, let me summarize the idea: I would like to attach to a woofer a device which measures this woofer's excursion. The signal is fed into the computer responsible for generating the audio signal. A software running on this computer compares the reading to the audio signal and shapes the audio signal accordingly.

This raises a couple of questions:
  • What to correct?
  • Is it at all possible to perform a useful correction, given the latency with which the correction arrives at the speaker?
  • How to accurately measure the excursion?

I've set up a small system with a digital crossover (BruteFIR) and applied digital room correction. Measuring the excursion of the woofers seems like a fun thing to go on with.

Grateful for any advice
Pagnol
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Old 30th July 2012, 02:00 PM   #2
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Quote:
Originally Posted by Pagnol View Post
I've been looking for websites and forum topics which deal with techniques to measure the exursion of a driver in real time, but I haven't found any. Probably I don't know the keywords.

In case it's not entirely clear what I want to achieve, let me summarize the idea: I would like to attach to a woofer a device which measures this woofer's excursion. The signal is fed into the computer responsible for generating the audio signal. A software running on this computer compares the reading to the audio signal and shapes the audio signal accordingly.

This raises a couple of questions:
  • What to correct?
  • Is it at all possible to perform a useful correction, given the latency with which the correction arrives at the speaker?
  • How to accurately measure the excursion?

I've set up a small system with a digital crossover (BruteFIR) and applied digital room correction. Measuring the excursion of the woofers seems like a fun thing to go on with.

Grateful for any advice
Pagnol
You are basically limited to low frequencies because of the phase shift betwwen the actual excursion and the driving signal at higher frequencies making the system unstable.
Also at higher freqs the cone will no longer be a single piston and have partial vibrations.
Google 'klippel' - Wolfgang Klippel has done a lot of (professional) work on that and has a functioning system.

jan didden
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Old 30th July 2012, 02:02 PM   #3
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This sounds a little like motion feedback too.
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Old 30th July 2012, 02:29 PM   #4
Pagnol is offline Pagnol  Germany
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Quote:
Originally Posted by janneman View Post
Google 'klippel' - Wolfgang Klippel has done a lot of (professional) work on that and has a functioning system.
His office is located in the city in which I live. Maybe I'll find an opportunity to learn more (I notice he's offering internships).
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Old 31st July 2012, 10:42 AM   #5
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Are you thinking of an adaptive feed-forward correction that tries to prevent the distortion that "should" be happening in the woofer.

Or do you want a feedback system?
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Old 31st July 2012, 12:13 PM   #6
forr is offline forr  France
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For excursion, a capacitive sensor does the job :

Capacitive motional feedback for loudspeakers
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Old 31st July 2012, 12:43 PM   #7
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Originally Posted by David_Web View Post
Are you thinking of an adaptive feed-forward correction that tries to prevent the distortion that "should" be happening in the woofer.

Or do you want a feedback system?
I think that is how Klippel does it. He 'measures' the driver parameters in a calibration cycle and then adapts the drive to compensate for the non-linearities.
It's pretty sophisticated - not only can he get much more undistorted output from a driver, but it also has a protection and soft-clip subsystem.
It's all software, of course

jan
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Old 31st July 2012, 12:53 PM   #8
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Find the "Reading driver displacement with radar" thread.

Voice coils provide velocity control over the control, which isn't as good as position control which is what (I suspect) is what the OP wants.
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Old 1st August 2012, 10:55 PM   #9
Pagnol is offline Pagnol  Germany
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Do you think something could be achieved using the Kinect? I suppose its frame rate is too low to make it apt for a real time application in audio. However if its resolution, accuracy and frame rate were remotely sufficient, one might attempt to measure such things as excursion and deformation of the cone. Maybe useful statistical data about a driver could be gained by running the setup (Kinect immediately in front of the woofer) many times.

I hesitated sending this off because the idea strikes me as heavily impractical. I do nevertheless because it might still provoke thought.

Quote:
Originally Posted by forr View Post
For excursion, a capacitive sensor does the job :

Capacitive motional feedback for loudspeakers
Thank you for the hint. That looks pretty much like what I'm looking for. The method he employs is rather elegant.

Last edited by Pagnol; 1st August 2012 at 11:09 PM.
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Old 1st August 2012, 11:00 PM   #10
jcx is offline jcx  United States
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cone velocity is what generates sound, not position
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