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Old 8th March 2004, 08:21 PM   #1
cm961 is offline cm961  Canada
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Default Transient Response Testing

Hi Guys,

I'm doing a school project (MFB woofer). Anyway, we're analyzing our measurements.

The blue signal is the input signal from the soundcard. The red signal is the recorded output signal from the woofer. The microphone is a Panasonic electret type as per Linkwitz.

I understand the ringing at the end is due to the resonance of the speaker. And I can clearly see the phase delay. However, what is causing the inital waveform to increase before decreasing (the spike at the beginning).

Thanks,
Pete
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Old 8th March 2004, 09:44 PM   #2
sreten is offline sreten  United Kingdom
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Other than a DC offset transient than can't be seen I've no idea,
this is implied by the assymetry of the soundcard signal, and
the assymetrical response.

There is no way the unit will travel in the opposite direction to applied voltage.

sreten.
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Old 8th March 2004, 10:07 PM   #3
Svante is offline Svante  Sweden
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A behaviour similar to this is what one gets from an all-pass filter. A second order linkwitz filter with a reversed tweeter behaves like an allpass filter. Was there a tweeter connected?
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Old 8th March 2004, 11:05 PM   #4
cm961 is offline cm961  Canada
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The connections are Soundcard to amplifier to speaker. The left input is taken from the soundcards output to input and the right is taken from the microphone in front of the speaker. The microphone is one of those panasonic electret types with a 9V bias and a non-inverting opamp (OPA227) with a gain of about 5.

I'm very confused.

Pete
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Old 8th March 2004, 11:13 PM   #5
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Can we assume there is no tweeter or crossover involved? Is the MFB disabled?
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Old 8th March 2004, 11:19 PM   #6
cm961 is offline cm961  Canada
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MFB is dissabled, no crossover at all.

Peter Binsted
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Old 8th March 2004, 11:45 PM   #7
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What is the freq. of the input 1 cycle tone burst? Is the box sealed or vented? If sealed, what is the Q and F3? If vented, what is the box tuned to? Or, come to think of it, is there a box at all? And, if no box what is the resonance of the woofer?
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Old 8th March 2004, 11:50 PM   #8
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For a start, I think the output signal is inverted on the graph. Remember that the voltage doesn't necessarily dictate the position of the cone, just as the microphone doesn't pick up air "movement" but air "pressure". Hence the odd phase difference.

CM
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Old 8th March 2004, 11:56 PM   #9
cm961 is offline cm961  Canada
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Nice, I think you might be right. Here's another graph. This is the first one we took, we thought the output was inverted so we changed it.

The odd thing here is that the speaker goes back towards the 0 position before the input. That's why we thought it was wrong.

Pete
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Old 9th March 2004, 12:05 AM   #10
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Imagine you've just started pushing a really heavy weight on a swing, before you've pushed it as far as you want it to go, it starts heading backwards - that's the initial step at the start of the waveform. It makes it appear that the cone is anticipating the voltage waveform. Maybe try longer pulse to get a steadier output?

CM
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