dual PR sub shake? break-in ? - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 04-19-09, 12:45 PM Thread Starter
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
Re: dual PR sub shake? break-in ?

I don't agree with much of what you have said.

Quote:
DrWho wrote: View Post
I don't think your test is accurately identifying the Fs of the PR's...you've got the phase angle to deal with and the effects of the enclosure around the PR. And who knows what your microphone is doing at those lower frequencies too...there's a lot of physics going on here.
There isn't any phase angle in my measurement method. If I were using another speaker playing sine waves to stimulate the resonance there would be a phase relationship between that speaker and the PRs output

As far as how I took the measurements the box has a large volume and 2 large openings so air is not restricted. I was simply using the box to hold the PR steady.

Quote:
DrWho wrote: View Post
Btw, the resonant frequencies you're looking at are well below the Nyquist rate of typical video cameras. I would recommend just filming the PR movement and then dumping it into windows movie maker where you'll be able to calculate the frequency with the timestamps.
I had thought about using this method. But when striking the PR, a low frequency oscillation is easily seen by eye as it decays over about 2 seconds. The box helps concentrate the backwave to maintain a large enough signal for the microphone in the subsonic frequencies. Filtering the recording and amplifying by only about 15dB showed this very clean oscillation that visually matches the decaying resonance observed.

Calculating the frequency from this figure matches the very clean peak at about 6.5Hz from the frequency domain plot given before .

Quote:
DrWho wrote: View Post
Also, you need to measure the PR Fs in the same manner that it was spec'd. I dunno if this is a half space measurement or calculated from a fixed cabinet volume or what. Changing the environment around the PR will changes the resonance you measure.
I feel that the way I measured the fs is accurate. The mass of the PR is much larger than the mass of the air inside the box. It is also much higher than any difference in air load between different fs testing methods. Its effect would cause only a minute shift in the PR resonant frequency.

Take for example when manufacturers give both baffled and free air fs measurements. They differ by only a few percent because the air load only slightly increases the effective moving mass.

Eventually I will try taking video of some weird modulation effect I get at high excursion. I am painting the box so that will have to wait. Also, maybe it will be gone once everything is properly sealed.

Last edited by SturmMD; 04-19-09 at 03:30 PM.
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Old 04-20-09, 08:09 PM
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Mike Bentz

Join Date: Sep 2006
Location: Chicago
Posts: 438
Re: dual PR sub shake? break-in ?

-Mike Bentz

"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"
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Old 04-20-09, 08:17 PM Thread Starter
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
Re: dual PR sub shake? break-in ?

Without some definition of a reference point (0 degrees phase) there is no way to determine the phase the way I took the measurements.
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Old 04-21-09, 12:35 PM
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Josh

Join Date: May 2007
Location: Louisville, KY
Posts: 2,431
Re: dual PR sub shake? break-in ?

Your impedence measurement looks normal to me. My guess is that there is a difference in weight on the PR's causing the oscillation effect. I'm not sure how much it would take to be noticeable though. Probably a fairly large %. I'd still like to see a measurement with the mic 2" from the driver cone and then 2" from each PR in separate graphs. It may show something...
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Old 04-21-09, 01:17 PM Thread Starter
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
Re: dual PR sub shake? break-in ?

Quote:
Ricci wrote: View Post
Your impedence measurement looks normal to me. My guess is that there is a difference in weight on the PR's causing the oscillation effect. I'm not sure how much it would take to be noticeable though. Probably a fairly large %. I'd still like to see a measurement with the mic 2" from the driver cone and then 2" from each PR in separate graphs. It may show something...
I just so happen to have a picture of this from some of my earlier measurements. I had done it quickly so its not very scientific. I believe this was without microphone calibration which explains why the low frequency output of the PRs doesn't "peak" very much.

I don't believe the 2 dips in the driver output mean anything. Observing the excursion versus frequency it is clear that there is a nice smooth excursion minimum. The peak within the tuning dip matches up with some of my in-room measurements and I didn't even bother moving the sub away from walls when I took this.
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Old 04-21-09, 01:26 PM
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Josh

Join Date: May 2007
Location: Louisville, KY
Posts: 2,431
Re: dual PR sub shake? break-in ?

Hmmm. I don't know SturmMD. That looks a bit suspect to me. Can you repeat it when you get a chance? you should nave a very defined low point in the driver response and a better defined peak in the PR's. If I were to guess what it would look like with the PR's tuned slightly differently it would look similar to that.

Mike,

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Old 04-21-09, 01:33 PM Thread Starter
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SturmMD

Join Date: Dec 2008
Location: Minneapolis, Minnesota
Posts: 165
Re: dual PR sub shake? break-in ?

Because of the log frequency scale there is actually a large difference between the frequency of those two dips. The difference in tuning frequency also corresponds to very drastic PR weight differences so that's why I don't think that's what they are.

Also, these were very hurried measurements - I didn't have equal distances, or take care to be perpendicular to the surface, or even near the center. Additionally, they box was still close to a wall and about 5 feet from a corner. Looking at the vertical axis scale this null is about 20-25dB which I believe is the "noise floor" because of all the stuff mentioned.

I'll definitely retake these measurements when I'm done painting.
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