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Old 6th June 2011, 11:47 AM   #1
Alex M is offline Alex M  United Kingdom
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Default Unexpected resonance

When I was working on a new passive crossover for my three-way speakers, I found a surprising peak in response at 230 Hz. I measured the impedance of my Audax HM210Z0 bass drivers in the cabinet, and found a peak at this frequency, which I ascribed to a longitudinal resonance in the box, whose internal height of 72 cm would match this pretty much perfectly.

Aha, I thought, I can tackle this resonance by putting in a couple of extra horizontal shelves in the cabinet to break up this mode. When I measured the impedance again, I found that the peak at 230 Hz had vanished, but to my puzzlement a new one had appeared at 188 Hz. Surely a cavity with its largest dimension 72 cm cannot support a mode of this frequency (half-wavelength 90cm)? Could it be some kind of interaction with either the driver suspension or the port?

Here is a plot of the old and new impedance curves, with the Audax data as a reference:

Click the image to open in full size.

The second peak at 425 Hz is relatively unaffected by the extra shelves, and may well be a front-back resonance (the half-wavelength is 40 cm, which is consistent with that).

Alex
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Old 6th June 2011, 11:57 AM   #2
SY is offline SY  United States
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Did you merely increase the path length by putting in the shelves?
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Old 6th June 2011, 12:13 PM   #3
Alex M is offline Alex M  United Kingdom
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Originally Posted by SY View Post
Did you merely increase the path length by putting in the shelves?
The thought did cross my mind, and I certainly wouldn't rule it out, but I couldn't intuitively understand how I could have increased the path length by that much at the same time as sharpening the resonance.

What do you think?

Alex
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Old 6th June 2011, 12:17 PM   #4
SY is offline SY  United States
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I think you increased the path length.

Check the effect of putting in some absorbent stuffing.
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Old 6th June 2011, 12:20 PM   #5
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Put damping (dacron etc.) in the box, a large thickness (10-20cm) in the bottom and the top. More efficient than shelves.
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Old 6th June 2011, 12:23 PM   #6
Alex M is offline Alex M  United Kingdom
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Originally Posted by jerome69 View Post
Put damping (dacron etc.) in the box, a large thickness (10-20cm) in the bottom and the top. More efficient than shelves.
I can certainly try that, but I always thought that the accepted wisdom was to avoid damping a ported system?

Alex
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Old 6th June 2011, 12:39 PM   #7
AllenB is offline AllenB  Australia
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200-400Hz is such an important region, I'd rather compromise in favour of this region. The damping will have a greater effect up here than down at the tuning frequency so I'd try to find just the right amount to use. I wouldn't be afraid to stuff a vented box if I had a good enough reason like this.
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Old 6th June 2011, 12:51 PM   #8
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Originally Posted by Alex M View Post
I can certainly try that, but I always thought that the accepted wisdom was to avoid damping a ported system?
An old rule of thumb, useful in the days before widespread availability of decent test equipment and software to avoid misalignment of system response. Now you can see what you're doing, not a great issue.
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Old 6th June 2011, 01:07 PM   #9
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A very distasteful comb filtering effect that may happen at low frequencies ...it happened to me ,and I just did like Jerome says , putting dacron at the top of the cabinet .
Now bass is good .
Nice work I see on the site !!!Mhh.mmh I see interesting basics for IC based filters
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Old 6th June 2011, 06:16 PM   #10
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My latest project, damping effect of the column :

Click the image to open in full size.

Too much stuffing kills the sound.
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File Type: jpg effect_amortisement.jpg (54.5 KB, 244 views)
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