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Multi-Way Conventional loudspeakers with crossovers

tuning aperiodic vents
tuning aperiodic vents
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Old 15th March 2019, 11:44 PM   #21
planet10 is offline planet10  Canada
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tuning aperiodic vents
If that impedance is a box with a hole in it (but an aperiodic “filter” over it), then it is either working pretty well as you have completely suppressed the 2nd peak or you may have too much filter and you have effectively sealed the box.

I have found that a filter at the terminus (ie VarioVent/Dyna A25 style) is harder to tune than more distributed damping (or a combo).

This is an aperidoically damped midTL. Free Air vrs in the box impedance. The impedance peaks are dramatically suppressed but you can still see a ghost of the bottom hump. Further refinement flattens it even more.

Click the image to open in full size.

dave
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Old 16th March 2019, 09:39 AM   #22
TimA is offline TimA
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The vent is 12mm thick and uses three compressed layers of 10mm thick dacron/polyester (as used in upholstery). Between each of those layers is a layer of fine-weave membrane as used for weed suppression among other things. The latter resists air flow quite noticeably when I blow through it, more so than the polyester. I've used gutter mesh to press it all together.

Soundwise the bass sounds OK to me (and it measures broadly in line with my aperiodic simulations) but as the design starts rolling off around 100Hz and the crossover isn't installed yet it'll be a few more days until I can assess it properly subjectively so for the moment my thinking is to leave further vent adjustments until then, particularly as the vent already seems to be attenuating it's own output quite effectively.
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Old 16th March 2019, 09:44 AM   #23
TimA is offline TimA
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This is the vent...
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Old 16th March 2019, 10:02 AM   #24
TimA is offline TimA
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I understand, but even with the vent completely open the impedance plot is the same albeit with a marginally lower peak, but the difference is VERY marginal. Of course much more sound is emitted through the open vent.
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Old 16th March 2019, 02:44 PM   #25
Galu is offline Galu  Scotland
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Quote:
Originally Posted by TimA View Post
This is the vent...
Thanks for your description and photos of your vent - it all seems good on paper!

Dave's impedance plot shows the typical aperiodic action of reducing the amplitude (Q) of the impedance peak while shifting it upwards in frequency.

Your impedance plot shows a small Q impedance peak at a frequency of around 95Hz.

This frequency is rather high for adequate bass reproduction. It would be interesting to know what the free air resonance of your 5.25" drivers is.

You say the impedance curve with the vents open is 'near enough identical' which is a rather puzzling. How does the sound compare with the vents open?
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Old 16th March 2019, 05:08 PM   #26
planet10 is offline planet10  Canada
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tuning aperiodic vents
Quote:
Originally Posted by TimA View Post
but even with the vent completely open the impedance plot is the same albeit with a marginally lower peak
Then i have to suspect your measurin kit, or that the box is as decided. You put a hole in thebox you should have a 2nd peak.

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Old 16th March 2019, 06:52 PM   #27
TimA is offline TimA
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Many thanks for the replies. I will remeasure soon both with and without vent membrane.
The drive units have an Fs around 67Hz when broken in but the measurements here use non broken in units with a higher Fs around 80Hz IIRC. Low bass extension is not required from this design as subwoofers will supply that.
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Old 16th March 2019, 08:27 PM   #28
Galu is offline Galu  Scotland
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Originally Posted by TimA View Post
. . . the measurements here use non broken in units with a higher Fs around 80Hz IIRC.
Thanks for that info' which is in accord with the aperiodic peak at 95Hz.

P.S. Is this is the first time you've mentioned you'll be using a subwoofer? If I'd twigged that I wouldn't have queried the bass extension.
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Old 16th March 2019, 09:03 PM   #29
TimA is offline TimA
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Thanks, that's good to know. No, I hadn't mentioned the subwoofers before. The crossovers are almost finished now so I hope be able to listen to them tomorrow.
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Old 19th March 2019, 10:24 PM   #30
TimA is offline TimA
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....I would have remeasured the impedance by now but I'm enjoying listening to these too much. The bass is really good. Initially I felt it wasn't great but the drivers have been unused since manufacturer in 1998 and perhaps needed some break in. After a few hours the bass sounded very natural and the speakers present a refined and effortless sound. I'll get round to the measurements eventually but in the meantime thanks to everyone for their helpful responses.
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