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Isetta - reflex port hissing
Isetta - reflex port hissing
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Old 4th December 2019, 06:44 PM   #1
cloudiq is offline cloudiq  Scotland
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Join Date: Aug 2018
Location: Stirling, Scotland
Default Isetta - reflex port hissing

Hi guys

I built a Paul Carmody Isetta and pretty much followed his design - same dimensions, same drivers and same filter design.

As an amplifier, I bought the 2.1 Sound Import EU (Europe's version of Part Express I believe) amp with Bluetooth.

For the outside, I used slate veneer, something we already use for our business, designing translucent slate veneer lamps.

The speaker sounds great and offers amazing volume and bass for such a small thing, exactly what I was looking for.

However, I have a problem and I don't know how to resolve it. With a few songs that have a high SPL at very low frequency but low volume for everything else (e.g. Robert Miles, Children, first 10 seconds), I get audible reflex port chuffing.

A few words about my reflex port. I bought one of those black, flared speaker port tube that wasn't long enough for the Isetta. So I took a 40mm OD / 36mm ID PVC tube and glued it onto the end of the flared port tube which is "1.5 ID (or 38mm), so about a 1mm step on each side.

Because I wasn't sure how sensitive reflex ports are for steps, I used a file and honed it down a bit, but there is still a step. Additionally, I added a bit of polyfill inside the speaker (not around the reflex tube) to dampen things a little.

Still, I have can hear chuffing under the circumstances described above, that hasn't gone away.

What could it be?
- Is it because of the step?
- Are 36mm simply too slim vs the 38mm Paul recommends?
- Or is it simply something you have to accept for this kind of extreme speaker design?

Any feedback is appreciated.

Cheers guys

Phil from Scotland
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Old 4th December 2019, 07:11 PM   #2
Galu is offline Galu  Scotland
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Join Date: Apr 2018
Hi Phil!

That's a lovely slate finish.

The step is likely to break up the air flow and cause turbulance in the port and this will be most audible during the extreme bass events you describe.

Eliminating the step is probably your best option, but I look forward to hearing what others may offer by way of a solution.
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Old 6th December 2019, 10:07 AM   #3
swann is offline swann  Germany
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Join Date: May 2010
Location: Berlin
Hi Phil,

thing'll be that the port diameter is in general too small for such a high excursion driver. If you take the general rule of one third to one fourth of the port diameter from the original driver (because of air vent velocity) than you end up by at least 5 cm of a port diameter. But this would require a vent with like 50 cm length...so impossible for this small speaker.
You can download WINISD an try to model it, with the given boxvolume and port and you you'll most likely see the vent speed is way to high for that given diameter...

In general i would say you should buy a passive radiator at Soundimports and put it on the back of the speaker and close the port.. Should be the easiest way to get rid of it. Because you can't change the diameter now :-D.

For example:

with this pr Dayton Audio DSA215-PR Tieftoner bestellen? | SoundImports - SoundImports

you would need an additional weight for the cone of roughly 83 gramms for the same alignment like the ported one

and with this pr

Dayton Audio DSA175-PR Tieftoner bestellen? | SoundImports - SoundImports
(in my opinion too less excursion, but may will work) you would need like 16 gramms of extra weight for the pr.

Greets Swany
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