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Help interpreting aperiodic membrane test results
Help interpreting aperiodic membrane test results
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Old 20th September 2018, 07:03 PM   #1
mojozoom is offline mojozoom  United States
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Default Help interpreting aperiodic membrane test results

I'm mostly a car audio guy so please bear with my lack of knowledge on some of these topics. I've got a pair of 6" Silver Flute drivers mounted in my doors and decided to build some AP type enclosures for them to see how they'd work. There's not a lot of AP guidance available on the internet, other than one site that states that:

"..the woofer's Qts (total system Q) that determines the AP candidates! ANY WOOFER WITH A QTS OF .45 or LESS CAN WORK VERY WELL IN AP DESIGNS! I made that bold so it will stick with you guys. So, if you want to try your hand at this type of enclosure, seek the Qts spec of the woofer. If it is less than or = to .45, you have a winner. You will find that it is mainly the subs designed for ported enclosures"

The 6" Silver Flute has a published Qts of 0.24, and Zaph's testing showed 0.29. Based on the criteria above it seemed like a good candidate for an AP enclosure. There's not a lot of space in a car door, so the enclosure depth was limited, and it also had to be water resistant, so the port had to be shrouded and point down. The membrane is fiberglass matte, in layers, and clamped between two grilles. Here's what it looks like:

Click the image to open in full size.

So I ran some tests using REW and my living room floor, to get a rough idea of the change in the impedance curve with varying amounts of fiberglass membrane. The tests were run in the middle of the floor with the driver in the enclosure and aimed up at the ceiling. It wasn't extremely precise, in that I was holding the test mic at approximately the same height over the driver in each test. I can see that adding more resistance to the membrane drops the low frequency impedance spike, but from what I understand that in itself isn't really critical unless using tube amps? I'm using all Class D in the car. Anyway the results of the impedance tests came out about as expected:

Click the image to open in full size.

It's also altering the frequency response of the driver, but that's not critical in my application either, as I have a DSP and can work with just about anything. Here's the amplitude plots:

Click the image to open in full size.

But the real questions come from the distortion runs. It seems that the AP membrane reduces the measured distortion levels by as much as 5 dB up to about 2000 Hz, then there's some real nastiness that wasn't in the open vent plot. Maybe that's from reflections inside the enclosure? But overall below 2000 Hz why would the distortion drop so much with an AP membrane in place?

Click the image to open in full size.

So, ultimately I'm trying to figure out what I figured out, and if I should bother applying it. Thanks -
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Old 20th September 2018, 08:00 PM   #2
Galu is offline Galu  Scotland
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Quote:
But overall below 2000Hz why would the distortion drop so much with an AP membrane in place?
The aperiodic loading makes the woofer think it's in a larger enclosure. Maybe that, and the reduction in cone excursion around the resonant frequency, could account for the reduction in distortion you have measured?
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Old 20th September 2018, 08:22 PM   #3
wolf_teeth is offline wolf_teeth  United States
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Aperiodic enclosures have a rep for having lined walls with a clear passage from driver to vent, not unlike vented enclosures. I think some lining/damping would definitely improve your results.

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Old 21st September 2018, 12:37 AM   #4
mojozoom is offline mojozoom  United States
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I just realized that the increased distortion above 2000 Hz has to be due to the membrane and not reflections, as the baseline distortion was measured with the driver in the same exact enclosure, just with no resistive membrane in play. Although i agree that adding some padding to the insides can't hurt at all, i don't think it would solve the sudden evil distortion above 2k.

Currently these are in a 2-way arrangement with Seas Neo's crossed at 2500 Hz with a steep slope, so that might be OK. In a 3-way setup I suppose it wouldn't be an isuue at all.
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Old 21st September 2018, 01:49 AM   #5
tsmith1315 is offline tsmith1315  United States
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Mojozoom, I don't know how much faith I'd have in criteria from a site which uses a scrotum as their logo.

However, YOUR handywork looks great. Did you build the entire enclosure yourself?
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Old 21st September 2018, 01:58 AM   #6
mojozoom is offline mojozoom  United States
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Thanks! Yes i drew them with Tinkercad and printed them in sections, as the bed of my 3d printer is only 5.9 inches. Then I epoxied the sections together. There's an adapter ring for the driver there that bolts into the stock location in the car door. In the pic it hadn't been bonded on to the enclosure yet, there's a recess in the back of the adapter ring that the enclosure part drops into.
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Old 21st September 2018, 03:52 PM   #7
tsmith1315 is offline tsmith1315  United States
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Very cool, I'm jealous of your 3D abilities!

For your reference measurements, was the membrane simply omitted or was the opening sealed?
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Last edited by tsmith1315; 21st September 2018 at 03:57 PM.
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Old 21st September 2018, 04:07 PM   #8
mojozoom is offline mojozoom  United States
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It was omitted, so the port was providing no resistance.

It's difficult for me to grasp why the distortion increased at higher frequencies where the membrane shouldn't have been doing much at all.
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Old 21st September 2018, 04:57 PM   #9
tsmith1315 is offline tsmith1315  United States
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Quote:
Originally Posted by mojozoom View Post
It was omitted, so the port was providing no resistance.

It's difficult for me to grasp why the distortion increased at higher frequencies where the membrane shouldn't have been doing much at all.
Except, perhaps, reflecting? It might be enlightening to have comparisons with the enclosure sealed. IMHO, that's a better "control" in terms of telling you how the membrane affects the system damping.

I will agree with wolf. Back in the older days (where the scrotum guys' information likely came from), it was recommended as standard operating procedure to stuff the enclosure well, leaving open only a direct path from the cone to the vent. That was more trouble to arrange in a car system, where vibration was bound to cause settling of damping material over time.
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Old 21st September 2018, 05:01 PM   #10
Galu is offline Galu  Scotland
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Quote:
Originally Posted by mojozoom View Post
It was omitted, so the port was providing no resistance.
Aperiodic loading is a means of fooling a driver into thinking it is in a sealed enclosure of larger volume. The action of the acoustic resistance should not be confused with bass reflex action. By removing the membrane, you have an untuned opening and not a tuned 'port'. Try taking measurements with the opening closed.

Last edited by Galu; 21st September 2018 at 05:03 PM.
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