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tuning aperiodic vents
tuning aperiodic vents
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Old 19th February 2019, 12:33 PM   #11
TimA is offline TimA
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I am currently building a pair of speakers with aperiodic vents and I have read suggestions of 'tuning' them via impedance curve measurements (which I shall do) and adjusting for the flattest result, but wouldn't the flattest impedance curve from a given drive unit and cabinet be with the vent completely open without damping material of any kind...or am I missing something?
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Old 19th February 2019, 03:14 PM   #12
Galu is offline Galu  Scotland
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Have you checked out the link below and the additional links it gives at the bottom of the page?

Aperiodic Loudspeaker Enclosure Design

A so called 'aperiodic' enclosure has more in common with a sealed enclosure than a bass reflex enclosure.

The idea of including a resistive vent is to reduce the amplitude of the single impedance peak which is characteristic of a closed box. This results in better amplifier matchng and control at low frequencies.
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Old 19th February 2019, 04:44 PM   #13
TimA is offline TimA
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Thanks for the reply. Yes, I've seen those web pages and I'm aware of the improved transient response and flattened impedance peak at resonance, but my query is regarding finding the optimum membrane density via impedance measurements.

As an example let's assume a starting point with a vent that is too densely filled and a highish impedance peak, so some stuffing is then removed from the vent, the impedance is remeasured and its now lower than before. All well and good, but then removing still more stuffing yields an even flatter curve so more and more is removed and the curve keeps getting flatter each time, so my question is at what point do you stop removing stuffing from the vent, because as I see it the flattest impedance curve would be with all stuffing removed, would it not?
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Old 19th February 2019, 04:51 PM   #14
TimA is offline TimA
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Is the optimum point the lowest impedance peak achievable without too much sound leakage through the vent? This seems to me to be the optimum point at which one would stop removing vent stuffing but it wouldn't necessarly result in the lowest impedance peak which some have suggested should be the criteria for adjustment.
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Old 19th February 2019, 05:27 PM   #15
Galu is offline Galu  Scotland
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I can't agree with your statement that removing the stuffing from the vent will result in the flattest impedance curve.

Reducing the thickness of the resistive material in the vent to zero will simply transform the closed box into a reflex box tuned to an inappropriately high frequency and which exhibits the characteristic double hump impedance curve.

There exists emperical guidance as to the area of the resistive vent, but the optimum amount of resistive material can only be determined by experiment.

The optimum 'tuning' point is when you obtain a single impedance hump of minimum Q.
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Old 19th February 2019, 05:35 PM   #16
tsmith1315 is offline tsmith1315  United States
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Quote:
Originally Posted by TimA View Post
as I see it the flattest impedance curve would be with all stuffing removed, would it not?
You're looking for a resistive implementation. As Nanook said on the previous page, the "flattest set of values" is what you're after, not a curve with a low local minimum.

edit: (what Galu said, he beat me to it)

I think you may find a good "membrane" density to be a bit higher than what you're envisioning. Try thinking of it as a "porous membrane".
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Last edited by tsmith1315; 19th February 2019 at 05:41 PM. Reason: late to the party as usual
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Old 19th February 2019, 06:18 PM   #17
TimA is offline TimA
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Many thanks to you both, I think I understand this better now.
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Old 19th February 2019, 10:44 PM   #18
Galu is offline Galu  Scotland
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Thanks Tim.

I think the crux of the matter is that a resistive vent is a modification to the closed box design which fools the driver into thinking it is in a larger enclosure, and should not to be confused with the bass reinforcement action of the port in a bass reflex speaker.

The insertion of bungs in the reflex ports of large floor standing speakers to prevent boom in small rooms does not convert the speaker to aperiodic loading, but merely suppresses the resonance of the mass of air in the port in order to reduce its contribution to the bass output.
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Old 15th March 2019, 05:11 PM   #19
TimA is offline TimA
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I have now finished building the aperiodic enclosures and have taken an initial impedance sweep with the vent filled with absorbent material. Attached is the impedance which is near enough identical to the vent completely open with no absorbent material. I think I need to increase vent filling density if I am to see any change in the impedance but as the output from the vent is already attenuated a lot compared with an open vent I'm wondering if there's much need for this.

The enclosure is around 28L, has four 5.25" woofers in a 2.5-way MMTMM arrangement. Total vent area for each enclosure is around 10 square inches.

I half expected to see a double peak with the vent open, but this didn't happen.
Attached Images
File Type: png Aperiodic impedance sweep.PNG (52.2 KB, 163 views)
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Old 15th March 2019, 09:34 PM   #20
Galu is offline Galu  Scotland
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Could you give more details about the form of your resistive vent?
It should consist of a thick layer of absorbent material compressed between two metal grilles.
P.S. How does the bass response sound to you?
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