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Old 3rd January 2006, 09:17 PM   #1
Puggie is offline Puggie  United Kingdom
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Default Aperiodic vents to flatten impedance to tweek passive Xovers

Been looking at high passing midbass at around 150Hz and was getting a very uneven response due to the resonant impedance peak. The aim was to reduce excursion (Full range driver so low XMAX) but the impedance peak was making the high pass ineffective (simple 1st order filter, single cap) and not reducing the excursion.

So my idea is to do an Aperiodic/MAPD enclosure to flatten the impedance and act as a high pass filter, then tune it further with the cap (which is now more predictable due to the flatter impedance curve).

Anyone else done this? anyone see any problems that I've missed?
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Old 4th January 2006, 01:43 AM   #2
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I had the same dilemma.
The way I got around it is to use a 2nd order highpass. The shunt L acts to supress the impedance hump and if L is big enough, it's close to a 6dB highpass filter around fc (actually it's a low Q 2nd order) .
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Old 4th January 2006, 02:00 AM   #3
soongsc is offline soongsc  Taiwan
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I think I saw a design somewhere in this forum a few months ago. It requires different tweeking. Much different from the main steam designs.
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Old 4th January 2006, 12:05 PM   #4
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The aperiodic enclosure will reduce the resonant peak, but it will still end up being higher in frequency and amplitude than the free air resonance of the driver.
If you really want toget rid of it and still use a 6db/oct xover cap, then stick an RLC circuit across the driver terminals.

F= 1/ 2pi * (sqrt LC) and the resistor is approx Re.

So if you're resonance peak is at 80Hz, you could use a 8mH coil and 500uF cap.

The coil can be small gauge laminate core, and the cap can be electrolytic to keep the cost reasonable.

Or do like David suggested.
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Old 4th January 2006, 01:35 PM   #5
Puggie is offline Puggie  United Kingdom
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Tobe honest the higher resonant frequency isn't a problem. The peak is about 100Hz, Fs is quoted at 65Hz and I want to low pass them about 150-300Hz (depending what sounds good) so the rise in frequency of the resonance isn't really a problem.

the aperiodic box, reduces the resonant peak and pushes up it frequency, this isn;t a problem for my aim, am I missing something (being full range I'd like to keep electronic components to a minimum).
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Old 4th January 2006, 02:57 PM   #6
soongsc is offline soongsc  Taiwan
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There seems to be a patent based on a Symetric Air Friction. Maybe that might work. The patent is really old, over 20 years. I first saw it in Popular Mechanics. Very Interesting. It sort of makes lots of turns while expanding and reducing cross section area to act as low pass filtering I think, but also adds damping to the driver.
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Old 5th January 2006, 06:40 AM   #7
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I guess I don't understand what the problem is with a Zobel.

None of the components are in the signal path and should create very minimal distortion of the signal itself.
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