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Old 17th August 2004, 12:20 PM   #1
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Default Membrane to limit "unloading" in ported-type enclosures?

What would happen to a ported enclosure system if you covered the subwoofer with a resistive membrane? Sort of a hybrid ported/aperiodic membrane enclosure. Would the membrane help dampen unloading under fs?

Is there a name for such an enclosure, or has anyone tried it?

If not, let me know if you think it's worth trying, and I may try this in my car. I'm not an expert at designing enclosures, but I can build with MDF and use WinISD.
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Old 17th August 2004, 01:22 PM   #2
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Basically, you have just mentioned one of the advantages of using the Passive Radiator system over the ported. When the Passive Radiator runs out of excursion, the enclosure acts something like a closed box and limits speaker excursion somewhat.

To my knowledge, there are no freeware programs that model when and how this happens. Or whether it occurs soon enough to do the speaker any good. I imagine it depends on the alignment and the excursion both of the woofer and the Passive Radiator.

Any low-cut filters, (20 Hz or so), on your car amp?
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Old 17th August 2004, 01:58 PM   #3
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Do you mean covered as in an Isobarik construction, but where the second driver is replaced with a passive radiator? Yes I have tried it already in a closed/sealed system. What happends is that the volume between the active and passive driver will make a resonansfrequancy quite high up in frequency (150 - 250 Hz or so), which will be the new resonance frequency for the active driver. At lower frequencies, the driver "believes" (Or actually does) it's moving a heavier mass, then the sensitivity drops and the lower resonance frequency will also drop. Twice the mass will drop sensitivity by 3dB, and lower resonance by approx 1/2 octave.

I do not recommend this solution.

Br.

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Old 17th August 2004, 04:09 PM   #4
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I didn't mean a passive radiator, I meant a permeable, yet resistive membrane such as found in aperiodic membrane enclosures.

I was thinking of something like a 4th order bandpass, but with a resistive membrane over the subwoofer, hopefully to reduce subwoofer "unloading" at low frequencies. Sort of like this:

Code:
+---------+--+  +--+
|         |  |  |  |
|        /*  |  |  |
|       / *  |  |  |
|    +-+  *        |
|    | |  *        |
|    +-+  *        |
|       \ *        |
|        \*        |
|         |        |
+---------+--------+
...where the asterisks represent the membrane. Has that been done, and does it have a name?

(What is up with the double-spaced monospace text? I couldn't get it to single space.)
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Old 18th August 2004, 06:18 AM   #5
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Eeek! Something to physically restrain the cone from moving too far?!!

How about something that restrains amplitude of the airflow through the port?

How about a cylindrical block of styrofoam that fits very smoothly into the port tube like a piston. Normally the styrofoam sits halfway along the length of the tube, and the air-pressure variations cause the piston to slide along. At each end of the tube there is a stopper of some kind, eg: a bit of springy metal, to stop the styrofoam from turning into a projectile.

Styrofoam is very light, and the original tuning of the port might not need to be changed that much (if at all). It readily accumulates a static charge, so experimenting is needed to see how easily the styrofoam will slide through the tube. There is no self-centring mechanism, so I'm not sure how well it would work. Perhaps the tube could be slightly curved so the styrofoam centres itself due to gravity.

CM
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Old 18th August 2004, 10:22 AM   #6
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There was a variation of what you propose with a couple of old bass-reflex enclosures from many years ago. In one of Gilbert Briggs (UK, "Wharfedale" fame) books he has a couple of designs which were reflex boxes and then he had put a thin plywood partition inside the enclosure which separated the upper part with the driver from the lower part with the port. The plywood was cut at regular intervals a few centremetres apart. From memory the cuts were each about 3mm wide and extended from one side of the box to the other and the purpose was to reduce the excusion of the driver below the resonance Fq of the box. I understand it was quite effective but without looking up my text I'm not 100% sure of the demensions. I can get the facts accurately if you want.
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Old 18th August 2004, 05:36 PM   #7
Ron E is offline Ron E  United States
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Adding resistance to the driver will only change the effective T/S params of the woofer - reducing Qms, and therefore Qts. This is one way of using a high Q driver in a vented box - by reducing Q with a resistive membrane. The problem is that there are no ready design formulas and you would have to cut and try and measure to see how the parameters are affected. One benefit is that you can get a really flat impedance curve, but solid state amps don't care about that anyway......

Adding the resistance in this way to a 4th order bandpass is really redundant because the sealed chamber behind the driver already controls woofer excursion below the lower F3.

You can put the resistive material on the port, but then you lose output No free lunches, I'm afraid.

BTW, an aperiodic is just a vented box with a lossy (resistive) port....
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