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Old 9th January 2003, 08:12 AM   #1
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Default porous dustcap

Some time ago, before vented magnets and spiders became fashionable, Peerless and Vifa vented the space in front of the pole piece by using a porous dustcap made from thin fabric. The only remaining devices are the Peerless CC line, to my knowledge.

Is there anything to be said in favor of this? After all, you get an acoustic short circuit in the center of the membrane.

Eric
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Old 9th January 2003, 08:56 AM   #2
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Default Re: porous dustcap

Quote:
Originally posted by capslock
Some time ago, before vented magnets and spiders became fashionable, Peerless and Vifa vented the space in front of the pole piece by using a porous dustcap made from thin fabric. The only remaining devices are the Peerless CC line, to my knowledge.
there are many drivers that vent out the front -- anything with a phase plug for instance. The Fostex FE103 with the "cyclops" eye is another.

I will often do a dustcapectomy to omprove the sound of a driver.

dave
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Old 9th January 2003, 07:19 PM   #3
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Yes, and I am a little sceptical about a short circuit from the front venting through holes in the VC former (if any) or through the gap along the back end of the VC former and then through the spider or spider venting holes. This might be worse in dipole apps because excursions are larger than in a box.

The disadvantage of a porous cap (compared to a phase plug) is that there is air behind the cap that gets trapped and has to move through the dustcap. I bet the flow resistance is not perfectly linear and may also have some temporal hysteresis.

Coming to think about it, does a phase plug have any advantage regarding flow dynamics compared to using no dustcap and no plug at all? After all, the only air that gets moved in the no cap case it that little volume that clings to the former because of laminar flow profiles. Using a plug will increase the resistance because there is a greater length of a narrow gap.

The only advantages of a plug compared to no plug/no cap are that it keeps some things from falling into the speaker and that it may improve the field if made from metal.

Regards,

Eric
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Old 9th January 2003, 07:27 PM   #4
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Quote:
Originally posted by capslock
The disadvantage of a porous cap (compared to a phase plug) is that there is air behind the cap that gets trapped and has to move through the dustcap. I bet the flow resistance is not perfectly linear and may also have some temporal hysteresis.
On drivers like the Fostex this could be a problem, but it also might act as a bit of damping to lower a Q peak somewhere (like an aperiodic enclosure). The vintage drivers i have seen with porous/transparent cloth have fairly low excursions and the cloth is so pourous i wouldn't think it would be much of an issue.

Quote:
The only advantages of a plug compared to no plug/no cap are that it keeps some things from falling into the speaker and that it may improve the field if made from metal.
With no dustcap & no phase plug (or a damping cylinder -- the PR-2 has a woolfelt plug) you will get a tin-can resonance from the air in the cylindrical voice coil. The phase plug also halves the path across the driver cone, pushing any side-to-side cone resonances up by an octave. They can also direct higher frequencies, disperse them, or even radiate them depending on geometry & construction. They are also often touted as having a cooling function.

dave
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Old 10th January 2003, 08:43 AM   #5
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Thanks, didn't think of those points.
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