Bracing a slotted port?

[50AE]

Hi folks,

Instead of letting a full width slotted port for reflex / MLTL , do you think it would be a wise idea to divide the slot into sections of smaller width, hence giving it somewhat of a bracing?

Best regards,
Alexander

 

[pkitt]

Why not simply change the dimensions of the slot, making it less wide and more tall but with the same area? For example, if you originally planned on it being 1" high x 10" wide, make it 2" high and 5" wide instead.
Paul

 

[planet10]

I do that as a matter of course, trying to avoid a full-width unbraced slot. We take it further in the full-on miniOnkens with the slots down the side having at least 2 vent spacers/braces.

Sometimes one still has resonances excited, can’t get at the machine with my drawings, i have a drawing of slot braces that take things even further.

dave

 

[GM]

Note that when subdivided it's like multiple vents, so tuning for a given duct length will rise in frequency with increasing # of dividers, so if going to internally brace, best to use a small diameter dowel or two if deep enough.

Normally though, making the duct out of 3/4" material is plenty stiff enough or use scrap for perpendicular rib braces at the duct-panel for thin material.

 

[planet10]

I call them vent spacers because their real purpose is to get the vent width right with a fixed height. That the spacer is also a brace is an appreciated bonus.

dave

 

[50AE]

Ok guys, thanks!

So instead of a whole 286 * 25mm, I'm dividing it into two ports of 143 * 25mm with a 18mm brace in between. The material is 18mm birch plywood and the port length will be made by gluing a sandwich of multiple routed pieces.

This is an attachment a template of the bottom plate with the port openings.

 

[tmuikku]

Have you made a prototype and measured the port performance? I mean don't put too much hard work to it only to notice it doesn't work as expected. Tuning frequency might need tweaking or there might be extra noises in the port be it schuffing sounds or resonance of the port or leakage of mid frequencies through the port.

I haven't figured out any other advantages of slot port than ease of build and can be done from the same sheet at hand. But it is almost impossible to tune after building the box. Not sure if it has any other pros to any other port type than maybe aesthetics. Better place for a port is in the back of the enclosure so even aesthetics is not important. These are my thoughts, each have their own goals and design principles. Have fun!🙂

 

[50AE]

Not really. It's only my second speaker build with a sloted, but bigger port. I was wondering if someone else did a comparison with listening or measurements.

Thanks!

 

[tmuikku]

You can build quick prototype box to check out the performance, then go fancy with the aesthetics and construction and what not 🙂 A tip to save you some trouble and money thats all. I found the tuning is almost always off from calculated and that there is extra noises coming through (mids leaking) that need tweaking of the port location inside the enclosure. Either of which might not be a problem but only prototyping ensures that.

 

[Harry72]

Model it using two ports instead

 

[planet10]

Not really. It's only my second speaker build with a sloted, but bigger port. I was wondering if someone else did a comparison with listening or measurements.

The miniOnkens, which now number in the hundreds, all take advantage of high ratio, preferably highR slot vents. They break all sorts of rules of thumb.

The extra R makes for a pushing of the box closer to aperiodic, makes it less sensitive to dynamic swings in T/S parameters (ie box tuning), but they only go as lw as they go. Bass has proven to be very articulate, finessed and nuanced.

The first real one:

An externally hosted image should be here but it was not working when we last tested it.

I don’t think i have any boxes downstairs (say a couple dozen pairs) with tubular ducts.

dave

 

[ICG]

So instead of a whole 286 * 25mm, I'm dividing it into two ports of 143 * 25mm with a 18mm brace in between. The material is 18mm birch plywood and the port length will be made by gluing a sandwich of multiple routed pieces.

This is an attachment a template of the bottom plate with the port openings.

The more inner surface the ports have, the more air turbulence losses you get. To halve the width doesn't increase the surface very much so the; increased losses are minimal. The advantage can be lesser cross resonances or, to be more precise, they are shifted upwards, about double the frequency, where they get less excited because the back radiation pattern.

The miniOnkens, which now number in the hundreds, all take advantage of high ratio, preferably highR slot vents. They break all sorts of rules of thumb.

The extra R makes for a pushing of the box closer to aperiodic, makes it less sensitive to dynamic swings in T/S parameters (ie box tuning), but they only go as lw as they go. Bass has proven to be very articulate, finessed and nuanced.

Uhm, no, that's not why it sounds better. The long ports got effectively a length increase because of the edge placement aswell as an extension at the 90° turn at the back wall. That lowers the tuning frequency, the drop off slope is less steep and typically fits better the room gain. The mid garbage gets reduced too because of the back radiation of the driver doesn't hit the port as much as a shorter, more open port (Yes, the back of the driver got a radiation pattern too!). Another reason for a better sound is the highly increased rigidity of the enclosure because of the braces.

Backside of it are higher losses because of air turbulences on the much higher inner port surfaces and the 90° turn of the ports. That can easily be improved by putting a port on the backside of the enclosure and stiffening the enclosure. Saves a lot of work and performs possibly/likely even better. But I've told you that some years ago already.

 

[planet10]

The long ports got effectively a length increase because of the edge placement aswell as an extension at the 90° turn at the back wall. That lowers the tuning frequency, the drop off slope is less steep and typically fits better the room gain.

All that is true and accounted for. A particular alignment shape in the modellers awhich is modified by the highR vents.

There are many things that go into my speaker designs that, i feel, help improve their performance.

The first one was a shot-in-the dark. We never realized how far afield the alignment would get.

dave

 

[ICG]

True. And it's never too late to improve things as we learn. 😉

 

[AllenB]

The advantage can be lesser cross resonances

One could also speculate that these 'cross resonances', or differing path lengths potentially offered by movement of part of the mass diagonally through the port, create a virtual analogue to a roundover or flared port and are not a bad thing.