Downfiring port question

[JasonB]

If a downfiring port is used will changing the distance between the enclosure and the floor significantly change the tuning frequency? I'm thinking that the area between the enclosure and the floor acts as part of the port.

Jason B

[ThomasW]

The area between the port outlet and the floor isn't part of the port, and isn't factored in as such. The only problem that can occur is having insufficient space between the outlet and the floor, which will create an unwanted resistive load.

[noah katz]

"I'm thinking that the area between the enclosure and the floor acts as part of the port."

It can; IIRC there was at least one speaker using it as an intended part of the port tuning.

I don't know how to predict the effect except by measuring.

[SteveCallas]

So you're saying the air in the immediate area between the port and perhaps a large baseplate would be coupled together and move as one? This would lower the tune considerably, would it not? Hard to see it working as a benefit though.

[ThomasW]

It can; IIRC there was at least one speaker using it as an intended part of the port tuning.

No one is saying it can't be done. There are however no modeling programs that will account for this effect unless one decides to model it as a bandpass.

So for the average DIYer, no it's not part of the port....

[JasonB]

The reason I asked is because I saw a possible advantage to it. I thought that the tuning frequency of the enclosure could be fine tuned by adjusting the height that the enclosure sits off the floor.

I just thought now..... my current speakers have a rear firing port, tomorrow if I get time I'll space one off the floor on it's back and vary the spacing a bit. I'll take measurements with a DMM and see what I can come up with.

Noah....The speaker manafacturer you are thinking of could be Polk, they had an idea called the Power Port where the bottom of the enclosure had a large flared port and the base below had the inverse(picture a flared cone pointing upwards) in this case the space between the flared port and base would definetly act as part of the port and tuning would change if that space were made smaller or larger.

http://www.polkaudio.com/education/tech_article.php?id=30

I got on to this idea looking for a unique way to make an enclosure with adjustable tuning. I don't really want a simple tube shaped port with a sliding section.

Jason B

[ThomasW]

The information for designing PowerPorts used to be in the public domain then Polk pulled it.

I have a copy of the Excel spreadsheet and program installer. If you want it I can email you a copy, in total it's a bit under 500k.

Designing and building one of these is a bit on the very complicated side. I hope you have access to a wood turning lathe and know how to use it, otherwise it's pretty much an impossible project....

I attached a couple screen shots from the spreadsheet.

[noah katz]

Yes, it was the Polk.

Thomas, not sure why you think it should be modeled as BP; the air between the bottom of the box and whatever is beneath it is just a continuation of the port.

The wrinkle is having a smooth enough path around the right angle turn, which could easily cause losses, and the effective length of the variable area.

[jonathanb3478]

The information for designing PowerPorts used to be in the public domain then Polk pulled it.

I remember when that came out. They were talking about bandpass enclosures using sealed rear chambers with really small volumes in combination with very high mass drivers. They indicated that if you combined the above with a power port (or more than one), you had the "ultimate" small box sub, as a powerport that would not "chuff" would fit in a pretty small box. I believe the "Db" series was released at this time as the "first" very high mass drivers for this type of application.

I remember that I downloaded some stuff, including a spreadsheet of some sort, at that time. I lost it when my only copy of all my archive data was lost due to an HD failure (I was not a Data Backup/Restore Admin at that time, lol)

[ThomasW]

Mike Keith, one of our members built a very elaborate PowerPort sonosub with a big rig window lift serving as the device to open/close the port plug.



More photos here....

http://kingdaddy.linaeum.com/Sonosub/

[jonathanb3478]

Mike Keith, one of our members built a very elaborate PowerPort sonosub with a big rig window lift serving as the device to open/close the port plug.

I have seen that image around, but did not realize the port end was adjustable. That is pretty amazing.

[ThomasW]

We brainwwashed him, now he has a IB sub with 4-18"s...:wink:

[jonathanb3478]

Holy cow!

That just looks awesome. I am sure it sounds about as good as it looks



I played with the bottom powerport concept a bit. What does everyone think of this as a bottom for a ported speaker with an elliptical enclosure starting at the top of this piece? The floor spikes would be attached to the bottom of the cross-sectioned base shown in these images

Below is a cross section for a detailed view of the actual port:




Now we have an overview shot for the look of the base, in general:



Some specs are:

Overall height of entire assembly: 7.25"
Tuning frequency: Barely over 27Hz
Top disc diameter: 9.75"
Bottom disc diameter: 13.6"

[JasonB]

The information for designing PowerPorts used to be in the public domain then Polk pulled it.

I have a copy of the Excel spreadsheet and program installer. If you want it I can email you a copy, in total it's a bit under 500k.

Designing and building one of these is a bit on the very complicated side. I hope you have access to a wood turning lathe and know how to use it, otherwise it's pretty much an impossible project....

I attached a couple screen shots from the spreadsheet.

Thomas,

I am interested in a copy of that spreadsheet.

Thanks

Jason B

[JasonB]

Had some time tonight to try a quick experiment. The speakers used were built in .5 cu ft. enclosures and have a 2" port on the rear with a 1/2" flare. I started by measuring for the frequency which drew the maximum current, in theory this should be the tuning frequency of the enclosure.

I started with the speaker well away from the wall in my normal listening position, the tuning frequency measured 41hz. Next I moved the rear of the speaker to 3" from the rear wall, no change. 2" from the wall, slight change, less than 1 hz. 1" from the wall, down to 39hz. 3/4" from the wall, down further to 37hz. Any closer than that and the port was becoming too restricted and SPL started to drop.

In this test I was able to drop the tuning frequency by 4hz, not quite as much as I had hoped. I might try experimenting with the "Power Port" further down the road, to see if changing the distance between the "Flow Guide" and the port produces significant tuning frequency changes without affecting SPL.

Jason B

[jonathanb3478]

I might try experimenting with the "Power Port" further down the road, to see if changing the distance between the "Flow Guide" and the port produces significant tuning frequency changes without affecting SPL.

Based on my understanding of how the powerport works, I would think you could modify the frequency, pretty much at will, simply by using the same flowguide with a different diameter external disk. Using a larger external disk makes the "port" longer by what ever the increase is for the radius of the disk.

You could make the flowguide with 1/4" dowels glued in, and make the disk with <1/4" holes of the same patern. Pull off one, put on a new one, and new tuning frequency achieved!

What do you think?



EDIT: Well, it does work, but is not as linear as I had expected so is not as effective. I would now guess that this is because, as it gets "longer" like I mentioned, it is also getting larger in effective diameter. This means a larger external disk means less increase in tuning effect, means larger increase in disk size, means even less increase in tuning effect... until some factor starts to kill the effectiveness of the whole thing.

[JasonB]

Thomas sent me the spreadsheet to model them so if I get some time I'll try this weekend. Your suggestion of changing the size of the external disk was a good one, but I see from your edit it didn't work quite as well as planned. What sort of tuning frequency range were you able to get out of it?

Jason B

[jonathanb3478]

What sort of tuning frequency range were you able to get out of it?

Well, I did not work with anything but the design I posted above. I remember it was dismal, but I thought it would be better if you started with an external disc that was much smaller than 13.6" in diameter.

So, I changed nothing in my design, but moving to a 7" external disk. Got a ~2hz lower tune, than with the 13.6" external disk.

The affects are not predictable, in my case. You need to experiment.



EDIT: I switched my design to external and internal disks that were the same diameter as the flowguide. This kicked the tuning frequency up just a tad more than 4Hz over the original design (another ~2Hz). It seems that the increased diameter effect dominates the increased length effect in my design. I have 1" roundovers, 1.5" disk to baffle height, and 4" ID port tube plugged into the spreadsheet. The flowguide ends up being 6" in diameter, with a 2.5" radius.