Port length and air speed?

Definitely need bracing. Unless you have a box that is an 8" cube.

MDF is strong but the pressure inside a cabinet of any type will push against the weakest area. So with a 24" length the weakest area will be in the middle. So with a longer piece the weakest area increases. The guideline I follow is to brace at least every 8".

What questions do you have on port air speed?

Hi I was using WinISD with the data for the Dayton, And it gives a port air speed of over 1000fps. Also the excursion graphs look ok with one watt but if i change the value to 500, It dosent look good at all. I dont seem to be able to get any size port to work. I'm going to try a different data source for the t/s's. I'm new at this program but something is off.

As far as port speed, it should be at or under 17fps or so? Is it feet per second?


So at 4" round,(1 or 2 or even 3 ports) how long does it need to be in 12^3 box at a tune of 19hz?

If you could model it out for me that would be great. If you get it to work can you give me a link to the source of the data. I like the program. its fun to play around with. I would very much like to get it to work for myself.

I used the search function and found a link to a file of data for the Dayton Titanic 15" and saved it into the WinIsd Drivers folder and it seem to work but the results seem strange. :scratch:

Thanks Joe

Ok i found out that the numbers were off due to my not noticing that they were in meters and not in feet. So I got it finally. I'm going with a 12 cubic foot box with a 16 Hz tune. It works out with two 4" ports 18" long. thanks for the response. Now I just have to build.:T

I don't understand why people are still using MDF. Cabinet grade pine is about the same price, but half and weight(more important for car audio), twice the strength, and easier on blades. Birch is even stronger, but more expensive. Use good material and common sense, you won't have to go too crazy with bracing and multiple baffles.(I'm not saying don't brace at all)

Good luck with the build!

That short of a port length could be possible since the box is so large, the length depends on both the size of the box and the desired tuning frequency.

What is your port area and are you planning on round or a slotted port? I'm guessing that your port is too small...

ach81090 said:

I don't understand why people are still using MDF. Cabinet grade pine is about the same price, but half and weight(more important for car audio), twice the strength, and easier on blades. Birch is even stronger, but more expensive. Use good material and common sense, you won't have to go too crazy with bracing and multiple baffles.(I'm not saying don't brace at all)

Good luck with the build!

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Lighter weight means a comparatively higher resonance frequency, bracing shifts resonance so that it is less or completely inaudible. So light weight really only saves your back when you have to move the sub. Also, MDF comes in 4'x8' sheets that can be found in about any corner of the modern world

fusseli said:

Lighter weight means a comparatively higher resonance frequency, bracing lowers resonance ideally below what is audible. So light weight really only saves your back when you have to move the sub. Also, MDF comes in 4'x8' sheets that can be found in about any corner of the modern world

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Sure, but depending on the specific application, it is likely that the benefits of a more rigid enclosure will more than make up for the lower density of the wood. I've used MDF for years, but never again if I can help it.

JWieczorek said:

Hello,

On a side note: do any ported boxes need bracing? If there vented, how could there be internal pressure high enough to flex 3/4" mdf?:scratch:

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There's a LOT going on when that sub is moving a lot. My 8^3 ft box will wobble back and forth with only ~400 watts going to it. Would flex like crazy without any bracing!

fusseli said:

Lighter weight means a comparatively higher resonance frequency, bracing lowers resonance ideally below what is audible. So light weight really only saves your back when you have to move the sub. Also, MDF comes in 4'x8' sheets that can be found in about any corner of the modern world

Click to expand...

Hmm. From what I remember reading, (and from a physics standpoint) bracing increases the resonant frequency of each panel above the spectrum of frequencies the subwoofer will see or produce. If you think about it, a large panel will have a lower resonant frequency than a smaller panel, so by bracing, you would actually be increasing the resonant frequency. That would also explain why boxes that aren't well braced sound "boomy"; there is some destructive resonance occurring there.

BD55 said:

Hmm. From what I remember reading, (and from a physics standpoint) bracing increases the resonant frequency of each panel above the spectrum of frequencies the subwoofer will see or produce. If you think about it, a large panel will have a lower resonant frequency than a smaller panel, so by bracing, you would actually be increasing the resonant frequency. That would also explain why boxes that aren't well braced sound "boomy"; there is some destructive resonance occurring there.

Click to expand...

You are correct, I stated that wrong. Adding bracing reduces a side of the box into smaller portions, which then resonate at a higher frequency. Once they are beyond what the subwoofer produces, the box will no longer interact so much.

The strength component is fairly important as well, unless the box is made of depleted uranium, there will be some movement that will result in an acoustic loss, AFAIK.

Even things we consider to be rigid oscillate when motivated by force. I'd like to see what the first panel in front of a tapped horn driver (S1) looks like on a high speed camera while playing a full power 20 cycle note.