How much area for sound to exit from downfire box

[racin69z]

I am building a downfire box for a single 10 for my side by side. I am down firing to keep it out of the elements and try to lessen the chance of damage from random flying objects I may encounter on the trail. I cut three 2.5" windows in the box for the sound to exit out of. I can make it into a slot if need be, but I was hoping this would work and help keep stuff out of the sub. The sub will have 600w RMS going to it. Is there any type of rule of thumb or formula for the area needed for such a thing?

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[Doxquzme Premium Member]

Actually, you've created a port - kind of a poor mans bandpass if you will.

It would be better to use chicken wire to cover the exit, this is going to cause issues. Wind noise turbulents, and other tonal issues.

You can't just create a slot, it needs to be calculated based on the mechanics of driver and enclosure interaction.

I can't really see the design, can;t tell where you are puting what, but there is a science to it which doesn't seem to be in order here.

 

[SlugButter]

A port has a length to it like the picture, based on the volume inside the enclosure and tuning frequency. Screw some wood over those holes and get some pvc pipe and cut a hole to fit it. Start long and shorten it until you like the sound and silicone it.

 

[racin69z]

Thank you for the info. I thought that as long as the area for the sound to exit was large enough that the air speed was not too fast that it would be ok.

The speaker sits 3 inches off the floor in an area just larger than the speaker diameter.

 

[racin69z]

I was not attempting to create a port, I was trying to build a sealed down fire box, but limit chances for the speaker to get damaged. I've only saw a few down fire boxes, and they all had a portion of the box that didn't touch the floor that the sound came out of.

 

[Doxquzme Premium Member]

I was not attempting to create a port

I understand the thought, unfortunately, that is a port, of sorts. Because all the sides are brought down and surround the driver, it's in affect, a 4th order bandpass, kinda like this.

All is not lost, and you need to make changes to that lower section. Is this going in a center console location?

 

[racin69z]

Yes, much like a console between the rear seats

 

[Doxquzme Premium Member]

Cut these sections out and install a netting of sorts (chicken wire comes to mind, grill cloth), much better and not a port.

View attachment 1705981197339.png

 

[Doxquzme Premium Member]

All in all, a nice box for sure.

 

[racin69z]

I am not for sure how to rotate the pics the correct way. I can cut the 3 areas near the speaker out, but the large area on the rear is part of the sealed enclosure. My pics are not great, and are before I finished assembling it. There are now 4 walls around the speaker and the speaker is in .65 cubic feet of sealed space. I did not have alot of room to play with and I had to cut clearance for seat belts in the side.

 

[racin69z]

I appreciate you folks helping me make it better.

 

[Doxquzme Premium Member]

Okay, that's even better. I thought it might be, wasn't sure. So yes, if you can clear out the three sides and use the corners as de facto legs, that's great. You can even run a little like 1/2"strip along the length if you need so that you have something to staple the netting to (whatever you use, make sure it is poress like chicken wire). Grill cloth the final if need be, otherwise, just do a roundover for the edge and leave them clear/open. Function needs to prevail over aesthetics in this instance.

 

[Doxquzme Premium Member]

Oh, and get a half pound (half the one pound bag the sell) of Acousta-stuf from Parts Express and stuff the cavity, improves the performance.

 

[racin69z]

I am sorry for the lack of clarity in the pics. The pic of it vertical shows the partition that was put it after the pic of me test fitting the sub in the hole was taken. I have some expanded metal that will fit nicely and still allow airflow. To be honest I never even thought of that. That will be even more protection for the sub. Thanks alot for the help.

 

[Buck 5,000+ posts]

How many in^2 of area do you have for the exit holes under the box? That's what I always look at, just like how you'd design a t-line, based off the Sd of the woofer, somewhat. Like, if you have a total bottom opening surface area that's at least the Sd of the sub or maybe the Sd + port area net, then it sort of shows a relative restriction won't happen, because the bottom of the box has more radiating area than all radiating surfaces of everything in/on the box. That's one way to look at it.

The box also needs to be high enough from the floor to not restrict the sub and port pressure, but it usually doesn't have to be more or much than a couple of inches. You can kind of compare surface area of sound wave travel vs surface area of radiating surface to make sure the non-tuned sound wave pathways have more "port" area than the radiating surface.