Importance of port placement?

[Grimfate126]

I just finished modeling my desired box in sketchup. It looked perfect, until i read this on DIYsubwoofers.org:

If your system calls for the use of ports, these can be placed on any side of the enclosure, once the intended location meets the following requirements:

1. The exit of the port should be at least one port diameter away from any external surface.
2. The entrance of the port should be at least one port diameter away from any internal surface, including the driver.

The port i am using is 4" in diameter. After measuring in sketchup, i realized that if i had port 4" away from the corner of the box, the inside opening would be too close to the driver (2").

So my question is, would it make a huge difference if my port was about 3" away from the sides of my box, and 3-4" away from the driver?

Sorry if all this is confusing. Here are some pictures:

thats the back view. the port is 3" from the sides, but the diameter of the port is 1" longer than that

Thats a side view, just to show the distance from the opening of the port to the (imaginary) driver. The mounting depth of the driver is 7.5", which will give it a 3.5-4" distance from the port.

Thanks

 

[TLS Guy]

I just finished modeling my desired box in sketchup. It looked perfect, until i read this on DIYsubwoofers.org:



The port i am using is 4" in diameter. After measuring in sketchup, i realized that if i had port 4" away from the corner of the box, the inside opening would be too close to the driver (2").

So my question is, would it make a huge difference if my port was about 3" away from the sides of my box, and 3-4" away from the driver?

Sorry if all this is confusing. Here are some pictures:

thats the back view. the port is 3" from the sides, but the diameter of the port is 1" longer than that

Thats a side view, just to show the distance from the opening of the port to the (imaginary) driver. The mounting depth of the driver is 7.5", which will give it a 3.5-4" distance from the port.

Thanks

I think you will be just fine.

 

[annunaki]

It will be fine although ports typically align best when on the same plane as the driver. Not a necessity, nor necessarily audible, but it can aide in measurable output. (tenths of a db or less)

 

[annunaki]

I would highly suggest the use of an aero-port for your application. It will significantly reduce the chance of port noise or "chuffing".

Rectangular vents can allow a larger vent area, to prevent port compression, while strengthening the enclosure in many cases. A 4" vent, for most 12" drivers, is typically too small as much port compression sets in limiting distortion free output.

What 12" woofer will you be utilizing for this project?

 

[Djizasse]

Don't want to disturb the topic, but what the heck is an aero-port?

 

[annunaki]

Don't want to disturb the topic, but what the heck is an aero-port?

An aero-port is a port that is flanged at both ends to smooth turbulence. Sometimes known as precision ports.

Here is an example.

 

[Grimfate126]

I would highly suggest the use of an aero-port for your application. It will significantly reduce the chance of port noise or "chuffing".

Rectangular vents can allow a larger vent area, to prevent port compression, while strengthening the enclosure in many cases. A 4" vent, for most 12" drivers, is typically too small as much port compression sets in limiting distortion free output.

What 12" woofer will you be utilizing for this project?

i will be using a precision port (the one from parts express). i just didnt know how to model it . I really dont feel like making a rectangular vent. i just dont like the look of them :/

will using a 4" precision port limit the output?

im going to be using the XO-12 DVC: http://www.torrentaudio.com/xo12-p-101.html

 

[annunaki]

i will be using a precision port (the one from parts express). i just didnt know how to model it . I really dont feel like making a rectangular vent. i just dont like the look of them :/

will using a 4" precision port limit the output?

im going to be using the XO-12 DVC: http://www.torrentaudio.com/xo12-p-101.html

I modeled the driver. A single 4" port will have compression, becoming audible around 25hz-30hzwith a 400 watt signal.

This is using the suggested enclosure volume of 4.15 ft^3 tuned to 21hz.

It models really well though.

What did you have figured for net internal volume at this point?

 

[TLS Guy]

Model for that driver on my web site

I have that Torrent driver modeled on my web site.

It definitely needs a slot vent and a long one.

 

[Grimfate126]

I modeled the driver. A single 4" port will have compression, becoming audible around 25hz-30hzwith a 400 watt signal.

This is using the suggested enclosure volume of 4.15 ft^3 tuned to 21hz.

It models really well though.

What did you have figured for net internal volume at this point?

well, i am going to use the oaudio 500 watt plate amp, and the speaker will be wired in series, making it an 8 ohm load, so the amp will deliver only 325 watts.

as for volume:

internal box dimensions = 1' 8" x 1 7" x 1' 7" = 4.178 cubic feet.
15 inch long port (4" diameter) = 188.495 cubic inches = 0.109 cubic feet.
brace = 0.059 cubic feet.
driver = 0.08 cubic feet.

all that comes down to a net volume of 3.93 cubic feet.

when i model using 325 watts, the air velocity at 20 Hz reaches around 40 m/s. I know this is not good, probably abysmal, but i think winisd shows a worst case scenario, and this sub will never, ever, be cranked up to 11. So, assuming i listen at moderate volumes, (70-80 dB), will it be noticeable? I will be sitting around 10 feet from the sub.

thanks

 

[Grimfate126]

i opened up winisd again, and this put i put in 150 watts as the input, which will still give me SPL's of over 100 (??), and the port velocity drops to 27 m/s at 20 Hz.

although, i think im doing something wrong, because i put in 1 watt for my signal and got 82 dB's at 20 Hz.

weird

 

[annunaki]

well, i am going to use the oaudio 500 watt plate amp, and the speaker will be wired in series, making it an 8 ohm load, so the amp will deliver only 325 watts.

as for volume:

internal box dimensions = 1' 8" x 1 7" x 1' 7" = 4.178 cubic feet.
15 inch long port (4" diameter) = 188.495 cubic inches = 0.109 cubic feet.
brace = 0.059 cubic feet.
driver = 0.08 cubic feet.

all that comes down to a net volume of 3.93 cubic feet.

when i model using 325 watts, the air velocity at 20 Hz reaches around 40 m/s. I know this is not good, probably abysmal, but i think winisd shows a worst case scenario, and this sub will never, ever, be cranked up to 11. So, assuming i listen at moderate volumes, (70-80 dB), will it be noticeable? I will be sitting around 10 feet from the sub.

thanks

With a net volume of 3.93 ft^3 what was the tuning frequency?

At a distance of 10 feet, you will have an overall reduction in output of around 3.5db if memory serves me correctly. To make up 3.5db at the listening position you need around 2x the power. The port comression could very well be audible, especially during dynamic passages, even though the out level at the listening position is somewhat moderate.

I would suggest a slot vent equal to or greater than 24 in^2 for this application to bring the port compression to a minimum.

I assume the plate amplifier is not compatible with the sub (i.e. it is not 2 ohm stable)? An Behringer EP2500 can be had from www.zzounds.com for as little as $260.00 (includes shipping), add $8.00 for the quiet fan mod. This would allow you plenty of power running it on a single channel. It will also let you upgrade to another sub down the road.

 

[Grimfate126]

With a net volume of 3.93 ft^3 what was the tuning frequency?

At a distance of 10 feet, you will have an overall reduction in output of around 3.5db if memory serves me correctly. To make up 3.5db at the listening position you need around 2x the power. The port comression could very well be audible, especially during dynamic passages, even though the out level at the listening position is somewhat moderate.

I would suggest a slot vent equal to or greater than 24 in^2 for this application to bring the port compression to a minimum.

I assume the plate amplifier is not compatible with the sub (i.e. it is not 2 ohm stable)? An Behringer EP2500 can be had from www.zzounds.com for as little as $260.00 (includes shipping), add $8.00 for the quiet fan mod. This would allow you plenty of power running it on a single channel. It will also let you upgrade to another sub down the road.

the tuning frequency ends up at 21.6 Hz.

ok then, i will do the slot however, is it possible for the vent to be placed such that it is not visible from the front? (ends up in the back.)

as for the amp, i really wanted the oaudio for its high pass filter, parametric EQ, and the fact that i really dont have space for another box on my rack.

Also, the oaudio is already pushing my budget (was set for 200), so i really dont think the EP2500 is a choice for me as of now

Thanks for the help ill post a new design soon.

 

[annunaki]

the tuning frequency ends up at 21.6 Hz.

ok then, i will do the slot however, is it possible for the vent to be placed such that it is not visible from the front? (ends up in the back.)

You could place the vent in the rear with no ill effects. Make sure to round over your edges on the slot vent (internal & external) to reduce turbulence. Check out my build thread if you want to see an example..

 

[Grimfate126]

ok, so i remodeled with a vent, and here are some pics:

without brace:

with new brace:

The internal dimensions of the port are 14" x 1.5". This puts the max air velocity at around 21 m/s at the 20 Hz mark (that's without the rounded-over edges). The port is 32.5 inches long.

However, the port takes up quite a bit of volume (around .78 cubic feet), and thus it drops the net volume of the box to 3.2 cubic feet, and brings up the tune to 22.22 Hz (which is still ok by me). If i had done a 12 x 2 port, it would have had to be 40 inches long to maintain the 22 Hz tune, and i dont want to have to do more than 1 bend.

There are a few questions a have about this design:

1. Will the brace still be strong enough?
2. Is 21 m/s good enough? Ive read on many forums that 24 is a good number to stay under, although some say 18.

 

[annunaki]

To make things easier, while increasing cabinet rigidity, I suggest using the rear wall of the enclosure as one of the port surfaces. It will also then take up slightly less internal volume.

The brace should be fine, though I am know to go a bit overkill in that department. If it were my enclosure, I would add more bracing. Just a personal preference.

I assume your external dimensions must be set in stone so as to limit the internal volume?

The port should be adequate as the air is at a more respectable speed. That is if you never exceed 325 watts with the sub. 2" x 14" would allow you up to 500 watts or so keeping it below 20 m/s.

 

[Grimfate126]

To make things easier, while increasing cabinet rigidity, I suggest using the rear wall of the enclosure as one of the port surfaces. It will also then take up slightly less internal volume.

The brace should be fine, though I am know to go a bit overkill in that department. If it were my enclosure, I would add more bracing. Just a personal preference.

I assume your external dimensions must be set in stone so as to limit the internal volume?

The port should be adequate as the air is at a more respectable speed. That is if you never exceed 325 watts with the sub. 2" x 14" would allow you up to 500 watts or so keeping it below 20 m/s.

it was my orignial plan to "attach" the back of the port to the back wall, but then i realized that there will be a plate amp there.

my outside dimensions are pretty much set in stone :/

another question: since the new port will require lots of cutting, i dont feel like getting the parts cut at a store (not to mention they are usually VERY off). What is a good tool to get good straight cuts? Will a router suffice?

 

[annunaki]

it was my orignial plan to "attach" the back of the port to the back wall, but then i realized that there will be a plate amp there.

my outside dimensions are pretty much set in stone :/

another question: since the new port will require lots of cutting, i dont feel like getting the parts cut at a store (not to mention they are usually VERY off). What is a good tool to get good straight cuts? Will a router suffice?

A good straight edge, clamps, and a circular saw are quicker than the router.

Build a small bump out off the rear of the enclosure for the plate amplifier. Or house it separately in it's own neat little enclosure.

 

[Grimfate126]

A good straight edge, clamps, and a circular saw are quicker than the router.

Build a small bump out off the rear of the enclosure for the plate amplifier. Or house it separately in it's own neat little enclosure.

that would look ugly no? :/

it is possible to do a good job with a router though right?

 

[annunaki]

Yes, but not any better than with the method I have suggested, simply more time consuming.