andrewsfm

iTrader: (3)

What kind of problems would that cause?

Dukk

The static tuning will be off, but that can be compensated for. The dynamic tuning will change though - ie as you turn up the volume the tuning of the box will change and that you can't compensate for.
It will also probably be noisy.

andrewsfm

iTrader: (3)

Do you have a technical explanation why? Sounds interesting that a certain ratio would change the port tuning, even though it has the same port area.

jalat

iTrader: (6)

1" wide port would cause so much port noise you'd hang yourself.

15 square inches of port per cubic foot is the rule of thumb.

turboeeyore

i must be on crack. the larger the opening the longer the port. must have too much on the brain. i figured the port calculator is the best advise anyways. sorry. i've changed my boxes and ports so many times i got confused.

godzilla1978

the 1" port would definatly be noisy. What happens when you move a lot of air through a small hole? If you had like maybe a 5" sub then maybe 1" port would be o.k. If you use WinIsd pro it gives you the port length(slot or round) and the vent air speed which you can use to figure out if the port will be noisy, and it's free and legal!(shareware)

andrewsfm

iTrader: (3)

Due to size restrictions, I just finished building a 1.0cu. ft. box with a 11.25"x1" slot port before this thread even came up.

I did simulations using WinISD beta 0.44 and the vent calculator shows any vent mach value over 0.15 mach in red, which I'm assuming means it's too high. The value I got for my enclosure design was 0.09 which I thought was fine.

Now you guys are saying it won't work being 1" wide because the air velocity is too high?

I'm confused and worried...

jalat

iTrader: (6)

if you mantained AT LEAST 15 square inches of port you should be okay, ut there will be port noise for sure. i've never made a slot pot enclosure with a port narrower than 1.5" wide

Dukk

How does that make any sense? Theoretically a sub (of any size and excursion no less) in 1cuft needs 15sqin but in 2cuft it needs 30squin of port? That's messed up..

AndrewsFM - it has to do with the ratio of port area to circumference or perimiter length of the port. Without delving to deeply into fluid dynamics, consider this:

- A 4" diameter round pipe port has an area of ~12.56sqin and a wall length (circimference) of ~12.57".

- A square vent of the same area needs to be 3.54" square on the inside. this results in a perimeter length of 14.16" or 12.6% more wall than the round port.

- A 1" wide slot vent of the same area has 27.12" of perimeter length, or a smokin 216% more wall than the round port

Now who cares right? Well consider this - the air moves through the vent at speed. The air near the walls has to slide along the walls and that causes friction so our example slot vent is literally introducing 216% more friction to that air than the round vent. That friction will cause noise, and if it is great enough, it will impede the air moving through it, causing it to act like a smaller, longer vent (lowering tuning) until it eventually chokes off and it behaves like a sealed box with a flatulence problem.

I'll leave it there but now a few things should be obvious:

- Round ports are the most efficient - although not always the most convenient
- A vent (which really is any shape other than a round port) should be as close to square as possible.
- The smoother the internal surface of the vent the better. This is why plastic ports are superior to cardboard. In the same way, vents should always have rounded corners, and if you really want to go the whole way, should be coated or arborited or something to get their coefficient of drag down. The smoother the air flow, the better the vent.

And my rule of thumb ratio of 6:1 actually comes from back when I was in the HVAC industry - a designer wants to be very careful designing air ducts less than this ratio as they cause trouble with both air delivery and noise. When desiging a rectangular duct, there is actually a compensation factor for the area compared to a round duct.


At then end of the day - your box is already made, so try it out. It may be fine. Your 0.09mach port velocity is at the high end though. If you don't have a car with a trunk, try to not point the vent towards your head.

phat_71

Well I'm no expert with enclosures, but when it comes to physics I'm not so shabby, so I think I can explain adequately enough.

We have an enclosure 1 cubic foot in size, a 10 inch driver, and a port. First things first, length is relative only to the port in question. Chances are that the displacement of air within all the port designs is the same, which makes sense. The function of a port is to allow air flow. Based on tuning frequency, the amount of air being displaced and allowed to flow changes, since the air within the port itself is considered neutral, and moves either inward or outward depending on the excursion and incursion of the driver being used. Higher frequency requires more rapid airflow, thus would require a smaller port than a low frequency tune in the same enclosure. Here's an example.


A 1 cubic foot box tuned to 35 Hz takes a 3 inch port that is 13.36 inches long. The same box tuned to 45 Hz takes a port that is 7.21 inches long. Again, the higher frequency requires air to be moved more rapidly, and thus the relative amount of "neutral" air must be reduced. The driver can obviously move the air in a shorter port much faster than it can with a longer one.


I realize that it doesn't really make sense to a lot of people at first, but basically what I just said is the way it works. It all comes down to the box and the amount of "neutral" air within the port itself, which determine the tuning frequency.