Ports - Size, Shape, Location, Quantity

[infamous_panda]

I have been scouring the forums on the subject of modifying ports and have a few questions. Totally new to this and have little practical or theoretical knowledge at this point.

From what I have read once your identify the diameter and length of your basic port tube you can basically place it anywhere it will fit in the box without significantly affecting sound quality or performance. This includes have the port physically located outside of the box so long as the volume of the box is compensated for. I did read some mention that front facing ports should not be located near the high frequency drivers but do not know if why or if there is anything to this. I have seen many designs that used such and arrangement.

Additionally I have also read that you can take your basic port tube and convert it to a slot so long as the aperture area of the slot is the same as the area of the tube hole and the length is the same. You can also divide a large port into two so long as the lengths are the same and the area of the two smaller ports are equal to the large one. Is this more or less accurate? I want to convert a tube port into two separate slot ports. But taking this a step further I know there are practical limitations to how many times you can divide a port, but what are the theoretical limitations? Can I take a 2 inch diameter port and convert it to 20 little tubes the size of straws?

Any opinions, rules of thumb, formulas etc, are greatly appreciated.

[Chris Roemer]

Re: Ports - Size, Shape, Location, Quantity

ONE 2" id tube has a x-sectional area of 3.14 sq.in., and a circumference (which the moving air "drags" against) of 6.28" (lineal).

20 little 0.45" dia tubes (OK, maybe not so little) would have an area of 3.18 sq.in., but a combined circumference of almost 28.3", or over 4 times a higher ratio of square area to circumference. Do you think that answers your question?

[infamous_panda]

Re: Ports - Size, Shape, Location, Quantity

It makes sense. 20 is an extreme example. If I understand this correctly then even if you divide one large port into two equal length and area you would be altering/degrading its performance? Can you compensate for the additional drag by varying the length or the size of the enclosure?

[Paul K.]

Re: Ports - Size, Shape, Location, Quantity

The port's "end correction" factor needs to be considered when using two or more smaller-diameter ports instead of one larger port. The effective length of a port is its actual length plus 0.6 times the port's radius. So two smaller-diameter ports whose combined area equals the area of the single larger port need to be a bit longer in order to have the same effective length. Going from a single round port to a slotted port needs to be carefully considered due to the circumference issue and resulting drag that Chris described and depends on the aspect ratio of the slotted port, and using a pair of slotted ports compounds the issue. As far as placement is concerned for any port shape, if you're building a floor-stander that's has a significant height, the 1/4-wavelength resonant frequency from that height will contribute to the system's tuning frequency and needs to be taken into account when settling on the port's tuning contribution and location.
Paul

[SirNickity]

Re: Ports - Size, Shape, Location, Quantity

Academics aside, if you just want the answer, WinISD will calculate it for you. (In case you weren't already aware.)

[Paul K.]

Re: Ports - Size, Shape, Location, Quantity

Will WinISD allow you to determine the best possible location for the port in a floor-stander, rather than simply base the port's dimensions on the volume in the box?
Paul

Academics aside, if you just want the answer, WinISD will calculate it for you. (In case you weren't already aware.)

[SirNickity]

Re: Ports - Size, Shape, Location, Quantity

Are you asking questions to which you already know the answer? :D

[Paul K.]

Re: Ports - Size, Shape, Location, Quantity

I honestly don't know what WinISD's capabilities are, but I suspect it cannot do what I asked about, and if it cannot, then your original statements incorrectly implies that WinISD has all the answers.
Paul

Are you asking questions to which you already know the answer? :D

[Taran]

Re: Ports - Size, Shape, Location, Quantity

I honestly don't know what WinISD's capabilities are, but I suspect it cannot do what I asked about, and if it cannot, then your original statements incorrectly implies that WinISD has all the answers.
Paul

WINisd doesn't have all the answers, but it definitely can calculate port sizes based on tuning and shape, as well as calculating port velocity.

[Paul K.]

Re: Ports - Size, Shape, Location, Quantity

The fact it takes port shape into account is really useful, but it's too bad it can't also allow you to determine the best location for the port or take into account the box's shape, both of which can be quite critical in a tall floor-stander.
Paul

WINisd doesn't have all the answers, but it definitely can calculate port sizes based on tuning and shape, as well as calculating port velocity.

[Taran]

Re: Ports - Size, Shape, Location, Quantity

The fact it takes port shape into account is really useful, but it's too bad it can't also allow you to determine the best location for the port or take into account the box's shape, both of which can be quite critical in a tall floor-stander.
Paul

You just said earlier you didn't know the capabilities...

[Paul K.]

Re: Ports - Size, Shape, Location, Quantity

Oops, left out a word. I should have said "it's too bad IF it can't also allow...". I assumed by you describing what it WinISD can do regarding port shapes and sizes, etc, it cannot do what I described. If it can, that's also useful.
Paul

You just said earlier you didn't know the capabilities...

[infamous_panda]

Re: Ports - Size, Shape, Location, Quantity

Thank you all so far for your input. This has been quite insightful. I have a current fixation with ports and how to use them as a design element / differentiation without adversely affecting their function.

Prior to this I thought all that was important was area and length. The rest was flexible. I never considered "drag" or 1/4 wavelength resonance as factors affecting design.

Speaking of 1/4 wavelength resonance. What is it? It was mentioned that this is to be considered for floor standers so I am assuming this is a factor of port location relative to the floor and not location relative to the speaker drivers?

Here is another idea which hopefully gets farther than my 20 small port concept. I wanted to place a port at the bottom of a cabinet that is sitting on floor spikes. I reconsidered after reading some of the above since there may be something to the sound having to travel in that small space between the cabinet and the floor. So I got thinking what if we do a slot port and using the surface the speaker is sitting on as the bottom surface of the slot. So for example if I have a subwoofer the box would be sitting above the floor at a height determined by the optimal height of the slot. Three of the four side walls would then extend down to the floor acting as feet.

[Taran]

Re: Ports - Size, Shape, Location, Quantity

You can experiment. Boundary reinforcement is a real factor. (it's far less important than actual enclosure tuning though)

[Paul K.]

Re: Ports - Size, Shape, Location, Quantity

The 1/4-wavelength resonant frequency comes into play for mostly 2-ways, whether MT or MTM, using floor-standing cabinets with the midwoofers mounted near the top of the baffle and the port near the bottom of the cabinet. The internal height of the cabinet will have a 1/4-wave resonant frequency that will contribute to the overall system tuning, with the primary tuning contribution coming from the port's dimensions. If you determine the port's dimensions based solely on the volume in the box, those dimensions, and thus the system tuning frequency, will not be correct or optimum. The taller the cabinet, the more its 1/4-wave resonance contributes to the overall tuning. Also, the port's location relative to the cabinet's height and midwoofer location, if not optimally located, will exhibit some unwanted resonant glitches in the overall response.
Paul

[QUOTE=infamous_panda;1931824]

Speaking of 1/4 wavelength resonance. What is it? It was mentioned that this is to be considered for floor standers so I am assuming this is a factor of port location relative to the floor and not location relative to the speaker drivers?