Ports - Size, Shape, Location, Quantity

[jhollander]

Re: Ports - Size, Shape, Location, Quantity

I’m having a problem understanding the concern about boundary layer or friction effects of ports. The port tuning I believe is a function of the mass of air in the port.

While I agree with the concept that infinitely increasing wall circumference/ area would create a problem. The formula for this must indicate the ratio must be quite large.

I would consider this a cliff function meaning almost anything goes with port shape until you create enough friction to limit the “calculated” mass of moving air. You would still have a mass of air resonating but it would be at a lower volume than calculated.

Please correct my thinking if I’ve gone astray.

[Zerkfitting]

Re: Ports - Size, Shape, Location, Quantity

Does anyone have thoughts on the port location, front, rear, side? Does the location matter?

[Wolf]

Re: Ports - Size, Shape, Location, Quantity

I read somewhere that airflow is better through a square opening rather than circular

anyway, question - is it not advisable to put the port opening inside the box nr the woofer - how much midrange can exit a port?

If you take the same circumference of material and fashion circular and quadrilateral cross-sections with it, the Circular type will actually have more x-sectional area than the other types. It also takes pressure equally at its circumference or inside, so I would not agree with the sentiment that quadrilateral shaped ports are inherently better.

As to midrange leakage, usually this effect is lessened if exited from the rear of the enclosure. Port resonance and midrange leakage can actually result in a cancellation of output in a small band of freqs if not done appropriately. It can be a very bad thing to place a port's inner exit right behind the woofer.

Later,
Wolf

[bill poster]

Re: Ports - Size, Shape, Location, Quantity

I read somewhere that airflow is better through a square opening rather than circular

anyway, question - is it not advisable to put the port opening inside the box nr the woofer - how much midrange can exit a port?

[Whitneyville1]

Re: Ports - Size, Shape, Location, Quantity

Not trying to complicate things more, but the characteristics of the woofer (mid-woofer) is part of the mix too. A mid-woofer I've used and like very much, behaves well when tuned to about 48 Hz. A simulation says you can go lower. This fine speaker turns on you like a cornered badger! "Stuff" starts to happen. I call it "stuff" because it's very complex (pages long) acoustical reflections and resonances and more. It really doesn't matter exactly what it is, but it makes the speaker unusable in the real world for lower frequencies. Sometimes, on a well designed speaker you're building, a change in the port may rise up and bite thee in the behind. If you're with-in 10%, you should be OK, but I wanted to throw that in here. Everyone wants more bass, but it always comes at a price. Today, often the "price" is a sub-woofer.

[Saturnus]

Re: Ports - Size, Shape, Location, Quantity

To add a few thoughts to consider. From what has already been written it would appear that higher circumference to area and thereby higher drag is universally a bad thing. That is not always the case. Higher drag is sometimes, even often, beneficial as anyone who has ever tried the stuffing-the-port-with-straws trick can confirm. Effectively this just adds drag which increase the dampening of frequencies below the port tuning where the driver otherwise sees an effectively open baffle. This helps to prevent excessive cone excursions which in turn improves performance and lowers distortion.

Round ports, typically when only one port is used, can also suffer from toroidal vortices which when they occur will act as a smaller port within the port which can create cuffing even at extremely low port air speeds. Furthermore round ports amplifies cuffing because being circular there is one dominant frequency and harmonics of this when cuffing occurs. Both things can be almost eliminated or at least severely reduce the effects of by having multiple ports or using slot ports, both of which have the aforementioned higher drag.

All in all, the right port for the right cabinet and the right driver is not something that is easily calculated as it is almost impossible to predict the exact performance of a port without testing it.

[SirNickity]

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.

Maybe I misunderstood you. I figured anyone here for more than a month has experience with WinISD, so I thought you were making a point about how WinISD does in fact NOT consider various factors -- such as 1/4 wave resonance, or the effect of having a port too close to the enclosure walls, etc.

My statement however doesn't incorrectly imply anything. If the OP wants to see the relative difference between round and slot ports, or one and multiple ports, WinISD will happily calculate that for you. It is not a complete physics simulation package, so it will unfortunately not describe the effects of leather couches vs. cloth, nor brick walls vs. glass doors. To be adequately thorough, I guess I should've covered all its deficits in the one-line reply I gave earlier. ;)

For the record, I tried to express the suitability of WinISD based on whether the inquiry was more "is there a difference between port types and sizes?" or rather "teach me more about how to calculate the differences".

OK, moving on...

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.

Having a port terminate into the floor would affect its tuning in that it would still restrict the airflow, but not necessarily as much as the port itself does. In other words, it would kinda act like an extension of the port, but to what degree depends on other factors -- such as the surface of the floor (concrete or a really shaggy carpet?), the area between the floor and enclosure, and so on. I'm sure it can be calculated, but if I were going to design-in such a thing, I'd probably chicken out and measure it empirically.

With a slot port, using the floor as the bottom of the slot, you could conceivably get close to the performance of a traditional slot port. IF you can assume the floor is of adequate hardness (similar to the enclosure material), and all enclosure-to-floor contact (with the obvious exception of the slot mouth) are reasonably well sealed, etc. Potentially doable, probably not advisable, but a neat trick if you pulled it off.

You asked about port placement. In general, you can put it wherever you'd like, with a few caveats. One, if it's directly behind a driver, that can have side effects -- for example, you'll hear more of the driver's direct backwave (midrange leakage) which is usually undesirable. Placing it elsewhere and lining/stuffing the enclosure can mitigate that. Next, if you place it close to an enclosure wall, the airflow at the port is restricted in such a way that the port appears longer than it really is. You can take advantage of this if you're careful, or it can be an unwelcome surprise. Slot ports are particular susceptible since they often take advantage of an outside wall. Another thing to keep in mind is wind noise, or chuffing. As the air velocity gets close to 20m/s, you might start hearing the turbulence. One workaround is to place the port such that it simply points away from the listener. If you don't hear it, it's not a problem. Otherwise, you just make the mouth larger (which requires the port to be longer of course), and/or flare the ends -- which helps, but only so much.

[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?

[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)

[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.

[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...

[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

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

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

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