Cornwall port dimension questions??

[bhendrix]

If one were to build a narrow version of the cornwall (cornscala), what are they design elements of the port that must be maintained? If the port is more narrow, can it be taller as long as port length is maintained? On factory Cornwalls, the port opening in the motorboard is a smaller area than the port itself. Which is the important measurement, motorboard opening area, or port area? Is there any advantage or disadvantage to the ports exhausting on the side of a K-Horn-shaped cabinet ( or like JW's DBB Cornscalas)?

I tried to search for these questions tonight, but the search engine is only returning posts from today. Sorry . . .

Thanks for your counsel !!

[colterphoto1]

You're looking for port VOLUME to be the same. I know what you're saying about the 2 or 4 cutouts in the motorboard being slightly smaller than the 'tunnel' of the port itself. I think that difference is nominal. But you must have the general DxWxL! of the port pretty close to the original for the K33 to work properly.

DrWho would have further info on the position of the ports relative to the driver and other elements.

I talked to Rick tonight, heard you bought parts to make a CW center box. Have fun!

Michael

[bhendrix]

Thanks, Michael. Did you get the LaScala's??

[colterphoto1]

he's sending me some photos. sound pretty beat up, but you know me. I like to stack up projects and keep old cabs in the family. I might 'glass them up and sell em as band PA speakers. Maybe put K43's in em.

Michael

[DrWho]

ummm, does the math actually come down to maintaining the same port

volume? If you make the mouth (cross-sectional area) larger, you need

to make the port longer too in order to maintain the same resistance to

air flow (it's easier to blow through a short straw than a long one).

as far as ports go, its better to go with a single large one versus many small ones.

[bhendrix]

Doc, do the port characteritics (tuning) stay the same if the port area and length remain the same? For example, would it work if the new port were a few inches narrower, but the port heigth slightly higher, maintaining the same total area and length (volume)?

[DrWho]

as long as the cross-sectional area remains the same, the length will

also remain the same. You can also divide the single large port into

any number of smaller ports as long as the area stays the same, but it

is to your high SPL advantage to use just a single one.

If you are going to be redesigning the cornwall, then I might suggest

going with a new alignment to get rid of a wide peak centered around

80Hz.

[bhendrix]

How would you realign the cabinet, Doc?

[hwatkins]

as long as the cross-sectional area remains the same, the length will also remain the same. You can also divide the single large port into any number of smaller ports as long as the area stays the same, but it is to your high SPL advantage to use just a single one.

If you are going to be redesigning the cornwall, then I might suggest going with a new alignment to get rid of a wide peak centered around 80Hz.

As to the high SPL advantage Doc - I think that you could model several variations that may include multiple ports that would not have a noticable (if any) effect on SPL. The overall port volume is still the primary issue and there are multiple options as long as you design away from situations where the port doesn't 'whoosh' (how's that for a technical term?), however, there is a general limit based on the speaker specs regarding the number of ports that would not lower SPL..

I wholeheartedly agree with the new alignment especially if you can take your own measurements on the woofer. That done accurately allows tremendous flexibility for design - quite often folks can do pretty good with the new design for a limited set of speaker boxes while the costs to manufacture such a change could be prohibitive.

[DrWho]

The "high spl advantage" has to do with the fact that a port actually

changes its tuning based on how loud the woofer is playing (basically a

function of the air velocity flowing through it). The louder the SPL,

the lower the tuning of the port. The magnitude of this nonlinearity increases as the numbers of seperate ports increases.

I went ahead and modelled two different ports, each with the same

volume just to verify what I said....the tuning point of a port has

nothing to do with its volume, but has everything to do with how

restrictive it is to air flow (making the cross sectional area smaller

or the port longer increases resistance to air flow). In an 8 cubic

foot cabinet a 5x10 inch port with a length of 5 inches corresponds to

a tuning of 41Hz. In the same cabinet a 5x5 inch port with a length of

10 inches (so same volume) has a tuning point of 25Hz.

Therefore the total cross-sectional area must be maintained if you want

to have the same length port. For example, in an 8 cubic foot cabinet a

2x25 inch port with a length of 5 inches also has a tuning point of

41Hz.

Dividing the 5x10x5 inch port into two 5x5x5 inch ports results in a

45Hz tuning. The reason for the slight difference has to do with adding

an extra 5" to the circumference of each port mouth (for a net total of

10" extra circumference). This extra edge space changes the flow rate

of the air as it hits the outside air. If you want to minimize this

effect then make sure the ports are positioned as close as possible to

each other (the modelling program assumes that there is no coupling

whatsoever between the ports).

[michael hurd]

I agree with Dr.Who that it is better to use a single larger port than several smaller ones, it will tend to be quieter and more linear at high driver excursions.

There has been mention that air is a compressable fluid, and at high velocities, the port tuning will change, the tuning frequency moving down.

One brand of subwoofer manufactuer uses triple 3" diameter ports, where they could use a single larger port. I assume that there was a reason behind this, possibly to give the driver subsonic protection at higher sound pressure levels.

One thing to consider is that a larger diameter longer port will actually displace less volume inside the enclosure, than mutliple smaller ports, at least in the case of circular ports.

[DrWho]

How would you realign the cabinet, Doc?

I would figure out the largest cabinet volume you could go with and then

pick a corresponding port tuning. For example, you could get rid of the

slight hump by going with an 8cubic foot cabinet with a 40Hz tuning.

[hwatkins]

Yo Doc - you are more right than I on the modeling. My point would be better stated as to say I could model to the same tuning point with a couple of ports. In my case as well as the other stated cases, the volume of air moved could be managed by port size and one could be sure to have adequate design to prevent any unwanted 'extra' sounds.

The variances folks talk about in the percieved sound, in my experience, based on the shape of a port is easily designed around for rectangle, square or oval.

2 cents - I have built multiple ways and when done properly the speaker boxes test out accurate within my acceptable variances - depending on the box it could be +or- up to 3% of the design for a woofer and I have not noted any relevant trend regarding either the number of ports or port shape. I screw up the crossovers much more often than the box...[]

[jwc]

If one were to build a narrow version of the cornwall (cornscala), what are they design elements of the port that must be maintained? If the port is more narrow, can it be taller as long as port length is maintained? On factory Cornwalls, the port opening in the motorboard is a smaller area than the port itself. Which is the important measurement, motorboard opening area, or port area? Is there any advantage or disadvantage to the ports exhausting on the side of a K-Horn-shaped cabinet ( or like JW's DBB Cornscalas)?

I tried to search for these questions tonight, but the search engine is only returning posts from today. Sorry . . .

Thanks for your counsel !!

The placement of the ports on the side of the dbb's was a matter of getting the two woofers close together vertically. I would agree that from principle, I would have chosen one port but that gets me into a lot of height that I didn't want to deal with.

The internal dimensions of the Cornwall port are 22.25 x 3.38 x 9 (HxWxD). I think you may have already seen my Tall Cornscala plans on the "ALK Cornscala with K401 or Trachorn". The real problem with that idea was the width of the midhorns that go down to 400Hz. The K401 is 18 3/4 inches wide. You are now talking about a 20 inch wide speaker at the minimum and that isn't too much narrower than a Cornwall. The other option was the vertically mounted K401. I haven't heard too many people giving much faith in that. The other horns such as the 511b and Trachorn are even wider.