Cornwall port dimensions

[tagger]

Hi all. I've been lingering around the forum for a while, but this my first post. I recently drew up some plans for a Cornscala type speaker that I plan on building. Actually, Chris (Popbumper) is going to help with the cabinets. It will consist of a separate bass bin and top hat, using the K-33 (or Bob's replacement woofer), the K-55 w/ Altec 511 horn, and the K-77 (or Bob's CT125). For aesthetic reasons, the 511 and K-77 will be mounted on a baffle in a top hat cabinet, rather than sitting freely on top of the bass bin. To accomodate the size of the 511 horn, and to keep the width of the two cabinets constant, I increased the internal width from 23 7/8" (ala Cornwall) to 25". Now for my questions...

First, what are the accepted dimensions for the Cornwall port? After searching the forum, I found two different answers. 22 3/8" X 3 1/8" X 9 1/2" ... and ... 22.25" X 3.38" X 9". Which one is correct?

Second, If I keep the same port dimensions found in a Cornwall, the port shelf will not extend from side wall to side wall due to the increased cabinet width. Can I simply decrease the depth of the shelf to achieve an identical port volume? My instincts say "no" and I think it has something to do with a change in air resistance with the larger cross-sectional area of the port, but I'm not quite sure. Would the answer be (as counter-intuitive as this may seem) to increase the shelf depth? Does anyone know a formula for this?

Thanks all,

Joe

[jwc]

22.25" X 3.38" X 9" is correct.

You need to read this!

http://www.jwcullison.com/csclicktoconvert/index1.html

jc

[tagger]

Hi jc. Actually, I got 22.25" X 3.38" X 9" from one of your comments in another thread (don't remember which one off hand). I noticed they were taken from an '84 Cornwall. The other dimensions I got from this thread:

http://forums.klipsch.com/forums/4/484769/ShowThread.aspx

If I remember correctly, they were taken from a '74 Cornwall. I noticed you participated in this thread, but did not dispute the port measurements. Did the port dimensions change in the later Cornwalls?

Great build log, btw.

Regards,

Joe

[colterphoto1]

The VOLUME of the port should remain consistent, and I would not vary any dimension too greatly from the original.

Michael

[DrWho]

Actually....the total surface area and length of the port need to remain constant. You can't make the port longer and the surface area smaller to achieve the same volume and have the same tuning.

Or to put it another way, "volume" doesn't matter. Granted, the volume remains the same when the length and total surface area remains the same (regardless of shape), but that doesn't mean volume is the only variable to consider.

[jwc]

Hi jc. Actually, I got 22.25" X 3.38" X 9" from one of your comments in another thread (don't remember which one off hand). I noticed they were taken from an '84 Cornwall. The other dimensions I got from this thread:

http://forums.klipsch.com/forums/4/484769/ShowThread.aspx

If I remember correctly, they were taken from a '74 Cornwall. I noticed you participated in this thread, but did not dispute the port measurements. Did the port dimensions change in the later Cornwalls?

Great build log, btw.

Regards,

Joe

OK. I had to wait until I got home to look at my notes.

Yes. I remember that Thread. I never agreed with the diagrams in that thread. Maybe at that time I should have spoken up but I didn't.

One correction. The inside dimensions of the port are 22 1/4 x 3 3/8 x 9 1/2. I just realized that my previous posting of the 9" depth was wrong. This wasn't posted in my build thread as it is correct.

Now. I don't know what the 77' Cornwall dimemsions are. But I will say that I am confident with my measurements of my 84' Cornwall (sold last year).

Some of the descrepancy can be explained by looking at the external dimensions of the CW 1 vs CW2 on the Klipsch website.

CW1

dimensions

35.75" x 25.5" x 15.5"

CW2

35.75" (90.8cm) x 25.25" (64.1cm) x 15.5" (39.4cm)

Now look at the total width difference of the two. Slightly different.

Now, I will tell you that the width of the port on the CW2 is 22.25. The side panels of the CW2 are 3/4 inch birch and the port side supports are 3/4 inch as well. SOOOO.....4 different 3/4" panels of birch come out to 3" total. Therefore.....25.25 minus 3" equals 22.25. That explains my port width.

The size of the 84' CW port shelf is a panel 23 3/4 x 3/4 x 9 1/2. The (2) 3/4" side supports for the port account the 1 1/2 inch loss of the port width to get the 22.25.

You with me here.

Now to the height of 3 3/8 is correct for my 84' CW. It is very possible the ports changes from the CW1 to the CW2. I'm not going further with that comment.

To be more technical about the depth of the port...well the port shelf is 9 1/2 inches deep but with the 3/4 inch baffle with port cutouts in front of the port.....really make the port a depth of 9 1/2 plus 3/4 inch giving 10 1/4.

Good luck man. The Cornscala is a no brainer. Sounds great.

jc

[colterphoto1]

Actually....the total surface area and length of the port need to remain constant. You can't make the port longer and the surface area smaller to achieve the same volume and have the same tuning.

Or to put it another way, "volume" doesn't matter. Granted, the volume remains the same when the length and total surface area remains the same (regardless of shape), but that doesn't mean volume is the only variable to consider.

Thanks Doc, I did not know that.

[tagger]

Using JC's port dimensions, you get a cross-sectional area of 75.205in^2 and a 9 1/2" depth.

Using the port dimensions taken from the "Forum Drawing" you get a cross-sectional area of 69.921875in^2 and a 9 1/2" depth.

I'm going to assume that both JC and Klipschguy (who confirmed the measurements of his '77 Cornwall for the Forum Drawing) are competent at taking simple measurements. So, do you think it would be safe to assume that the difference in cross-sectional area is not significant enough to alter the tuning of the port? I'd love for someone knowledgable in this area to chime in. Doc?

I've attached a rather crude hand-made drawing of my bass bin. If I extend the port shelf from side wall to side wall (taking into account the 3/4" shelf supports) you get a cross-sectional area of 73.4375in^2 and a 9 1/2" depth. This puts it right between the two documented port dimensions but closer to JC's. Apparently, cabinets have been produced from both JC's and the Forum Drawing with excellent results. I think my design should be OK then. What do you guys think?

Joe

[jwc]

I need to know a few things to give you my opinion.

What is the depth of the side panels?

How much recess is your baffle from the front edge of the cabinet?

Your back panel......will it be flush with the back edge of the cabinet?

[DrWho]

Assuming a 6.190 cubic foot internal volume:

75 sq in x 9.5" depth = 45.85Hz

74 sq in x 9.5" depth = 45.58Hz

70 sq in x 9.5" depth = 44.57Hz

NOT a big enough difference to worry about - you're talking like 0.3dB differences in output over a very narrow range.

I would recommend you download this free program that lets you do all the calculations and models for yourself:

http://www.linearteam.dk/default.aspx?pageid=winisdpro

[capo72]

How would this translate into round ports? I may build a cornscala , but narrower and taller. If I do, a couple 4" dia. ports would be easier than trying to build a shelf with the same surface area.

Jeremy

[jwc]

How would this translate into round ports? I may build a cornscala , but narrower and taller. If I do, a couple 4" dia. ports would be easier than trying to build a shelf with the same surface area.

Jeremy

I have done that. 2 x 4" ports work well. I used 3 different lengths

10" ,7" and 5 ". I think I went with the 7". Of course my cabinet was slightly bigger on purpose.

http://forums.klipsch.com/forums/thread/705763.aspx

jc

[tagger]

JC,

These are going to be decorator style, with baffle and back panels flush with the side panels. The entire cabinet, including front baffle, will be veneered. The side panels are 21.75" deep (20.25" internal depth). I started with the dimensions of Bob's test box and adjusted for the extra width needed to fit the 511's. Internal volume excluding woofer and bracing is 6.5918ft^3.

Doc,

Thanks for the link. I'll download it when I get home tonight.

Joe

[jwc]

JC,

These are going to be decorator style, with baffle and back panels flush with the side panels. The entire cabinet, including front baffle, will be veneered. The side panels are 21.75" deep (20.25" internal depth). I started with the dimensions of Bob's test box and adjusted for the extra width needed to fit the 511's. Internal volume excluding woofer and bracing is 6.5918ft^3.

Doc,

Thanks for the link. I'll download it when I get home tonight.

Joe

If you want the Cornwall sound, I would suggest a slightly smaller cabinet.

10641.51650144 cubic inches or 6.158285012407408 cubic feet. This is the internal volume of the Cornwall minus the mid/high drivers and crossover.

Keep us posted on your progress.

jc

[dtel]

Just checked the Cornwall lll ....22" wide and 2 1/4" high, the shelf on the inside goes in 7" tuned a little different ?

The bass speaker is 9" up on the motherboard, that could also why they changed the port size ?

[jwc]

Thanks Dtel. The rise of the woofer was for other reasons besides the porting.

Thanks for the port info. Say when you say 2 1/4 high...do you mean from the from the bottom of the shelf to the topside of the bottom of the cabinet or is 2 1/4 the height of the holes in the baffle?

Also....22" wide seems a little weird as the external width of the CW3 is 25.31". That would mean there is 3.31 inches of wood thickness to make up some where. Is there a chance the width of the internal port could be 22 3/8"?

The 7" from the back of the port shelf to the fron tof the baffle...is this right?

Thanks for the input. As you can see....I'm nutty over details.

jc

[Coytee]

10641.51650144 cubic inches or 6.158285012407408 cubic feet.

Is it ok to round UP to 10641.51650145 cubic inches, or 6.15828501240741 cubic feet, or would it be better to round DOWN?

[] []

[IB Slammin]

JC,

These are going to be decorator style, with baffle and back panels flush with the side panels. The entire cabinet, including front baffle, will be veneered. The side panels are 21.75" deep (20.25" internal depth). I started with the dimensions of Bob's test box and adjusted for the extra width needed to fit the 511's. Internal volume excluding woofer and bracing is 6.5918ft^3.

Doc,

Thanks for the link. I'll download it when I get home tonight.

Joe

Joe,

First of all, could you tell me about your avatar? Creeps me out a bit, but then the acid I took could be comming on.[]

I would like to make a few suggstions if you don't mind.

1) IMHO, a top hat on a 3/4" to 1" riser that is a bit wider than the bass bin could look pretty cool and simplify the bin build. (remember to subtract the volume of the K600/K55)

2) Dump the K55's and replace them with Altec 902's on the 511b.

3) Cross them at 400hz (disregard the published spec's for the low cross) and run up the next cross point to 9000 or so.

4) Keep it three way for the sparkle at the top.

5) Brace the bin. Extra 3/4" plywood on the back. Cross brace the motor board.

6) Allow enough room in the top hat to accommodate a JBL baby-cheek tweeter.

For me, the ultimate Cornwall spec's are listed above.

I and others call it Alcorn. There ain't no Scala to it.

Welcome to the madness, and good luck with the build.

Regards,

Terry

PS: You have a super nice guy in your corner. Pop will be a great asset to you.

[DrWho]

10641.51650144 cubic inches or 6.158285012407408 cubic feet.

Is it ok to round UP to 10641.51650145 cubic inches, or 6.15828501240741 cubic feet, or would it be better to round DOWN?

[] []

lol, seriously....how the heck does one measure down to such significant digits? []

[tagger]

Terry,

First of all, could you tell me about your avatar? Creeps me out a bit, but then the acid I took could be comming on.[]

Hey! That's my poor wittle ol' piggy bank. He's just a bit malnourished!

I would like to make a few suggstions if you don't mind.

1) IMHO, a top hat on a 3/4" to 1" riser that is a bit wider than the bass bin could look pretty cool and simplify the bin build. (remember to subtract the volume of the K600/K55)

I thought of this from the get-go, and ran it by Maria (who has taken a surprising interest in this project for fear of it ruining her living room) and we both agree that if anything, the top hat might look good smaller than the bass bin, but not wider. Just my opinion.

2) Dump the K55's and replace them with Altec 902's on the 511b.

Funny you should mention this. I have both a pair of K-55M's and a pair of 902-8B's, and was planning on trying both to see which I preferred.

3) Cross them at 400hz (disregard the published spec's for the low cross) and run up the next cross point to 9000 or so.

I will definitely have some X-over questions, but that'll be for another post when the time comes. Expect me to pester you a bit since you've got a similar setup if I remember correctly [].

4) Keep it three way for the sparkle at the top.

5) Brace the bin. Extra 3/4" plywood on the back. Cross brace the motor board.

Hmmm... Do you mean make the back panel twice as thick? And cross brace the motor board where? I'm just having trouble picturing it.

6) Allow enough room in the top hat to accommodate a JBL baby-cheek tweeter.

What are the dimensions of the JBL 2404 and what size cutout would it require? For now, that'll put me over budget, but I was thinking of going with the APT-200, which is the same driver as Bob's tweeter, but with a baby-cheeks horn lens. About the same cost, too.

For me, the ultimate Cornwall spec's are listed above.

I and others call it Alcorn. There ain't no Scala to it.

I still kinda like "Voice of the Corn." Always makes me think of Isaac and Malachai standing atop a giant Cornwall in a huge cornfield!

Welcome to the madness, and good luck with the build.

Thanks!

Regards,

Terry

PS: You have a super nice guy in your corner. Pop will be a great asset to you.

After seeing pictures of his work, the "nice guy" is just a pleasant added bonus!

Joe