Cornwall with Two 4inch Diam.Tube Ports?

[SkyDover]

Hi, I'm hoping to get more expert advise here with my problem, posted in the 2-channel but this is probably more the correct area to post this in. I had a set of Cornwall speakers built, they used two 4inch Diam. ports with 4 1/2 length.

These ports aren't working out, as they said they usually do but because I had him move the woofer up 9 inches from bottom motor board, like Klipsch did with the cornwall III, that he thinks I need to add another port most likely.

So over the weekend, I took the 7 inch shelf (really 6 1/4) out of my cornscalas and put in a 10 inch self (really 9 1/2) to see what it sounded like, the reason for doing this is because my cornscala has the woofer moved up 9 inches from bottom motor board too. What I discovered is this:

Klipsch had to change the port shelf for the Cornwall III after moving the woofer up because the bass chamber builds up pressure faster than it can be released. The 9 1/2 shelf sounds just like how it makes the cornwall's with the 4inch port sound, not enough air released for the ports. In Moulin Rouge when the wind mill blade passes through the view of a window.... it cannot be heard in the 9 1/2 shelf but with the 7 inch shelf it's back and can be heard, same thing with the cornwall with two 4"diam. ports, it can't be heard. It's like the woofer is getting too much pressure build up and being choked down, I guess.

I'm not sure how to covert the 7 inch port shelf into what it would come out to using for 4" diam. ports and was hoping someone could give some insight here. Do I need to add one more port, or two? If I plug one port, the problem is worse... if I plug both, there isn't any bass. If I remove both ports out of the holes, things get a little better.

Any help would be appreciated if someone can help me out here!

Thanks, Sky

[pzannucci]

SkyDover,

If you look at a good box simulator, having the T/S parameters for the woofer and the box size, you can model how low or how high the cut off of the box is when changing the length of the shelf or the area of the ports. You are changing the Q of the box and thus the response, making the port react a either a higher or lower frequency. Depening on how tight and extended you want your bass will force you to look at box size, port area and port length.

This page will give you the basics though it will not model the box such as boxplot or bass box pro with predicted responses.

http://www.diyaudioandvideo.com/Calculator/Box/

http://www.ht-audio.com/pages/BassBoxPro.html

The tuning isn't as much with the location on the box but the volume. Typically the closer the drivers, the better the integration.

[SkyDover]

Thank you very much for the info and links! I checked it out and used it, I've used several online calculations and WinISD-Pro and everything seems right on from what it gives me but I'm thinking the woofer moved up is throwing off it's expected calculations, probably the very reason Klipsch had to shorten the shelf for the Cornwall III, to let the woofer breathe more. I'm still stumped! I'm going to talk some more with the speaker builder tomorrow and see what we can come up with. Thanks!!

[pzannucci]

There have been different specs over the years. Perhaps they never changed the shelf size and moved the woofer up to better integrate the woofer and mid, especially with going to Ti. If you use the current woofer specs from Emminence, you will find a 5.25 or so box with the same area as two 4 in ports will tune to 34 hz, if the length is 7 inches giving a excellent Q to a box with a 34hz fo woofer.

If you add a third port, to get the same low fequency tuning, you will have to make all the ports longer - almost 10.7 inches along with taking that volume of are from the box, probably putting you closer to 11+ inches for three ports. Might not give good clearance to the back of the box.

If you stayed with the 7" ports and used three, you'd likely get much better mid bass punch at the loss of definition and extension. You could although plug one or two ports when you have multiples depending on speaker placement and music preferences.

[Marvel]

and moved the woofer up to better integrate the woofer and mid, especially with going to Ti.

IIRC, the woofer was moved up to help with the front baffle resonance issues. I would guess it helped moving it closer to the mid driver as well.

Bruce

[SkyDover]

Thanks for the advice and help! The two 4in. dia. ports are working out fine now, I see what you are talking about. I played around with different port lengths... this really gave me a very good idea how it changes the sound. It was very interesting! I've got the ports tuned very good now. None of the bass is missing. I think two things I didn't count on was helping it lack in bass. One, after removing the damping material off the floor near the ports, it helped the air velocity better for the ports, and Two, I didn't count on new woofer break-in period. Seems that the new woofers needed some hours on them to maybe loosen up the spider and surrounds to get to it's better potential of it's bass capabilities. I've been amazed by the large bass sound now. Thanks guys!!