Here are the basics, gathered from some users who have built this mod to the La Scala bass bin. thanks to djk for all the design, input and figures.

First, let's assume you only want to try this out and not make any permanent changes to your precious LS cabinets. Sounds fair to me. Build a box to go underneath the cabinet, after first removing the bottom cover. This box will have a matching opening to mate with the LS bottom, and two ports, using 4 inch pvc for your favorite building supply store (Home Depot, Lowes, Menard's, etc).

Considering the LS is about 125 pounds, I certainly wouldn't use anything less than 3/4 inch. Dennis says this for the size:

"I'm looking for a total net volume of about 4.8 cu ft. I figure the stock back volume is about 2.25 cu ft. Call it an internal height of about 9" (22.5 X 22.5 X 8.70 = 4404 cu in = 2.55 cu ft). Two 4" ID by 10" long ports displace about .15 cu ft so 10" outside is close enough."

Comments made by some folks who have built this, have lead them to shorten the ports to around 7 inches. This retains the snappiness of the horn but gives the lower bass.

Here's a simple drawing:

Without a filter between the pre and power amp, Dennis says the bass can sound a bit lean.

" The net volume was found to be correct for 6th order tuning for the K43 woofer (B6) or quasi 6th order tuning (C6) for the K33 woofer (in the stock LaScala). Vents are typically two 4" diameter by 7"~10" long. Longer is tuned lower, in general the larger magnet woofer can handle the lower tuning.

    Boost is applied ahead of the amplifier and is usually Q=2 at Fb.

    Fb is sort of experimental with the smaller magnet woofer liking the shorter ports. [i.e., the K43 could use the 10 inch ports, while the K33 works better with the 7 to 7.5 inch ports]

    Part of this has to do with room modes. The small sized bass horn does nothing below about 100hz, but floor to ceiling gives us boost in the 70hz~75hz region (7-1/2'~8'), and wall to wall (short wall) in the 47hz~51hz region (11'~12'). An Fb around 31hz~35hz seems to work best with these room sizes and speakers. If you have an actual wall for the third wall (most newer homes don't, they have a large opening into a 'dining' area) and it is around 16'~20', you may have to lower the Q of the boost. It should be pointed out that most recordings are rolled off in this area and a little extra boost is nice."

Nice picture.  

For the newbie's on the block, remember you can cut through the top of the dog house from the xover chamber with a router and seal the back area to gain the volume and add some ports there.

Thanks Marvel.

Exactly what I was looking for.

 Wrinkles

speakerfritz:

For the newbie's on the block, remember you can cut through the top of the dog house from the xover chamber with a router and seal the back area to gain the volume and add some ports there.

Speakerfritz,

This is for all the cowards out there who would like to hear what it sounds like without putting any marks on their present cabinets. Also, some have said they wanted to raise up the cabinet some. This is ten inches, which to me would make them too high. Of course, since most people don't have their LS all the way against the wall, you could adjust the depth and lower it some. If you don't like round ports, you could do a slotted port as well. I might find that more attractive.

Here is the mod done to the top:

Pete Fowler and his buddy built a new set of cabs with the added space. Testing, they came up with the following figures (I think with no eq filter between the pre and power amp):

200 160 125 100 80 63 50 40 31 25 20 HZ

 77  80  79  73 72 78 68 60 61 52 51 dB Stock, mouth braced
 76  78  78  73 69 75 65 60 66 56 52    4.5 cuft, no ports
 76  78  77  75 74 80 70 60 68 54 54    Two 4"dia 10" ports
 76  79  78  76 75 80 71 60 68 54 53    Ports cut to 7"

The interesting thing with 10" ports is the boxes went deep, but the snappy, fast horn sound was lost! Cutting the port length to 7" brought the snap and speed back to same as stock, plus the lows remained, esp. at the 31Hz box tuning point. This data is with K33E driver and light stuffing. We think a K43E in this box would be superb, but haven't tried it yet.

Neither of us is going back to stock.

Pete

I wish I had access to a tablesaw. I love the LaScala bass....and if I could get another 10 Hertz lower I'd be happy as hell....

I wonder if you make a pair with one 4 x 8 sheet of 3/4" plywood, or if it will require more than one sheet.

With a quick calculation it should fit on one sheet. And if you would rather do a slotted port, Dennis gave me these figures:

The vented LaScala as I described runs to about 4.8 cu ft net. Two 4" ID by 7.25" long ports gives an Fb=35hz.

A 15" by 1" port would need to be about 5" long for a 35hz Fb.

A 13" by 2" port would need to be about 8-7/8" long for a 35hz Fb.

A 15" by 2" port would need to be about 10-5/8" long for a 35hz Fb

It helps to have a good software port model, the long narrow aspect ratio requires a different length than the same area in a square or round port. It also helps to start out a bit long, and trim it shorter to reach your target Fb.

Marvel:

 

It helps to have a good software port model, the long narrow aspect ratio requires a different length than the same area in a square or round port. It also helps to start out a bit long, and trim it shorter to reach your target Fb.

Right, start with a long port and adjust accordingly. You never know how the room itself will effect one's target Fb.

Cool...one sheet could do it...

Marvel:

Testing, they came up with the following figures (I think with no eq filter between the pre and power amp):

Do you know the details of this testing? I went ahead and plotted the numbers on a graph, which seems to be showing that the port is essentially doing nothing compared to simply making the rear volume larger...the measurement also looks to be swamped by the modal distribution of the room the speakers are in.

Does anyone have accurate numbers for the area expansion rate on the lascala? I would like to dump them into hornresp and model what the port should theoretically be contributing to the system (and then compare to the measurements provided). I should have whipped together a graph way earlier on, because it's showing mostly what I would expect from the system...

DrWho:

Marvel:

Testing, they came up with the following figures (I think with no eq filter between the pre and power amp):

Do you know the details of this testing? I went ahead and plotted the numbers on a graph, which seems to be showing that the port is essentially doing nothing compared to simply making the rear volume larger...the measurement also looks to be swamped by the modal distribution of the room the speakers are in.

I have no idea how they did the testing, but a 2db gain at 31hz is not bad, on top of the 5db from the box volume change from stock to Pete's 4.5 cu ft. Dennis is figuring 4.8, probably not enough of a difference to change the results much.

Does anyone have accurate numbers for the area expansion rate on the lascala? I would like to dump them into hornresp and model what the port should theoretically be contributing to the system (and then compare to the measurements provided). I should have whipped together a graph way earlier on, because it's showing mostly what I would expect from the system...

Dennis posted this before: The taper rate of a LaScala is 100hz,The mouth area is good for 125hz. Below this it is a big woofer in a small sealed box. If we plug the T/S parameters for the K33E into a box program we will see that the Qtc=.85, the Fc=82.5hz, and the F3=70.9hz. If we close in the back of the high frequency cabinet and open the woofer rear chamber up into this volume and fill with fiberglass we now have Qtc=.577, Fc=58.2hz, F3=73.6hz .

I think Dennis has said that under 125hz, you only need to think of it as a sealed or ported box. The horn has little to no effect. He did post about room gains when using the filter ahead of the amp:

Boost is applied ahead of the amplifier and is usually Q=2 at Fb.

    Fb is sort of experimental with the smaller magnet woofer liking the shorter ports.

    Part of this has to do with room modes. The small sized bass horn does nothing below about 100hz, but floor to ceiling gives us boost in the 70hz~75hz region (7-1/2'~8'), and wall to wall (short wall) in the 47hz~51hz region (11'~12'). An Fb around 31hz~35hz seems to work best with these room sizes and speakers. If you have an actual wall for the third wall (most newer homes don't, they have a large opening into a 'dining' area) and it is around 16'~20', you may have to lower the Q of the boost. It should be pointed out that most recordings are rolled off in this area and a little extra boost is nice.

 "the measurement also looks to be swamped by the modal distribution of the room the speakers are in."

 It looks like there is a room mode (null) at 40hz.

"which seems to be showing that the port is essentially doing nothing compared to simply making the rear volume larger..."

Simply making the box larger gains at 31hz, but you lose 3dB at 50, 63, and 80hz. The vented (without EQ) has about 3dB more output than stock at those same frequencies. While the vented has only 2dB more output than large sealed at 31hz, it's 7dB more than stock. When EQ is applied things change.  EQ applied at 31hz will cause distortion and increased cone motion on the sealed system, on the vented system the EQ is at Fb and thus does not eat up cone motion or increase distortion. It also protects the system from excessive infra-sonic inputs. The Q=2 at around 31hz will bring up 31hz by about 6dB, and the 50hz by about 3dB, little change above there. The response with EQ fits in a 6dB window (as measured in the example here) vs 19dB for stock (ignoring the 40hz null).

Fletcher/Munson contours of equal loudness are 10dB apart midband, but only 3dB apart at 31hz in the 90dB region (where most people listen). That means the un-EQ'd response difference at 31hz is going to sound much, much louder.

The original concept was based on the computer model for the Eminence 15711 (a custom-made driver based on the K-43) as a 6th order equalized system. The lighter magnet K33 is a reasonable 'fit' with some ripple in its response (±1dB or so in the model), but sounds a bit 'heavier' than the K43. The concensus is that the 7" ports sound better, they model within 1dB. When measurements are that close I always go with what sounds better, all but one pair I have supervised are now 7" (the owner of the lone 10" pair likes the 'heavier' sound on his RAP music).

"" Do you know the details of this testing? I went ahead and plotted the numbers on a graph, which seems to be showing that the port is essentially doing nothing compared to simply making the rear volume larger..""

Interesting....but consistent with reports from folks who enclosed the xover area and used no ports.

The plot also looks to be showing the peaks of standing waves at 30 and 60Hz. If the microphone moved in the room or the speaker moved (which at least one of them moved 10"), then there is no verifiable change in the frequency response. In other words, the measuring conditions inside the room can easily be responsible for most of the differences shown in the frequency response.

That's not to say that there is no difference from the mod - just that the measurements provided so far have been unsubstantial to draw good conclusions. I think the only thing apparent to me is that the mod requires the peaking highpass before it is worth the effort.

Is there anybody nearby that has done this mod? I would love to run some ground plane measurements comparing the before and after. Of course I would also like to hear it too