Backstage Class: The Means To Do It Yourself
A variation of the LAB Subwoofers successfully deployed

By Dale Shirk


3-D renderings of the LAB Sub, like this one, are hosted on the forum.

A horn is rather simple. It’s just a reasonably rigid barrier to direct the sound wave.

The only trick is getting the shape right. This line, a concept at least in part attributable to one of the giants of the past, Paul Klipsch, gets to the heart of how simple yet complex a horn can be

Bass horns, when done right, have always been impressive in size and output. But they are also impressive for clean, tight sound at any level.

With that in mind, some of you might be familiar with the Live Audio Board (LAB) Subwoofer project. Tom Danley, of SPL/ServoDrive fame, agreed to shepherd the design of a type of “ultimate” bass horn for the regional touring market.

This is aimed at the “Do-It-Yourselfers”, to give them the means to build world-class bass horns that are quite affordable. In the spirit of open-source software, the design was a collaborative effort among a number of talented individuals, meeting agreed-upon goals for size, output, and low frequency extension.

END RESULT BACK

In an interesting twist, Tom engineered the system from the outside in. Rather than sorting through various drivers to find one that best fit the need, he chose to design starting from a desired end result back to the driver parameters required to meet those goals.

Tom then approached component manufacturer Eminence about building the custom drive unit. After being convinced that the market was there, Eminence came on board, and the LAB-12 driver is their contribution to the project.


This space now gets a whole lot of low-frequency courtesy of a large sub concealed entirely within the organ “window” on the left.

A single LAB Subwoofer is a 45-inches (h) by 45-inches (w) by 22.5-inches (d) folded horn box containing two 12-inch cone drivers. If flown in free space, 12 of these units are required to make one complete 29 Hz horn.

Utilizing one surface reflection, such as a ground-stacked system, six boxes are required, while used at a floor and wall corner, three boxes are needed, etc.

If a smaller number of boxes, or more precisely, a smaller total mouth size, is used, the result is a slight roll-off of the lowest frequencies as well as a ripple in the frequency response.

Because of the many angles and the requirements for a tight, sealed fit, along with the need for strength to hold up on the road, properly building these subwoofers presents a challenging project for even an experienced woodworker.

Which brings us to the point of our discussion here. It would be great if there were a simpler way to achieve the performance of a LAB Sub without the complicated woodworking. In a permanent installation, we can lose the portability requirement and can simplify the woodworking by not having to fold the horn. After all, we aren’t trying to achieve a specific shape for directivity; we just need it to expand at the specified rate.

This is exactly what I, a middle-aged, semi-skilled, lone-wolf contractor, was able to accomplish. The application was a fairly large (1,000-member) contemporary church that wanted to turn an old sanctuary/gymnasium/multipurpose room into a rockin’ youth center. The room is also used for children’s church, meetings, aerobics, and lots of other things as well.

Surviving from the room’s past life as a sanctuary are two large organ chambers. The organ is long gone, but the acoustics of the chambers are not wasted. Air handlers have been put in them to grace the room with their noise!


Looking straight down at the bottom of the horn gives a better view of the expansion shape at the mouth. After the outer wall breaks towards the corner, there is a hump built from the inner wall to keep the flare from expanding too fast.

CONCEIVING A DESIGN

The first order of business was to close off the openings, except for one: the 4-feet by 8-feet space facing the rear of the room. This became the mouth of our horn. We conceived a 6-driver bass horn, based on the LAB Sub, largely built flat against the wall.

The same front and rear driver chamber volumes were used, as was the same 29 Hz hyperbolic flare. One exception is that the mouth is extended to fit the remaining organ space opening. This is slightly larger than the combined mouth area of four LAB Subs.

The vertical expansion is basically straight line, from 6 feet at the drivers to 8 feet at the mouth. By the way, the arrangement of the drivers in no way implies line array behavior. The dimensions are too small relative to the wavelength.

Horizontal expansion starts at about 3.5 inches at the throat and loosely follows the hyperbolic curve to the 4-feet width at the mouth. As with most horn implementations, minor discrepancies, with dimensions much shorter than the wavelengths involved, cause errors only at frequencies much higher than the pass-band of the horn.

This “beast” was built entirely out of dimension lumber and ordinary 3/4-inch plywood. While purists will certainly have a field day because we didn’t use expensive imported wood with more plies than I have fingers to count, remember - this thing never gets trucked. It’s liberally braced and glued, and weight is not an issue.


Leading to the opening, the completed horn mouth, throat channels visible.

Six or eight large construction tubes of Liquid Nails adhesive and at least 10 pounds of deck screws went into this thing to seal it and keep it tight.

And, it’s built against a rigid, hard plaster wall.

Throughout the building process each panel was evaluated by pounding on it with a fist or dead-blow hammer. The goal is to have it not just be strong, but also very rigid.

If the panel moved or produced “rings” at any frequency within or below the pass band, it got more bracing: “hit and listen, hit and listen”.

The mouth end was the hardest to make rigid, but by bracing it to the opposite room wall (which is masonry), and using internal cross braces as well as thread rod, the appropriate stiffness was attained.

A BIT OF OVERKILL?

So how does it work? How about 140 dB at 1 meter? That’s not estimated or calculated, that’s measured with a Gold Line TEF system. Actually this output is massive overkill for this 60-feet by 43-feet room. Sensitivity measured at about 108 dB (1 W/1m), as expected. The low-end response falls off and ripples a bit, because the mouth is still undersized, but was equalized flat to 30 Hz very nicely.


With six drivers in a line, only the back cover is yet to be put in place.

The rest of the this system consists of two Frazier Cat 59 loudspeakers in a stereo configuration, Electro-Voice Evid 6.2 loudspeakers for near fill, QSC CX series amplification, an Allen & Heath iDR 16 by 16 digital signal processor and an A&H GL2200 mixing console.

The DSP handles 4 mic inputs in an automatic mixer configuration for simple automated speech use, stereo line and wireless mic inputs for aerobics use and the main and monitor sends of the main console.

This console is on a case/cart and can be disconnected with a multipin and stored away when not needed, and the other inputs function without it.

So would I do a project like this again? Probably, in the right application. I’d certainly use fewer drivers and less power in a room this small. Materials cost about $500, plus the drivers.

I spent almost three weeks building it, with a fair amount of that time spent mumbling, head scratching and running the numbers through the spreadsheet over and over. Not to mention trimming and shaving and test fitting and gluing and bracing.


This grille cloth hides the “secret weapon”. Also note the absorption/diffusion panels on the wall, installed throughout the room to improve the acoustic signature.

Please don’t ask for step-by-step assembly instructions! Every situation will be different.

In this room, I may well have used assembled LAB Subs, except that I could not have gotten them through the organ chamber hatch. The doorframe had to be removed just to squeak a sheet of plywood through diagonally.

In the final analysis, I want to contemplate other, simpler ways to build “a reasonably rigid barrier to direct the sound wave”. Perhaps sewer pipes could be adapted to the job.



Editor’s Note: To learn more about the LAB Sub, as well as to download plans and gather data and more information, go to the ProSoundWeb home page, www.prosoundweb.com, and click on “LAB Subwoofer Forum”.

 

Dale Shirk heads up Shirk Audio & Acoustics, a systems contracting firm based in Terre Hill, Pennsylvania. He can be reached at sales@shirkaudio.com

December 2003 Live Sound International

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