Fixing the Labhorn?

[just a guy]

About the Labhorn

This first post is for people that don't know about the Labhorn. If you are familiar with the Labhorn you can skip to the second post.

Labhorn plans, details, pictures and a forum can be found at Live Sound: LSP Documentation Page - drawings, photos, specs and more - Pro Sound Web.

The Labhorn is a dual 12 inch front loaded horn subwoofer. It was created by inventor Tom Danley as a free gift to diy'ers 8 to 10 years ago on the old Live Audio Board (hence the name LABhorn). This was long before I got internet access so a few old forum posts that a forum member graciously forwarded to me recently are all the insight I have to relate.

Danley set out to design a bass horn that would stomp the commercial offerings available in a live audio industry friendly 45x45x22.5 inch package, meant to be used in multiples. He started by using (I assume) Leach's math to determine the optimum driver parameters to cover the intended bandwidth - approximately 30 - 120 hz. Danley also used (I assume) Leach's math to design the optimum front loaded horn (within the size limitations) to mount this optimum driver to.

After a bit of modelling trial and error, he adjusted these theoretically "perfect" driver specs and the "perfect" horn design "for maximum smoke", as he puts it, and Eminence built the driver - the original Labhorn driver - which was never available to the public. He designed the snailshell folding scheme and it wasn't long before someone built it and provided some measurements. Using this information, he switched up the driver specs a bit and the famous Labhorn Gen. II driver was put into production as a regular Eminence product. Currently available for about $160 each plus shipping in Canada and the US.

Somebody drew up plans in CAD and the diy world started building Labhorns. Lots of them. Since this design does not hit xmax until about 2000 watts (if adequately protected with a high pass filter) and the drivers are buried in small back chambers, people started reporting driver thermal failure. Shown below is a vague approximation of the "push push" loading scheme inside the Labhorn, both drivers enclosed in their separate small back chambers and firing into a central front chamber attached to the horn throat.



In addition to the thermal overload issue, people were reporting a nasty dip in response around 40 hz, as shown below. (It gets worse as the drivers heat up too.) This simulation of a single corner loaded Labhorn - or 4 Labhorns on the ground outside - is shown at xmax (2000 watts) so this representation of maximum potential spl is a bit optomistic considering the aforementioned thermal issues.



Very quickly, modified Labhorn plans began to surface. There were at least two modified versions hosted on personal websites by members of the Live Audio Board. These websites no longer exist so I have no idea what changes were proposed. Also, Wayne Parham (Pi Speakers, a commercial company) came up with modified plans and sell their 12Pi basshorn as a commercial product. I haven't seen those plans either but I gather the Pi Speakers version is much larger and incorporates a push pull driver mounting scheme to reduce the driver's inherent non linear distortion. Pi Speakers also invented an aluminum heat plug specifically for this design and although they claim it works well in both their version and the original Labhorn, I'm not really interested in such a fancy ($$$) solution to the thermal issue.

[just a guy]

Fixing the Labhorn?

My first question is this. Does the Labhorn need to be fixed? It's an immensely popular cult classic and a lot of people enjoy it the way it is. My simulations show that it can be better though, at least the simulations of my modifications look better to me.

The first issue is the frequency response, particularly the big dip at 40 hz. A big hint from the Live Audio Board posts is that the back chambers were originally supposed to be 25 liters each. It was not until the design was finalized and drawn up in CAD that it was discovered that the actual volume of each back chamber was only 21.5 liters - about 14% smaller than planned. The model below shows the same Labhorn as pictured above, same conditions, but the back chambers in this model are 25 liters each, as originally intended. The result of this is that the dip at 40 hz is now mostly gone. Not a huge change maybe. but still a visible improvement. Problem solved. (When Danley discovered the back chambers were too small he suggested adding stuffing, claiming that this method alone would gain back the lost space. I have no reason to doubt that but I don't know how to simulate it, and I'm guessing that this solution would only make the thermal issue worse. It might be better to increase the volume instead, if possible.)



Now the issue is how to increase the volume of the back chambers, especially in such a way as to not modify the CAD plans too much or else the complicated inner chamber will be impossible to build. My solution is the picture below. It probably won't gain back the entire 7 lost liters but it's a step in the right direction. Additionally, it incorporates a push pull loading to eliminate non linear driver distortion. The plans don't change much at all. One driver is simply reversed and the big black thing in the picture can be visualized as a tube joining the chambers together. I won't actually use a tube but can't draw my exact proposed modification. Now, the back chamber is not only bigger, but only one driver motor is enclosed in this new larger single sealed back chamber.



My simulations showed almost no change whatsoever in dramatically reducing the size of the shared front chamber, so sticking a driver and a connecting tube between the chambers seems to make no significant difference. My next question is whether joining the separate back chambers together will help much. I'm not familiar with airflow behavior, so I wonder if the air will travel through the whole space freely or not. I'm hoping the whole connected chamber will heat uniformly, keeping the driver cool. Will this happen or not?

[just a guy]

To recap my questions - and add a couple -

1. Does the Labhorn need to be fixed (in your opinion)?
2. Will my proposed change (connecting the 2 back chambers) serve to make a single larger chamber that will heat uniformly?
3. How much difference will these proposed modifications make?
4. Does anyone have copies of the other 2 variations on Lahorn plans modifications from the dead websites? (I believe they were called Labhorn 2 and Labhorn 3.)
5. Should I bother with the Labhorn at all? Is there another 30 hz sub I don't know of that outperforms it significantly?

Thank you to everyone who bothered to read all this.

[Andreas]

labhorn V.3 used a higher compression ratio see:
Hörner
allthough the links within the above links are dead, some information is still there.

Regards
Andreas

[Don Snyder]

I started a thread on prosoundweb to document the history of the LabHorn. While it is accurate, it could have been more diplomatic.

PSW Sound Reinforcement Forums: LAB Subwoofer => History of the LABhorn!

Also, the first 5 inches expands too quickly, then the next 5 inches contract. Fixing this would make this part of the horn much more difficult to build.

[soundhead]

Go a head modified it, make it better and also to work with other drivers out there

[Don Snyder]

1. Does the Labhorn need to be fixed (in your opinion)?

If used as intended, no "fix" is needed.

2. Will my proposed change (connecting the 2 back chambers) serve to make a single larger chamber that will heat uniformly?

Connecting the two chambers would provide little air movement. The most popular stack is shown in the attachment. With the
exception of the two aluminum covers on top of the stack, there is little possibility of convection or radiation anywhere.
Wayne Parham's "fix" (getting the heat into the aluminum covers) doesn't go far enough. How do you get the heat out of the stack?

3. How much difference will these proposed modifications make?

If you use them as they were intended, little difference.

4. Does anyone have copies of the other 2 variations on Labhorn plans modifications from the dead websites? (I believe they were
called Labhorn 2 and Labhorn 3.)

Search ProSoundWeb archives on Peter Sylvester and Brad Litz

5. Should I bother with the Labhorn at all? Is there another 30 hz sub I don't know of that outperforms it significantly?

Danley's new stuff

[just a guy]

Originally Posted by Andreas

labhorn V.3 used a higher compression ratio see:
Hörner
allthough the links within the above links are dead, some information is still there.

Regards
Andreas

Thanks for the link. It gives some basic specs on the v.3 detailing the modifications, more info than I had before.

[just a guy]

Originally Posted by soundhead

Go a head modified it, make it better and also to work with other drivers out there

My proposed modifications are very small changes, it will still be very similar to the original and driver selection will still be the same. From Danley's original Live Audio Board posts about using different drivers:


Other drivers could be used if similar in acoustic density and motor strength.

Density = Mms and area, look for a 12" with about 140-150 Gm Mms.

Motor strength = BL / Sqr root of Rdc, look for a driver where motor strength
is about 6.4

Excursion to power is a squared function, going from a 13 mm Xmax to 6,5 mm
Xmax lowers the max output -6 dB.

Greater motor strength, increases the efficiency and upper cutoff and may (or
may not) increase ripples. Greater inductance lowers the high cutoff. (actually
the R/L corner). Greater density lowers the high cutoff but may increase lf
efficiency. Less density results in more response ripples and less lf output.

[just a guy]

Originally Posted by Don Snyder

If used as intended, no "fix" is needed.

Ok, I respect that, but before moving on, allow me to post another graph to better illustrate the point. My previous FR graphs were based on info from Danley's Live Audio Board posts.

St = throat area = 80 sq ins (40 sq inches per driver)
Vb = rear chamber volume = 1530 cu in per driver (separate rear volumes for
each driver)
Vf = front chamber volume = 170 cu in per driver (volume between cone & throat)
Flare = 26.4 Hz, hyperbolic t = .5, path length = 126 inches (inc front volume).

I showed the Labhorn FR with the intended 25 liter back chambers, and I also showed a more realistic 21.5 liter back chamber. But that's without the drivers installed. Several sources (like the link in post 4) claim the back chambers are only 17 liters with the drivers installed. Here's a new graph to illustrate that point.



Technically, that dip is only 3 db deep but it looks pretty nasty. And it gets worse as the drivers get hot - this is shown with no power compression at all - and there will be plenty of power compression if you try to juice these with 2000 watts. Increasing the back chamber volumes fixes the FR hole right up.

Connecting the two chambers would provide little air movement.

I suspected that might be the case. My connecting "ports" would have a cross sectional area almost the size of a 4 inch diameter round port though, so I was hoping for good airflow through the conjoined back chambers.

Search ProSoundWeb archives on Peter Sylvester and Brad Litz

I've done that. There's plenty of links to their dead websites but not much actual information besides that.

5. Should I bother with the Labhorn at all? Is there another 30 hz sub I don't know of that outperforms it significantly?

Danley's new stuff

I'll look into it but not much of his new stuff is designed to excel between 30 - 120 hz and I'm not sure the new stuff would outperform the Labhorn significantly. I'll look into it though. Thanks.