20 Hz HT bass horn

[paulspencer]

I have plans to move into a new place that will allow me to (finally) build a bass horn

The room: 3.9 x 4.2m with ~2.7m ceiling

Drivers: AE Speakers AV12 x 2
Hornresp parameters for both drivers:
SD 996
BL 15
CMS 1.25E-04
RMS 13.70
MMD 456
LE 1.48
RE 1.49

I've modelled quite a few different drivers, and these drivers not only model similar to the Lab driver in the Lab horn, but for this application I get better results than I can model for any driver.

My best results are with these numbers:
compression ratio of 2:1 hence throat of 500 cm2 and a mouth of 12000 cm2 and an axial length of 6m, a rear sealed chamber of 80L (40L per driver)

Shown in the image is the design against the wall:



Lab horn shown for comparison. On the right a side elevation is shown. A fixed screen will be included as well as a concealed AV rack. Distance from mouth to side wall is ~840mm.

Hornresp model:



Excursion is shown lighter. IIRC that is 100w input power.

Hornresp input parameters:



I've modelled it as a single horn, but it will be built as two horns stacked, each with 6m length but half mouth size.

Now for some questions ...

How do I work out how close I need to be to the corner for it to be considered corner loaded?

I have tried to keep some space on the sides as I want some clearance around the mains.

Can anyone else get a better result with some changes? This is the best result I've got. I'll consider making it bigger if I get significant improvement, but so far I've found it has to be a LOT bigger to be significantly better.

Can anyone point out any weaknesses in the design?


Is it worth push pull mounting? If I do this I will have to have a throat chamber due to the magnet - does this have any acoustic penalty?

Any suggestions on how I might better utilise the corner loading situation?

Is 18mm MDF enough? Yes I know ply would be better, but this is huge and ply is going to be expensive. Hence I've opted to brace extensively, use curves where possible and make the parts connected closely to the driver ULTRA stiff (not shown in the schematic yet).

Please note: schematic drawing only - I will simplify it for construction, although I intend to keep the gentle curves for rigidity.

[Workhorse]

Hi Paul,
Which amp would you use do drive these massive Horns

regards,
K a n w a r

[lndm]

Great stuff, I like to see horns like this.

I have found success building a horn so it fits into the corner (as if the corner was an integral extension of the horn). I also assume that eighth space means just this.

[paulspencer]

My amp is overkill for this horn - Europower EP2500 which supplies 650w to each driver. With that kind of power, my sims show 135db within xmax limits, but it only really needs 100w to reach 120db at 20 Hz.

For a future upgrade I have considered later adding some other small sealed subs in locations to smooth out room response, and could use one channel of that amp. They would need to be very good to match the low distortion of the bass horn. This is the approach that Earl Geddes of Gedlee uses and recommends.

[e-side]

Is it worth push pull mounting?

I think this depends on the design. When you calculate Vb so that it'll provide the same pressure to the diaphragm as the horn (so equal pressure on both sides of the driver), then you could probably use isobaric configuration. But when you try to get Vb as small as possible to reduce excursion, then the drivers will see different (unequal) pressure, so that the isobaric configuration won't work as intended.

Is 18mm MDF enough?

When comparing 18mm birch plywood with 18mm MDF, then ply will provide more damping around the 30-90 Hz region. Anyway, 18 mm MDF is enough in my opinion (also, the horn will be very heavy when using thicker material).

[pneuma]

Originally posted by paulspencer
My amp is overkill for this horn - Europower EP2500 which supplies 650w to each driver. With that kind of power, my sims show 135db within xmax limits, but it only really needs 100w to reach 120db at 20 Hz.

Holy smokes! How much extension do you have at 135dB? Still down to 20hz? And is that accounting for room gain?

That thing could really make you say "Oops, I crapped my pants!"

P.

[cowanaudio]

G'day Paul

This is a VERY interesting project. It will likely be your last sub, if you get it right, so it's worth doing properly.

A few small changes I'd suggest, firstly, include the front chamber volume in your model. There is at least the cone volume that should be taken into account.

Secondly, the spacing to the right wall should allow the expansion to continue as smoothly as possible into the room. It appears your sub is a little far from the right wall to accomplish this. Comparing my models with real horns in real rooms suggests this is a valid approach and should also be included in your hornresponse model. Have a look at the 18" basshorn on my webpage at http://diy.cowanaudio.com I have not yet built this horn, but many others have and their measurements show it behaves at least as well as the model. I have built the 40Hz basshorn on my webpage, and in a real room it works better than modelled, despite having included the last conical section (which is not in the box) and is modelled for 1/8th space. You have modelled your horn as radiating into an eighth space environment, when it is actually more constrained than that. My guess is that you will have an f3 in the mid/high teens.

Don't forget to delay your mains by your horns path length for proper integration. Also you might want to start planning your new main speakers now

Cheers

William Cowan

[peace brainerd]

May I ask anyone interested to jump in here with their take on the pros and cons of horn loaded subwoofers? I've been dying to ask this and waiting for an appropriate thread.

There is a mind-numbing amount of subjective nonsense on the web devoted to (relatively) different approaches for home subwoofer designs. Any real measurable facts that can be brought to light here? I'd love to make one of these my next project.

...or is this a hijacking that should be taken elsewhere?
(dammit - now where's my sock-puppet?)

[lndm]

Originally posted by cowanaudio
the spacing to the right wall should allow the expansion to continue as smoothly as possible into the room. It appears your sub is a little far from the right wall to accomplish this.

I agree. e.g. A tractrix expansion turns over around itself, but an exponential continues outward/requires continuity.

Originally posted by bluebeard
May I ask anyone interested to jump in here with their take on the pros and cons of horn loaded subwoofers?

Not meaning to hijack, but quickness/effortlessness, and if done a certain way you can beat the room by joining with it (if you take the meaning).

[cowanaudio]

G'day Bluebeard

Here's my take


Pros:

Low distortion (Harmonic, IMD, compression)
Low group delay
High output
High efficiency
Small size for given output

Cons:

Large size for LF cutoff
Complex to build
Harder to design than SB/VB
Fixed delay that must be accounted for
Hard to go back to non horn bass

Cheers

William Cowan