Bass horns in progress

All sheets now cut for two bass horns - F20 (lilmike's design on AVS) and T20 (dual Rythmik tapped horn). 

T20:

F20:

What you see is low cost Chinese form ply purchased for the same price as MDF. Not recommended as a rule but my T20 will be installed out of sight - it simply needs to be light and strong. It is quite a bit lighter than MDF and needs less bracing. 

Acoustic Elegance TD18H active woofers

100L sealed boxes in progress for Acoustic Elegance TD18H active woofers. Built with 18mm marine ply. Being low cost imported ply there are obvious shortcomings - voids can be seen in the cuts, but at around $80/sheet there isn't much to complain about.

The bracing is very simple. Just one horizontal and vertical matrix brace with a driver cut out. It isn't necessary to cut lots of holes in the braces, the volume gained is minimal, the braces are weakened and more effort is required.

Cutting braces:

Clamping and gluing bracing together:

Cutting driver holes:

If one cares about the appearance of the cut out that will not be seen, a router can be used to cut the full depth. However, these days I prefer to do things quicker so I start with the router, but don't go full depth. I then cut with the jigsaw. (That isn't me above, as always I'm the one taking the photos.)

Progress so far:

This box is very simple. The outer baffle needs to be added, then the other fives sides and the box is assembled.

Two 20 Hz bass horns

1. Lilmike's F20 modified to suit dual AE 10" drivers (PB10) - a front loaded 20 Hz bass horn
2. Rythmik T20 - a dual opposed tapped horn with dual 12" drivers

Black: T20 (Rythmik) 2 x 12"

Grey: F20 (AE) 2 x 10"

Black: T20 (Rythmik) 2 x 12"

Grey: F20 (AE) 2 x 10"

T20 has more output but it is due to greater cone area and excursion. F20 needs 80w in total and a high pass at 25 Hz for max SPL. T20 can use all of the power in the plate amps (2 x 370w) and needs a high pass at 20 Hz. 

Both of these will be constructed from formply at the same time, measured and compared. 

Rythmik dual opposed tapped horn sub - sims

Black: simulated
Magenta: measured sealed sub (right corner)
Orange: measured sealed sub (rear corner)
Grey: measured sealed sub (left corner)

80L CSS SDX15 sub - part 4

Edges trimmed:

Edges were trimmed with the hand plane. You might notice the gouges where I've used the plane. Unfortunately I was in a bit of a hurry at the time and didn't check the blade. On closer inspection later I found that it had been damaged and needed grinding back. The gouges aren't a serious concern because they will soon be covered with bog anyway. 

Filling holes:

Bog filler applied over butt joins, then sanded:

Next up, I applied a roundover to the edges, 3mm on the bottom edge, 6mm to all others. 

I also cut a groove at the join. The idea here is to conceal the join so that if any cracks appear over time, they will be hidden in the groove.

Clamping the feet. Fixed with polyurethane glue, chosen for its very high strength. It is four times as strong as ordinary PVA and expands when curing to ensure a good bond. The oozing glue was wiped over the edges to seal them up.

Preparing for painting over MDF butt joins

Have you ever seen an MDF box where paint has been applied over butt joins? In virtually every case where great care isn't taken, a hairline crack appears. It is caused by the ends of the MDF soaking up paint and then expanding more than the rest of the material. How am I handling this?

Firstly, I've cut a groove at the join so that any cracks will be concealed if they appear. Secondly I have buttered up all the edges with two part bog. This will tend to avoid the edges soaking up paint. I've also applied polyurethane glue over the edges as a second layer. The plan is to spray the box with automotive paint with a satin black finish. 

80L CSS SDX15 sub - part 3

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Rythmik dual opposed tapped horn sub

I have always been happy with the sound of my Rythmik subs. I have a pair of sealed 12" servo kits that have been in 70L sealed boxes. They are equal to the best I have heard. I achieve 15 Hz extension in room without any more EQ than is included in the servo system. I have found that I can find their limits thermally with music and have managed to drive them to thermal shutdown twice (since 2006). This is a limitation inherent in nearly all subs. Low efficiency combined with amps intended for home theatre with a lighter duty cycle results in subs that can overheat with music turned up.

I'm very reluctant to let these go so my solution is to start with active woofers running down to 40 Hz. I have 18" pro woofers driven by a pro amp - no chance of running out of steam. The Rythmik subs will then simply add extension and carry a lighter thermal load. Tapped horns will make the box carry more of the load and increase the sensitivity so that instead of around 88 dB from a pair I get around 94 dB.

Here is the concept:

These are around 400L and yes that is big! Boxes will sit under the floor and be attached to it, hence the need for dual opposed mounting. The box is around 400 x 600 x 2250mm. 

1w1m simulated:

94 dB with some sacrifice of anechoic extension to achieve higher efficiency. I need only to get down to 25 Hz and room gain gets me down to 15 Hz with a sealed box. I'm aiming for sensitivity and headroom over extreme extension. 

Max SPL as limited by the amp:

Excursion:

The excursion peaks at 31 Hz (16.7mm) - xmax is 18mm. 

How to avoid over-excursion

The driver will bottom as with a vented enclosure if not carefully protected. The trick is to add a high pass. The amp includes a first order HP which won't be enough when pushed. I will add a 2nd order HP, probably around 20 Hz. I can then test with tones, using a 30 Hz sine wave and noting excursion. Then applying a 10 Hz sine wave with a check on excursion. The excursion below the cut-off should never be able to exceed that above. When that is achieved, the output will be limited only by amplifier clipping and I will be sure that xmax will not be exceeded. This means when unexpected heavy bass hits in a movie, I won't have to worry about damage to subs. I will have set up the system so that the drivers are protected from all conditions. 

Achieving headroom

I don't want to create the impression that Rythmik subs are lacking output. However, asking 12" subs to match the headroom of a high sensitivity horn system isn't realistic. To get down to 20 Hz it is necessary to move a lot of air. This could mean one giant super hero woofer with a very powerful amp, but I prefer a number of more modest subs. So for those who want SQ loud, a number of Rythmik subs is a good way to go. I may in fact build two of these, one in each corner.