Back Loaded Horn with Pro Coax Driver

[djdan]

Can you tell me your opinions about this design ?
I want to make a back loaded horn with coaxial pro speker for use at home and powered by a small class A amplifier ( 2 - 5w / channel ) .

Can you give me some help to simulate the response of this cabinet ?
I have some experience to calculate simple horn but I want to know other opinions.

Some specs :

Driver : PAUDIO BM-12CXA

Fs = 55 Hz
Qms = 8.19
Qes = 0.43
Qts = 0.41
Vas = 103.54
Xmax = 3 mm
SD = 510 cm
Re = 5.6 ohm
Z = 8 ohm
Pe = 200w
SPL = 100 dB/1w/1m
Bl = 11.59
Cms = 0.28mm/N
Mms = 29.87 g
Rms = 1,26 Kg/s

I want to use MDF - 19mm for this project .

[BillFitzmaurice]

Download McBean's Horn resp program. To model a rear-loaded horn leave the rear chamber volume and length at 0; what this will give you is the predicted response of the horn only (rear wave). The front chamber volume of the program becomes the rear chamber of the actual horn. To approximate system response you combine horn output with the manufacturers standard baffle frequency response from about 100 Hz on up. The folding scheme is up to you, but don't waste a lot of effort with complicated curves or reflectors as below 200 Hz they have little if any effect as long as you keep pathways between bends less than 1/4 wavelength long (which at 100 Hz is about 2.8 feet). In fact, no reflectors is a good idea assuming you want to maximize the low pass function of the horn.

[Geek]

Originally posted by BillFitzmaurice
......as long as you keep pathways between bends less than 1/4 wavelength long (which at 100 Hz is about 2.8 feet).

Outta curiosity, why is that?

[BillFitzmaurice]

If the wave coming down a horn section hits a bend and reflects backwards you will get a phase cancellation mode at 1/4 wavelength ( or additive mode at 1/2 wavelength). To prevent this you use an angled or curved reflector to direct the wave around the bend. But if the path section is too short for the phase shift to be of consequence then using a reflector isn't required. It's precisely the same phenomena as you get with room boundary reflections, only in this case the horn is the room.

[dwk123]

Hmmmm, interesting idea. I'm still very much learning about horns, but my quick take is that you might want to look at the BM10-CXA instead (assuming the PAudio specs are accurate) - lower Fs, higher BL and smaller Sd mean (at least I think they mean) that the horn can be rather smaller than for the 12. I'm sure someone will correct me if I'm wrong.....

You've got me thinking now, dammit. I'd been intending to try Bills AutoTuba, but now I'm thinking that a corner-loaded rear horn based on the same basic idea as the AutoTuba with either the BM10CXA or even the Eminence Beta10CX might be an even better project. Arrrgh - like I need more project ideas......

[BillFitzmaurice]

Lower fs, higher Bl and lower Qts can be desirable, but not always. You have to tailor the driver and the horn to the desired passband. Too low an fs robs efficiency, especially in rear loaded horns where there is no ability to reactance annul to compensate. Despite the availability of programs such as Hornresp the art of horn design is still that, an art, where experience is the key ingrediant.

[Geek]

Thanks Bill

[djdan]

Would be a good idea to place this B.L.H. in room corner ?

This placement will act like a bigger mouth for horn ?

[BillFitzmaurice]

Would be a good idea to place this B.L.H. in room corner

It depends on the reponse of the horn. If it is flat in half space down to 40 Hz or lower the added bass response gained from boundary loading may be excessive; on the other hand, if it is flat freestanding down to perhaps 60 Hz then boundary loading can be effectively used to take response lower. I'm in the final tweak stages of a horn loaded tower that uses boundary loading to take useable response down to 20 Hz while keeping the cabinet size at 6 cu ft; without boundary loading this wouldn't be possible. How a horn will work in a corner is primarily a function of whether it was designed to be in one or not.

This placement will act like a bigger mouth for horn

Yes.

[djdan]

Thank you Bill !

I've made some simulation and the response is very good .
Some nasty peaks apear but I hope to solve that with a good dumping on thr first horn segment .

What do you think ? If I will make a bigger compresion chamber , the overall response down to 60 Hz will be better ?