Wave tracing and other experiments

LittleCar_w/12s

note: I use "driver" to refer to the speaker/woofer, etc

I have for some time been looking into building a custom enclosure style for my car that has the drivers inside the enclosure and the sound output focused through a large opening.

The purpose of this experiment is to try and focus most of the sound through an opening to the cabin of the car while at the same time directing it forward. The driver will have a sealed enclosure behind it and a sort of focussing element in front.

Thinking about this I have proposed it would have a resonant frequency (not the desired effect) aligned to the distance from the drivers mean surface to the lip of the opening. This is essentially a bandpass box and will be used for frequencies 70 or 80 Hz and lower. The idea is to get it to perform close to having the driver directly facing the cabin without eating up the sound qulity and having the drivers and their sealed enclosure half hidden in the trunk area.

What I need is ideas on how to do wave-tracing by hand or a program to figure this out. I'd rather use the math myself. Given the obscure design of this enclosure, I havent found any program that can handle even the idea. I can use vectors to find the appropriate angles and then find the distance of the sound's path inside for the resonant frequency part, but this does not tell me what the enclosure will do to the shapeing of the sound.

Thanks for any help,
Jerry

LittleCar_w/12s

Ok, I have done the vector tracing and found the folowing:

the driver is 12" in diameter, and the opening is about 4" in the small dimension. to squeeze the sound I have two options:
1- One reflective surface must be concave to focus the sound through the opening -- this produces a moderate angled vertical fanning of the sound through the openning.
2- the first surface must be concave and the second reflective surface must be convex -- this is very efficient and will let the sound go straight through the openning
3- A series of flat reflective surfaces are placed for the second reflector in the shape of the concave -- this affords all of the sound to go straight through the opeinning but has some loss due to imperfection of cuts.

So my two options seem to be 1 or 3, as 2 is a little too much work for now.

Therefore my question now becomes:
Is it better to have the sound fanned vertically into the vehicle cabin from the rear or to have it fire straight forward? The first option removes any cabin resonance, which is ok for me as I'm an SQ person but it may reduce the concentration too far; and the third option will essentiall focus the sound too much creating zone of loudness and extreme cabin resonance.

#1 seems to hover aoround a 60* fanout and #3 has 0* fanout.
This fires forward from the rear of the vehicle.

Ideas?

neomagus00

if you're tuning exclusively for the driver you can simply eq or tune away those resonances, but that leaves your passenger mildly screwed. or, you can lose a bit of the original idea to gain uniformity; that's what i'd do.

if that makes sense, i think i kind of get what you're trying to do... do you have a drawing or picture that might clarify the situation, it sounds interesting?

LittleCar_w/12s

I have my vector sketches.
They're not perfect or measured yet or even close to what I'm sure it'll end up as.

The closest thing I can think of is think of a Cassegrain telescope with the eyepeice on the side. the subwoofer is at the objective end. The hard part is there's no wasy way to focus sound using a lens, so I have to construct one of these three reflector systems.

I'll sketch it into the PC. I am also going to do a mockup of it out of some scrap lumber for a single sub first just to get the feel of the output. It's complicated and the final design is going to be monstrous in weight.

Another reason I want to know when the Polk HQ tour is goin on is so I can have this thing up and going. Hopefully the final design will have windows into how it works from the back.

-Jerry

MacLeod

I would just like to say that this thread is waaaay above my head.

neomagus00

if i understand correctly, he's going to make an enclosure system that focuses the sound from the sub into the cabin, rather than just letting it filter through. of the 3 ways he listed, which i believe are 3 different ways of guiding the sound out of the box, he can do 2, and there are tradeoffs between them. resonance and focused sound? or a more dispersed sound and no resonance?

i think that's an appropriate summary.

MacLeod

Isnt that what a ported box would do assuming you stuck it in the trunk and routed the port thru to the cabin, or is this different?

neomagus00

no, this is different in that the sub is in a sealed box and the entirety of the output is guided using carefully-shaped panels. vaguely like the horns on HLCD tweeters.

LittleCar_w/12s

I guess it's somewhat similar to a compression-hone enclosure used for tweets, the only diference is the sound isn't going straight forward, but bouncing off two surfaces before entering the cabin.

Neo, I am going to see about getting that mockup made, it might not be as wide as the final design, but it'll allow me to fine-tune the angles. Maybe I can figure a way out to swap the two reflectors and decide real-time. It should be fun to build, as I literally have to build the box around the sub.

The final product is going to be a feat of marvelous engineering in the field of bracing and reinforcing the reflective surfaces. The first reflector is a large, wide, flat surface. I may end up using something like 1" MDF with a sheet of harwood plywood for back-bracing. That's why the mock-up is important: the real thing will cost me some moola.

Anywho, I'll keep yall updated.

LittleCar_w/12s

OOh, sory for DP, but I was wondering if anyone can rec a good fre-ware cad prog? I'm not going for machinist caliber, but just rudamentary 3d-ing.

neomagus00

i don't know of any free 3d stuff, every cad program i know of is pretty expensive. i'd offer to let you borrow mine, but sending two CD images could get a little annoying.

neomagus00

how's this coming along? any progress to report?

LittleCar_w/12s

Not as of yet, it's been a busy week of work. That and I still need to rip some sort of cad program or dl one. Sucks to be broke. I could start hacking something up, but free material runs out quick.

Of course I could quit being lazy and find the drafting paper...

neomagus00

just on a whim, i tried www.freecad.com ... and it worked! take a look.

LittleCar_w/12s

Kick ****!!

Thanks Neo.