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Old 6th August 2009, 01:12 AM   #1
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Default MTM + woofer in a car

My buddy just acquired a Porsche 356 Speedster kit car. He told me he wants to have a great sounding stereo that will be loud enough with the top down. He was looking in a Crutchfield catalog and was thinking of going the standard door mounted route but I stopped him and threw the catalog away. This car has a rather large and open firewall between the front trunk area and cockpit, its rear engine. The only thing on the firewall are the pedals and steering column on the drivers side, nothing on the passenger side.
I envisioned where feet will go and it seems like the most logical way to arrange the drivers is a vertical MTM on both left and right outside edge of the firewall, sorta near the kickpanel area, and a woofer just inside of it high enough off the floor that feet won't hit it and clear the pedals and steering column. Any necessary enclousure would be very easy as the trunk is on the other side.

Has anyone done an MTM arrangement in a vehicle either on the door or otherwise? Driver selection can some later if this doesn't have any inherent flaws I'm missing.
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Old 6th August 2009, 03:16 AM   #2
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Neat project.

I hope the car was properly equipped for fun. If not, it looks like you can help him out with that, too!

As far as I remember, MTM's gained momentum in the mid 80's in D'Appolito form. This used odd order crossovers and the MTM arrangement to control the polar response of the system.

In a car, the advantages of vertical symmetry are lost. However, there are no incompatibilities or reasons to stay away from an MTM if it fits well. MMT or TMM would be just as viable if the tweeter doesn't get cramped into a poor location.

I would suggest that if you're choosing between, say, a single 6" midrange and a pair of 4", that the 6" would be a better choice in terms of output to compete with wind and road noise. Simply put, larger will likely be better in this case.

Been a long time since I looked inside a 356. Is the firewall open to the trunk where the woofers will be located, so that they're using the trunk as an enclosure?


BTW, I almost did an MTM in the kick panels of my Alfa Spider many years ago. The front edge of the kick panel was wide enough for a 4", and there looked to be a half a cubic foot of enclosure space behind it. Perfect.

I didn't follow through because the engine already made beautiful driving music.

Maybe your friend just needs some good headers and a set of hot cams and big carbs?
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Old 6th August 2009, 03:40 AM   #3
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The "firewall" is solid and maybe not technically the "firewall" as the engine is in the back. The fuel tank is in the trunk but I'm sure it can all be positioned to work.
I was just amazed at how much open space there was as the dash does not extend down very far like most cars. I sat in one of the temporary seats and put my feet where they would normally go which left alot of area, plenty of room for multiple drivers and large ones if necessary.
An MTM is not designed to work in this type of environment but it would fit perfectly so it came to mind. We have time to think about it as he has other assembly to accomplish. He is reluctant to cut holes in the door panels so he was happy to hear my idea.

He's gonna run a stock bug engine which is pretty mild and wimpy. I hope it has enough horsepower to turn the alternator and power an amp. Otherwise I'll suggest he get a battery powered boom box so as to not kill the engine.
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Old 6th August 2009, 04:20 AM   #4
Glowbug is offline Glowbug  United States
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Open firewall? Need for lots of undistorted volume? This car sounds like it's begging for horns up front

http://www.audiojunkies.com/blog/107...horns-for-2008

Either ID or USD, Illusion Audio, or Veritas.

They'd need to be actively bi-amped with a fair amount of EQing, but they sound like they'd be perfect for that kind of install.

Rear engine would also lend itself well to a front-mounted IB sub
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Old 6th August 2009, 04:45 AM   #5
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Well, with a beetle engine it'll still be true to 356 form and fun if he tosses it around! I hope it's a 1600...

I've never looked closely at a kit, does it use a stock VW pan?

I can only remember working on one 356, a very nice coupe, probably 20 years ago for an upholstery shop that specialized in European cars. There was no problem with tight working quarters.

Stay with a moderate power amp and there shouldn't be any alternator issues. There's really nothing else for it to power.

BTW... 901's? Aren't they a bit scarce?
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Old 6th August 2009, 08:44 AM   #6
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IMO there is no need for double set of mids. Just get a great pair 6'' mids that can take 150W Rms each (look at Morel raw drivers) and build a sealed or aperiodic enclosures in kick panels. You can place 1'' dome tweeters in kick panels crossed at about 2 khz and another set of tweets in solar panels crossed at 3-5khz to draw the soundstage up. Use space in the the trunk to make sealed fiberglass enclosure for the sub facing the cabin through firewall. This should sound really good
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Old 6th August 2009, 02:05 PM   #7
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So is it better to have the drivers facing inwards or facing backwards towards the occupants? I realize kick panel mounting usually involves angling the drivers for proper imaging.

I'd like to use full range drivers and cross to a tweeter much higher out of the telephone band like say 8khz or so. The doubling up of a mid/fullranger will overcome inefficiency. A 6" is wider than what I had in mind but I'll take another look and see. A single centered sub is also a possibility as long as the main drivers can cross low enough.

Horns would fit too as they are nice and thin, I'll have to give that some thought as well. Surely its possible to build my own horns to fit if the orientation of the driver is not ideal on the premade items. I'd also prefer one that could cross to a woofer or subwoofer, good down to say 100hz.

The engine is a 1600 dual port and he may even go with a dual carb set-up! Woo Hoo, what is that now 70hp instead of 60hp? It should be able to power an amp no problem though and is so light it will still be plenty quick. This kit is on its own custom tube chassis, not a Beetle pan. Which 901's were you referring to as being a bit scarce?
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Old 6th August 2009, 04:27 PM   #8
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Personally, I'd prefer a mild inward cant to lessen off-axis issues.

The automotive horns have a lower limit in the 800Hz-2kHz range depending on the model and crossover slope used. You'd still need a mid. A 100Hz horn would likely be prohibitvely large for a car.

70HP? That would make it a Speedster! On a tube chassis at that, very cool.

I was referring to the first run of 901's that were actually labeled as 901, not 911. Ohhhh, silly me. All this Porsche talk has me confused... Bose, now I get it.
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Old 7th August 2009, 03:52 AM   #9
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I found this site trying to figure out how to make a passive crossover for a pair of Bose901 I purchased rather cheaply, actually a trade for an irrigation pump. I'm now addicted to all of the info and builds the members here do and all of the other discussions that I only half way understand.
Yes, the 911 was originally supposed to be designated the 901 but Peugeot beat Porsche to it. Ironic that the 911 is still such a fantastic car where as Peugeot is unheard of except for Rally racing. Such is life.
This project is gonna be a bit slow going as he has 3 small kids, a wife and a job that get in the way so as long as I can keep him from buying any off the shelf crap and convince him to do it my way it will work out ok. I need to get a more accurate picture of what space he will have available once the pedals are in so I know what we are working with.
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Old 10th September 2009, 07:29 PM   #10
FE3T is offline FE3T  Norway
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Click the image to open in full size.

Macrom SE-TV28 tweeters
Macrom 58.10 Midranges
Macrom 59.17 Woofers

(Morel made components)
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