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Old 25th February 2012, 06:21 AM   #1
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Default Dallas II modifications for an Alpair 10?

Hi, I'm new around these parts but have done a chunk or two of reading here - you guys really know your stuff.

I'm in the process of designing a set of speakers for my car audio revamp, and I'd like a little input. So far I think I've narrowed down the driver choice to a set of Alpair 10.2's. I'll probably end up adding a CSS subwoofer later - but that's the only other driver I'll be running in the room.

As for the enclosure I think that the Dallas II (EDIT: I now realize it'd be just a small BLH with a front-facing mouth, sort of like the Dallas II) would be a great fit for the space I have as the mount location for the speakers are right at the point where the windshield meets the dash (as seen in the pic) Click the image to open in full size.
The opening of the horns would be where the driver's and passenger's feet sit.

So my questions are 1. What's wrong with this design plan? 2. What sort of math do I need to understand or where can I read more on the design of horn loaded enclosures like this so I can make adjustments without screwing up fundamental acoustics? 3. Are there other options that would make more sense?

Last edited by stochastic; 27th February 2012 at 07:21 AM.
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Old 25th February 2012, 08:11 AM   #2
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1/ Dallas II was very carefully designed & optimised by Ron Clark for the Fostex FE206E, in-room conditions, and with the physical configuration and shape of the enclosure taken into account.

2/ You'd need a new design. Ron's boxes were designed & modelled in his own custom software on workstations generally used for designing the compressor stages etc. of jet turbines, industrial gas lines etc., so unless you're up on this, you'll need to gen. up on horn design. Here's a thread with a variety of texts suggested by GM & myself: Physical explanation of how horn speakers work

3/ Well, personally I wouldn't be using BLHs in a car; you'd be better off IMO with simpler internals (sealed & supported with dedicated woofers).
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Old 25th February 2012, 08:44 AM   #3
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Thanks, that points me back on a better understanding of the designs I was looking at. I wasn't really thinking the exact dimensions of the Dallas II, but rather the pattern of the internals - I guess that just means a BLH (is that bass loaded horn)?

Quote:
Originally Posted by Scottmoose View Post
3/ Well, personally I wouldn't be using BLHs in a car; you'd be better off IMO with simpler internals (sealed & supported with dedicated woofers).
Can you explain why?
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Old 25th February 2012, 09:19 AM   #4
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Correct.

Several reasons. Generally speaking, BLHs require a certain amount of distance to allow the two radiation sources to become reasonably cohesive. Then, the A10.2 is unlikely to need any help from a BLH loading in a car environment in any case (recall that a back-horn, like any rear-load, only operates over a relatively narrow BW). If you give it a low enough upper corner frequency (Fhm) & high order roll off, then you'd be more likely to get away with it, but then it begs the question: why? What special advantages would you be hoping to obtain, that you couldn't achieve with a dedicated bass unit running in that BW (which would almost certainly provide a superior quality than the 10.2 with a horn could under these particular conditions)?

Granted, the challenge of design & construction would be interesting, but I suspect in this case, it would be making life considerably harder than it needs to be, for dubious benefit (& IMO likely inferior performance).

Last edited by Scottmoose; 25th February 2012 at 09:23 AM.
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Old 25th February 2012, 04:50 PM   #5
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to add to Scott's response, most home audio drivers will not withstand the elements of car installation for very long. Vibration, dirt and humidity would destroy or severely "discolor" the sound from these very quickly i think. most higher end home drivers are a bit "delicate" for automotive use.

most of the enclosures we focus on here really rely on little to no background noise to be audible in the bass regions.
if you are really insistent about horns...there are a few folks out there that make them for autosound use.

these cats pioneered them SpeakerWorks/USD Audio Products
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Old 25th February 2012, 05:00 PM   #6
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As far as car audio goes if your looking for pathlenght equalization in pursuit of good soundstage and imaging a kick panel install is your best option. That puts the nearside driver as close to an equal distance from you as the far side driver, and both as far from your ear as you can get without using a HLCD.

At that point a sealed enclosure and steep highpass filter, 24 or 48 db LW filter, are your best friend. Set the cutoff to a point where the sub stops sounding like its behind you or as low as is possible at reference level without doubling in the midbass.
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Old 26th February 2012, 09:57 AM   #7
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My question is "where are you going to find the room to fit a BLH"? Rip the engine out and put it in the trunk? You'd alo need to pull the dash out, and figure out how to live without the steering wheel.

Modern cards are designed to use all the available space. I think you'd even have trouble finding space to mount an A10 where you have shown even with no box.

They might fit into the doors, but i suspect they wouldn't last long.

I'd be looking to a 3", but even then it could be a challenge to fit it (unless there are already speaker cavities in your dash)

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Old 27th February 2012, 06:49 AM   #8
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Thank you all for the critiques. I'm not sure of the math behind BLHs yet (though I'm reading up on it) so I wasn't sure if a smaller length BLH would do the speaker justice. I also am interested in giving the 10.2's (I'm also considering the 7.3's) the best box possible and thought the natural shape of the dash's lower side could help act as a horn mouth. I don't want to use the kickpanels or the doors for speaker placement for various reasons (primarily the stage-height problem), so pods above the dash or built into the dash would be the best option - I do plan to cut sections of the dash away and re-fabricate with fiberglass, including adding some diffusers and absorbent sections to the dash.

Part of what drew me toward the BLH design was the number of people in car audio that talk high-praises for horn-loaded and wave guide setups.

Are the alpair speakers really not going to last on the dash of my car in the Vancouver climate (dave, you might be the best to answer that one)? A number of people in the diyma forums talk about using home-audio drivers in their car with great praise. Across the board car audio driver seem to be four-times the price of home-audio, so even if I have to run through a couple sets of them....


I am really doing this as a fun acoustics project and am not expecting award winning car audio here, just trying out a couple different ideas to improve the OEM muddy system so that I can enjoy my audio a little more on long drives (I've already created a drop birch ceiling between the roof and the headliner, and am in the process of putting holes in my headliner in the pattern of the RPG BAD panels).

Last edited by stochastic; 27th February 2012 at 07:08 AM. Reason: cell-phone only posted first section of my reply
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