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    Posted: 28 October 2021 at 10:55am
Hello everyone.

I've got a pair of spare 8" woofers kicking around that I was unable to use in the current iteration of the system and I was toying with the idea of trying to build them a folded horn to use as a sub in my car. The system is SQ oriented so I'm looking for something that can sound accurate with excellent low end extension. I've tried to figure out whether they would be suitable for this type of loading but I'm a bit over my head and thought maybe some of you could help me out.

The T/S parameters of the drivers in question are the following:
Overall diameter: 215mm (8.5-inch)
Mounting depth: 92mm
Bolt Circle Diameter: 195mm
Mounting hole: 185mm
Pnom (without crossover) 75W (AES Standard)
Pmax (without crossover): 150W
Pmax (with recommended crossover): 250W
Frequency range: 40Hz – 2,000 Hz
Sensitivity: 87dB (2.83V/1m)
Mms: 31.5g
Cms: 454 um/N
BL: 5.99 T*m
Voice coil diameter: 35.55mm (1.4-inch)
Impedance: 4 Ohm
DC Resistance: 3.4 Ohm
Fs: 40Hz (free air)
Qms: 7.35
Qes: 0.693
Qts: 0.765
Xmax: 8.5mm (one way)
Vas: 26L
Sd: 0.02 m2

I'm afraid the Qts might be on the high side for horn loading because the speakers are designed primarily to work IB, but any feedback from someone more versed in the matter would be greatly appreciated.

Thanks in advance,
Stefano


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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 28 October 2021 at 1:54pm
Front-loaded horns aren't a good choice for car audio IMO. Likely to be louder outside the car than inside!

Tapped horns work better - however, no car audio driver is designed for horn loading really; if sound quality is your goal, you're likely to get lower distortion from a simple, well braced sealed box. Sure the horn might have more output, but at the expense of lower bandwidth and higher distortion.

The last time I tried a car audio driver (a Vibe 12") in a tapped horn (an MTH30) it did not go well! It played three notes, farted and died. On a 250 watt amp channel. So no good at all.

I had better success using a Peerless 10" hi-fi sub driver in a longer tapped horn. That sounded nice in the car, and could take enough power to rattle the windows without blowing. However, it is very heavy, was complicated to build and took up half the boot, so now it's in the lockup...
Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dodrive Quote  Post ReplyReply Direct Link To This Post Posted: 28 October 2021 at 2:05pm
Thanks for your input mate.

I've always had sealed subs in my car setups but I've already bought these speakers and liked the idea of doing something different with them, if possible. I really liked the idea of the greater efficiency of horns since I'd rather not have to purchase a dedicated sub amp too. My plan was to use 2 bridged channels off a 4 channel amp to power this sub (about 350W RMS into a 4 ohm load), so relatively little power to play with...
My fallback plan is to buy a new subwoofer and use it in a sealed box if this route just doesn't make any sense, just wanted to explore a less common solution before spending any significant amount of money... 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 28 October 2021 at 8:19pm
Horns are most effective in open spaces because of the way they provide a better coupling of the driver to the air between you and the speaker.  This isn't an issue in a car because they are enclosed spaces so slightly different rules apply.

You could try a 4th order bandpass cab but there are some good reasons why car audio uses sealed cabs.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 28 October 2021 at 8:38pm
a well designed car sub uses a sealed box with the box roll-off being exactly matched by the cabin gain of the vehicle. gives you good transient response and goes down to 20Hz or until your amp gives up. if you want to try something a bit different do a dual clamshell sealed box so that mechanical box vibration is minimised.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bitSmasher Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2021 at 1:25am
Low BL, high QTS, I'd guess those speakers are designed to flap about and wouldn't have the strength or control required in any bass horn
https://www.instagram.com/batteryacidsoundsystem/
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dodrive Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2021 at 7:41am
Thanks a lot for all the feedback.

At this point I guess I'm going to start looking for a dedicated sub/amo combo and I'll keep these speakers for a potential future project where they can be put to use the way they were designed to perform.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2021 at 12:49pm
Originally posted by studio45 studio45 wrote:

Front-loaded horns aren't a good choice for car audio IMO. Likely to be louder outside the car than inside!


Why would this be the case? I guess you're talking about including the cabin of the car as an extension of the horn geometry, but that still doesn't make sense. A horn is louder between the mouth and throat than it is once radiating into a given space. Inverse distance law still applies...

Originally posted by ceharden ceharden wrote:

Horns are most effective in open spaces because of the way they provide a better coupling of the driver to the air between you and the speaker.  This isn't an issue in a car because they are enclosed spaces so slightly different rules apply.


Does the car's cabin not have air inside it?

Assuming it's a sealed space (windows and vents closed), the characteristic impedance of air isn't going to be different. In fact, the biggest issue is that it's an acoustically small space, so the entire bass region is dominated by the modal response of the volume.

Unsurprisingly, car acoustics is a field of heavy interest and research. There's a lot of cash in making them sound good, and the market is many times bigger than pro audio. Near-field acoustic holography is super cool, and that's based on acoustic impedance measurements. This means the behaviour of the medium in a car is generally well understood.

Here's some plots using BEM in a COMSOL Acoustics example model for low frequencies:


You can see that standing waves exist, as a function of the car's shape and size.

Here's an example of finding the first mode, again a function of the cabin that will affect any sub:


There are a bunch of reflecting and absorbing surfaces in close proximity, sure, but if you know even the rough arrangement of sound hard boundaries then you can design a horn's geometry to consider the mirror image sources and potential reflections that may occur at the 'new mouth' or discontinuity at the transition from cabinet to cabin.

The rest of the COMSOL presentation PowerPoint is here:
https://www.aes.org/conferences/2017/automotive/downloads/AES_Modeling_Car_Cabin_Acoustics_2017.pdf

ALTI also hosted some cool webinars on this process recently, which might be free to watch if anyone is interested.

Sure, if you shove a horn designed for free-field or even simple half-space conditions into the boot it might not work as expected.

You don't even need COMSOL similar to do that. The free version of Akabak would work, even with a lumped element model of a base horn coupled to a flat plate as a radiator at the boot. You can even play with placement and size. Just need to be a bit handy with SketchUp to draw a basic shape of the car cabin at roughly the right volume. Leave out the seats and whatnot.

Or, just stick to lumped elements and approximate the car volume as a conical expansion at the end of a given horn.

It's just a case of designing the right thing rather than a fundamental "x doesn't work". Frankly, efficiency in car audio is an admirable goal, especially if you're considering going electric where every watt counts towards your mileage...


If you own the drivers already, then it won’t hurt to throw together a Decware Wicked One out of scrap.
https://www.decware.com/newsite/whorn.html

Edit: looks like there’s plans on SketchUp’s 3D Warehouse for them, saving you twenty bucks…

https://3dwarehouse.sketchup.com/model/823f0b42679a7c13f6045e4ebc9df52f/Decware-WO32?hl=en

https://3dwarehouse.sketchup.com/model/30d8f0a1b6678993d9dfbc520b0d9752/Decware-Wicked-One?hl=en

It’s a very forgiving design (technically not purely a horn), aimed at car use, easily scaled, and makes a lot of noise. You might even have fun doing it.

Edited by toastyghost - 29 October 2021 at 1:05pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dodrive Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2021 at 2:17pm
Thanks a lot toastyghost.

I've had a quick look and it seems like the WO32 version might be more appropriate for my requirements. I'm looking through their forum to figure out if my drivers would be suitable for this type of loading but haven't had much luck yet TBH... It seems like they should have low Qts and Fs (around 30Hz I think I read), and the drivers in question have neither... I'll do a bit more research before trying to figure out how to actually scale the design down from 2x10" drivers to 2x8" drivers and check that the size is actually compatible with my car. lol

I've also recovered another pair of 6.5" woofers I had in the basement that I'll try and dig up some specs for to see if those might be better suited for this use case.

I'm really liking the idea of trying something different making the most of equipment I already own rather than going out and buying a new sub to stick in a sealed box, which would be a tried and tested solution to reach my objective.

Unfortunately I have no woodworking skills, tools or even space (I live in a city and have a garage that has just enough space for the car and a few bicycles), so I'd have to pay someone to build it for me, hence doing as much research upfront as possible to check that there's a decent possibility of it actually working well in my application.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 29 October 2021 at 9:07pm
You put a car in your garage? There's your problem mate😆
Keep pushing on, things are gonna get better.
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