Please take the time to read the first page at the very least, as it describes the whole theory behind the build. Patrick describes the diffraction issue and its effect on soundstage realism far far better than I ever could.
As you’ll see, it will cost a little more than $2. Surprise surprise. However the results will be, as they say in the ad, priceless.
First of all, you’ll need every installer’s key tool: a decent beer. Today we’re drinking Hoegaarden, which is a very nice quality Belgian wheat beer. If you keep your costs down on the build, you too will be able to afford a better quality beer. Thanks to Crosspug for the beer.
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• Packet of 10 polystyrene balls. http://shop.ebay.com...-All-Categories
Cost $5.50 inc postage.
• Exterior PVA. I used Selleys Aquadhere, available from Bunnings. It's important to use the exterior stuff, as the normal stuff might just melt into a gluey puddle on your dash in the middle of summer. Boooo!
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o Cost $10
• 5 minute epoxy resin. Something like Araldite works well, I used the Bostick stuff cause it was cheaper. 2 packets will be plenty!
o Cost $7 per packet
• Flocking powder. I purchased mine from Carrols woodcraft supplies.
o Cost $18 delivered.
Total cost: About $40.
• Automotive body filler
• Drill and holesaw attachment
• Bamboo skewers
• Latex gloves
Determining the size of the polystyrene ball:
The equation used to work out how large the radius of the sphere needs to be is this:
required sphere radius in cm = 34029 / lowest frequency / 2 / pi
This equation uses 340.29 metres per second as the speed of sound. If you have another equation, feel free to use it, although it may not be as correct as the one above.
For my sphere, I chose 5kHz as my frequency, so:
34029 / 5000 / 2 / 3.14 = 1.08cms.
For 2khz (most people’s lowest cutoff frequency)
34029 / 2000 / 2 / 3.14 = 2.7cms.
Remember, this is for the radius, so the diameter will be……BING! Twice this, or in other words 54mm for 2kHz. In other words, your main issue will be finding a suitable sized polystyrene ball to suit your tweeter rather than worrying about the lowest diffracted frequency.
I chose a 65mm diameter ball for my Hybrid Audio L1v2’s. This was a perfect fit.
Step 1: Covering the balls with epoxy.
• Mix up a decent amount of epoxy. Make sure it’s well mixed.
• Stick 3 skewers into the ball, fairly close together. This will act as your handle so you can get crap everywhere without getting it on your fingers.
• Using a spatula or a piece of cut up plastic (I cut up the thin flexible plastic from the epoxy packaging into rectangles), cover the ball in a thin layer of resin. Make sure there are no runs. Wear latex gloves for this bit.
• Put the skewers with the ball on top into an egg carton, and leave to dry for 12-24h.
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• Sand off any runs and recoat.
• Now the ball should be fairly hard. If there are any runs, you will need to sand them down until it is nice and flat.
Step 2: Cutting the mounting hole
• Now you need to work out approximately where to cut the ball, depending on the size and shape of your tweeter. A few mm past the centre line is what I aimed for.
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• Now you need to cut the inner hole for the tweeter to sit in. I used a hole saw attachment just a bit bigger than the tweeter cut out.
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• Drill down enough for the tweeter to sit in, then scoop out the polystyrene with a blade or knife.
• Clean up the inside with a dremel sanding attachment or just don’t worry too much about it.
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• All you’re looking for at this stage is a clean fit with not too much movement.
• Once this is done, you can sand the face with a sanding block to get a perfectly flat surface.
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• Tape around the outside, and epoxy the face and inside.
• Once dry, use bog to get a nice flat face. The reason why we use epoxy first is that the bog won't stick to the polystyrene.
• Now you need to make sure that everything is sanded pretty nicely, and the whole thing looks ready to trim.
Once ready to trim, you need to work out how the tweeter will attach to either the dash or the a-pillar. Crosspug uses a ball swivel mount on his pods, which give a completely adjustable and very sturdy mount. Props to Sydmonster for the build.
I got a bit fresh with mine , and made up some adjustable mounts which go over the wooden cowl on my dash. I used a t-nut on the inside of the pod which bites into the polystyrene, and a very short bolt. I drilled a second hole for the wires. See below for pics.
Another way is to use the method described here:
which uses hollow threaded rod. This is great ‘cause you can run the tweeter wire up the inside of the rod. Nice.
Trimming can be done in any number of ways. I suck the big one at trimming, so I went with the easiest option I could find – flocking. As always, the exterior of the pod must be as perfect as possible before trimming.
• Skewer the pod again very carefully to make a handle, as it’s easy to put the skewer through the back. You could also use a Stanley knife, but make sure it’s secure.
• Mix up PVA with about 2-4 teaspoons of black laser printer toner in a plastic cup. This should turn the PVA a very dark grey. You can pinch this out of the photocopier at work by taking the toner cartridge out when no one's looking, undoing the end and emptying some into a glass jar. Be careful cause it will go everywhere. This is good stuff because it is inert and mixes into the PVA very well.
• Paint the pva onto the pod until you have a light coat. If it’s too thick, it will run and you’ll be stuffed. Make sure the PVA totally covers the pod and the face. Take your time here.
• Put the flocking powder into the sieve and, over a box lined with clean newspaper, shake the flocking powder over the pod. Make sure you use plenty of flocking and cover every surface– it can always be collected after the pod is dry and reused.
• Leave to dry for AT LEAST 24 HOURS.
• Shake off the excess flocking, collect it and put it back in the bag.
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and just cause its a cool car...
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Tips for the n00bs:
1. Don't use polyester resin to cover the polystyrene ball. It makes napalm. Waaay to 70's for what we want.
2. Don't use spraypaint on polystyrene.
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3. Don't use bog straight onto polystyrene. It sort of rips it up and makes it very hard to get a nice sphere. Plus it doesn't stick very well.
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4. Don't try and use modelling clay to make a teardrop shape. Usually it ends in fail.
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5. Don't try and vinyl a sphere. Get a pro/shop to do it or just paint or flock it.
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6. Careful when using Araldite, as it sticks stuff together. Like your stanley knife.
BUT HOW DO THEY SOUND?!!
They sound fantastic. In my original pods, the sound was very raw and "in your face", or "hot", with a large amount of sibilance and harshness at around 5-10kHz (hard to tell without measurements). A combination of the pods themselves as well as tuning (phase, rather than EQ) and the ability to change their aiming has resulted in a much more relaxed, listenable sound. For the first time ever, I actually drove for 5 hours (on the way home from nationals) without dicking about with crossover slopes, phase, eq, time alignment, levels etc etc etc etc. For those of you with a high level processor like the hxd2 or similar, this is no mean feat.
I just simply listened to the music.
Anyway, these were my results. Feel free to post up anything of relevance, including a discussion about diffraction and any improvements that can be made. My next step will be to fill in the gap between the tweeter and the pod, and paint them. When I can be bothered.
* I'd like to thank Patrick Bateman for posting this stuff up. Whilst he is not necessarily the final word on acoustics and physics of sound, he is someone who thinks about stuff, theorises, goes out and tests his hypotheses, and posts up his results. I am merely riding on coat tails here, or, as Newton said, standing on the shoulders of giants.
Edited by Matt VIP, 14 December 2009 - 10:08 AM.