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Old 22nd September 2012, 08:23 AM   #1
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Default cabinet box internal space

I'm a good while away from building speakers as I've only really just become interested in the whole diy speaker idea. However, in my research so far, which is limited i'm not seeing certain things explained, i'm sure when loudspeaker cookbook arrives these things will be answered.

I see a lot of people build square cabinets with specific litre volumes and dimensions, then followed by what looks like dodgy internal Swiss cheese bracing and then foam stuck here and there to dampen. Why go to all the trouble of a calculated cabinet size but then just kind of throw the rest together? What exactly is happening inside the cabinet that requires specific litre dimensions and does the foam placement really matter?

wouldn't a messy internal structure be counter productive, what about hanging cables, do these interfere with soundwaves?

Another question, some tweeters have zero cabinet space while others have an entire box... I'm just a little confused. Can anyone help clarify?

cheers.
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Old 23rd September 2012, 12:58 AM   #2
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So I just made a 3 ply cardboard cabinet and lined it with chunky styrophome. It's twice the size of the original sub cabinet and it sounds about the same even with very dodge non air tight construction.


Did I learn anything? No not really, however is there a reference somewhere to find out the optimal litre volume ? What about the height of the cabinet being the largest dimension, is there any advantage to a larger height ratio than depth? Or vise versa.
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Old 23rd September 2012, 02:19 AM   #3
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1. Why do you assume people do not account for the volume taken up by the internal bracing when they design a cabinet? The difficult part is guessing how much volume is taken up by the driver its self. It's rarely listed in the specs. Also, don't forget to subtract the volume taken up by the port, if there is one.
2. Many people believe there is a "golden ratio" for rectangular cabinets, which evenly distributes the frequency of standing waves. Look here: Enclosure Calculator -Closed Back - 1 speaker
3. The purpose of lining the walls with foam is to dampen higher-frequencies from the woofer. It's typically done in ported cabinets to minimize sound that gets through the port. It has little or no effect on cabinet volume. Some people like to play around with placement of the foam.
4. I don't know what you mean by "messy internal structure". I doubt that a wire will have any effect on sound.
5. Most tweeters have the back closed. The volume behind the tweeter is built into the tweeter its self, which is not practical with woofers. If a tweeter or midrange does have an open back, it needs to be isolated from the space behind the woofer.
6. As far as your cardboard cabinet sounding much like the original cabinet: I can only assume it must have been an awful cabinet.

I'm glad you ordered the Loudspeaker Design Cookbook. It's a great book, and you'll learn a lot.
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Old 23rd September 2012, 02:24 AM   #4
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boy some can of worms you wanna open here..well a cuboid cabinet is not ideal there is such a thing as a optimum "Prism" (rectangular construct) for an enclosure this limits standing waves whithin the structure at the outset but it will still need internal damping which in general has the effect of making the same behave as if slightly larger dims but this is dependant on its structure ie density.
as for internal bracing this does subtract from the given volume and a design actually requires a theoretical diaphragm in a specific given volume so any subtractions from this must be allowed for inc the volumes displaced by the driver(s) ports and bracing but Aperiodic chambers are far more complex and I will leave this for now.
as for your venture into experimenting with cardboard if your results are inconclusive I would suggest your criteria of and for analysis is somewhat lacking.
for a basic reflex design the purpose is to achieve an equal ripple impedance modulus to damp at the system resonance via use of a tuned port it is not so possible to get a load to behave linearly so we endeavour to do the best possible within these limitations.
my best suggestion is to read as much as you can find on the subject or use software which I use often as a targeting tool leaving final design to testing, measurement and "realworld" analysis to fine tune any design. there are many out there for this and have used them for many proffessional product installs here in the U.K. an easy quick one is the Harristech "Bassbox" as it is pretty "point n click" it can get you fairly close to a result but not a finished design. I hope you find this useful and points you in the right direction
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