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Old 15th July 2004, 01:51 PM   #1
seewot is offline seewot  United Kingdom
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Default help.. cabinet volume, shape, aghh!

heyo

I'm currently at the design stage, building my own hi-fi speakers. no problem with electronics side (crossover, amps and drivers) however it tends to get very confusing designing the cabinets.

I plan to have an 8" base/mid driver and a tweeter in each box (ported cabinets), and have tryed a few of the programs on the net for calculating volume of the box. With the TS parameters given with the driver i am getting volumes of 97ish litres, which is huuge!

I want to make funky shaped boxes, which will be way too big if they are to have 97 litres inside, and am wandering how important and how accurate the volume inside must be, and if there is anything i can do to enable me to have smaller box??

here are the TS parameters given with the driver:
Impedance: 8 Ohms
Sd: 216 Cu Cms
Rdc: 6.9 Ohm
Vas: 61 Litre
Le@1kHz: 0.86mH
Mms: 19g
Fs: 38Hz
Cms: 0.94 mm/N
Qms: 5.5
Rms: 0.82 kg/s
Qes: 0.5
Bl: 7.9 Tm
Qts: 0.46
Xmax: 4mm

any suggestions or information on the theory behind box size and shape would be very helpfull

thanks.. charlie
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Old 15th July 2004, 02:51 PM   #2
4real is offline 4real  Netherlands
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In deed, this driver need 94 Liters in a standard BR alignment. You could about 60 Liters and tune a little higher (around 33 Hz), and still get a nice curve and an exceptable group delay. If that is still to large, look for another woofer!

What woofer are you planing to use? And what tweeter? Since you are going to use a 8" woofer, you schould cross as low as possibel (if possible below 2 Khz), because the large woofer wil start beaming at higher frequencies! The tweeter should have a low fs (I would recommend below 1 Khz) to be usable.

Ow, I forgot about the shape... Well, there are no real rules for that. Basicly any shape is possible. Of cource the speakers need to be lined up correctly on the baffle . Every change of shape of the baffle, or movent of the speakers has in impact on the response of the speakers. So when desiging the crossovers, first measure the units mounted on the speaker, and use that as a reference for designing the crossover. Volume is the most important. A few liters off is not so terrible. To mutch can be filled with extra wood, to little can be compensated with more stuffing.
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Old 15th July 2004, 03:09 PM   #3
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97L sounds much too big for such a driver! You should be able to get 25 - 50L for a good midbass driver. The 8" vifa P21 will get down to 40 Hz in a 50L box.

What driver are you thinking of using?
What program did you use?


However, as suggested, it is harder with an 8" driver crossover wise as you generally need to cross at 2k to avoid over stresssing the tweeter. Could you consider an MTM with 6.5" midbass drivers? That would typically need to be about 40L and you can cross over higher, which gives your more choice with tweeters. This is not to say you can't pull off a 2 way TM quite well, in fact it will be cheaper, but limits your choices more. Also you will have a wider cabinet, I'm not sure if that's what you want.

What kind of shape of box are you looking at making?
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Old 15th July 2004, 03:18 PM   #4
4real is offline 4real  Netherlands
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Quote:
Originally posted by paulspencer
97L sounds much too big for such a driver! You should be able to get 25 - 50L for a good midbass driver. The 8" vifa P21 will get down to 40 Hz in a 50L box.

What driver are you thinking of using?
What program did you use?
Well, his measurements were corect, I checked with winISD. Strang woofer...

Quote:
However, as suggested, it is harder with an 8" driver crossover wise as you generally need to cross at 2k to avoid over stresssing the tweeter. Could you consider an MTM with 6.5" midbass drivers? That would typically need to be about 40L and you can cross over higher, which gives your more choice with tweeters. This is not to say you can't pull off a 2 way TM quite well, in fact it will be cheaper, but limits your choices more. Also you will have a wider cabinet, I'm not sure if that's what you want.
Well, to make a decent MTM, without lobbing, you still need a low crossover when using a MTM with 6.5" midbass units. You still need a crossover between 1.6 and 2.3 Khz (at max) to make a good MTM (distance from tweeter to woofer needs to be at max 1x or better 2/3x the wavelength of the crossover frequency, to prevent lobbing). You could make a 2.5-way system, with overcomes the problem, but then again, a normal 2-way with 6.5" midbass woofer will do just fine.
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Old 15th July 2004, 03:40 PM   #5
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Depends on your priorities as they are lot of different trade-offs. I prefer a higher xo point to get more power handling out of the tweeter. Female vocals in particular are very strained when crossed too low.
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Old 15th July 2004, 04:19 PM   #6
4real is offline 4real  Netherlands
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Quote:
Originally posted by paulspencer
Depends on your priorities as they are lot of different trade-offs. I prefer a higher xo point to get more power handling out of the tweeter. Female vocals in particular are very strained when crossed too low.
I agree! But choosing a right tweeter can help a lot. Tradeoff's are alwyas difficult and can almost never be prevented (often a budget problem). Better a good 2-way system, that a not so good MTM system, I guess. If there is a small budgert, I'd go for a normal 2-way system with a 6.5" woofer and an 25 of 27 mm silk tweeter. If you choose a good midbass woofer, bass response can still be very good, with not to much cabinet volume.
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Old 15th July 2004, 04:43 PM   #7
GM is offline GM  United States
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Using a bit more accurate program I get 91.61L/33.6Hz Fb. Using the same tuning, 63L is about as small as I would go, but of course it will be much more rolled off. For midbass duty, 28L/31Hz Fb looks good.

Size is relative, mine are ~566L, so ~92L is a large bookshelf size to me. Anyway, making it a ML-TL, I get a ~116.6cm high cab (inside dim), so it does not have a large 'footprint' even after factoring in other drivers/bracing/vent.

Not being a very creative person, I do not know how 'funky' a tower design can be made, though it seems reasonable to me that mimicing anything of a similar proportion seems doable.

For my records, which driver is this?

GM
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Old 15th July 2004, 06:32 PM   #8
seewot is offline seewot  United Kingdom
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okk thanyou vewy much everyone.. rather a lot there to think about and understand!

The drivers are AC 8 HE 's by Alcone (from www.bkelec.com) and unfortunately i have alredy got them so different drivers are out of the question.. no going back! as reguards to tweeters i havnt really thought much and any suggestions would be very helpful (say to cross over at 2kHz)

60L sounds ok ish and tune a little higher.. jus how do i do that? and do you mean a wider cabinet as in wider? duh me i know.. but what would go wrong with tall and narrow?

not really sure what shape it will be.. was thinking about a big egg or something UFO shaped.. who knows! shall try and get some pictures when i get it all done!
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