Speaker cabinet project -- need help!

[coxy1214]

I want to make a passive speaker cabinet for the final project of my audio-tech course, but am a little lacking on a lot of the necessary know-how. The cabinet will be used for vocals in places ranging from my band's small practice room to medium sized bars. I am leaning toward a two-way, assuming that a second crossover network will add a bit more difficulty. Regardless, my instructor emphasized 8 ohm speakers. I think I'll have the enclosure under control because my grandpa is a pretty skilled hobbyist in wood working. I was leaning towards MDF board for the material. The professor initially expressed concern for the cost and work of this project. But since I want to buy cabinets for my band anyway, couldn't I get good sounding ones for cheaper if I just made them?

Main questions:
For the intended use, what overall wattage do I want?
What wattage would the woofer and tweeter each have to be?
The only internal components for passive speaker are the speakers and crossover network, correct?
When speaker size is determined, how do I figure out the necessary dimensions of the enclosure?
What enclosure design should I use?

Any advice on components is appreciated!

Thanks!

[chris661]

Hi,

I'm not sure why your instructor has emphasised 8ohm speakers, most amplifiers will drive 4ohm comfortably.
Anyway, for vocal use, it needn't be a large set-up... How big is a medium venue?

The thing with DIY is that it's rarely cheaper than something off-the-shelf. But, when you get it right, it'll blow anything off the shelf for the same price out of the water. Easily. It'll give things worth much more a decent run for their money.
Your system will be optimised for exactly what you want. In this respect, anything off the shelf will never be as good for what you want it to do. It might well do other things better, but if you won't be using those other features, they're irrelevant.
For band use, I'd say ply is a must over MDF - MDF splits when drops, soaks up water like a sponge and falls apart when it does so. If you had to replace the cabinets a few times, would it still be cheaper than MDF?

To try to answer some of your questions:
1 - depends on how big a large venue is, and what you'll be driving them with
2 - see above, tweeter wattage varies with crossover anyway.
3 - correct.
4 - when you've modelled the bass response in winISD or similar, and found a compromise in response you're happy with, look at the minimum dimensions you'd need to fit the drivers, ports etc on, after that, play on something like Google SketchUp to find something that looks "right" in terms of aspect ratios.
5 - most PA speakers are ported. For purely vocal use, there would be no need: a fully sized ported system will give a more extended bass response, sacrificing cabinet size. Bass extension isn't of huge importantance for vocal use, so using a sealed cabinet would certainly mean portability.

Eminence make some decent stuff: aparently the co-axial 10" (Beta 10CX) sounds good.

Chris

[18Hurts]

I'm with Chris on this one

Vocals don't need deep bass so the 10" co-axial should be perfect in a sealed box. Since you said instructor--sounds like the vocal box is an assignment?

MDF is great until you have to pick it up (weight), drop it, strip out a screw or it gets wet. 3/4" ply works well when cross braced from side to side, front to back and top to bottom with a common 1x2 or 2x4s--I used 2x6's since I had the space. If you put handles on the sides, the mounting screws are great to mount the side-to-side braces with and makes it a stronger mount for the handles. Use metal edge hardware, rubber feet and grills since the first rule of PA is it will be dropped!

I'll take a guess that the vocal box is there for the singer only with the band monitors for the instruments? SWAG it at around 115 to 120 dB max of output at one meter so around 200 watts into the 10" co-axial. If there is any change on people patching into the "vocal box", it would be a good idea to spend the extra $5 for the 12" version. The 12" is 3.4 dB louder and will hit 120 dB at 200 watts. The box will need to be twice as large as the 10" but should be around 40 pounds of weight.

[coxy1214]

Alright. Thanks guys! I know it's a bit strange that I'm an audio student and don't know too much, but I want to learn! So if I got the 10" what tweeter would be useful? Also, should I even be considering this is I don't have an amp to power it? I'm a guitarist but I have a Marshall combo so I don't understand really understand the speakers that need separate powering.

[chris661]

Okay, your Marshall amplifier is designed for guitar use, so will have response-shaping in the pre-amp (cutting out <60Hz that the speaker can't handle is an example). You could plug a microphone into it, but it would sound pretty bad. I certainly wouldn't use it for that. So, you need to give these speakers their own (clean, ie no altering the signal) amplifiers. 200w/ch would be plenty, but you'll need something to get from microphone output to line level (most power amps aren't sensitive enough to take a microphone in directly - it will be really quiet).
Maybe look for an amplifier with a mixer built-in - I've never used this particular one before, but it has the right features, the price isn't too bad, but look on ebay for mixer amplifiers - there's some times some older models that have 4 inputs, some speaker outputs and little else - they sell pretty cheap, but finding one with enough power may be tricky.

I'd match the co-axial drivers with their recommended tweeter.

Chris

[coxy1214]

Okay, guys. Here is my proposal so far:

woofer: Eminence 12" BETA-12CX
**RMS 250W, 8 ohms, 43Hz resonance, range 66Hz~5kHz
tweeter: Eminence APT-50
**RMS 35W@3.5kHz/85W@kHz (some explanation wanted, but I assume this is what you meant about crossover considerations), 8 ohms, 1.6kHz resonance, range 3.5kHz-20kHz

All in a sealed box of birch plywood (does baltic birch matter)
26"x14"x10" (HxLxD) [outside measurement]
3/4" thickness???
calculated volume:
1.45 ft^3/40.96 lts (meets recommended volume of woofer)

Does this all sound good so far? Opinions and concerns?
Next, how do I go about designing the crossover network?

Thanks for everyone's help so far!

[coxy1214]

And, of course, I want a crossover network best designed to protect the tweeter. So a second order?? But what variant of this?

[chris661]

Sounds good so far, I would say the steeper the crossover on the tweeter, the better protection it will have.

Looking at the response curve for the 12" driver, it falls off quite smoothly after 3kHz, so I'd aim to bring the tweeter in there - you might be able to get away without a crossover on the woofer.
For the tweeter, 2nd order at 3 to 3.5kHz would be okay. When you wire it all up, have a listen then reverse the phase of the tweeter, then listen again.
Make sure you put a resistor in to pad the tweeter down to a similar level to the woofer - a quick calculation would suggest 20 ohms - make sure you put that before the crossover, or it will interfere with your crossover point of choice. An aluminium clad resistor will have sufficient power handling.
If you want to protect the tweeters again, a filament bulb (not sure how many watts, can someone who knows say?) in series would mean when you get a lot of power in, the bulb glows, increasing resistance so the tweeter receives less power than it otherwise would. Make sure you include the cold resistance of the bulb when you're adding the series resistor.

Chris