College Dorm speaker system...on the cheap

[joeminden]

Hi everyone. I wanted to ask a few questions regarding a dorm speaker system I'm setting up.

-First of all, what kind of power would I need to really fill the dorm with sound? I know it depends on efficiencies I have in each speaker, but what generally works. I have a 5 channel, 100W per channel tuner deck in mind. I then plan to either send in MP3s from my computer or use a portable CD player for the input.

-I already have an MTX Thunder 4000 car subwoofer that I'm looking into using for the system. Basic specs are 30-150 Hz... 89dB sensitivity. What would one need to do to make it suitable for a dorm system?

-I plan to have 3 boxes; two of them will have a midrange and tweeter, and the third is a dedicated subwoofer box. How will I ensure that the mid/tweet boxes play at the same level at the subwoofer? I know an easy way is to get a sort of attenuation circuit on the subwoofer.

-Final question is what have you found that looks cool in a speaker system that I should add to this one? I was thinking an internally lit sub box with a plexiglass window... any suggestions?


THANKS! Appreciate any and all input. I'm a mechanical engineering major so I don't anticipate many problems with this project.

[khaosman]

For the subwoofer, you can just build a box for it and power it off of a plate amp. Granted, it probably won't be the best solution but for a dorm room... it'll work fine

Partsexpress.com has a good selection of plate amps, and so does apex jr. Your receiver should have an adjustable output and the plate amps have a level control so matching it with your bookshelves shouldn't be a problem.

I would make the box quite a bit bigger than MTX specs it. Especially for sealed. If you want to go ported... again I would go larger and tune lower. This will help out with efficiency being as you have a good amount of space in houses (although not dorm rooms )

Check out some threads in this forum about cheap speakers... there was one with some Audax drivers that are on sale that looks decent.

[BAM]

I hope you're not planning to wire two channels of the amplifier together into the subwoofer, are you? This will kill your amplifier. I did it to one of mine when I was first getting started with speakers and sound. I thought I'd found a clever way to get a mono signal. Then I let the magic blue smoke out of the amplifier. What you really need is a dual-voice-coil subwoofer driver if you are planning to use it without its own dedicated amplifier. Parts Express sells some good dual voice coil subwoofers which are 8 ohms/coil. Then you just need a passive crossover network to get the bass to the sub. They can be tricky to design and build, and soldering experience is necessary. However, if you just buy a plate amplifier, this will be a lot easier, and a dedicated amplifier for the subwoofer can make a world of difference in sound quality and output levels over a passive subwoofer with a crossover. Plate amplifiers for your project can be had very cheaply. Parts Express is a good place to find them. I think one plate amp, the 100w model, is only $80.00. Pair it up with two dayton 8" woofers at about $19 each, and you've got a subwoofer.

[joeminden]

Alright khaosman, thanks for your reply. I'll look around for those Audax drivers. I'm thinking about doing a sealed box as it's the easiest to construct and I don't want to do much port tuning. That'll be for another, better system.

And BAM, thanks for your insight! I never thought that it would be bad to wire up two channels together. I'm going to then buy a dual voice coil subwoofer and take 2 of the channels for the sub, so I can give it 200 watts. This leaves me in a bit of predicament though. How many watts for midranges and tweeters is sufficient? Because if I have a 4 channel amp then I'm only going to have 2x100W channels left for them. I don't think that could be enough... but maybe I'm wrong. Please correct me if need be.

I have a relatively high understanding of crossover networks as one of my classes dealt with them... also I'm pretty good with soldering etc. I'm really excited about this project!!

Thanks, and I welcome any more input.

[khaosman]

You should have plenty of power. You'll run the sub off the rear channels, and the stereo speakers off of the front two channels.

One thing though--many speakers you build might have an overall impedance that is lower than 8ohms. Some receivers do not take too kindly to this.

I suggest you check out the Adire Audio Shiva or Tempest... they are dual voice coil 8ohm subwoofers and would work perfectly. Dayton also makes similar DVC subwoofers.
Daytons: www.partsexpress.com
Adire: www.adireaudio.com

I finally found that Audax thread too!
Audax on sale again...

[Dave Jones]

I don't think there is any time of day or night when it would be appropriate to play a subwoofer in a dorm room. Others might be studying or sleeping. (Don't laugh. It can happen.)

I have an office that's about the size of a typical dorm room. I'm using a 45 watt stereo amp. I can't turn the volume control more than an eighth of an inch or so when there are other people in their offices. One watt can do a lot more than you might think.

[khaosman]

Dave Jones, you are correct. The subs I recommended would absolutely guarantee you getting yelled at by your R.A. and floormates if they were up much at all

Perhaps an 8" subwoofer would be a better choice. That is what most people with "box" type systems had. Believe me, those things are quite loud in a dorm. If you have self-restraint, you could probably get away with a bigger sub though.

Also some floors are "loud" floors meaning you can have your stereo up loud during certain times of the day. Some of my friends were on those and it was interesting to say the least.

[joeminden]

For the record, I have utmost respect for others. I'm only going to play it loud during parties, etc. It will be fine. Besides, I plan to have this for wherever I live upon graduation as a small home theater system.

Thanks for the advice people.

I'm rethinking this; for simplicity I'm going to do two boxes. I think I'll do a pair of 2 or 3 ways. Just have to find the tuner.

Another question: is there any acceptable way to drive a single 2 or 3 way speaker on 2 channels of the tuner deck?

[Greg B]

Subwoofer shmubwoofer. Aren't you forgetting something? Woofers?

In my view a subwoofer is something that operates only below 50hz, but I'm in the minority these days. Anyway, that would just annoy the entire building, especially if it's a concrete dorm like the one I lived in.

My preference would be a simple 12"/5"/1" 3 way in a sealed box. Something like an eminence woofer for the bass driver. It would play loudly for parties on the weekend, without ringing everyroom in the building like a bell, and sound good at low levels too.

If you'd like a compact bass module and satellite system, I'd recommend quality ~5" full range drivers and a 10", ahem, subwoofer. The one you have would be fine. Just get a cheap plate amp for it. Forget about passive.

This would be more than a little SPL limited for what I remember from dorm living.... I guess I lived on a loud floor.

GB

[khaosman]

Originally posted by joeminden
Another question: is there any acceptable way to drive a single 2 or 3 way speaker on 2 channels of the tuner deck?

<edit: sorry if I misinterpretted your question by the way>
Yes, the drivers in the speakers are usually wired in parallel or series. Here is an example that explains it better:
You have a two way speaker with a tweeter (16ohms) and a midrange (16ohms).
You wire them together in parallel, achieving a final impedance of 8ohms, which your receiver likes. This is a very simple example though.

A crappy picture:
amp + -----------------> tweeter xo network -------> tweeter + -----> woofer xo network ------> woofer +
amp - -----------------> tweeter xo network -------> tweeter - -----> woofer xo network ------> woofer -

That probably makes no sense, so on second thought, look at this crossover design:
http://www.adireaudio.com/Files/KIT51Plans.pdf

It should give you an idea of what I'm talking about.

Sorry if this is confusing, it is late and I'm pretty new to this. Hopefully someone else can clarify it more for you

Also, what you are thinking of with a pair of 2way speakers needing four channels is called running the speakers "active." This means there is no crossover like shown above--each driver is powered off a seperate channel and an active crossover (a set top component you buy) determines the frequency and slope.