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Old 19th April 2010, 11:15 AM   #1
Oil is offline Oil  United Kingdom
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Default small boombox speaker selection and port design

Hi All

I know that there have been a number of threads about building a powerful, outdoor 'boombox' for casual listening --- I have decided to do a similar project. I have read through them, but I am looking for guidance on the arrangement with regard to ports and drivers in my enclosure (see drawing). The speaker placement is not definite.

Goals: to take a line in input and produce a decent, loud and clear reproduction (with solid, powerful bass). I understand there will be many compromises in this design since it could be used in so many different environments and because of the size of the enclosure.

Enclosure:

I have a definite size I want to work to - 350 x 187 x 187 mm (13.7" x 7.36" x 7.36") and I will be making the enclosure from laser cut plywood, probably around 6 - 10mm.

Amplifier:

I'll be using the Sure Electronics TK2050 2*100W board. I chose this because of the high efficiency, good value, simplicity and low distortion in the ranges I'll be using it. I'll be bypassing the power inputs with a large electrolytic and a decent sized film capacitor.

Power:

I'll be using a 12V 4 - 7 Ah Sealed Lead Acid battery


Speakers:

This is where I need help - I have very little speaker / enclosure design knowledge. I thought a pair of 4" full range drivers could be suitable - but everything I seem to read suggests that they may not be competent enough at the high end without spending £££. So perhaps a set of 4 - 5" 4 Ohm drivers with a tweeter? (I had the idea that the dayton RS100-4 drivers could be suitable). I am posting in the full range forum because I'm unsure which it should be!

I know there is no perfect solution and alot of this will be prototyping and experimentation - What I would ideally like though:

1/ A suggested set of speakers (and reasons why). Ideally they should be available from this site (Europe Audio) as I'm in the UK. My budget it around £70 maximum for the complete set (whatever configuration that may be)

2/ A suggestion of porting arrangements on the enclosure and brief reasoning behind this.

I intend to use the suggestions as a starting point for experimentation leading to the development of a solution, which I will document here for everyone to see.

Many thanks in advance.
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Last edited by Oil; 19th April 2010 at 11:32 AM. Reason: added picture
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Old 19th April 2010, 03:41 PM   #2
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Decent bass isn't an easy thing to come by. Using a couple of 4" woofers, you have no chance, using a ported or sealed box.

So, what's the priority? Decent bass or small size?
I also expect that the battery you've selected will mean you'll need some high efficiency speakers. 200W@12V means a very high current draw, so your battery won't last long. The alternative is a different battery/amplifier. I like the amp6b from www.41hz.com - it gives 15w/ch, which is plenty if you've got some efficient speakers. It also means you won't flatten your battery in 15 minutes.
If you were to use 4x 6" woofers, 2 on the front, 2 on the back, then add a pair of tweeters to the front, I expect you'll get something pretty loud, with reasonable bass. You won't need to modify the box size either.

How are you going to brace the cabinet? I'd consider this essential, having heard cabinets that aren't braced.

Chris

PS - I know this is a very mixed up post, so feel free to ask for clarification.
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Last edited by chris661; 19th April 2010 at 03:42 PM. Reason: typo
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Old 19th April 2010, 06:48 PM   #3
Oil is offline Oil  United Kingdom
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Thanks for your reply Chris - really appreciated!

Small size - the dimensions that I've chosen have to be the final dimensions, so this is the critical specification. I'm trying to get the biggest sound possible from this size!

Obviously the small size of the speakers (4") won't help the efficiency - I guess a 5" - 6" driver would help this somewhat.

With Class-T, how much power it could potentially drive doesn't mean it will use that power (unless I'm missing something that you're not mentioning). The two chips on the board I'm using quote < 200mA MAX combined @ 28V quiescent current and driving at 81 - 90% efficiency. I'd be running at 12V so the total would presumably be less. Shorting the battery into a 4 Ohm speaker direct would only use 3A - that gives more than two hours off a 7.2Ah!

I imagine 10 - 15W would be sufficient for most intended uses of this. The main reason for choosing this board was that distortion really cranks up above the 10W level on the smaller class-T chips ( 15W @ 4Ω, 10% THD+N on the TA2024 ) - since their efficiencies are similar and the quiescent current minimal, I don't see a reason to use them.

I can certainly find a way to brace the cabinet in the design.

I'm really asking for advice on the acoustics of the enclosure - I don't want to take the route of calculating everything precisely but I would really like some basic advice on things like port size / speaker reccomendations and so on for this particular application and some reasoning behind it.

Thanks again in advance
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Old 19th April 2010, 07:14 PM   #4
polop1 is offline polop1  United Kingdom
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my knowledge is far from absolute, but i have designed and built something similar. i went with 6.5 inch component speakers and small tweeters from fusion car speakers (£50 to uk but now can be had for around £40) calibur 150 watt amplifier, 2 7AH 12v batteries. conclusions bloody loud (i'm up to 4 house parties now bass is very reasonable - batteries will last less than an hour on full power but 3 or so hours on chunky volume, and have used for around 10 hours on casual chill around a campfire. the box is just the smallest possible box that all the components would fit in, hence almost no cavity volume (box size is about 35cm * 25cm * 15cm ish - the amp takes up allot of that as it is an amp designesd for car audio)

i am keen to follow your build as i would like to redesign the box with more efficient smaller class t amp, and better extension in the low end.
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Old 19th April 2010, 08:02 PM   #5
jwmbro is offline jwmbro  United States
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At that price range for drivers, when it comes to fullrange, I'd suggest looking at the CHR-70 by MarkAudio, they are quite reasonably priced.

I did some quick math, and I'm getting that the boombox would have two 4.5l compartments, or is it supposed to contain the battery and amps as well? 4.5l would be a quite acceptable volume for a medium size fullranger, but if it's less than that, you're going to be entering a suboptimal volume range.
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Old 19th April 2010, 08:27 PM   #6
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Quote:
Originally Posted by Oil View Post
...

Obviously the small size of the speakers (4") won't help the efficiency - I guess a 5" - 6" driver would help this somewhat.
.....
.....
I imagine 10 - 15W would be sufficient for most intended uses of this. The main reason for choosing this board was that distortion really cranks up above the 10W level on the smaller class-T chips ( 15W @ 4Ω, 10% THD+N on the TA2024 ) - since their efficiencies are similar and the quiescent current minimal, I don't see a reason to use them.

I can certainly find a way to brace the cabinet in the design.

I'm really asking for advice on the acoustics of the enclosure - I don't want to take the route of calculating everything precisely but I would really like some basic advice on things like port size / speaker reccomendations and so on for this particular application and some reasoning behind it.

Thanks again in advance
Someone on here made a larger version of something you're trying to do. He used 4x PA 10" woofers, and a couple of tweeters.
WRT the amplifier - the amp 6b that I mentioned does 15W per channel at 0.007% distortion. 10% is around 25W. There is simply no need for a big amplifier like that. Remember that even just one watt is loud through efficient speakers... Class-D Amplifiers

If you got hold of some PA speakers with a low (ish) resonance, (60Hz), that would be good. 4 of these would be good for a kick-off. You'd end up with 100dB @2.8v per side if you used 4 of them
EMINENCE|ALPHA6A|SPEAKER, 6" MID RANGE 100W 8R | CPC
Efficiency this high would mean minimal power use on the batteries, because 106dB (2.83V per channel) is incredibly loud. However, the resonance is quite high, so, while there won't be much low bass, a mid-bass hump will be present, which is commonly used by the manufacturers of PA cabinets to give the impression of bass. It can be quite effective, too.
I've just noticed that you could only afford two of the drivers I mentioned, with little spare for tweeters. Still, you could afford a couple of these
--|53-650|HORN TWEETER | CPC
Efficiency would need to be rematched for having only 2 woofers, but a resistor in series would be fine for this.

I think it would be easier to use a sealed cabinet. Ported ones are difficult to do if you want them small - the ports themselves can take up far too much space.

For the bracing, a few 2"x1" pieces will do, joining the centres of each panel.

Chris

PS - A few quick simulations show the driver I linked to won't go that low, but will be fine in a sealed or vented cabinet.
For vented, a single port, 70mm diameter, 20mm long will suffice, with the box size you've given (around 12 litres, by the way).
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Last edited by chris661; 19th April 2010 at 08:35 PM. Reason: Additional info...
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Old 20th April 2010, 05:10 AM   #7
kagolu is offline kagolu  United States
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Quote:
Originally Posted by Oil View Post
Shorting the battery into a 4 Ohm speaker direct would only use 3A - that gives more than two hours off a 7.2Ah!
You shouldn't count on that battery providing 7.2 amps of power. You should expect approximately 80-95% of the rated capacity over the first few cycles then the performance will slowly degrade form there with each cycle. This is especially true with sealed agm or gel batteries. Most sealed batteries shouldn't be discharged more than about 50% in a cycling application if you want them to last any period of time.
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Old 20th April 2010, 10:23 AM   #8
Oil is offline Oil  United Kingdom
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Thanks for all of the replies.

It seems a multi-driver setup is the way to go. I have a couple of questions:

1/ If I choose a mid range driver and a tweeter that has x and y sensitivity, once I have added a crossover how does this affect these figures? How is it calculated / estimated?

2/ I am considering buying a second hand pair of bookshelf speakers and ripping the drivers and crossovers out of them - thoughts?


Could this be moved to the Multi-Way forum please?
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Old 20th April 2010, 11:01 AM   #9
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by Oil View Post
..The speaker placement is not definite.

..I have a definite size I want to work to - 350 x 187 x 187 mm (13.7" x 7.36" x 7.36") and I will be making the enclosure from laser cut plywood, probably around 6 - 10mm..

,,I'll be using the Sure Electronics TK2050 2*100W board..

..I'll be using a 12V 4 - 7 Ah Sealed Lead Acid battery..

..I intend to use the suggestions as a starting point for experimentation ..
Hi Oil,

If your given dimensioning requirement is for an internal volume: See the suggestions made in my submitted picture:

b
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Old 20th April 2010, 02:00 PM   #10
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You certainly have your work cut out for yourself, given the constraints...

Since you will not get any sense of stereo from speakers so close together, I would consider making a mono system. This will allow you to get deeper bass from the constrained box volume. Your dimensions limit you to a 6.5" driver, so I would look at them. Port size is not an independent variable here - it is driven by the driver, so that is your first priority.

Another thought would be a single Jordan JX92S - the unshielded version is $US 135 - in a ported set up with a ca 3 x 11cm port (per the Jordan website). This is not very efficient and will not play super loud, but will make deeper bass than any 2 x 4" configuration I can think of.

I suggest you play with one of the online box calculators and plug in different driver's specs, to get a sense of the parameters required of a driver that will work for you.

John
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