Making Speaker Cabinets

[chargerboy]

Hello Fellow Speaker-heads,
I'm constructing cabinets for some non-boxed Zenith 3-ways I've got. I don't know a ton about boxes for speakers. The speakers are complete and in good cond. They already have crossovers (!). So, is there anything i should know?
For instance: Why are speaker boxes usually made of pressboard? Does it have to be made out of pressboard? How does the hardness of the wood affect the sound made? How much space should be inside the speakers around the drivers? And, what is speaker shielding?
Well, that's pretty much it for now. Thanks in advance for any and all your help. Any advice I would greatly appreciate.
Thanks!
Michael

 

[gregz]

Hi Michael,

You ask some darn good questions!

First and foremost, you need to know some specifications about your speakers so you can build the boxes the right size. Each speaker model is different, and some like big boxes while others prefer small. If you build a box that is too small for the speaker you have, you will not get any low bass and it will be kind of boomy. If you build the box too big, you won't get enough of any bass, and the speaker will get easily damaged if you try to play it too loud. Those specifications are called the Theile Small parameters, and it's three numbers that the manufacturer can (hopefully) supply you. If not, perhaps you can calculate the size of the original box the speakers were made with?

The rest of the questions are less crucial. You don't need to use pressboard, but it makes a great enclosure material because it's dense and cheap. Dense is good because it won't resonate very easily - you want the speaker cone to make all the sound by moving air, not from a cabinet that vibrates or flexes. Pressboard is too heavy for DJs, so their speaker boxes are usually made of plywood - which is rigid but not as dense and if there's a large void between layers it can develop a nasty rattle (I've never had troubles with plywood in my car boxes). No matter what material you use, you can make up for lack of hardness or mass by damping the inside of the box and/or installing extra internal bracing to make the largest walls more rigid.

Finally, speaker shielding is only necessary if you want to place your speakers close to a tube television or CRT monitor. Without shielding, the speaker magnets will mess up the screen on cathode ray tubes. It's a pain to add shielding, but it can be done.

 

[Goatman]

does the box size have to be calculated?
how's a 54litre box for a 10 inch sub and a tweeter?
the box will be made ot of 16mm thick chipboard.

what difference does a vent make?
what kind of material should be used on the inside?
what difference do different materials make?

 

[gregz]

Hi Goatman,

There are several box designs you can use depending upon what type of performance and efficiency you want. Generally speaking, vented (ported) boxes are more efficient than sealed boxes, but sealed boxes can give a tighter sound and extend to lower frequencies with their slow rolloff.

No matter what type of box alignment you choose, the box size needs to be calculated based upon the driver (speaker) you are going to put in it. Each speaker needs a different box size, and changing the box size changes your response characteristics.

With a sealed box, you can be off in size by +/- 20% and still work well, but with a vented box you really need to be accurate or you'll get terrible sound.

Materials are the least important, as long as your final design doesn't resonate, flex, or leak air. Weaker, lighter materials need more bracing or the longest sides can act like drums that muddy the whole sound or even cancel the sound coming from the speaker cone. You can test the walls by knocking on the finished box. If any of the sides sound loud and hollow, it needs bracing. A well built box should be dead when you knock on it, giving a very soft high pitched sound almost like you're knocking on a tree.

If the speaker is going to handle upper bass frequencies (200Hz and up), then it's a good idea to put sound absorbing material inside on one side of each facing wall to keep reflections from resonating. If the speaker will only operate as a subwoofer or lower woofer, then the sound waves are too long to resonate inside a box and you don't need material on the walls. Stuffing the box loosely with polyfill can increase the effective size of the box, allowing you to make it slightly smaller if size is an issue. I forget offhand if it was 10% or 20% size savings, but I don't use polyfill anymore. I just make 'em big.

 

[Goatman]

If the speaker is going to handle upper bass frequencies (200Hz and up), then it's a good idea to put sound absorbing material inside on one side of each facing wall to keep reflections from resonating.

which one side? the back?
can i have a 48 litre sub share 48 litres with this tweeter?
http://www1.jaycar.com.au/productVi...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

BTW, here's a link to the sub
http://www1.jaycar.com.au/productVi...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

 

[gregz]

Tweeters are enclosed in the back, so there's no problem with the tweeter sharing a box with a sub. I took a look at the link - I take it you're making the 48 L design from the website? That looks like the ideal box size and type for that speaker. That's nice that they provide such useful info.

The idea of dampening is to prevent a standing wave from bouncing back and forth between two walls facing each other. So you put sound dampening material on the back wall, one of the side walls, and on the bottom. You can also help prevent standing waves by designing the box so it isn't shaped like a cube.

Frequency vs Wavelength:

1.5KHz = .23meters
1KHz = .34meters
500Hz = .69meters
250Hz = 1.4meters

As you can see, it's the frequencies between 250Hz and 1KHz that have the best chances of wavelength resonance between two reflecting walls. If you make the box a rectangle, then you don't get all six walls bouncing the same resonance point, you get several different resonances which aren't as intense. Damping material helps kill the resonances as well, and cross braces helps keep the walls from vibrating at resonance.

Are you buying these speakers as a kit with crossover? If so, I recommend it. Getting the crossover right can be a pain in the butt.

 

[Goatman]

well, no. lol
i thought a 6db lowpass and a 6db highpass would be just fine.
I wasn't originally plannibg to build a box to those specs, but i can change my design easily to mach those specs.
since you said a sealed box has tighter bass, could i build that box the site reccomends but without a vent?

 

[gregz]

That speaker is really tailored for a vented enclosure with such a stiff suspension. For a sealed box design, you'd actually use a smaller box and your -3db cutoff would be around 45Hz and you'd need to pump more than twice the power into it just to get the same amount of bass you'd get from the ported design. Ah, everything comes at a price, huh?

If you want to make a sealed box design, you're better off selecting a different speaker that's made for it. Then you'll get a lower frequency response, and if you choose a more efficient speaker (SPL of 91 dB or more), then at least you can get the same amount of bass as the 10inch Re/Sponse Titanium Subwoofer without needing a bigger amplifier.

 

[Goatman]

http://www1.jaycar.com.au/productVi...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=
this sub is rated at 92db SPL and they reccomed a sealed box.
the only thing i don't don't get is "-3dB point: 71.07Hz". Isn't that a bit high for the 10 inch mofo this sub is?

 

[gregz]

Ouch, 71Hz??? Just looking briefly at the specs on that speaker, it looks even less suited to a sealed box than the other one - for a sealed box, you want a low VAS, a low Q (forties or thirties), and a low resonant frequency (in the twenties or low thirties).

Have you looked at speakers from Crutchfield or Parts Express? There's a website that has built-in calculators to figure out box volume and resonant frequencies if you're not familiar with them:

http://www.carstereo.com/help2/Articles.cfm?Car_Audio=Subwoofer Box Volume Calculator

 

[Goatman]

I think i'll just go with a vented box then. I'll get this sub http://www1.jaycar.com.au/productVi...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=