Enclosure sizes

[megamuel]

Hi there,

I am currently working on my first speaker building project and I am after some advice. I was under the impression that enclosure size largely effects the sound of the driver enclosed. Unless it has its own enclosure, like a dome tweeter. So I was trying to find out the optimum enclosure sizes for the driver and the mid I have selected and I emailed a guy at parts express and he said it doesn't matter what size the enclosure is for the mid, whether it be tiny or the size of a house it will sound the same. Is this really true? If so why does the size of the enclosure for the woofer matter so much? Sorry if this is a silly question, as I said I am new to this! Cheers,

Sam

[ThomasW]

The size of the baffle determines the boundry loading.

For a true midrange the size of the enclosure is fundmentally irrevelevent since the passband is so high.

[SQdude]

It depends. If you are using a conventional cone driver, say in a 2-way design, the box size will effect the lower octaves. It would be best to play with Unibox, WinISD, or some other program to model the low end response of a driver in a particular enclsure volume using the TS parameters of the driver.

[megamuel]

Basically I had conflicting information. On one website I read:

Why Individual Chambers? The mid and woofer both need their own separate chambers in the box. Both the mid and woofer are designed to work in an enclosure of a specific size. If they are both in the same chamber, like most cheap speaker systems, then the enclosure size for the mid will be too large and performance will be lost. Also, the sound waves from the woofer can overpower the mid and distort it. The tweeter is indepentently sealed and doesn't need it's own chamber.

Yet on the other hand the guy at parts express said to put the mid in the same enclosure as the woofer and when I asked if this enclosure would be too big he said that the size of the enclosure doesn't matter for the mid, it could be the size of a house and still sound the same. As long as the backwave from the woofer doesn't effect the mid. So who is right?! Cheers

[cjd]

Size matters. Unless it doesn't.

Things like dome mids come with chambers - hell, tweeters have chambers. Chamber is another word for "very small box built into the driver"

If you're using a cone mid, it'll still have some relevance - and the lower you take the driver, the more relevance it has. For best results you always want to keep drivers covering different passbands in separate enclosures - it's more than just isolating backwave (though I suppose the term "backwave" could include a lotta stuff). Box size is worth considering, not just choosing whatever is convenient.

C

[Deward Hastings]

It’s a simple-seeming question, but there are several answers . . .

A mid driver needs a box of its own because the back wave (pressure) from a woofer in the same box would pump the cone (somewhat like a passive radiator), driving it into non-linear regions of operation. Some mid drivers, like the RS52 dome, come with their own enclosure (as do most tweeters) . . . they don’t need an additional box.

The minimum size of the box is mostly determined by its effect on the lf resonance of the enclosed driver . . . too small pushes the driver resonance up and limits low frequency extension. For some woofer designs the box itself is deliberately made resonant to extend low frequency response . . . in those designs box size is a critical design feature. That does not apply to mid drivers, where resonant (ported) boxes are never used.

It is possible to make the enclosure for the mid driver too small, but it is unlikely, almost difficult if you allow working clearances. Typically a couple liters is more than sufficient . . . if the box is big enough to clear the basket and deep enough to clear the magnet structure you’re probably there. An exception might be a design with a low (300-400 Hz) crossover with a shallow slope . . . there you would want the driver resonance below 100 Hz and a mid-box of 3-5 liters might be needed. You’ll find mid enclosures in that range in some Thiel speakers, for example . . . the mid driver enclosure in the Thiel 3.6 is over 3 liters.

There is no maximum size for the mid enclosure except as a practical matter. Bigger boxes (with adequate stuffing to absorb the back wave) just act more and more like infinite baffles.

There are commercial speakers which do not isolate the midrange driver, using instead a very stiff suspension on the mid to resist back pressure from the woofer. They typically don’t sound very good.

[Deward Hastings]

an afterthought:

the mid enclosure does need to be big enough (in addition to the reasons noted above) to contain enough absorptive material (stuffing) to absorb the back radiation from the mid driver. Otherwise it will re-radiate, delayed, through the cone, which is not a good thing.

[cjd]

FWIW there *are* systems that go with a TL on the midrange, and some horn designs back-load the mid along with the woofers. But if you're messing with that stuff, you need a little beyond the average clue.

C