Opposite of optimal speaker box

box is needed to contain radiation from back of the driver cancelling radiation from front of the driver, in other words it prevents low frequency sounds annihilating, in other words it allows bass. There is even possibility to utilize sound from the back to augment sound from the front, like a Helmholz resonator.

Worst box is likely ugly to look at, hard to fabricate and expensive, makes noises ruining quality of midrange and doesn't augment the bass so doesn't full fill it's main purpose. Could be too big that it doesn't fit to proper position in the room. Positioning and how the room affects sound is perhaps most important thing for any speaker system. You can imagine box radiating arbitrary sounds to arbitrary directions doesn't exactly help achieve good sounding listening setup.

Some problems you can have with a box:
  • too small, no bass
  • if it needs to be big, it affects inside and outside dimensions, don't make too big because
  • box structure / walls resonate, their surface area is much much more than that of the driver, hence even small vibration can be quite loud.
  • standing waves inside the box, hears through, depends on inside dimensions. The longer a dimension is, the lower in frequency modes are.
  • there is a port that resonates mechanically, or has audible noise due to fast particle velocity, or has pipe resonance to it, or midrange noise inside the box leak through
  • there is a leak in the box, which alters it's intended operation, perhaps adds audible noises
  • outside dimensions of the box are such that do not support use case, baffle step, edge diffraction, both affect off-axis response which matters how the system interacts with the room for example or what the response is at quite narrow optimal listening window.
  • outside of the box is basically a waveguide. If it's not considered as one but randomly chosen then it is very likely not very good one (diffraction related backwave)

Since its fullrange forum you can be sure all of these potential issues are on the pass band of the transducer and should be evaluated if they are audible. Enclosure effect to system performance is very important, you must juggle as much bass out of it as possible since the driver is likely quite small, hence the enclosure is likely relatively large and has some port or horn, sound from back of the driver is used to augment bass output. All of the above, except the first one, are something to consider.

If it was multiway speaker, its possible to juggle the system so that some of the issues fall out of band of each transducer and be dealt that way. Basically only the lowest frequency transducer really needs "optimal box", ways higher up can skip on "bass augmentation" and have no enclosure, or enclosure that is as small as possible reducing many of the issues, optimized for wave guiding.

Imagine you were in fullrange heaven and had ideal driver, flat disk with no distortion and ideal radiation pattern what ever that was, flat disk made of rings which only progressively smaller contribute to increasing frequency, or what ever you can think of, what ever is the holy grail of fullrange drivers. Still all of the above would affect system performance. Same goes for multi-way speakers, think the system with (any)ideal drivers and see how your structure affects the outcome in your room.

Perhaps there is more, missed something?:)
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^That would actually be quite optimal closed "box" for a transducer that is not required to play bass. Given the baffle was no bigger than the transducer itself so that diffraction performance wasn't any worse than that of a plain driver. The cup would be very stiff and surface area very small and thus radiate very little sound. Also it would be small, no bigger than the driver motor (imagining) so distance from cone to the surface inside would be quite short, perhaps few centimeters with small driver so any resonance inside would be very high in frequency, approximately as high as any reflection from driver motor / frame itself. Issues of the box would be similarly high frequencies where cone breakup already makes the response wonky anyway and begs a crossover. Such small enclosure would not help provide any bass though, it would prevent it cancelling out but system resonance would be likely quite high, no real bass.

So, could be worse. Even if the half sphere was very big so that the resonances inside fell more on pass band and baffle was big to introduce diffraction for wider bandwidth the half sphere would not resonate too much because its much more stiffer than a flat panel so similar size cubicle would be worse :D I think. I'm not sure which is worse, sound radiating back out from the cone, or walls radiating sound all around (while also having some sound come through cone)?

Well, fun topic :) It's basically the same as how to make a good speaker, both requires listing issues and them ranking them by audibility.
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What if the half sphere was about 2 foot diameter, or "optimised" for the worst dimentions - the rear reflected wave peaking right in the mid-range.
Presumably a box with and internal reflection or standing wave contributed to a front baffle diffraction would be bad.
On one of my early forays into tinkering with speakers, I added a weight to the rear of a driver and mechanically isolated it from the front baffle, then stuck it on a cube enclosure; interestingly, taking away the mechanical resonances (from the reaction of the driver) highlighted the echoes from inside the box, and made it sound worse.
I also experimented with an open baffle by hacking out the rear panel of some speakers, resulting in an awful resonance between the back of the driver and the open back of the enclosure, I stuck some foam in the back, and it when from terribly coloured to very natural.
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Any rectangular box including paralell walls (standing waves=energy loss) and absorbibg material (absorbing "energy") will in fact act like a
exhaust muffler or silencer. That`s the main reason why dynamic speakers doesn`t sound too dynamic. Another reason is off course energy-choking passive filters. A third reason are thin stranded wiring.
Any rectangular box including paralell walls (standing waves=energy loss)
I find MLTLs rather effective myself.
and absorbibg material (absorbing "energy")
'Unwanted reflections and / or eigenmodes above tuning'
will in fact act like a exhaust muffler or silencer.
It will in fact if it's designed to.
That`s the main reason why dynamic speakers doesn`t sound too dynamic.
I'm not sure anybody who's heard, say, a pair of 15in HE woofers per channel in a suitable cabinet + compression mid-tweet would entirely agree with that. ;)
Another reason is off course energy-choking passive filters.
Why 'of course'? How are you defining 'energy choking'? Passive filters can only attenuate, but you lose just as much dynamic headroom by increasing amplitude via active filtering, and a properly designed passive with decent quality (which doesn't have to mean 'massively expensive') components is not automatically losing out to any significant extent -see the above example, or something like a JBL 4450 in that context.

A third reason are thin stranded wiring.
Sure, if the resistance pushes voltage drop over the loop length to audible levels. An appropriate wire guage is a good solution for that. ;) A little bizzare though how often what is completely unimportant (or at least of very little importance) in wire gets elevated to 'obsessive' levels, while what typically makes the greatest difference is just as often overlooked. Go figure on that one. :scratch1:
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How to make the worst box for a driver?
Can you give me the worst conditions for a driver?
The box does many different things, inside and out. I might guess you're talking about loading the driver resonance. If the box is too small the bass rolloff happens higher in frequency and there is a peak in response.

If this is not what you meant, could you expand on your question?
I read it as a blanket 'everything possible to cause maximum ill effects', but I might be wrong.

Assuming that's the case though, and without any specific driver type given, I'd be inclined to GM's view on the box load, probably with a nominal square internal form-factor, and if that isn't viable, the nearest possible rectangle -if that puts the internal rear-wall in extreme proximity to the driver, so much the better. If we're assuming a nominal direct radiator, I'd also suggest some form of stretched cavity in front of the enclosure with its dimensions and profile chosen to cause the highest amplitude of edge diffraction at 3KHz possible, at whatever listening axis is intended. Details such as loose wiring positioned to rattle against the panels would be a given, especially if you can contrive the positioning to foul the suspension / leadouts, and if you can also ensure mechanical failure of the insulation to result in shorts at high amplitude, you should have ticked many of the necessary boxes.
A driver mounted part way down a tube? terrible edge diffraction and resonances from the rear.
About a year ago I was making an omni with a driver mounted with a vertical axis, I thought I'd break the drivers in using a partly finished enclosure that had an open bottom, they sounded dreadful.