What makes the SPL of a speaker?

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Box (mostly) only affects low frequencies. A box can provide gain at LF and extend flat response further down but usually at the compromise of faster rolloff and vulnerability to “flapping” when the box no longer provides a load.

The box will not affect overall SPL. A horn can increase SPL over a limited range.

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The driver design is the major factor - basically the efficiency of the transducer - high efficiency provides high SPL, but cannot handle small enclosed boxes well (can't push as hard). Low efficency drivers have more moving mass, and most of the power goes to accelerating that mass rather than the air, but they can push harder against small enclosure volumes and are less affected by them. High efficiency drivers tend to be more fragile and can handle lower acoustic powers before breaking up, because of the lower mass.

All the above is gross generalization of course. Horns are a law unto themselves really I think.

There's a rule of thumb about speaker design - you can have upto 2 out of 3 of:
o) enclosure compactness
o) high efficiency
o) good bass extension.

If you want high efficiency and good bass performance, large enclosure is the way to get it.
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I may learn something here. When it comes to horns, I would understand concentration of sound, not amplification.

I would use the same thinking with ports or boxes. The fact that they may concentrate certain parts of the audio spectrum, but not amplify it.
Would it also be because some frequencies may not have the same acoustic impedance compared to other frequencies resulting in what may appear as gain to a band of frequencies? The trouble that I am having with this is my belief that there really is no free lunch. Energy can be made, but it doesn't make itself.
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Horns are often called acoustic transformers. What is gained is from fewer losses coupling the small driver area (high impedance) to the low impedance of the air.

Think about the area of the diaphram vrs the area of the mouth. There is a transformation from high impedance/small area to lower impedance/larger area. It is a trade that leads to less energy being turned directly to heat.

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Consider the compression ratio that the driver works with on a horn. Think about it like the difference between throwing a very light plastic ball vs launching a shot putt. You can not invest pressure into the light ball to throw it further, the only thing that works is swinging your arm at a higher velocity, but you can push hard against the shot putt. Similarly the driver can build pressure against a restriction using it's motor strength.
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What do you mean by this? Do you want a box design that allows for the greatest SPL from a given driver?

And do you want a box that is the most efficient, or a box that allows the greatest amount of power to be sent to the driver without exceeding X-max?
To squeeze more power the box usually made smaller.
But involves the usual tradeoffs.

Speaker is , what it is.
Higher SPL can be obtained with
higher sensitivity speakers.
and the usual tradeoffs with those driver types.

otherwise linear excursion is what it is.
Some drivers have more linear travel.

can also add more drivers for more SPL
but up against center to center spacing.
and where you get directionality.

comparing similar drivers with similar suspension.
usually the larger speaker has more sensitivity.
But again you working against directionality.
such as a typical 6.5 inch might be a more
efficient midrange. Compared to similar 3 inch.
But 6.5 becomes directional at a lower frequency.
So puts more demand on a lower crossed tweeter.
The box determines how the low end rolls off. Horns are a mixed blessing and not recommended for hifi because they have a lot of self-resonances, distortion and rarely great deep bass. Matching the box to driver characteristics was well described by Thiel and Small.

The driver efficiency is mostly about precision manufacturing, ie minimal air gap, and quality material. And the larger the cone, the more efficient it is, for similar reasons that horns are efficient, ie better impedance match to the air. But very large cones start to break up at higher frequencies so one size does not fit all. There are issues with the magnet construction. Ie a strong magnetic field and a shorting ring to prevent the magnetic field from ~yielding.
https://celestion.com/blog/demodula...ece being the primary arm of the transformer).