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I've been reading a bit about speaker design but I haven't been able to find what purpose the internal volume of a speaker cabinet serves. From what I've read, having an internal volume causes extra considerations like needing to deal with internal reflections and requiring bracing to reduce the cabinet resonance; though I am sure there are good reasons for it I haven't been able to find specific answers addressing this.

For example, why can't a speaker enclosure simply be a block of MDR with just enough space cleared to fit a driver and maybe a small vent to for air to be pushed/pulled? Is internal volume just like an expanded vent that improves low-frequency rendering by allowing a larger volume of air to move, and it's better/more efficient to make than say just a very long vent, which could resonate at much more detrimental frequencies (but couldn't those resonances be mitigated by bracing)?

I'm pretty curious about this so I'd love a good answer.

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It's actually a complicated thing, how much space to put behind a driver. The air mass behind the driver actually affects how the driver moves. If you put the right volume of air behind the driver, you can even out the low end. If you put the right volume of air and the right size port, you can change the frequency response of the system (with a few tradeoffs). Enclosure size, share, and porting/venting/etc. is possibly the most significant part of speaker design

See: http://www.eminence.com/2011/06/sealed-vs-ported-enclosures/

For sealed enclosures:

The air inside the enclosure acts as a spring, which helps control the movement of the cone. When the speaker moves out, the pressure inside is decreased. When the speaker moves in, the pressure inside is increased.

Ported/vented enclosures:

This design requires a more scientific approach, and there is less room for error in design and construction. Ported cabinets allow for an extended bass response. The result sonically is more “rumble” and deeper bass tone. A port (or vent) is used to tune the enclosure to a specific frequency (Fb). The surface area and length of the port are crucial to the tuning. The Fb of the enclosure does not change with speaker selection, but F3 does. The port uses the speaker’s rear output to enhance the speaker’s front output, which increases bass output (or SPL) above F3 (see Figure 3 again). This minimizes the movement of the speaker cone, so mechanical power handling at and above the tuning frequency is very good. The port is actually producing most of the output at the tuning frequency and the speaker’s excursion is minimal. Distortion is lower at this point due to less cone movement.

The article also discusses pros and cons of each design type. There are many other articles on the web. You might also do a search for "thiele small parameters".

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