Materials Science and Speaker Enclosures

[BKr0n]

Lately I've been reading up on different materials that they use to make speaker enclosures... which lead me to even more materials NOT used in speaker enclosures (yet). I've even jumped down the composites rabbit hole... it's deep. But something sticks out to me: there doesn't seem to be a clear consensus on design philosophy. It seems it all comes down to "it depends" but I do know that some materials are better than others. Some say it's density that's the most important aspect, others say it's ridgitity. Some say acoustic lining is good, others say it's bad. But this is audioSCIENCEreview dammit! What does the science say on materials selection for enclosures?

 

[DVDdoug]

It seems it all comes down to "it depends" but I do know that some materials are better than others.

It's true! Speaker design involves many decisions & trade-offs, not just the cabinet material. Whatever a manufacturer chooses will be touted as an advantage/feature.

In a good design the material shouldn't have any effect on performance. Speaker design software usually doesn't ask you what you're making the cabinet out of.

If you want a particular shape, some materials are easier to shape than others. Portable PA speakers often use plastic (and neodymium magnets) to reduce weight.

Some say it's density that's the most important aspect, others say it's ridgitity.

I don't see much advantage to density if it doesn't give you rigidity. But density/weight would reduce mechanical vibrations which are sometimes transmitted to a shelf or desk.

Some say acoustic lining is good, others say it's bad.

I don't see how it could be bad. It helps dampen internal resonances. In the the case of a subwoofer (or depending on the crossover frequencies) the wavelengths are much longer than the internal dimensions so internal resonances are out of the operating range and lining may not make a difference.

 

[BKr0n]

In a good design the material shouldn't have any effect on performance. Speaker design software usually doesn't ask you what you're making the cabinet out of.

I mean wouldn't certain materials push you closer than ideals than others? Also, wouldn't some materials allow you to, in some cases, overcome structural limitations you wouldn't be able to without them?

I don't see much advantage to density if it doesn't give you rigidity. But density/weight would reduce mechanical vibrations which are sometimes transmitted to a shelf or desk.

So then would you say ridgidity is more important than density? Or does one simply predicate the other?

I don't see how it could be bad.

Yeah that one I don't get myself, but it seems to be a factor in bass response in some designs, as well as a factor in things like coil heating and overheating electronics... I also think if it's designed well those things are nonissues

 

[DVDdoug]

I mean wouldn't certain materials push you closer than ideals than others? Also, wouldn't some materials allow you to, in some cases, overcome structural limitations you wouldn't be able to without them?

If you were to build multiple "identical" speakers with different materials, I doubt you'd hear or measure a difference as long as none of them resonate, rattle, or vibrate.

If I was buying a speaker, I'd only consider cabinet material as it affects appearance (and possibly weight).

There's not a lot of "structural stress" in speaker. It's not like a compressed air tank, or a concrete block holding up a house.

 

[Timcognito]

Open baffle, in wall, cardioid designs only mitigate the vibrations from the speakers where as closed box and to a greater extent ported designs combat fluctuations of internal air compression vibrations and standing waves leading to the need for rigidity. There several schemes, bracing, material properties, thickness and others that are easily accomplished becoming more difficult as cabinet size, and volume lead to larger spans and more weight. It's not biggest issue in design except in designs like in-wall speakers where the wall construction is not speaker construction from the start..

 

[BKr0n]

There's not a lot of "structural stress" in speaker.

What about for things like subwoofers, where all that energy becomes omnidirectional? Wouldn't you need to design around that fact?

Open baffle, in wall, cardioid designs only mitigate the vibrations from the speakers where as closed box and to a greater extent ported designs combat fluctuations of internal air compression vibrations and standing waves leading to the need for rigidity. There several schemes, bracing, material properties, thickness and others that are easily accomplished becoming more difficult as cabinet size, and volume lead to larger spans and more weight. It's not biggest issue in design except in designs like in-wall speakers where the wall construction is not speaker construction from the start..

What about for things like active speakers, where the enclosures are really small? I know there's lots of speakers with cardioid design, but I would imagine you would still need to be meticulous in the enclosure design to function as intended.

 

[terryforsythe]

Lately I've been reading up on different materials that they use to make speaker enclosures... which lead me to even more materials NOT used in speaker enclosures (yet). I've even jumped down the composites rabbit hole... it's deep. But something sticks out to me: there doesn't seem to be a clear consensus on design philosophy. It seems it all comes down to "it depends" but I do know that some materials are better than others. Some say it's density that's the most important aspect, others say it's ridgitity. Some say acoustic lining is good, others say it's bad. But this is audioSCIENCEreview dammit! What does the science say on materials selection for enclosures?

My perspective is that whatever material results in the lowest cabinet vibrations is best. How I reached that conclusion is playing with my own speaker cabinets. I added additional bracing and glue to reduce cabinet vibrations, and added a lot of damping material (I used fiberglass, but rock wool probably is safer). Those modifications served to reduce harmonic distortion quite a bit, 7 or 8 dB at the most offending frequency.

I'm not a materials scientist, but if I were to speculate, I think you would want both high density and high rigidity for the cabinet material. To some extent the two go hand in hand. Granite would work well. There are some speakers make the cabinets out of granite slabs, but they are very expensive.

 

[Timcognito]

What about for things like active speakers, where the enclosures are really small?

Small means rigid without the longer spans. Take coat hanger wire and bend with your hand 1 ft apart then 4 inches, the same applies to vibration.

 

[BKr0n]

So ridgid, dense, and inert. Sounds a little like Hoffman's Iorn Law. I don't think you could get all three of those materials in one. Which I'm assuming then is why speaker manufacturers mix and match materials for their enclosure.