Concrete speaker cabinets

[The Grand Wazoo]

I thought you were going to say that they sounded quite pour.

[vinylspinner]

Ideal if you are into the Stones.

Nigel

[lurcher]

That's a great idea as concrete would not be resonant like wood.

More than you might imagine. There is a chap who makes concrete speakers who brought a pair to Owston last year, Simon who just happens to be a Engineer responsible for motorway bridges had a very interesting conversation, and the outcome was that concrete is a lot more resonant that you might expect, and at a highish frequency. His suggestion was to mix rubber particles in with the aggregate to help damp them.

[chris@panteg]

I thought you were going to say that they sounded quite pour.

very good.

[StuN]

I have a book somewhere in my archive about making enclosures out of low-density concrete. I seem to remember that the enclosure was poured in a mould and the front panel was wood and fixed with bolts. I'll try and find it and see if there are any tips about construction methods.

[Barry]

More than you might imagine. There is a chap who makes concrete speakers who brought a pair to Owston last year, Simon who just happens to be a Engineer responsible for motorway bridges had a very interesting conversation, and the outcome was that concrete is a lot more resonant that you might expect, and at a highish frequency. His suggestion was to mix rubber particles in with the aggregate to help damp them.

Concrete, like any other material, does not have a resonant frequency as such. Structures have resonant frequencies, the frequency of which depend on the geometry of the structure with the Q-values (how sharp the resonance is, and how well it is damped) depending on the material properties of the material from which they are made.

[Wakefield Turntables]

Do a search under Alexander accoustics

[Reffc]

Concrete, like any other material, does not have a resonant frequency as such. Structures have resonant frequencies, the frequency of which depend on the geometry of the structure with the Q-values (how sharp the resonance is, and how well it is damped) depending on the material properties of the material from which they are made.

Correct. Even if it was a homogeneous material which DID have a resonance at HF, providing that the resonance occurs above the critical mid range and high into HF response, then that is where we want it....not down low or within mid band. Every structure, be it a laminate or single material has a harmonic resonance point. The speaker designer's job is to ensure that the resonance is low in SPL terms (preferably well under -30dB if panels are considered as a radiator)and high enough in the frequency range (preferably above 10 to 15KHz).

This is where computers have helped in speaker design as finite element analysis is a very good tool for modelling cabinet response, although it can be done longhand (and as a young graduate engineer I remember spending many hours undertaking long hand finite element analysis on various structures). The interesting thing is that with the average cabinet, there are two basic rules when considering how to stiffen an enclosure: Firstly spacing between bracing should never be equi-distant and secondly, there will be an optimum number of braces beyond which you'd get the same result by doubling up cabinet panel thickness.

Concrete's behaviour and stiffness can be vastly improved by introducing glass fibres into the mix (using 10mm aggregate and making walls 50mm thick). Doing this, the average floor stander, let alone stand mount, will need no bracing. It will be stiff enough.

Another way to achieve similar results is to use 18mm Baltic Birch Ply (slow grown and dense) and line the inside with thick ceramic tiles glued to the interior panels. The resulting laminate is incredibly stiff and dense.

Be that as it may, most constructions still benefit from some form of damping to excel as speaker cabinets, just to limit the duration and amplitude of resonance. Whilst some manufacturers tout "tuned" cabinet resonance as part of the bass reinforcement, such designs usually sound too coloured and muddied for my personal preferences.

[Macca]

There are those Bosendorfer speakers that actually have vibrating panels attached to the speaker. Very odd way of doing things IMO

[nat8808]

Except that if you have resonant panels which resonate at a frequency different to the natural resonance of the cabinet it can kind of dampen the cabinet resonance.

There was a sound insulating material talked about in New Scientist once which was only a few mm thick - it consisted of a resonant rubber-like membrane attached to a grid in which each square there was a weighted disc suspended. The weighted discs and membrane each had natural resonant frequencies which cancelled each other out (along with all their harmonics) enough so that the result was a massive attenuation of sound trying to pass through it across a wide bandwidth.

3D printing made it possible for economic manufacture which otherwise was too complex.