The acoustical nature of concrete depends largely upon its surface texture; an open surface such as concrete building blocks being considerably more absorbent than a smooth cement-rich cast/moulded surface as seen in paving slabs or garden ornaments.
Sounds like an excellent project. It always surprises me that composites and alternative cast materials are so rare in speaker DIY as the experimentation and possibilities of reinforcement and damping filler materials is almost endless and could yield some very special properties. As an aside, Kevlar is know for its damping properties compared with other fibres.
From a quick search:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7830111/
https://www.arup.com/projects/acoustic-concrete
https://www.sciencedirect.com/science/article/abs/pii/S2352710219325847
Basically it seems to be a deadening material, you can read the articles.
No ties to any of the above.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7830111/
https://www.arup.com/projects/acoustic-concrete
https://www.sciencedirect.com/science/article/abs/pii/S2352710219325847
Basically it seems to be a deadening material, you can read the articles.
No ties to any of the above.
I think it is a brilliant idea. Stiff, dense and cheap; also the possibility of adding damping materials such as rubber crumb/Kevlar pulp and special aggregates giving it metal-like densities (5g/cm3) endows it with unique and I suspect beneficial qualities.
The thing is that most of the additives will be small, less than 3 mm, and to get strength you need bigger than 8 mm or so materials into the aggregate.
Strength of this material in thin sections could be an issue.
Those people standing on slabs may be on steam cured slabs, or with special additives.
And you can safely use 50-50 epoxy and neat cement to get speed and strength.
We used to fix grinding wheels, 800 mm OD and 150 thick, ID was about 600, so they were like a ring, to the plates on a special grinder, two plates at 1500 rpm each driving a wheel, as to grind the ends of leaf spring eyes.
We used Araldite at first, expensive and 24 hours, later on the advice of a good friend, we switched to a mix of epoxy and neat grey cement, it stayed strong, setting time became 6 hours, all were pleased with the result.
The leaf spring factory belonged to my late uncle, and I worked there in my vacations from engineering school.
Not many practical engineers here in India, able to turn and weld metal...or set up an engine valve and timing job.
So what happens when you make speaker housings with epoxy / cement aggregate will not be easy to predict.
You can always go for sandwich construction, put foam or similar in the gap, or cast one layer, add foam, cast another layer.
And bear in mind that the normal speaker materials are a little more flexible than concrete, so the acoustics in concrete will be more treble, lots of bass will be lost, in my opinion.
As an example, think of the acoustics of a speaker made of marble or granite.....you can make one of cut pieces of stone, joined with cement or epoxy.
I also remember speaker housings made of acrylic sheet, those were never very popular, again the sheet has excess damping properties for some frequencies.
Strength of this material in thin sections could be an issue.
Those people standing on slabs may be on steam cured slabs, or with special additives.
And you can safely use 50-50 epoxy and neat cement to get speed and strength.
We used to fix grinding wheels, 800 mm OD and 150 thick, ID was about 600, so they were like a ring, to the plates on a special grinder, two plates at 1500 rpm each driving a wheel, as to grind the ends of leaf spring eyes.
We used Araldite at first, expensive and 24 hours, later on the advice of a good friend, we switched to a mix of epoxy and neat grey cement, it stayed strong, setting time became 6 hours, all were pleased with the result.
The leaf spring factory belonged to my late uncle, and I worked there in my vacations from engineering school.
Not many practical engineers here in India, able to turn and weld metal...or set up an engine valve and timing job.
So what happens when you make speaker housings with epoxy / cement aggregate will not be easy to predict.
You can always go for sandwich construction, put foam or similar in the gap, or cast one layer, add foam, cast another layer.
And bear in mind that the normal speaker materials are a little more flexible than concrete, so the acoustics in concrete will be more treble, lots of bass will be lost, in my opinion.
As an example, think of the acoustics of a speaker made of marble or granite.....you can make one of cut pieces of stone, joined with cement or epoxy.
I also remember speaker housings made of acrylic sheet, those were never very popular, again the sheet has excess damping properties for some frequencies.
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The standard speaker cabinet material is wood-based sheet goods (MDF, plywood, etc). It works really well, its easy to construct with, and it is not expensive. Any other material will need to be compared to this.
In order for me to consider GFRC for cabinets, I would need to understand what problem or unmet need is this material going to address? in other words, I would have to identify a deficiency in wood-based sheet materials, and then understand how GFRC addresses that deficiency. I would also need to think about those aspects in which GFRC is inferior to wood-based sheet materials, so I could understand what performance aspect I am gaining, and what I am losing.
At this point, I can't think of any serious deficiency with wood-based sheet materials. So to the proponents of GFRC, can you identify what the advantages would be ?
j.
In order for me to consider GFRC for cabinets, I would need to understand what problem or unmet need is this material going to address? in other words, I would have to identify a deficiency in wood-based sheet materials, and then understand how GFRC addresses that deficiency. I would also need to think about those aspects in which GFRC is inferior to wood-based sheet materials, so I could understand what performance aspect I am gaining, and what I am losing.
At this point, I can't think of any serious deficiency with wood-based sheet materials. So to the proponents of GFRC, can you identify what the advantages would be ?
j.
While not using concrete the late Max Townshend made a loudspeaker called the Glastonbury using a metal cabinet lined with fibre
reinforced plaster. The plaster expanded slighty when it set making the cabinet exceptionally rigid while the fibre provided some damping.
The poured mix took some considerable time to dry out so in spite of a forced warm air drying process some speakers still retained some
of the moisture which caused internal corrosion over time which resulted in expensive warranty claims.
I'm sure GFRC doesnt suffer from this.
reinforced plaster. The plaster expanded slighty when it set making the cabinet exceptionally rigid while the fibre provided some damping.
The poured mix took some considerable time to dry out so in spite of a forced warm air drying process some speakers still retained some
of the moisture which caused internal corrosion over time which resulted in expensive warranty claims.
I'm sure GFRC doesnt suffer from this.
Those articles prove the opposite of what you are claiming. They are all investigations into making concrete more acoustically absorptive, precisely because it so rigid and reflective in the first place.
So do you favor materials that are so non-absorptive that they radiate every bit of sound energy put into them, back in to the environment? So high mass, or CLD methods would be bad?
It would look cool! Also I could imagine an economic benefit to a small-volume manufacturer when the cabinet could be poured in one piece and little or no finishing required. Compare that to the time it takes to build a cabinet and finish it with wood. I've actually been thinking about trying this lately and see how it compares to the other cabinet materials I've been measuring.
Transport will be a difficult topic, say sending speakers to Darwin by road train from Perth.
I think this topic is not well studied, which is why I pointed you at those articles.
And the properties are variable, if different sizes of aggregate are used.
Also, how do you bond the sheets, or do you cast the whole thing as one piece?
If you feel it would be a good idea, go ahead and make some units for trials.
The thing is that I have extra speakers, and no, I am not a speaker designer, just a listener.
I did say compare the materials, and their properties. Cement is too rigid and reflective for speakers, until modified, and the modification methods are not well studied.
My wall mounted speakers must be 2 kilos each, I suspect the cement version would go to 10 kilos.
That extra rigidity will affect the sound, good or bad needs to be investigated.
I think this topic is not well studied, which is why I pointed you at those articles.
And the properties are variable, if different sizes of aggregate are used.
Also, how do you bond the sheets, or do you cast the whole thing as one piece?
If you feel it would be a good idea, go ahead and make some units for trials.
The thing is that I have extra speakers, and no, I am not a speaker designer, just a listener.
I did say compare the materials, and their properties. Cement is too rigid and reflective for speakers, until modified, and the modification methods are not well studied.
My wall mounted speakers must be 2 kilos each, I suspect the cement version would go to 10 kilos.
That extra rigidity will affect the sound, good or bad needs to be investigated.
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My idea would be to cast all but the bottom as one piece. The bottom would be cast separately. That should make the form easy to construct and detach from the concrete. I hope.
I would be looking at modern methods used for countertops and such. So GFRC, polymer and flow additives, and pumice for aggregate. I'm thinking about 5/8" walls and maybe 1" for the baffle. I've wanted to try this for countertops for years, just haven't had a chance, but I really like what they are doing with concrete these days.
I would be looking at modern methods used for countertops and such. So GFRC, polymer and flow additives, and pumice for aggregate. I'm thinking about 5/8" walls and maybe 1" for the baffle. I've wanted to try this for countertops for years, just haven't had a chance, but I really like what they are doing with concrete these days.
Read up on concrete additives for waterproofing and form release, that will help you a lot.
Ready sheets are available in cement aggregate, so you could try those instead of casting.
Ready sheets are available in cement aggregate, so you could try those instead of casting.
This is how I did mine back when I used epoxy. Otherwise it's almost impossible to get rid of any air bubbles. The joint in between the cured and fresh epoxy has to be wetted so there's a uniform bond. Some casting agents don't like to have a junction to adhere to. There are definitely some engineering challenges working with casting materials and getting a predictable end product.
That’s correct. They were made from concrete, with no reinforcements like steel or glass fibre.
And I remember the Onkel Focal, where the “egg” was indeed GFRC.