Silly me.
The foam is a sponge and air is a fluid.
As pressure increases the sponge holds more fluid.
Under fluctuating pressure conditions the sponge alternately absorbs and releases fluid.
For Plummer's sub the release is controlled by the pressure differences in the waveform. That's why only one side of the duct is lined with foam.
BTW interesting picture here:
http://www.pmcloudspeaker.com/transmission.html
The foam is a sponge and air is a fluid.
As pressure increases the sponge holds more fluid.
Under fluctuating pressure conditions the sponge alternately absorbs and releases fluid.
For Plummer's sub the release is controlled by the pressure differences in the waveform. That's why only one side of the duct is lined with foam.
BTW interesting picture here:
http://www.pmcloudspeaker.com/transmission.html
An externally hosted image should be here but it was not working when we last tested it.
doing a proof of concept
Hi Moray, things have gonetohellhere last couple of days so I'm a bit scattered but
I was thinking about proof of concept.
I live in an apartment; don't have direct access to a shop or have much equipment, so I have to plan.
................................................
Need to prove:
That a narrow duct open at one end (the tap), lined with open cell foam on one side,
and pressurized by the tap leading from a sealed chamber pressurized by a speaker,
will reradiate measurable* pressure back into the chamber.
*Measurable defined as differences in speaker operation with and without the duct, and with and without the foam applied to the duct.
.................................................
A box like one of those in the attached drawing?
Adjustable? Have to measure with and without foam, so it has to open. Possibly have to experiment with various depths and PPIs of foam.
I not too keen on the type of box in the picture I attached to my previous post because the size of the pressure chamber and tap aren't really defined - they're all run together. But might it do as a proof of concept? It's simpler to build for sure, and I'd just have to take the side off to change foam, etc. Im not sure whether coarser or finer celled foam would be best at capturing pressure at longer wavelngths - this is different than using it as an acoustic filter.
I'm thinking the duct has to be reasonably narrow to optimize pressurization effects in the foam.
I'm thinking this experiment should be small - using maybe a 3" speaker, but not junk. I think I saw something suitable with an aluminum cone in the Parts Express catalogue at @ $10 or under.
What do you think?
It's getting late. I'll go to bed and dream of transmission lines and measurement requirements
Hi Moray, things have gonetohellhere last couple of days so I'm a bit scattered but
I was thinking about proof of concept.
I live in an apartment; don't have direct access to a shop or have much equipment, so I have to plan.
................................................
Need to prove:
That a narrow duct open at one end (the tap), lined with open cell foam on one side,
and pressurized by the tap leading from a sealed chamber pressurized by a speaker,
will reradiate measurable* pressure back into the chamber.
*Measurable defined as differences in speaker operation with and without the duct, and with and without the foam applied to the duct.
.................................................
A box like one of those in the attached drawing?
Adjustable? Have to measure with and without foam, so it has to open. Possibly have to experiment with various depths and PPIs of foam.
I not too keen on the type of box in the picture I attached to my previous post because the size of the pressure chamber and tap aren't really defined - they're all run together. But might it do as a proof of concept? It's simpler to build for sure, and I'd just have to take the side off to change foam, etc. Im not sure whether coarser or finer celled foam would be best at capturing pressure at longer wavelngths - this is different than using it as an acoustic filter.
I'm thinking the duct has to be reasonably narrow to optimize pressurization effects in the foam.
I'm thinking this experiment should be small - using maybe a 3" speaker, but not junk. I think I saw something suitable with an aluminum cone in the Parts Express catalogue at @ $10 or under.
What do you think?
It's getting late. I'll go to bed and dream of transmission lines and measurement requirements
Attachments
from a gut instinct level...
I like the version with the larger cavity. Don't really know why. Figure that the cavity acts a a perssure eq between the driver and the line. Would think that if we could find an engineer who has a major in the area of fluid mechanics that this sort of thing could be proved on paper no saw needed. That said a small unit (2-3 inch driver) should work to prove or disprove theory. Probably easier to measure response or impedance of the driver rather than pressure at the driver. Your guess is better than mine. Thanks for thinking about it. Regards Moray James.
I like the version with the larger cavity. Don't really know why. Figure that the cavity acts a a perssure eq between the driver and the line. Would think that if we could find an engineer who has a major in the area of fluid mechanics that this sort of thing could be proved on paper no saw needed. That said a small unit (2-3 inch driver) should work to prove or disprove theory. Probably easier to measure response or impedance of the driver rather than pressure at the driver. Your guess is better than mine. Thanks for thinking about it. Regards Moray James.
I made a mistake in my first post. I thought open cell and reticulated foams were the same thing.
They're not.
Reticulated foam is the kind with all the cell walls blasted out. All that remains is the edges and corners. It makes fabulous filters but it doesn't hold on to fluids very well.
Open cell foam is the kind with holes in the cell walls. Common application is cushioning - force displaces air from the application area to adjoining areas. Thus the cushion or mattress conforms to the body shape and force is distributed evenly. Open cell foam does hold onto fluids well.
Areas of application for the two types overlap. This was the source of my confusion.
There is also closed cell foam.
http://www.steplaw.com/opencellfoam.html
http://www.steplaw.com/reticulatedfoam.html
The TBI patent specifies open cell foam.
This makes sense to me because even just on inspection, it's clear reticulated foam is pretty well transparent to low frequencies, and Plummer is talking about his waveguides having a spacing (in the examples given) of between 12mm and 9mm ([0028])! This doesn't leave a lot of room for foam.
The stuff in the picture I posted above sure looks like open cell mattress foam. That's what made me go back and look at things again.
Off to the foam shop tomorrow to see what they have. The guy who used to cut wood for me who was just around the corner has retired and gone - have to find somebody else.
They're not.
Reticulated foam is the kind with all the cell walls blasted out. All that remains is the edges and corners. It makes fabulous filters but it doesn't hold on to fluids very well.
Open cell foam is the kind with holes in the cell walls. Common application is cushioning - force displaces air from the application area to adjoining areas. Thus the cushion or mattress conforms to the body shape and force is distributed evenly. Open cell foam does hold onto fluids well.
Areas of application for the two types overlap. This was the source of my confusion.
There is also closed cell foam.
http://www.steplaw.com/opencellfoam.html
http://www.steplaw.com/reticulatedfoam.html
The TBI patent specifies open cell foam.
This makes sense to me because even just on inspection, it's clear reticulated foam is pretty well transparent to low frequencies, and Plummer is talking about his waveguides having a spacing (in the examples given) of between 12mm and 9mm ([0028])! This doesn't leave a lot of room for foam.
The stuff in the picture I posted above sure looks like open cell mattress foam. That's what made me go back and look at things again.
Off to the foam shop tomorrow to see what they have. The guy who used to cut wood for me who was just around the corner has retired and gone - have to find somebody else.
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