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Unless you fill it with something other than air. I don't see it making a difference. The sound would largely pass right through the inner tube and use the air inside as air volume just the same.
Does it matter that the inner tube's air content is acoustically translucent/transparent more than than the cabinet's volume is changed by it's displacement?
Will the inner tube's air volume change the impedance of a driver mounted inside the cabinet?
Things I think about while tuning up my table saw . . .
Here are my thoughts. The air volume inside the cabinet is a damper, like a shock absorber. While sound can pass through the rubber membrane, the air itself cannot, therefore it will effectively reduce the air volume needing to be compressed by the speaker itself. I.e., the rubber membrane is no less "transparent" to the air molecules than a piece of MDF. I think this will work, but I think you might see something in the impedance curves that acts a bit like a ported enclosure, depending on the pressure of the bladder inside the box. I also think you have a good chance of blowing out your box with the pressure if you overdo the inflation. 50 psi on a panel that is 100 in2 is 5000 lb force.
I have not fact checked this, but I found that 170db SPL (super crazy loud) is about 1 psi. 110db is about .001 PSI. The offset pressure in your bladder to reduce the effective volume seems to be very low, less than 1 psi.
https://youtube.com/watch?v=Ocoi_Nh0u-c
Thanks for your excellent thoughts, rstillin.
Got a DATS, Steve? If you do this, it would be interesting to see before & after impedance plots.
They say, in physics, everything is a spring.
Personally, I don't think it'll work as a volume displacement method unless that inner tube is filled with something incompressible. I suspect it would act more like an aperiodic vent or perhaps simply an enclosure with a variable amount of stuffing.
I mean the entire "bladder" will expand and contract in response to the change in pressure from the woofer.
As mentioned, the air inside the innertube will compress just like the the air outside the innertube. The only change it should make is whatever percentage difference in air density results inside the innertube from the tension of the innertube, which will probably not be much.
You would need to put something non-compressible (for the most part) in the bladder, like water. It may jiggle and do weird stuff from the sound waves but should remain constant in volume.
Yes, you effectively have two springs. If the bladder is inflated to 1 psig, and the box is pressured to 0.001 psig, the bladder volume will change very little. It's like a ballpoint pen spring on top of a car strut spring. Compress the ballpoint spring 1/2 it's length, and the strut spring appears uncompressed. Place the pen spring on another pen spring and they both compress equally.
Relatively easy experiment for anyone to try. Take a 2 litre bottle and put a 3-4" speaker on the big end. Measure with bottle cap on, cap off, and balloon inflated inside using up 1/2 the volume. You should have a 2l ported, 2l sealed , and 1l sealed enclosure measurement you can compare to modeled results. You will also see if the balloon compresses much in the process.
I think the pressure inside the tube/bladder needs to exceed the maximum pressure that the woofer can deliver in order to be effective. I found a suitable innertube and low pressure gauge on ebay:
https://www.ebay.com/itm/404771948857
https://www.ebay.com/itm/322004932237
I may order them in the future as I feel boredom sinking in or just get tired of building more test boxes of differing internal volumes . . . fun to think about - thanks for all the replies, guys.
Are the walls of a hot water bottle thicker than an inner-tube? That might work too.
I can also pressurize the bladder with air and pump water into it if needs be and invert the cabinet to allow the air pressure to expel the water . . .
Hot Water bottle should work as well - just need a plug and fill method that can withstand some serious air pressure.
Make yourself some wooden blocks to stuff in.
I use to dump wood pellets in the port to make test changes. Now I just use them to measure volumes that are irregular.
Use blocks of closed cell styrofoam, i.e. blue or pink insulation foam to adjust the volume. Not as easy as and inflatable object, but more known volume.
To change the volume of a cabinet I would use styrofoam blocks also. Note, Steve wants to put the inner-tube valve through the terminal plate so he can adjust the net volume WITHOUT actually getting back inside the cab by removing a driver, baffle, etc.
Consider it electro-mechanical Q adjustment - your song list could have a different PSI setting programmed with an Arduino controller via compressed air supply, pressure transducer and E/P {electric to pressure} valve . . . Just kidding, but it could be done.
A4e has it right - just want to tweak the volume of a cabinet +/- some volume to get the perfect Q for each driver under testing without all the tear-down and reassembly - BTDT too-oooo many times already.
I can't recall a time I actually wanted less volume in a cabinet. Most of my challenges have been tuning the port to match the cabinet volume
Dan N made an adjustable port on one of his speakers. In my opinion, an adjustable port makes more sense than an adjustable volume.
A switch to go from ported to sealed would be nice in some situations. I could see it working well for a speaker system that had to do home theater and 2 channel listening. Sealed with subwoofers for HT duties and ported no subwoofer for 2 channel. Something like exhaust cut outs.
if it is for test cabinet and you want to try out a bunch of 6-7 inch drivers on a 9" baffle, then make a removable baffle, that you can use to mount the different drivers and keep a bunch of standardized wooden blocks that you can chuck in along with an adjustable port to try out different box tuning - the PE adjustable ports with masking tape holding the joint will do fine for testing.
I'm messing about with 12" woofers right now and building WinISD modelled DSP boxes . . . 6 ~ 8" MW's are next.
It was just a passing thought I may play with later.
This thing has my attention at this price point for some reason too --> https://www.madisoundspeakerstore.com/sb-audience-pro-sound/sb-audience-bianco-8mw125-8-mid-woofer/
I did an experimental cabinet many years ago to with adjustable "volume". I used wood blocks and would fill the cabinet with varying amounts of the wood blocks to change the internal cabinet volume. There wasn't much bracing in the cabinet so I could reach my hand inside the cabinet through the woofer hole easily. I really don't think an air filled tube would have anything resembling a predicable outcome. MOST of the sound waves would easily go through the rubber bladder of the tube. // I also implemented the idea of a removable baffle cabinet. I did this many years ago and still occasionally use this test cabinet.
So I decided to tackle a project to make a plant stand woofer box, or possibly the bottom end of a 3way. I wanted to compare sealed, ported, and bandpass arrangements before finalizing the design, but I didn't want to build a bunch of boxes, so I decided to make a rig that can do all without any glue or destruction to the enclosure.
Figured I would share here in this thread as I think this is easier than a bladder or volume stuffing arrangement.
I use three ratchet straps to tighten down the tube on each ring, and I created a volume chart on the outside, so I can measure and set the volume in 1L increments. Using a yard stick to measure inside and out, I set my straps and tighten slightly, adjust the ring to be perpendicular, and voila, variable volume cabinet that you can swap out ports, and slide various volumes.
Here are some pics of the rig, currently set for 4th order bandpass testing with a single 8" woofer.
I have about 23L on the sealed side, 9L on the ported side, and a 3" port tube to tune to ~65Hz:
Brilliantly done, rstillin!!
Thanks for posting this as I need some sonotube anyways for baffle edges . . .