How can I build a better pulsed pressure generator?

The compressor, or the Cobra ?:-)

I had a mechanical engineer friend, Laird Pettit, who was re-building a '64 Mustang... working on the engine on an engine stand in his living room ;-)

His wife, Carol, finally threw a hizzy and said out with it. I bought it, literally a Mustang in a box, and gave it to 16-year-old-son Duane, who had the engine assembled and running in two days... in our carport, started it up, came running into the house, "Can you hear that, Mom?"

Who couldn't? A 289CID engine running with no muffler... shook the whole house. ...Jim Thompson

Both, but the Cobra was louder. When he started it up the leaves in the driveway flew off.

A friend did that with his motorcycle engine.

Same here, except my car back then was a Citroen 2CV with a whopping 16 horses. Someone had tried to restore the engine which brught it to zero horses. I had it running the afternoon after dad towed it home with me in it. Since it had no battery I couldn't honk when a swarm of bees decided it was better to move from the air filter to the cabin.

When it ran my mom came running onto the balcony, "I thought a helicopter just landed". Without the exhaust it sounded like a super-loud Harley.

Which reminds me. Back in the mid '80's I had a chip probe station in my lab at GenRad... required a vacuum pump to hold the silicon wafers down on the stage.

Noisy sucker (literally ;-)

So I made my own "muffler" for the exhaust outlet from iron pipe fittings... sort of a mechanical equivalent of a multi-stage L-C-R filter. "R" was glass wool packed in the parallel large diameter "C" sections, "L" was small diameter series pipe sections.

Knocked the noise wa-a-a-ay down.

Might be something for you to try if you have to do this regularly. ...Jim Thompson

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If you take a 2" ID tube, close one end, and then drill a hole in the wall of the tube about 1.5" from the closed end, then insert a piston in the tube until it just closes the hole, the volume of air contained between the top of the piston and the closed end of the tube will be about 4.62 cubic inches .

Push the piston 0.36 inches farther into the tube and the volume of the enclosed gas will decrease to about 3.5 cubic inches while its pressure will increase to about 4.3 PSIG.

Now, if you connect a solenoid valve into the compression chamber, and open it quickly, you'll get that 4.3PSI as a pulse to your DUT.

Not really, because you've got the volume of the output hose and the solenoid to contend with, but that can be easily overcome by changing the length of the stroke or the diameter of the bore.

Here's a sketch:

news: snipped-for-privacy@4ax.com

("Joerg air pump" on abse)

If you run the crankshaft at 60 RPM you'll get 1Hz out of the output port, and by opening the solenoid valve at TDC quickly you can maybe get the spectral components you're looking for.

Another way would be to use a linear solenoid to push the piston.

That way you could build up a pressure head, hold it, then use the solenoid valve to shape the leading edge of the pulse and the pusher solenoid to shape its decay.

-- JF

Check the exhaut on this here machine:

Exhaust made from standard pipes, nipples and other fittings. When starting you had to wear eye protection and jump away from the engine the millisecond you think it catches. Because a lot of stuff could be flying out of there with gusto.

I understand you'll need to split a lot of logs this winter season. ...Jim Thompson

I would imagine if you sealed the flap valve in the bellows it do the suck bit as well.

No, we became decadent and "hire it done". It's actually orchard wood, cust and split near Merced and trucked up here. Less expensive than the local guys. Due to ... ahem ... "global warming" our consumption has gone for about 2 cords to 4-5 cords over the last decade.

I like the fuel injector suggestion, open and close times of around 1 msec and designed for 40-50 psi. Usually work from 20-ish up to 70 or 80-ish psi so you can pick a size then tune the delivered volume with pressure. They are designed to be cooled from the flowing fuel so you might have to watch that, just use a peak and hold scheme like the Cherry CS452 or 453, the one that does a linear ramp from zero to 4 amps in 1 msec then pulls back to 1 amp to hold until closing. About the compressor - does it have to be air or would CO2 work? If so, just get a 50 lb cylinder of CO2 and it will last a long, long, long time. CO2 is liquid at room temperature so that tank holds way more than a cylinder the same size of compressed gas like air or nitrogen. From shaky memory CO2 expands about 500 times going from liquid to gas, but the pressure at room temp will be about 500 psi vs. 2200 psi for full pressure on a steel compressed gas tank so the CO2 will last about 500

• 500/2200=113 times longer.

Carl Ijames wrote:

Interesting, thanks, maybe the auto parts store is part of the solution. But that still doesn't rid me of that loud compressor IIUC.

Technically, a long-throw woofer IS a linear motor (I've certainly used them for that). The 4psi thing, though, is gonna mean backpressure on a 6" speaker in the range of 100 lbs of force. So, a voice-coil will have to be wound on a fiberglass former and connected to a serious piston (not just a paper cone) to do this at the speaker face.

Maybe use a phased-array of speakers arranged around a small target volume?

If Jeorg doesn't need the volume of a 6" cone, he might be able to do something like coupling the voice coil of a (reach in hat, pull a common/affordable size out) 15" "musical instrument" woofer to a syringe or other piston (probably one with less resistance than a syringe - perhaps a section of 3-4" diameter accordion tubing acting as a bellows) so that the force is only acting on a small area to get the desired pressure. If you cut the voice coil free of the cone, you will need to fabricate some structure to hold the voice coil in position - which might look a lot like the cone you started with...just don't apply the desired air pressure to the whole cone.

Unfortunately I don't have abse access anymore and such links also do not work in Thunderbird. But IIUC what you described I had tried something similar early on, using a very large syringe. It wore out very fast.

Right now I have a McGyver-style fast valve in there. It is a pressure switch that I can either operate morse-code style by hand or via an excenter disk fasten in the chuck of an electric drill. It's very tired by now.

That's exactly what I am looking for, but as a nice reliable unit with a metal piston, rings and the ability to run oil-less for some tens of hour.

I may need to build something like that but trying to avoid it, or use a local machine shop. Plastic pistons get hot and don't last long. If I had a lathe, well then ...

Not very well. We used "reversible" bellows to get the air out of our huge rubber boat when I was a kid, when vacation was up. That was a frustrating job.

Thing is, fuel injectors handle much, much smaller volumes per "spritz".

I had designed something like that, with even more gusto. But the power needed for this is likely well over 100W, no matter how I apply it.

Cooling is not critical, I can do all sorts of things like immersing modules in water, taking enough breaks, letting ice cubes melt on things.

That's one thing I'd like to avoid, schlepping a CO2 bottle into the office.

So use racing injectors and even more pressure?

Make an array and send different pressures to them; make a pneumatic R-2R DAC. ;)

Tim

Tom Danley is the guy to go to for high SPLs. The Servodrive subwoofer was a product of an earlier company, Intersonics.

Here's a picture:

Here's a link to the patent. Mr. Danley has several for high SPL generators.

Kevin Gallimore

I don't know what brand / part number the solenoid valve was, it was 10+ years ago. It worked up to probably 2 or 3 Hz. It was very high pressure (MPa) & noisy (both solenoid & the exhaust). IF you have slow valves, perhaps two or more could be manifold connected in parallel to increase the operation rate.

It may be global warming, averaged over the whole globe, but it's climate c hange in any specific place. There's been about 1C warming over the last 100 year s - more in the Arctic, and presumably less in some other places - and that means 5% more water in the air over the oceans (on average), and 5% more l atent energy of vapourisation to make 5% more interesting weather when that water vapour condenses out as rain or snow.