Differential Pressure Measurments = Confused

[sr512]

Picture a loudspeaker cone in a water bath. The entire cone is submersed (fluid on both sides of the cone). The face of the cone sees pressure as it tries to compress the water (when driven by the speaker motor), which distorts the shape of the cone. The back side of the cone sees the weight of the water behind it on the return stroke (top of water is open to atmosphere), which distorts the cone in the opposite direction.

I've placed gauge pressure sensors on either side of the cone, as I want to see the signal from each on the scope.

I need to measure the pressure introduced to each side of the cone as it:

1. tries to compress the contained fluid
2. returns and lifts the open fluid

I'm over my head a bit in understanding what the pressure actually is and where it is.

When the cone is not moving, I can measure the pressure of the fluid (weight of the water) as PSI_B. Once I move the cone in the direction of compressing the fluid, PSI_B increases as expected. As I drive the cone the opposite direction, PSI_B will decrease passed the static point.

The PSI_T sensor (on the back side of the cone, which the top of the water is open to air), seems to be almost useless in measuring anything except noise in the system. Of course the water is not trying to be compressed, so there is no real pressure gain behind to measure.

I can see the cone deform in the direction of force being applied to the back of it on its return stroke.

It may be important to note that the cone does not perfectly separate the fluid above and below it. Therefore, if I push the cone by hand in the compression direction and hold it at the peak of excursion, the pressure builds then falls back to its static point as some fluid escapes to the other side of the cone.

I'm pretty confident in the compression stoke measurement from the sensor. I'm not confident at all in the return stroke measurement. Looking at the cone, I can see what appears to be about equal distortion of the cone, yet the measurement from the sensors is quite a bit less. I must be missing something (or somethings) here.

Three pictures are attached from the scope. The Yellow line is the pressure sensor output PSI_B (mounted on the face of the cone which is trying to compress the fluid). They Blue line is the PSI_T sensor output which is on the back of the cone. The Red line is the math between the two inputs (PSI_B - PSI-T).

Thanks for reading this long post. If you can help me figure out what I'm actually measuring, I'll be extremely grateful.

Scott

 

[DuncanM]

What are you attempting to prove/do/discover, and why?

 

[1gibson]

Just based on the shape of the cone, it will capture the fluid and push most of it forward on the "out" stroke. On the "in" stroke, the fluid will be displaced to the sides and generate much less pressure, probably not enough to measure.If you had a driver with a flat diaphragm instead of a typical cone shape, I really doubt you would see much difference at all.

How's this: let's both grab a cup, sit next to the pool, and have a water fight. I'll use the cup normally, you have to hold it backwards and try to throw water without using the inside of the cup. Good luck!

 

[sr512]

I'm trying to prove that the cone can handle x amount of pressure over x cycles.

The cone is not quite as rigid as a standard speaker cone is. It has soft sections throughout the surface. With the water fight example, the cone used in the "normal" direction will be able to throw much more water than the cone used backwards. Both of us would get wet though.

On the "in" stroke, the cone does deflect quite a bit. I can't quite figure out how to measure this though!

Thanks,
Scott

 

[IRstuff]

Your cone is not symmetrical with respect to the stroke axis, so there should be no reason to expect symmetrical behavior.

You could potentially use strain gauges to measure the deflection of the cone

TTFN
faq731-376

 

[IRstuff]

Another thought might be to glue a a thin stick to the cone such that the stick protrudes above the water. You can then compare the motion of the stick to the gross motion of the cone. The difference would be the deflection of the cone

TTFN
faq731-376

 

[MintJulep]

I think you stated the problem exactly in your OP.

The PSI_T sensor (on the back side of the cone, which the top of the water is open to air), seems to be almost useless in measuring anything except noise in the system. Of course the water is not trying to be compressed, so there is no real pressure gain behind to measure.

You are trying to measure a transient pressure pulse as the water is trying to move, but essentially the pressure gradient is unchanged.

 

[racookpe1978]

How have you accounted for cavitation at the surface of the "loudspeaker" where it is pulling back from the water?

At all but the lowest acoustic frequencies, the speaker membrane is going to be moving faster than the water will respond due to gravity.