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Electrostats vs conventional drivers
Electrostats vs conventional drivers
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Old 10th February 2019, 07:42 AM   #141
golfnut is offline golfnut  New Zealand
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Pressure source vs volume velocity source... a bit tricky, here goes...

You could think of the electrical equivalents as a voltage source (pressure) and a current source (volume velocity), with both driving the acoustic resistance of the air.

With a conventional speaker, the motion of the cone is almost entirely determined by the force from the electromagnetic motor and the mass of the cone. This motion would be the almost the same in the absence of air. Think of the cone sweeping out a given volume per cycle (hence volume velocity). So the conventional speaker has the same displacement independent of the atmospheric pressure. As the atmospheric pressure goes up, the acoustic resistance goes up, and the acoustic pressure generated by the speaker increases.

The electrostatic motor applies a constant force per unit area to the membrane, and if the membrane is massless, 100% of the pressure is applied to the air. If the atmospheric pressure drops, acoustic resistance drops, the acoustic pressure must still be the same, so the displacement of the membrane increases.

So at high altitudes (Denver or Black Hawk say - low atmospheric pressures), the SPL from conventional speaker goes down. With an ESL, the SPL stays the same, but the membrane displacement goes up, and they cannot play as loud without the membrane hitting the stator.

Note - I've omitted the complicating factor of different frequency dependence of the acoustic resistance for the two speakers, but hopefully you get the idea.
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Old 10th February 2019, 06:50 PM   #142
kazap is offline kazap
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Quote:
Originally Posted by golfnut View Post
Pressure source vs volume velocity source... a bit tricky, ...... hopefully you get the idea.

Thanks. That's very clear and helpful.
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Old 10th February 2019, 07:03 PM   #143
bentoronto is offline bentoronto  Canada
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Electrostats vs conventional drivers
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Originally Posted by golfnut View Post
...Obviously it depends on the extent of your acoustics training....
Before my copy of "Einstein at Breakfast" reaches me, I'd like to ask a physicist a question.

Forever I've been ridiculing cone-drivers air mismatch with "... shaking a heavy piece of cardboard at thin air..." as compared to light-as-air ESLs. But can you or Bolserst provide an everyday tactile analogy to impedance mismatch?

Like in the form of: riding a see-saw with a 2 lb bag of sand, an automobile going up a hill, 3 ice cubes in a bucket of water....

B.
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Old 10th February 2019, 11:43 PM   #144
tacit_tactix is offline tacit_tactix  Canada
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Gears on a bike, going up a hill. The transmission of energy is optimized with things like horns. Bass ports acoustically amplify through resonance, but horns couple the transfer of volume velocity to acoustic pressure, like gears they optimize efficiency in energy transfer by matching the load at the point of transfer.
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Old 10th February 2019, 11:44 PM   #145
tacit_tactix is offline tacit_tactix  Canada
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golfnut I have no problem with what you said. I was not talking to you about being dissapointed.
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Old 11th February 2019, 07:36 AM   #146
golfnut is offline golfnut  New Zealand
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Hi B

“…provide an everyday tactile analogy to impedance mismatch?”

Hmm, another tricky one. I’ve not checked this, but the following is what I expect to happen….

Imagine you have a longish piece of rope, and you shake one end and send a pulse down the rope...(silicon rubber tubing might be better if you want to do the experiment – something with not much loss)

If the far end of the rope is waving free in the breeze, it will shake about and the pulse will return down the rope back towards you, it will have been reflected.

If the rope is anchored on a wall, the wave will again be reflected, and it will come back towards you, but the shape of the pulse will be upside down from before.

If the rope is connected to another piece of rope of the same mass per unit length, the wave will not be reflected at all but continue down the second piece of rope.

You can also imagine that when the second piece of rope has a different mass per unit length, that the wave is partially reflected, and partially transmitted – also the phase of the reflection depends on whether the second piece is heavier or lighter that the first. 100% of the energy is transferred only when the impedances are matched. The reflectance or transmittance of the interface between the two regions depends on the ratio of the characteristic impedances.

The same thing happens with all sorts of different waves when they cross boundaries into regions of different characteristic impedance. An acoustic wave in air for example is reflected off the surface of water, because of the impedance mismatch.

An interesting trick is to insert a third piece of rope between the first two, but with a carefully chosen intermediate mass per unit length. Then there is a reflection from both interfaces. If the spacing between the two interfaces is correct, the two reflections will be out of phase and cancel each other– you get no reflection and 100% transmittance - this is how antireflection coatings on glasses work. But it only works over a narrow range of frequencies – the wavelength has to be right.

Another trick is to have a graded connection between the two section of rope, say several different sections, all with the same impedance ratio to their neighbours (ie an exponential match) then there will also be very little reflection. This is how stealth technologies work, e.g on a submarine – multiple layers of rubber of different densities to provide an impedance match between the water and the steel. In this way there will be very little acoustic reflection from the surface and it will be invisible to sonar. The rubber is also lossy so the energy is dissipated in the rubber. The big rubber wedges in anechoic rooms do the same thing.

Hope this makes sense
R
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Old 11th February 2019, 01:54 PM   #147
tacit_tactix is offline tacit_tactix  Canada
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And you can model the reflected portion, the transmitted portion and the absorbed portion, with two port anaylsis and putting the pressure equations in a matrix, and solve in matlab to plot the imaginary terms to show the phase shift. The classical way to measure absorption is with an impedance tube. It might be an interesting exercise to plot the radiation impedance of a flat panel vs a cone in matlab. Have you ever done anything like that?
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Old 11th February 2019, 04:03 PM   #148
tacit_tactix is offline tacit_tactix  Canada
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Originally Posted by bentoronto View Post
I gather you didn't like that I described your posted statements about acoustics as rudimentary and hidebound to a flawed grasp of real-world room behaviour.

But I will stick with that assessment.... as would the late and great Linkwitz.

B.
This is what I consider disappointing. "Rudimentary and hidebound to a flawed grasp of room acoustics." please say something constructive, explain why you said this, or just remove it.
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Old 12th February 2019, 12:10 AM   #149
bentoronto is offline bentoronto  Canada
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Electrostats vs conventional drivers
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Originally Posted by golfnut View Post
Hi B

“…provide an everyday tactile analogy to impedance mismatch?”

Hmm, another tricky one. I’ve not checked this, but the following is what I expect to happen….

Imagine you have a longish piece of rope, and you shake one end...
Thanks for elaborate metaphor. But not quite quite... visceral or intuitive.

Perhaps impedance matching to air can be pictured using a circuit that needs a transformer but aint got one (duh). Or an airplane wing lift angle. A Rice-Kellogg driver is like...

Not any kind of direct analogy, but as an example of inelegant design, I liken an R-K driver to powering a vehicle with an internal combustion engine (that has zero torque at stall speed) to an electric motor that can have max torque at zero and no stall speed.

B.
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Old 12th February 2019, 12:20 AM   #150
tacit_tactix is offline tacit_tactix  Canada
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He literally described the three phenomena, transmission, reflection, and absorption, and gave simple visual analogies of real world cases.



Impedance matching is about optimizing energy transfer, doesn't matter if its electrical, acoustical, or mechanical, the simplest visual analogy is literally gears. Low gear, up hill, high gear down hill.

The reflection, absorption, and transmission was a nice touch, and the rope is an easy way to picture it.


You should just thank him.
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