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Old 15th December 2008, 02:25 PM   #1
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Default What power do I need to move vertically 20kg with 4 speakers ?

Hi everybody !

First, I apologize for my poor english (it isn’t my first language).

I would like to move vertically (in oscillation) 20kg with 4 speakers in phase.
Frequency : between 20 and 200 Hz. Amplitude : less than 1 mm (50 microns).

My questions are :
- What power do I need ?
- What cheap amp and speakers can I buy ?

---

Some explanations :

I would like to move a giant mixture of non-newtonian fluid to make Faraday waves.

- This non-newtonian fluid is made of cornstarch and water (so, it is edible by children).

- Faraday waves are a very funny phenomenon. Here is a famous video about it ("fingers" start at 2:00) : http://www.youtube.com/watch?v=nq3ZjY0Uf-g

---

Every year, I build a machine for children. Last year, it was a vortex cannon called "boum".
(Examples : http://www.youtube.com/watch?v=lMLop6MIwUU)

If I succeed, the new machine will be called "life".

Thank you very much for your help !
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Old 15th December 2008, 03:38 PM   #2
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Hi,

Amplitude < 1mm does not mean a lot because anything
can be moved a small amount by a small amount of energy.

There are bass shakers :

http://www.parts-express.com/pe/psho...28&ctab=1#Tabs

But these exploit efficiency around a resonance to
be effective which is why they are not widerange.

Doing 20Hz to 200Hz at whatever "g" is required is a different matter.
I'm assuming here you are not talking about 20Kg of mixture, it is
fairly obvious no cheap amplifier or speakers can move around 20kg,
given a typical 200mm subwoofer cone mass might be 20 grams.

For a resonant system, i.e. at a fixed frequency, e.g. 100Hz, it will
be a lot easier to build up the amplitude and the related "g" force
of the related acceleration, velocity and amplitude.

The higher the frequency the lower the amplitude you need.

If you do literally mean 20Kg of mixture ("giant") you also need
to specify your target g force and the frequency. Without a
resonant system at that frequency you will struggle.

Power requirements are difficult to estimate becuase it will
depend on system losses, not a direct brute force input.

/sreten.
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Old 15th December 2008, 03:46 PM   #3
HK26147 is offline HK26147  United States
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If bass shakers don't work,
Google "shake table" and you'll find thousands of references and DIY articles.
Shake tables are used for physics demos. and more practically for testing the sturdiness of consumer products.
The low frequency and strength they provide may be what you are after.
Interesting project...
Syd
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Old 15th December 2008, 04:02 PM   #4
HK26147 is offline HK26147  United States
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May I assume you had something like this in mind:

http://experiments.magnify.net/video...-in-a-Shaken-C

I have seen many physic demonstrations on line ( similar to this one ).
I am sure if you e-mailed them they would tell you how they set up their demos.
I noticed the mass they used was a fraction of what you requested. The demo can be performed with less mass.

Syd
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Old 15th December 2008, 09:54 PM   #5
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Thank you very much for your answers.

Sreten : I do try to move 20 kilos of fluid, not 20 grams. I would like to build a table big enough to have 5 or 6 people around, not a small laboratory plate.

This size causes 3 problems :

- The stability. The table has to stay horizontal, and move only vertically. If I have more than one support, they have to move in phase.

- The adjustment. I need to change frequency AND amplitude because the phenomenon is unstable. I didn't find any DIY shake table which allows the two parameters.

- The noise. This is just for comfort, but this is important. I think electro-magnets are more silent than a engine. That is why I tried to do with speakers.

As you notice, I dont know how to estimate Power requirements. Any ideas ?

---

Yes HK26147, I started with this video (In my first post, I give the youtube link). My calculations are based on their parameters :

At 2:00 on the video, I read : f=120Hz and a=25g.

If the movement of the table is : A(t)=A.sin(w.t)
Acceleration is : A''(t)=A.w^2.sin(w.t)
a=25g means : A.w^2=25
w=2.pi.f=750
so A = 25 / 750^2 = 4 E-5 m = 40 microns.

I hope I have the good parameters.
The problem is how to do it on large scale, in concrete terms...
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Old 16th December 2008, 12:05 PM   #6
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK
Hi,

I cannot estimate the power requirements except for the brute
force approach, which with 20kg is simply not going to work.

If you want variable frequency I'd say you have to make it
mechanically adjustable (e.g. number of springs) and also
make the electronics self oscillate at the natural frequency.

/sreten.
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Old 16th December 2008, 12:30 PM   #7
HK26147 is offline HK26147  United States
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This is not a job for 4 speakers,
I suggest
http://www.dataphysics.com/products/...mily/index.htm
http://www.dataphysics.com/products/...kers/index.htm
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Old 16th December 2008, 02:27 PM   #8
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While looking for my tweeter diagram, I found this...

Made this to shake a 7.5 ton granite table. 25 watts per unit.

at 90 hz, it was possible to feel some vibration, but can't tell you if it meets your needs..

Cheers, John
Attached Images
File Type: jpg aura bass shakers.jpg (57.7 KB, 307 views)
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Old 16th December 2008, 06:23 PM   #9
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A industrial "shaker table" is basically a loudspeaker without a cone. Maybe one of the table manufactures would have a chart.
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Old 17th December 2008, 01:49 AM   #10
Ron E is offline Ron E  United States
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F=ma

m=20kg
a=25g
F=20*25*9.8=5000N

For a loudspeaker motor F=B*l*i

So for one speaker Bl=20 (some pro drivers - most drivers have Bl~10) you need 250 amperes. That's a big amplifier

You might wish to also calculate the SPL of whatever area of fluid you are thinking of moving at the excursion you are planning. Might be a fair amount of noise the larger the container gets...

The larger the container gets the more likely you will have modal resonances in the container that may or may not be interesting to watch...
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