Acoustic magnetic generator.

[itsu]

The resonant frequency is not the bandwidth of the piezo transducer, it is the frequency at which an unloaded transducer has the lowest impedance.
An unloaded transducer will be the loudest a this frequency (and is most likely to be damaged at it), but it will reproduce lower frequencies, too ...albeit at lower amplitudes. 
When a mass (load) is attached to a piezo its resonance frequency decreases proportionally to this mass - just like it does in a mass-spring* mechanical system.

The frequency response of a piezo is very jagged, see the graph below for a common 3cm diameter piezo disk.
This jagginess is why I don't recommend relying on electrical amplitude measurements for determining the standing wave frequency in the piezo and any loads attached to it.

Those pictures are quite revealing, thanks for them.
Not sure what the mass (load) of such a transducer would be, most likely a fluid as in those "mist machines" or cleaning apparatus
I wonder if i could make a frequency resonse graph of my transducer, it seems they where using some (good) microphone (distance 10 cm),
perhaps i could use a pickup coil at that distance.

The literature states, that the pre-loading force pressing on the piezo in rest mode should be 10% of the extension force generated by the flexion of the piezo.  It also recommends some type of glue or gel in the interface gap.
Also, remember that those piezo disks extend flexurally like in Fig.3a and Fig.3b  (somehow most of my students come wrongly convinced that they extend as in Fig.2).

pre-loading force he, so the rods should be inbetween some (flexible) fixture!
Also the both rods would show a 180° out of phase response looking at those pictures.
I could try to glue them to the piezo as the mechanical solution i have now is not really perfect.

Thanks,  regards Itsu

[verpies]

Those pictures are quite revealing, thanks for them.
Not sure what the mass (load) of such a transducer would be, most likely a fluid as in those "mist machines" or cleaning apparatus.

Yes, air, water droplets, water baths, gels, patients' abdomens, a steel beams of a building, welding joints, tungsten gold bars and metal skin of an airplane are all examples of massive loads to the transducer.  In your case the ferrite rods are the load.

I wonder if i could make a frequency response graph of my transducer, it seems they where using some (good) microphone (distance 10 cm), perhaps i could use a pickup coil at that distance.

That would be hard to do, because the unshielded electric field from the piezo would capacitively couple to your microphone and jam its signal.
Even if you managed to measure the frequency response of an unloaded piezo (e.g. optically), it would change immediately upon putting any mass on its surface.

pre-loading force he, so the rods should be inbetween some (flexible) fixture!

That's only needed for optimum mechanical coupling.  A piezo will work even without this pre-loading, albeit not as well.

Also the both rods would show a 180° out of phase response looking at those pictures.

Yes, the compression waves will travel in opposite directions from the piezo, reflect from the free ends of the rods and later come back to the piezo attempting to affect it from both sides (rods have equal lengths so reflections come back simultaneously from both sides*). That's why one narrow pulse would produce an echo across the piezo after ~90µs round-trip delay.  See this animation  and  this applet.

When you visualize this as in Fig.3b, you can see that the amplitude of these compressions will be in-phase on both sides of the piezo but their direction of travel will be opposite. 
Also, ferrite rods are very massive compared to the piezo and these rods represent equal masses on both sides, thus the piezo will stay equally in the middle between the rods as it pushes on them.  If it were not so, then Newton's 3^rd law would be violated or one rod would have to be "more equal" than the other.  - a quote from "Animal Farm".

I could try to glue them to the piezo as the mechanical solution i have now is not really perfect.

If you do that, make sure that the glue is weak enough so the rods can twist-off without breaking the piezo ...or use an ultrasound gel.


* When reflections return to the piezo, the piezo must move in-phase with these reflections in order not to destructively interfere with them.  Constructive interference with reflections is the principle behind standing wave resonance.

[itsu]

Ok,  great info,  also the applet is great to visualize what suppose to happen.

or use an ultrasound gel.

like this?:  http://www.youtube.com/watch?v=m2w_hKvtY5E    (now where is my guar gum powder)


Here a video of my setup as i have it now, sweeping through the frequency range.

Funny is that via the video the audible sound, for me, is reaching much higher as real time.
I can via the video hear the upcomming of these spurious (4-6KHz) audible signals much better too.

No activity in the 12KHz range, so i will glue the rods to the piezo and see if anything changes.

Video here: https://www.youtube.com/watch?v=U0nIM89E2FA&feature=youtu.be


Regards Itsu

[MileHigh]

Itsu:

Gel used in non-destructive testing (NDT), "juice," is a brown goop with a texture something like slightly warm molasses.  I have never looked it up online but for sure you could order it from somewhere in cyberspace.

If you use the juice then you don't have to put any pressure on the rods with the clamps.  If you are lucky it will all stick together with the thin layer of juice acting like a glue.  It should be quite solid.  Note that some of the piezo power is being "shorted" through the clamp system, and it becomes another impedance discontinuity causing reflections.  If you assume that the juice does the job, then the two rods sit happily on the Styrofoam bed, with your piezo in the "well."  I think you will get better "power beaming" into the rods like that.  Note the "juice" is specifically designed for what you want to do, and will do what the glue will do, without all the disadvantages of the glue.

Have you seen clips of those "vibration tables" on YouTube?  Sometimes they just put sand on the table and as the frequency is swept there are multiple "resonance points" where the sand grains gather at the vibratory nodes on the square vibrating apparatus.  You see all of these amazing patterns.  Other people put "goop" on the tables and at various frequencies it looks like the goop is "alive" and doing various things.  "The Blob"

If you set up a standing wave, I wonder if you could feel it if you slid the tip of your finger down one of the rods.  I doubt it.  If you wanted to, you could buy an ultrasonic microphone, like one used in non-destructive testing, and sweep that down the rod and listen for the peaks and the nodes.  That might be very expensive.  However, that sounds like a cool project - making your own ultrasonic microphone.  Perhaps you can find out what the architecture of a real NDT ultrasonic microphone is like and then build it on the cheap. 

I realize that the waves are primarily going to be longituidinal waves, but there will still be transverse waves generated also.  So a microphone sliding down the rod won't be able to hear the longituidinal waves, but it would be able to hear the transverse waves.

For your big magnets lying flat, are they polarized "up-down?"  I noticed that there are pole designations on those big flat surfaces.  I thought the goal was to make a magnetic flux loop?

MileHigh

[synchro1]

@Itzu,


Quote from Milehigh:

"For your big magnets lying flat, are they polarized "up-down?"  I noticed that there are pole designations on those big flat surfaces.  I though the goal was to make a magnetic flux loop"?

Milehigh's making an extremely important point on the magnet polarization. You would need to stack that type of ceramic magnet like dominos in a box, not end to end, to construct a magnetic flux loop. Why not just try and position stacks in opposition on each free end of the ferrite rods for initial testing if you don't have a sufficient quantity?

Stacking them side by side would still take a couple of iron keepers on the ends to contact squarely with the ferrite ends. Otherwise you would have to begin to fan them out to form a horseshoe so they met just right. Pretty tricky to get get to work good. My advice is to just go with the end stacks in opposition. There's hardly any field strength at the ends of those side to side magnetized ceramics.


Bar magnets, polarized end to end would be ideal!