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Drivers for ultrasonic frequencies

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PeterE View Drop Down
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    Posted: 23 June 2016 at 10:59am
Hi,

I am creating acoustic levitation demo rigs.  I need as much sound energy as possible (within price constraints) at around 25-30kHz.  The levitation occurs on axis, so I'm only concerned with the sound intensity on axis.  

I'm currently using a BMS 4540ND which delivers a decent amount of power, I wondered if anyone had suggestions for anything more powerful at these frequencies?

Also, since the sound only needs to be issued in a single direction, does anyone have any thoughts about what kind of horn/reflector might be used to focus the energy back onto the axis at a given position (typically about 5cm from the tweeter front).  However, I wonder about phase issues (ie more near field effects) if this solution was pursued.

In case you are interested, the basic setup has a glass reflector about 10cm from the tweeter (I want this to be longer).  Driving a sinewave around 25kHz sets up a standing wave that can hold polystyrene balls in the nodes of the standing wave. At this frequency some children can still hear it, but it is generally OK.

Thanks,
Peter

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MattStolton View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2016 at 11:39am
Piezo transducers.

At university a lecturer friend of mine, was playing with acoustically vibrating the surface of a plane's wing, to change the skin effect to change (lower) the stall speed of a plane. Covered the wing in thin peizo material and bunged in ultrasonics.

Other sources are from other stuff that uses ultrasonics. Often used in Chemistry labs to investigate cavitation in solutions, or to heat solutions - again they use piezo transducers.

Piezo material is cheap, and easy to drive? Pretty reliable, and can be made quite efficient.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2016 at 11:42am
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
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PeterE View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PeterE Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2016 at 2:55pm
Thanks,

I have been considering piezo based options too, but so far have not found similar power levels in off the shelf air-based transducers. Specific designs are needed to couple efficiently into the air.  The most power I have found around this frequency is here:

http://www.prowave.com.tw/english/products/ut/open-type/250s240.htm

However this only gives 121 dB SPL (at 30cm, 20Vrms max) compared to ~128 dB (at 1m?, 60W) for the 4540ND.

So unless anyone knows of much stronger piezo transducers, I continue to think that a more powerful audio driver might still be the solution if I could find one.

Peter
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 24 June 2016 at 3:14am

You will need to consider the dB level at the given frequency to determine if indeed the BMS will surpass the Piezo or not.

 

 

Here is another contender

 

http://www.fostexinternational.com/docs/speaker_components/pdf/ft17hrev2.pdf

 

Best Regards,

Elliot Thompson
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PeterE Quote  Post ReplyReply Direct Link To This Post Posted: 24 June 2016 at 10:45am
Thanks Elliot, I shall have a look at that one. The figures I gave above are my best guess for 25kHz  (about -4db compared to the peak for the 4540ND).

While the FT17H is less sensitive and handles less power (25kHz: 98dB,30W  vs 110db,60W), it has a much more extended frequency range (up to  40 or 50 kHz !), and may be useful in projects where I want to work closer to 30kHz.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Meat Quote  Post ReplyReply Direct Link To This Post Posted: 24 June 2016 at 1:20pm
Another vote for piezos. This is the go-to choice for ultrasonic frequencies.

I think our acoustic levitation rigs in the lab use piezos. Scanning probe microscopes use piezos for small displacements at very high frequency.

You'll probably want a horn and you probably also want to look at horns suited/opimised for ultrasound rather than audio frequencies.

Coils are fine for audio but ultrasonic frequencies not so much.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 24 June 2016 at 2:36pm
SO at present are you just using the bare driver with the exit hole pointed at a sheet of glass?
 
A focused beam of sound is the opposite of what most audio horns are supposed to produce. You could make a very narrow dispersion horn (zero degrees, haha) but it'll still be a diverging beam. I think the device you'd really want would be a complex 3d structure of waveguide channels to approximate a collimated beam like a laser. The very high frequency will make it marginally easier but still, it'd be a three-pipe problem!

Agreed that a piezo is probably your best bet for efficient conversion of power into ultrasonics. A shame that good ones are difficult to get now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 24 June 2016 at 4:25pm
Have you looked at LRAD circuits? I remember seeing a really good DIY circuit on the net a few months ago. I canĀ“t remember exactly where, but I imagine that a bit of googling would find it.

 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PeterE Quote  Post ReplyReply Direct Link To This Post Posted: 24 June 2016 at 8:29pm
Thank you for more food for thought...

I do definitely consider piezos, I work with them for liquid based levitation at higher frequencies.  I have seen custom made piezo designs with solid horns for air levitation at 40kHz that work very well. I might go that route, though I was hoping to come up with a simpler solution that used off the shelf components, and could be driven with typical audio amps compared to avoid getting stuck into the electronics of driving piezo loads at high powers (simple ccts do exist, but buying commercial piezo power drivers is v. pricey).

LRADs (Long range acoutic devices) have actually been used in this kind of application already, with phased arrays of commercial ultrasonic transducers.  Again a definite possibility, but needs reasonably complex electronics to control the phasing. I have made simple ring arrays of these already but these only tend to work near the focus point rather than over the whole levitation gap.

My current system actually points the tweeter at a glass lens to gently contain the acoustic energy.  Reading around more over the past few days I realise that my tweeter is likely to be creating a reasonably tight beam (20-30 degrees?) at this frequency, so I'm probably getting reasonable acoustic focussing already.

All comments and more are really welcome - I'm exploring the parameter space to find what is possible.  Partly I'm driven to find cheap systems that use off the shelf solutions for reliability for use in science exhibitions, etc.  Reading around in response to your comments, I'm realising that the driver I have is probably one of the most powerful out there, so perhaps I'm already close to optimum for an audio component based system.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote madboffin Quote  Post ReplyReply Direct Link To This Post Posted: 25 June 2016 at 12:58pm
You may have done this already, but otherwise:
Have a look at the research papers and internet message board articles on this subject by Tom Danley. He worked on acoustic levitation systems for NASA (for positioning items in zero gravity).




Edited by madboffin - 25 June 2016 at 1:00pm
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Greg32 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greg32 Quote  Post ReplyReply Direct Link To This Post Posted: 27 June 2016 at 1:58pm
maybe this ultrasonic transducer http://www.altronics.com.au/p/z1690-40khz-ultrasonic-transducer/
seems powerful at 60w 
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