High power ultrasonic standing wave.

[NoiseMachine]

Hi there,

I expected this to be very simple but I cant seem to get it working properly.

Basically I would like to create a high power standing ultrasonic wave (I am using a 3W amplifier). I have tried using a tweeter but with no joy. At the moment, I'm working at 21kHz and am confident that the frequency range of the tweeter is able to handle this.

In short, its not working.

Can anyone suggest a good transducer that could be used to generate a nice clean standing wave or something that should be done at higher powers?

I got this working with a low power ultrasonic transducer, just cant get working at higher powers. Any suggestions welcome.

 

[btrueblood]

"I would like to create a high power standing ultrasonic wave "

In what medium, over what distance?

 

[NoiseMachine]

In air at room temp at 21kHz with an expected wavelength of about 16cm.

It sounds too simple not to work!

 

[SomptingGuy]

How high is high? Are you getting non-linear?

- Steve

 

[NoiseMachine]

At the moment high is only using a 3W amplifier but I hope to bring that closer to 100W once I get it working.

When you say non-linear, what do you mean?

 

[SomptingGuy]

Are your pressure fluctuations on the same scale as your mean pressure?

What is your SPL?

- Steve

 

[NoiseMachine]

Are your pressure fluctuations on the same scale as your mean pressure?

>>I'm not exactly sure what you maen by that (I'm not an acoustic engineer). At the moment, I'm doing this in open air. Not in a tube etc. This might make things a little difficult for me (i.e. making sure that the reflecting surface is at exactly 90 degrees to the sound waves). Since this is in open air I expect that pressure fluctuations would be on the same scale as the mean pressure but I may be misunderstanding this.

What is your SPL?
I cringe at saying this but at the moment I dont know! I dont have a proper mic at the moment. Will have one tomorrow though.

 

[MikeyP]

The wavelength would be more like 16 mm than 16 cm.

M

--
Dr Michael F Platten

 

[NoiseMachine]

Apologies, you are of course right, the wavelength is 1.6cm

I will try a slightly different approach and see if I can get some more results for tomorrow.

 

[MikeyP]

If you want to set up standing plane waves, then you will need to do it in a tube. Open air won't work. The diameter of the tube must be small compared to the wavelength (say 2 mm in your case). Trouble is then how to get your sound from a 20 mm ish diameter tweeter into a 2 mm diameter tube without losing most of the efficiency of your tweeter...

M

--
Dr Michael F Platten

 

[NoiseMachine]

I expected to have to do this in a tube at some stage but did not expect to have to put into such a small diameter tube.

Could you explain why the diameter needs to be so much smaller please (even a web link to something on the topic would be brilliant).

Also, I have already managed to get a standing wave from a small low power ultrasonic transducer (about 40kHz) in air. In saying that I was only a few wavelengths away from the transducer.

The transducer in the tweeter is buried inside a casing about 20cm from the port. There is also a grill at the port which I am wondering if it is effecting the way the reflected wave may enter the port on the way back. I cant remove the transducer from the casing as the casing provides a rigid base for the cone mounted on the piezoelectric plate.

Does anyone know if there are any transducers that are particularly suited to this type of job?

Thanks for everyones response so far. I really appreciate the fact that you have taken the time to respond and be so helpful.

 

[NoiseMachine]

I should probably clarify....

Also, I have already managed to get a standing wave from a small low power ultrasonic transducer (about 40kHz) in air. In saying that I was only a few wavelengths away from the transducer.

Should have said, the reflecting surface was only a few wavelengths away from the transducer.

 

[Strong]

The standing wave distance to the reflecting surface should be from the rigid radiating surface of the tweeter and not to the grille surface. How can you tune the frequency and verify a standing wave without a micorphone and spectrum analyzer?

Walt

 

[NoiseMachine]

The standing wave distance to the reflecting surface should be from the rigid radiating surface of the tweeter and not to the grille surface.

Yes, I understand. I only mentioned the grill to point out that maybe it had an effect on waves bouncing back from the reflecting surface into the tweeter surface.

How can you tune the frequency and verify a standing wave without a micorphone and spectrum analyzer?

I have a strange setup where I have a laser scanning along a plane (effectively a cross section of the soundwave). Using particles (as you might see smoke in a wind tunnel), I can actually see the movement of the air waves when the wave becomes a standing wave.

It's a way of studying the various air pressures within the soundwave in real-time without simulating it and getting errors. I should have a spectrum analyser and proper mic setup tomorrow.

 

[MikeyP]

http://www.bruel-ac.com/tr/tr9701/TR9701.html

OK, maybe 2 mm isn't necessary, but 8 mm diameter is.
I am assuming when you say standing waves, you mean 1-dimensional standing plane waves.

M

--
Dr Michael F Platten

 

[NoiseMachine]

Yes, they would essentially be 1D standing waves.

The root of the problem I have is that I can create standing waves with a cheap low power ultrasonic transducer at about 40kHz in air without a tube (eventually this will be placed in a tube). In this case, the wire mesh grill was removed from the transducer. The waves were bouncing off a surface straight back to the transducer with nothing between.

But I would like to create high power ultrasonic standing waves (up to 100W and preferably closer to 30kHz) and I can’t seem to find any transducer that will easily allow me to do this. For one reason or another, the 100W tweeter does not seem to work.

Also, the diameter of the tube - does this need to be half the wavelength?

 

[MikeyP]

It is explained in the link.
M

--
Dr Michael F Platten

 

[NoiseMachine]

Sorry, I missed that link. Just looked at it now.

Many thanks, that article will be a great help. Much appreciated.