resonance!

[epilot]

Hello,

I need to measure the resonance frequency of an ultrasonic transducer,
I had a test with a transmitter and a receiver ultrasonic sensor that I knew
their resonance freq (about 40kHz ,)
I connected the receiver ultrasonic sensor to my oscilloscope probe while my transmitter sensor was near it, the oscillator circuit for transmitter had a volume and it was able to generate a freq between 30 to 50kHz., so I was able to see the change in the wave on the scope, when the volume was set at 40 kHz I had the max of wave form.
Well unfortunately with this way I am able to measure the resonance only at 40kHz because the T and R ultrasonics are designed to make resonance at that freq,
But I have a piezo and a tweeter that have another freq (between 22 to 50kHz)
Now I have a big problem to measure their resonance freq,

Any idea about solving my problem would be very very appreciated

 

[Papabravo]

I'm confused about something. A piezo and a tweeter need to be excited by an AC signal. Each of these devices will have a "response" across a range of frequencies. The frequency at which a maximum response occurs may or may not be a resonance. Is this correct?

That said there are instruments called Gain-Phase Meters which will automate this process, by sweeping a signal source across a frequency range and measuring the response. This is exactly analogous to measuring the response of an RC filter, or any other filter for that matter.

The results are often displayed on a Bode Plot which separates the magnitude of the response from the phase of the response.

 

[audioguru]

In an audio speaker or a piezo membrane usually a resonant frequency results in a peak in the output level.
A woofer resonates with the air in its enclosure at a low frequency where lower frequencies quickly drop off, then another resonance occurs at about 4kHz to 6kHz where the higher frequencies quickly drop off.
A tweeter resonates at about 18kHz so that it still responds to 20kHz.
A Piezo membrane resonates at many frequencies and its resonance produces the highest output if the enclosure resonates at the same frequency.

 

[Nigel Goodwin]

Assuming these are audio devices?, the resonance will either be fairly low amplitude, or outside it's normal operating frequencies. It's it's a piezo beeper, these are sharply resonant, and should be operated at that frequency.

As for ultrasonic transducers, stick the scope across one of them, and feed it from the signal generator via a resistor - this way you can test each one individually. Notice that the transmitter is series resonant (minimum amplitude at resonance), and the receiver is parallel resonant (maximum amplitude at resonance).

 

[epilot]

Assuming these are audio devices?, the resonance will either be fairly low amplitude, or outside it's normal operating frequencies. It's it's a piezo beeper, these are sharply resonant, and should be operated at that frequency.

As for ultrasonic transducers, stick the scope across one of them, and feed it from the signal generator via a resistor - this way you can test each one individually. Notice that the transmitter is series resonant (minimum amplitude at resonance), and the receiver is parallel resonant (maximum amplitude at resonance).

Sorry Nigel i could not understand your mean with my broken english can you explain your mean in other way please?

 

[Nigel Goodwin]

Assuming these are audio devices?, the resonance will either be fairly low amplitude, or outside it's normal operating frequencies. It's it's a piezo beeper, these are sharply resonant, and should be operated at that frequency.

As for ultrasonic transducers, stick the scope across one of them, and feed it from the signal generator via a resistor - this way you can test each one individually. Notice that the transmitter is series resonant (minimum amplitude at resonance), and the receiver is parallel resonant (maximum amplitude at resonance).

Sorry Nigel i could not understand your mean with my broken english can you explain your mean in other way please?

Which part, about tweeters or ultrasonic transducers?

 

[epilot]

Which part, about tweeters or ultrasonic transducers?[/quote]

you said "It's it's a piezo beeper, these are sharply resonant, and should be operated at that frequency"

like "audioguru" said the main part for a piezo to make resonant is the enclosure, even an ultrasonic sensor uses a piezo with a "suitable" diaphragm or something like that ( don't know its name) i mean the piezo in the ultrasonic sensor has a resonant freq near 40kHz because of that diphragm.


by the way i could not understand this:
"
As for ultrasonic transducers, stick the scope across one of them, and feed it from the signal generator via a resistor - this way you can test each one individually. Notice that the transmitter is series resonant (minimum amplitude at resonance), and the receiver is parallel resonant (maximum amplitude at resonance)."
i made a test but with no results becuase i think i could not understand something in your text

 

[Hero999]

Piezo beeper elements are usally have a resonance at 2.8kHz which they are most comonly operated at, but I've tested this and there's also another very sharp hi Q peek at 32kHz so you could also use it as a cheap ultrasonic transducer.

 

[Nigel Goodwin]

you said "It's it's a piezo beeper, these are sharply resonant, and should be operated at that frequency"

Sorry, a typo! - should have read "If it's a piezo beeper, these are sharply resonant, and should be operated at that frequency"

by the way i could not understand this:
"
As for ultrasonic transducers, stick the scope across one of them, and feed it from the signal generator via a resistor - this way you can test each one individually. Notice that the transmitter is series resonant (minimum amplitude at resonance), and the receiver is parallel resonant (maximum amplitude at resonance)."
i made a test but with no results becuase i think i could not understand something in your text

Connect it like this:

 

[audioguru]

I have some piezo transducers (speakers) taken from alarm clocks. Their sound output varies very much with frequency, but they are much louder at certain frequencies when the back of the transducer is near an acousic chamber like a thimble or a big bottle cap.
Piezo beepers that I have are mounted in an acoustic chamber for resonance.

Piezo beepers have a built-in oscillator with feedback from the piezo, so they oscillate at the high output resonance of the piezo transducer in its acoustic chamber.

 

[epilot]

Piezo beeper elements are usally have a resonance at 2.8kHz which they are most comonly operated at, but I've tested this and there's also another very sharp hi Q peek at 32kHz so you could also use it as a cheap ultrasonic transducer.

how could you make a test at that freq?

 

[epilot]

Connect it like this:[/quote]

ok i'll do the test and see what will happen, thanks.

do you know what is the standard device to measure the resonance of a transducer?

 

[epilot]

Piezo beepers have a built-in oscillator with feedback from the piezo, so they oscillate at the high output resonance of the piezo transducer in its acoustic chamber.

can you give me more info about that?

by the way i have read that there is something called "PVDF"(Polyvinylidene Fluoride) that has a property like a piezo(produce sound when there is a voltage and produce power when there is a pressure),
i heard it can handle lot of frequencies, with a good resonance
i wanted to use it because i heard that it is easy working with but could not find any source here for it.

it would be good if someone could give us more info about it.

 

[audioguru]

Piezo tweeters in cheap speakers take a lot of abuse without burning out, but sound like a whistle. Only their few resonant frequencies are heard. People with tin ears think they sound great!

Take a piezo beeper with a built-in oscillator and change the size of its acoustic chamber. The frequency and loudness of its beep will also change. If the resonance of the piezo's element matches the resonance of the acoustic chamber it is mounted beside, then it will be extremely loud.

 

[Dr.EM]

Yes, I remember they demonstrated that effect in physics once with a speaker and measuring cylinder. It becomes louder because standing waves are formed which is where the orignal and reflected waves are perfectly in phase with each other, and so produce constructive superposition. This effectively doubles the amplitude of the signal.

 

[Nigel Goodwin]

do you know what is the standard device to measure the resonance of a transducer?

As far as I know I've just drawn it! :lol:

 

[epilot]

ok thanks for all inputs,

yes Nigel it works and i think the best postion is when i get a sine wave with highest amplitude(i have triangle waves mostly and it becomes a sine wave with highest amplitude at a specific freq)
my small piezo has a resonance freq at 29kHz(i wish i could get a FLAT resonance for it to 40kHz!?

Nigel, what did you mean with parallel and serial resonance in transmitter and receiver?

 

[Nigel Goodwin]

Nigel, what did you mean with parallel and serial resonance in transmitter and receiver?

The transmitter becomes a low impedance and ABSORBS power at it's resonant frequency, as required for maximum output, the receiver becomes a high impedance at it's resonant frequency, so lets the signal through, rejecting higher or lower ones.

So for the transmitter you adjust for MINIMUM signal at resonance, and for MAXIMUM signal on the receiver.

 

[epilot]

Nigel,
you have showed a pic to measure the resonance freq,
i want to know if it is possible using it to find out the bandwith of a transducer?
i saw a data sheet about an ultrasonic transducer that is able to work with 10kHz of bandwith,
is it possible to measure its bandwith with that circuit?

i think the conical of an ultrasonic sensor is "the most important" part that makes the piezo to has a resonance at a specific freq(i mean if you cut part of conical the the freq range will be changed) am i right?
if so then how that ultrasonic sensor is able to work at a flat freq of 10kHz??

here is the link:
**broken link removed**

 

[Nigel Goodwin]

Nigel,
you have showed a pic to measure the resonance freq,
i want to know if it is possible using it to find out the bandwith of a transducer?
i saw a data sheet about an ultrasonic transducer that is able to work with 10kHz of bandwith,
is it possible to measure its bandwith with that circuit?

Yes, I don't see why not, just plot the amplitude for a range of frequencies and draw a graph from them. Bear in mind, the Q will be a LOT lower for 10KHz bandwidth, so the tuning won't be anywhere near as pronounced, and you will probably need a LOT more gain to make up the difference!.

Presumably this is still about your audio over ultrasonic project?, 10KHz bandwidth from the transducer should just about give you a low quality voice channel!.