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An ultrasound transducer is specified to have two standard frequencies; 820 KHz, and 2.5 MHz, does this mean that any frequency in-between can be used?

If not, what would happen if a generated signal with a different frequency, let's say 1.5 MHz, was connected to the transducer?

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    \$\begingroup\$ Please post a link to the datasheet for the device. That should contain graphs/specification which will answer the question, and we may be able to help you find the answer to your question using it. Without a datasheet, we'd just be guessing. \$\endgroup\$ – gbulmer Feb 9 '16 at 13:37
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    \$\begingroup\$ It's either resonant, or broadband. It will behave differently to an intermediate frequency depending on which. The datasheet should provide enough information to say which. \$\endgroup\$ – Neil_UK Feb 9 '16 at 13:40
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820kHz and 2.5MHz (=3*820kHz) are eigenfrequencies of the mechanical system. You will be able to drive the transducer at any other frequency, but the damping will be greater by orders of magnitude, so it won't be usable.

Some ultrasound transducers can oscillate at both longitudinal and transverse mode. It is possible that that 820kHz and 2.5MHz are the natural frequencies for transverse and longitudinal mode while n*820kHz and n*2.5MHz are their harmonics. In this case, the transducer can be operated at both 820kHz and 2.5MHz with comparable amplitude but will produce different wave patterns. That does not neccessarily mean that any of the harmonics' damping is low enough to be usable.

Modes of operation Image from german wikipedia, public domain.

  • The transducer in the top left image creates longitudinal waves by pushing and pulling a solid surface, liquid or gas it is pressed against.
  • The left center one creates transverse waves in a solid fixed or pressed to its bottom (transverse waves cannot exist in liquids and gases).
  • The lower left is the same as the one above it, but operated at the third harmonic.
  • The three modes to the right are not used in ultrasound transducers.

There are also broadband transducers that do not operate in resonance. They can be used equaly well over a broad fequency range, but a resonance transducer operated at a correct frequency is much louder.

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  • \$\begingroup\$ What would happen if I ran the transducer at 2*820 kHz, or 4*820kHz?. What is the upper limit? what would happen if I applied the maximal frequency of my generator, in my case 12*820kHz = 9.84MHz? \$\endgroup\$ – Keine Feb 9 '16 at 13:49
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    \$\begingroup\$ I expect it to work at any frequency that is a multiple of 820kHz, but the harmonics' amplitudes decrease with higher order. 820kHz should be the dominant one. Could you test and confirm that using two transtucers mounted at a fixed distance and an oscilloscope to measure the result amplitude? \$\endgroup\$ – Rainer P. Feb 9 '16 at 14:00
  • \$\begingroup\$ Will do once we acquire it, which will unfortunately take some time. I do not know if this is allowed in StackExchange, but do you know any producer or distributor of ultrasound transceivers / generators?. \$\endgroup\$ – Keine Feb 9 '16 at 14:05
  • \$\begingroup\$ How can the piezoelectric material in a transducer oscillate at both longitudinal and transverse mode, and still emit the waves in the same direction (out from the face of the transducer itself)? \$\endgroup\$ – Keine Feb 10 '16 at 8:29
  • \$\begingroup\$ @Keine I updated my answer to cover that. In the image, the wave´s direction of travel will be downward for both modes (if you attach something to the transducer´s bottom), but the oscillation will be either push-pull or left-right. \$\endgroup\$ – Rainer P. Feb 10 '16 at 21:23

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