FAQ: I designed some probes and I had difficulties regarding the frequency and length arrangements. In some construction, the length is not long enough and I want to extend it. When I test some construction with TRZ® Analyzer, length is 40 cm and frequency 20 kHz, when I analyze the same type probe, whose length is 20 cm and frequency 40 kHz, both works with 20 kHz driver. If I set the frequency at 20 kHz or 40 kHz, when I extend the length to 40 cm, will it work with 20 kHz driver? Can a 20-kHz probe work with its double frequency, 40 kHz? Mr. H.B.
Illustration of sonotrodes and tips with multiple lengths of lambda/2 and the same frequency.
It is possible to extend the sonotrode / probe length and keep the same frequency, however it should be done by adding multiples of half-wave length (λ/2). It is not possible to have arbitrary lengths at will. The image above shows sonotrodes with lengths equal to λ/2 (half-wave), λ (full-wave) and 3λ/2, all operating at the same frequency. The blue line illustrates the maximum amplitude of vibration.
When you have a sonotrode with length longer than half-wave (λ/2), than you also have subharmonics frequencies. For instance, a 40 kHz full-wave sonotrode may be able to also operate at 20 kHz. It worth to note that to drive a sonotrode at subharmonics will affect amplitude, gain, stress distribution and nodal lines position.
Learn more about tuning of ultrasonic horns.