Good morning gentlemen, I'm new to the forum and I hope I haven't entered the wrong section. then a simple theoretical question considering ultrasounds with frequencies between 20 khz and 100 khz, how can I reduce or eliminate the acoustic impedance between two media? do they reflect too much, or do most of the ultrasonic waves pass through?
Welcome to the forum!
This sounds more like a physics question than a multi-way loudspeaker question, but here goes:
Reflection of ultrasound occurs when the wave encounters a change in acoustic impedance (the resistance to molecular vibration).
In simple terms, ultrasound travelling in a less dense medium is mostly reflected when it encounters a more dense medium.
Acoustic impedance is directly related to material density, so to minimise reflection the density change must be minimised.
That's why gel is used in an ultrasound scan. The percentage of energy reflected is almost 500 times smaller going from gel to tissue than going from air to tissue.
If the above doesn't answer your question, please tell us more of what lies behind your question.
This sounds more like a physics question than a multi-way loudspeaker question, but here goes:
Reflection of ultrasound occurs when the wave encounters a change in acoustic impedance (the resistance to molecular vibration).
In simple terms, ultrasound travelling in a less dense medium is mostly reflected when it encounters a more dense medium.
Acoustic impedance is directly related to material density, so to minimise reflection the density change must be minimised.
That's why gel is used in an ultrasound scan. The percentage of energy reflected is almost 500 times smaller going from gel to tissue than going from air to tissue.
If the above doesn't answer your question, please tell us more of what lies behind your question.
hi thanks for your answer, yes my question is exactly that, that is, can a beam of ultrasound of 20 or 40 khz pass from air to solid (not metal) without being completely reflected? so even with low frequency ultrasound it is possible not to force them to reflect all, are there other ways besides gel?
yes, of course I need it for a botany experiment to hit the roots of plants with ultrasound. the transducer must overcome 60 cm of air and so far everything is ok and then a few cm of ground and here is the problem because there is acoustic impedance. the frequency, however, is based around 20 khz and therefore more easily penetrable, but I don't know how much the solid can reflect the ultrasound
hi allenb so yes the resistance problem can be solved by increasing the power, while the impedance is a little more difficult to overcome. I need ultrasound because it is a new method for eliminating pesticides in plants. so I have to use ultrasound, I just need to understand how to overcome the problem of impedance between the densities of two different materials
Galu I can't bring the transducer too close to the ground because there are herbs and then if I bring it too close the plant doesn't grow, the distance is carefully calculated
So there are actually 2 major interfaces: transducer to air, and air to soil?
If the transducer is buried directly in the problem area, the amplitude should be proportional to distance squared. But if we are talking about potted plants, that could help to confine the vibrations to the pot and soil volume. The transducer could also be attached to the side of the pot via a similar gel-like medium as discussed.
If the transducer is buried directly in the problem area, the amplitude should be proportional to distance squared. But if we are talking about potted plants, that could help to confine the vibrations to the pot and soil volume. The transducer could also be attached to the side of the pot via a similar gel-like medium as discussed.
By the way, the amplitude adjustments would have to be redone, with a view to significantly reducing them if the waves are conducted into the plants with greater efficiency.
Alternatively: attach the transducer to the base of the plant, I'm thinking. That would guarantee good conduction to the roots, even if the soil is a bit hit and miss because of air pockets or variable moisture.
Alternatively: attach the transducer to the base of the plant, I'm thinking. That would guarantee good conduction to the roots, even if the soil is a bit hit and miss because of air pockets or variable moisture.
hi,abstract so the transducer is positioned 60 cm above the ground, so the air must first penetrate and then a few cm of soil, so the transducer is not buried. however, the frequency is low, around 20 khz or a little more.
That sounds like a ground penetrating (GP) radar. The usual ones in the field use EM radiations (vs ultrasound) though.
https://www2.whoi.edu/site/coastalgroup/about/how-we-work/field-methods/ground-penetrating-radar/
Have you considered something like a wand-style laboratory sonicator? These are often used to homogenize samples, and systems are available where nearly all the energy is transmitted at the tip. You could easily penetrate the soil without doing much damage at the surface or putting significant energy into the above-ground plant structures (if those are your main concerns with this approach). Doing this to freshly irrigated soil should help improve coupling as well.