Best Practice Guide to Measurement of Acoustic Output Power (Additional Information)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
Focused fields
In a focused field the relationship between force and power is different. For a circular, spherically transducer incident on a totally absorbing target, the relationship is as follows:
Fm = (P/c)(1+cos γ)/2
where γ is the half-angle subtended by the transducer calculated from γ = arcsin a/Rc, a is the transducer radius and Rc is the radius of curvature of the element.
For a conical reflecting target the following formula is valid
P = 4cF fct(γ, θ)
where
fct(γ, θ) = (1-cos γ) / [(1-cos 2γ)(1+cos 2θ) - (2γ-sin 2γ)sin 2θ]
The formula is valid for certain conditions usual with a reflecting surface. The acoustic rays of the circular, spherically curved transducer should progagate along a straight line and be totally reflected at the surface of the target. In addition, the target should be a perfect reflector that covers the whole transducer field with no rays reflected back to the surface. The apex of the target should be positioned on the field axis between the transducer and the focus.
References
- Ultrasonic power measurement in liquids in the frequency range 0.5 MHz to 25 MHz International Electrotechnical Commission (1992), IEC Publication 61161, IEC, Geneva