Best Practice Guide to Measurement of Acoustic Output Power (Introduction)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
What is being measured?
In providing information on the total power of an ultrasonic beam (a 'bulk' characteristic of the field under test), the phenomenon of radiation force is utilised. Ultrasound waves cause a mechanical displacment of a medium passing through at a fixed speed and is characterised by localised changes in pressure in the medium.
When a solid object is placed in the path of a plane progressive ultrasound wave, momentum is transferred from the beam to the object, and it experiences a force along the direction of propagation - the radiation force. If the solid object, or target, is larger than the beam, the force experienced is proportional to the total acoustic power.
In the simplest case, assuming plane waves, and using a target that totally absorbs the incident momentum, the ultrasonic power, P, is directly proportional to the radiation force, F, is given in (1) where the constant of proportionality, c, is the propagation speed in the medium, usually water.
At 20 °C, the value of c for water is 1482 ms-1, so that an applied power of 5 watts (a typical output level for physiotherapy equipment) will produce a force of approximately 3.4mN (2). An equivalent change in mass can be calculated (3). This change in mass can be measured readily using a balance. From the typical changes in effective mass that have been given, it can be seen that for diagnostic beams, the forces produced can be small, which in turn places greater demands on the refinement of the detection techniques used.
References
- NEMA UD-3 Standard for Real-Time Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic Ultrasound Equipment
- FDA 21 CFR 1050.10 Performance Standards for Sonic, Infrasonic and Ultrasonic Radiation-Emitting Products: Ultrasonic therapy Products
- NEMA UD-2 Acoustic Output Measurement Standard for Diagnsotic Ultrasound Equipment
- IEC 61689 Ultrasonics - Physiotherapy systems - Performance requirements and methods of measurement in the frequency range 0,5 MHz to 5 MHz
- IEC 61161 Ultrasonic power measurement in liquids in the frequency range 0,5 MHz to 25 MHz
- IEC 60601-2-5 Medical electrical equipment - Part 2-5: Particular requirements for the safety of ultrasonic physiotherapy equipment
- IEC 60601-2-37 Medical electrical equipment - Part 2-37: Particular requirements for the safety of ultrasonic medical diagnostic and monitoring equipment
Note: General references for standards have been given and are subject to updates and changes. The reader should view the relevant website regularly for the latest version.