Best Practice Guide to Measurement of Acoustic Output Power (Radiation Force Balances)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
Suspended target balance
Two configurations exist for a suspended target balance; top loading (A) or bottom loading (B). Although reflecting targets are represented here both systems can be fitted with absorbing targets.
Top loading balance
The target is suspended in water from a gantry using a minimally perturbing material such as a fine wire to minimise the effects of reflection. The gantry is connected around to the weighing mechanism of the balance. The transducer is positioned over the target and radiates down during operation. This type of configuration is most stable over the therapy range and is commonly used in commercial power measurement devices.
The advantages of this system lie in its simplicity of use and set-up. As the total mass of the target and gantry is small, a low capacity balance can be used which reduces the cost for a user who is developing a system. Its stability over the therapy range produces low random uncertainties during measurement. However, the range of operation is limited to the therapy level as the balance is not as stable at outputs below 500 mW.
Bottom loading balance
For a bottom loading balance a compensation system is incorporated. The transducer is positioned and sealed in the base of a water tank so that it is radiating upward when in operation. The target is suspended over the transducer and is connected to the balance. Weights are used to counter the buoyancy of the target in the water. When the target is moved the change in mass is compensated for using a servocontroller in a feedback system. This compensating effect is indicated on the balance display as a change in effective mass.
The main advantages of this system are its resolution. Measurements of acoustic power down to 1 mW are achievable under stable conditions. A low capacity balance is required which generally is less costly than the higher capacity counterparts of equivalent resolution.
However, when measuring at low output levels, the system is prone to environmental effects such as convection currents and draughts. Additionally, the transducer can be inconvenient to mount and without the appropriate seal (self-amalgamating tape, o-rings) can be prone to leakage. Also, the range can be limited. The NPL Primary Standard Radiation Force Balance is a compensating system with an upper output level limited to 1 W.