Best Practice Guide to Measurement of Acoustic Output Power (Radiation Force Balances)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
'Weigh all' balance
In this configuration the target is coupled directly to the balance pan therefore the relatively small change in effective mass is measured along with the total mass of the target. The balance used must have a much higher capacity than for a conventional suspended target system, usually in excess of 1kg.
The target consists of a lightweight tank fitted with a piece of absorber to cover the base and act as the target. The transducer is applied from the top and radiates downward. Although, this balance is not commercially available, its potential lies in its simplicity; the user is able to obtain the relevant parts and construct the system at a reduced cost compared to purchasing a commercially available balance.
The main advantage of this configuration is its stability. Compared to a suspended target system, the range of operation can extend down to around 50 mW with a reasonable random uncertainty and extend upward to 10 W or higher. The main disadavantage is the high cost of a higher capacity balance with the appropriate resolution. The 'weigh all' approach is also susceptible to higher drifts due to evaporation of the water which can potentially lead to errors in the calculation of the output power. However, this can be compensated for by using an appropriate manual method of obtaining results or by using a PC to acquire the data.