Acoustical power
Why specify this parameter precisely

The capacity to generate a high level of acoustic power is one of the essential characteristics of a transducer for therapeutic applications.

One of the main factors limiting transducer’s power is the rise in internal temperature caused by the production of heat. This heat production results from losses during the conversion of the electric energy into acoustic energy.

Transducers for therapeutic applications are designed in such a way as to minimise losses and to evacuate the heat produced towards an external medium, directly or by the means of a coolant.

However, even after defining the design, the internal temperature of the transducer still depends on the configuration of the excitation cycle and the cooling conditions.

The influence of excitation conditions

The temperature of the active element increases during the ON excitation cycle, then cooling takes effect according to the environment of the transducer. The absolute and relative durations of these ON and OFF cycles therefore establish the maximum temperature during the ON cycle and thus determine the maximum acoustic power during that cycle.

Evolution of the temperature rise of the active element vs. duty cycle
(at a given acoustic power)

a/ Duty cycle at 100% leading to an unacceptable level and the destruction of the device
b/ Duty cycle at 50% (shorter time ON) leading to an acceptable situation
c/ Duty cycle at 25% leading to low heating

Evolution of the temperature rise of the active element vs. ON & OFF times
(at a given acoustic power and same duty cycle)

a/ Duty cycle at 50% (time ON 20s) leading to destruction of the device
b/ Same duty cycle at 50% (time ON 2s) leading to an acceptable situation

The influence of cooling and the direction of the transducer



By evacuating heat by forced cooling, the maximum temperature reached during the operating cycle can be reduced, thus increasing the maximum power achievable. When a high power level is required, a cooling solution can be developed on a case by case basis. The figure below, showing the case of a transducer with an internal cooling circuit, illustrates the evolution of the temperature according to the cycle, at different coolant flow levels.

Temperature of the active element according to the flow
(Measurement conditions: 7 Wac/cm2 20s ON / 50s OFF)


The direction of the transducer in the coupling bath also influences the conditions of thermal exchange. The example below illustrates this influence on the internal temperature of a transducer cooled only by its contact with the coupling bath. The impact on the internal temperature of the orientation conditions of the transducer in the bath should be noted:

  • optimal conditions in a straight up shot (upward 180°),
  • deteriorated in a slightly angled,
  • downward shot (downward 10°)
  • and a drift leading to the destruction of the transducer in the condition of a vertical downward shot (downward 0°).

Temperature rise vs. orientation of the transducer in the bath

Control of the acoustic beam

It is essential to ensure that the power delivered is used to treat the targeted tissues. A lack of control in the distribution of energy and the location of the acoustic beam can burn tissues at undesired locations or can reduce the efficiency of tumour destruction. Problems of various sources can lead to this situation.

A reversible or irreversible deformation of the active surface can modify the focal distance and thus the location where tissues are insonified (see hereafter). A potential solution to this problem is the thermal management of the transducer and its environment, especially acoustic coupling media. Another possibility is the regular checking of transducer surface geometry and acoustic beam geometry. The geometry of the beam at the focus should be controlled carefully (-6 dB focal area, for instance) and not only at a low power level as is usually done. However, at present, precise methods of characterizing beams at high level are still not available despite the relevance of such methods. A promising method based on acoustic holography is under evaluation


Evolution of temperature and reversible deformation of the front face of a focused transducer (F number close to 1) measured at 4 W/cm2 during 200s.
The maximum deformation results in a change of 1.2% in the focal distance.