The acoustic noise pressure of sensors with very small apertures

Abstract

For medical diagnostic modalities like intravascular ultrasound (IVUS) and intravascular photo acoustics (IVPA), it is paramount to have small, sensitive ultrasound elements for detecting the reflected pressure pulses. The development of one and two dimensional arrays for such applications will call for even smaller element sizes and advanced microfabrication techniques. In search for miniature receiving elements we developed an optical ultrasound sensor with an optical strain detector integrated on a thin acoustical membrane [Leinders et al., Sci. Rep. 5, 14328]. To predict the lowest detectable pressure, we wanted to determine the noise level of this sensor. Unlike a piezoelectric sensor, the noise in our sensor is not dominated by the electrical impedance and will only be caused by the thermo-acoustical noise of the sensor’s internal mechanical impedance, and the noise caused by thermally agitated medium particles that hit the sensor surface. To expand the existing knowledge, we will analyze both noise mechanisms and show that in thermodynamic equilibrium these give rise to the same noise pressure at the sensor surface. Moreover,we will show that for sensors with vanishing aperture area, the noise pressure will reach a well-defined finite limit, and not go to infinity as predicted by some literature.