01/11/2019
Ultrasonic receiver

Air-coupled ultrasonic receiver, with modules (a) cellular polypropylene, (b) bias voltage and (c) preamplifier

Source: BAM, Acoustic and Electromagnetic Methods division

High sensitivity is an important requirement for air-coupled ultrasonic sensors applied to materials testing. The main challenge in constructing these sensors is the low transmission of acoustic energy from air to any solid. For this purpose, common air-coupled ultrasonic sensors are provided with layers of soft material (so called matching layers) glued onto a sensor made of a harder piezoelectric material. In our work we have focussed on constructing sensors using softer materials, to increase the sensitivity and to simplify the construction, so that matching layers are redundant. Most promising material in this sense is polarized cellular polypropylene (PP). The polarization plays the same role as bias voltage for capacitive microphones. In this paper, we show that the sensitivity of the sensor can be increased by applying additional DC bias voltage. This work presents the first ultrasonic sensor based on charged cellular PP including a high-voltage module providing DC bias voltage up to 2 kV. This bias voltage led to an increase of the signal-to-noise ratio of up to 15±1 dB, which is equivalent to tripling the sensitivity. The measurement of the received signal depending on the applied bias voltage is proposed as a new method of determining the internal voltage of charged cellular polymers (called ferroelectrets). The sensor combined with a cellular PP transmitter was applied to non-destructive testing of a rotor blade segment and glued laminated timber, enabling imaging of the internal structure of these specimens with a thickness around 4 cm.

Air-coupled ultrasonic ferroelectret receiver with additional bias voltage
Mate Gaal, Rui Caldeira, Jürgen Bartusch, Florian Schadow, Konrad Vössing, M. Kupnik
published in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 2019, Vol. 66, Issue 10, Pages 1600-1605
BAM, Acoustic and Electromagnetic Methods division and Non-destructive Testing Methods for Civil Engineering division