Jiri Brozovsky,  Calcium Silicate Bricks – Ultrasonic Pulse Method: Effects of Natural Frequency of Transducers on Measurement Results.   journal   = {International Journal of Architectural and Environmental Engineering}, [online]. World Academy of Science, Engineering and Technology.
    September 2014, vol. 93(9). 971 - 974
    [viewed 19 October 2019]. Available from: https://publications.waset.org/pdf/9999379.