I am not sure but .you can try piezoelectric material. for matching layer.
Maybe, you can try the commonly used matching materials , such as epoxy resin, vinylic/acrylic copolymer, cellulose ester etc. These materials have the lower acoustic impedance to be easily used as one-layer or multi-layers matching for achieving the optimum one-fourth or half wavelength acoustic impedance design of your air-coupled transducer
Dear Cheng-Che Tsai: i am so thank full for your useful answer.I have fabricated an air-coupled ultrasonic transducer(as a send/receive sensor) with PZT-5A and layer of paperboard as a matching layer with thickness of about 0.5 mm.I could drive my sensors with v=5V and i have 5mV of amplitude at distance of 10cm.
my sensors are designed to use in an ultrasonic gas flow meter.
How is paperboard as a matching layer compared to above material(epoxy resin ,etc)?
How can i find the wavelength of paperboard to determine one-forth thickness?
Regards
Hojat Nasseri
Hojat, you need to know the ultrasound speed of the materials, at the specified frequency. Then quarter wavelength is determined by λ/4 = c/(4f) where f is the frequency, c is the speed of sound.
Hi Hojat Nasseri ，
What's kind of paperboard you used? I have the same question like yours?
Creating a matching layer is not hard. It needs to be 1/4 of the wavelength in thickness and ideally an acoustic impedance between the 2 layers needing matching.
A good reference with step-by-step instructions is the text at:
https://www.amazon.com/Ultrasound-Piezoelectric-Physics-Engineers-Applications/dp/1793995389/ref=sr_1_3?keywords=henry+savage+ultrasound&qid=1560259435&s=gateway&sr=8-3
Matching or coupling into air is hard. Usually you put the piezo film over a cavity so that when the signal is received it passes through the film and stretching happens as the wave transits through the film.
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