Airborne Ultrasonic Transducer

Nagahara (El)and M. Hashimoto Advanced Technology Research Laboratory, Panasonic COTporatim, 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan 

Acoustic impedance is the most important parameter for configuring the acoustic matching layer. Using acoustic velocity c and density p of the propagation material, acoustic impedance Z can be represented by the following (33.1). 

X is the ultrasonic wavelength in an acoustic matching layer. If (33.2) and (33.3) are satisfied, the transmission rate of the plane wave will be maximum. If the acoustic matching layers are of a multilayered structure, it is necessary for (33.2) and (33.3) to be satisfied at any continuous three layers. 

Assuming the transmission and reception of acoustic waves in an air atmosphere, the acoustic impedance of air is about 400 kg/(m s) (density 1.2 kg/m, acoustic velocity 343 m/s at 1 atm, 20°C), and the acoustic impedance of piezoelectric ceramic as a typical 

In fact, solid materials with such low acoustic impedance are extremely rare. For the acoustic matching layer of an airborne ultrasonic transducer, non-woven fabrics made of polymer or polymer materials packed in small hollow glass spheres are generally used. 

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From these basic experiments, it was found that silica aerogels had very low acoustic impedances compared to those of the current matching layers [7-9], and are therefore very promising as materials for the acoustic matching layers of high-sensitivity airborne ultrasonic transducers. 

With the above features, AC-ML is suitable for the matching layer of the airborne ultrasonic transducer. AC-ML with the above advantages was fabricated as follows ... 

As shown in Figure 33.15, the sensitivity of the aerogel ultrasonic transducer is about 20 times as high as that of the conventional airborne ultrasonic transducer. Thus, high sensitivity at the same level as the calculation result was verified. 

Broadband microphones (from the audible range to ultrasound), which can be achieved in a converter using the piezoelectric properties 33 in the thickness direction, have aheady been applied as transducer elements in an airborne ultrasound transducer (Yasuno et al. 2010). Application to ultrasonic probes for medical use is also expected. 

For large scale (200 L capacity plating tank) it proved impossible to apply airborne ultrasound and therefore the ultrasound was introduced into the plating tank (Fig. 6.13). The transducers consisted of two banks of three mounted in dummy tanks and delivered in total 1.4 kW into 135 L making the overall ultrasonic intensity 0.01 W cm The cathode consisted of a steel bar (5 cm diameter and 20 cm length) and 4 large lead anodes (diameter 3.7 cm and length 39 cm) were used. On sonication... 

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