Aerogel Ultrasonic Transducer

Red line and black line are the calculated results of aerogel ultrasonic transducer and ultrasonic transducer with conventional matching layer respectively. The properties of conventional acoustic matching layer are 0.54 x 10 kg/m density, 2,400 m/s acoustic velocity, and 1,296 X 10 kg/m s acoustic impedance. 

Calculation was performed under the following conditions the driving signal was a rectangular wave with three wavelengths at the frequency of 500 kHz, and the input impedance of the receiver was 50 O. 

The thickness of each layer of AC-ML and conventional acoustic matching layer were optimized so that it could efficiently transmit and receive ultrasonic waves at a fi equency of 500 kHz. 

Altringham JD (1996) Bats Biology and Behaviour, Oxford Oxford University Press p.79-113 

Thomas JA, Moss CF, Vater M (2004) Echolocation in Bats and Dolphins, Chicago University of Chicago Press p. 3-27 

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Figure 33.7. Simulation result of aerogel ultrasonic transducer.

From Figure 33.9, when the acoustic impedance of the aerogel is about 10-20 x 10 kg/ (m s), the sensitivity of an aerogel ultrasonic transducer may be 45 times as high as a conventional ultrasonic transducer. 

Table 33.1 shows the material properties of the main stmcmres of the aerogel ultrasonic transducer. 

Figure 33.14 shows the calculated results of the time-domain response of the aerogel ultrasonic transducer using the parameters shown in Table 33.1. 

Figures 33.15 and 33.16 show the experimental results of the time-domain and frequency-domain responses. The red line and the black line show the aerogel ultrasonic transducer and conventional ultrasonic transducer. An acoustic matching layer of...

The vertical axis in Figure 33.15 is the relative value based on the amplitude of the time-domain response of the conventional ultrasonic transducer. The vertical axis in Figure 33.16 is in dB, based on each maximum response level of the aerogel ultrasonic transducer and conventional ultrasonic transducer. 

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. 

Moreover, the time-domain responses of the transducers were evaluated. The duration ( -20dB) of tho aerogel ultrasonic transducer was about 86 ms in Figure 33.15. 

A fundamental vibration of about 500 kHz (about 2 ms) and a beat vibration of about 60 kHz (about 16 ms) can be observed for the aerogel ultrasonic transducer in Figure 33.15. It was considered that the beat vibration was based on the difference (about 65 kHz) between the two peak frequencies at about 455 kHz and 520 kHz shown by the red arrows in Figure 33.16. 

Acoustic impedance of these materials is around 1,000-2,000x10 kg/(m s), more than ten times the value of ideal acoustic impedance. That is, the current performance of ultrasonic transducers with an acoustic matching layer is restricted by the limitations of the acoustic characteristics of its acoustic matching layer. Silica aerogel has an extremely low density as a solid. We focused on this fact and researched the potential regarding the acoustic matching layer of ultrasonic transducers, as we describe in more detail in the next section. 

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. 

The acoustic properties of ultrasonic transducers with an aerogel acoustic matching layer were studied using a one-dimensional Krimholtz-Leedom-Matthaei (KLM) equivalent circuit [10, 11] to simulate using computer. Figure 33.6 shows the structure of the... 

The porous ceramic portion, which protects the aerogel of the second matching layer and regulates its thickness, makes it possible to fabricate stable ultrasonic transducers. 

At lower ultrasonic frequencies (0.5-10 MHz), Raman-Nath diffraction of laser light by plane acoustic waves was used to measure the decay of acoustic energy inside an aerogel specimen [57], This method relies on the density variations produced by the sound wave that create a phase pattern inside the specimen. The intensity of the first order of the diffracted light beams is proportional to the sound intensity at the intercept of light and sound beam. In a log plot of sound intensity versus distance from the transducer, the slope is a measure of the attenuation. 

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