## 2 Materials and Methods
### Measurement of Frequency Characteristics  
To measure the frequency characteristics of the ultrasonic receiver using the proposed ultrasonic parametric audio modulation approach, a data acquisition system (DAQ, NI PXI-4461 in NI PXI-1042; National Instruments, Austin, TX, USA), LDV (laser Doppler vibrometer) (OFV-5000 and OFV-545, Polytec, Karlsbad, Baden-Wurttemberg, Germany), and a probe microphone system (ER-7C, Etymotic Research, Elk Grove Village, IL, USA) were connected to form the device shown in Figure 2a. After taking the root square of the audio signal generated by the DAQ system, the SRAM signal was obtained by producing it with the carrier signal. For this, two multiplier ICs (MPY634, Burr Brown, Tucson, AZ, USA) were used. The modulation rate was set to \(m\) = 0.8, and the characteristics of the \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Target Pressure** & **Modulation** & **Envelope** & **Air Density** & **Sound** & **Absorption** & **Non-** & **Radiator** & **Object** \\  & **Index** & **Freq.** & **Velocity** & **Coefficient** & **Linearity** & **Radius** & **Distance** \\ \hline \(p(t)\) & m & \(f_{m}\) & \(\rho_{0}\) & \(\varsigma_{0}\) & \(\alpha_{0}\) & \(\beta\) & \(a\) & \(r\) \\ \hline
100 dB SPL & 0.8 & 1 kHz & 1.21 kg/m\({}^{3}\) & 343 m/s & 1.37 Np\({}_{\text{p}}\)/m & 1.2 & 0.3 cm & 0.5 cm \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters for the calculation of primary beam sound pressure. parametric modulator, in which THD (total harmonic distortion) increases when it is too high, were considered. The output signal of the 1 Vrms SRAM modulator was amplified to a level capable of generating a parametric audio signal and supplied to the PU receiver. For this purpose, RF power with a voltage gain of 30 dB, an output of 5 W, and a cutoff frequency of 1 MHz was used. For ease of experimentation, the amp used was a push-pull amplifier composed of 2 SA1012 and 2 SC1814 power transistors, without a class D amp, which requires a switching stage and low-pass filter. Figure 2b shows the configuration of the experimental setup installed in a soundproof chamber with a noise level of less than 50 dB. It shows the tube of the laser beam head and the probe microphone of the LDV installed to measure the sound pressure and vibration amplitude generated from the radiator by supplying the SRAM signal to the PU receiver. Figure 2c presents the oscilloscope waveform captured at this time. The first image is a diagram of the SRAM voltage (yellow) applied to the receiver, the second presents the ER-7 microphone output voltage (cyan), and the third presents the LDV output waveform (magenta). Figure 3 shows the sensitivity characteristics of the PU receiver measured at distances of 3, 6, and 10 mm, and is plotted along with the frequency characteristics of the BA receiver at a distance of 10 mm for comparison. A 130 kHz SRAM signal with a modulation rate of 80% was applied to drive the PU receiver. In order to avoid changes in vibration characteristics due to the limitations of the amplifier and the heat generation of the PZT element used, a 10 Vpeak SRAM signal was applied for the test, and the power supplied at this time was 1.3 mW, and the same power was supplied to the BA receiver. When applying 10 V peak to the receiver, the voltage amplitude required to obtain 100 dB SPL sound was calculated as 33 V; to avoid changes in the vibration characteristics due to the limitations of the amplifier and heat generation of the PZT element used, a 10 Vpeak was applied and tested. Figure 2: **(a)** Configuration of the RAM-type ultrasonic receiver test device; **(b)** photo of the probe microphone and LDV system setup for the PU receiver; **(c)** from the top, the waveforms of the applied receiver voltage with SRAM (yellow), ER-7 microphone output (cyan), and LDV output (magenta). For use as a practical PU receiver, a flat frequency characteristic from low frequencies to 10 kHz is desirable.