## 3 Discussion  
Ultrasound hearing aids are similar in principle to middle-ear implants or light-driven contact hearing aids in the sense that they amplify sound and transmit it directly to the eardrum, avoiding the effects of the external auditory canal. More in-depth research is needed to explore whether ultrasonic hearing aids have the performance and effectiveness of typical hearing aids. This area of research is in its infancy, and the main focus here was to determine how much sound pressure can be delivered to the eardrum with a parametric modulated ultrasound beam and how much power the PU receiver should supply at that time. Our results confirmed that the frequency-specific directivity of the audible sound obtained after SRAM modulation at 130 kHz was within the spread angle of the theoretical beam pattern of the ultrasound beam, and that all the acoustic power was concentrated within 8 mm--the size of the eardrum--at a distance of 6 mm. To clarify, the ultrasonic transducer emits a 130 kHz carrier that is amplitude-modulated by a 0.5-10 kHz audible signal. As the modulated wave propagates through air--a nonlinear medium--the ultrasonic carrier component undergoes progressive attenuation, while the audible signal is recovered via self-demodulation. Therefore, although the emitted waveform is ultrasonic, the effective acoustic output perceived and measured at the eardrum is in the audible range. The frequency characteristics revealed a larger-than-expected difference between the low- and high-frequency gain, and revealed that the high-frequency gain with gain fluctuation after 3 kHz was higher than that of the BA receiver. Additionally, the transient time of the ultrasonic receiver for burst sound is significantly shorter than that of the BA receiver, and this is expected to improve the sound intelligibility of hearing aids and benefit individuals with sensory neural hearing impairments. The transient time of the ultrasound receiver for the tone burst experiment was significantly shorter than that of the BA receiver, which is an important advantage. Ingeneral, however, the sound generated through parametric modulation was not very good in terms of THD, and this needs to be improved. The receiver consumes more power than an EM receiver at the same sound pressure; to produce sound greater than 80 dB SPL, it generates heat despite the use of multilayer PZT with high vibration efficiency. To solve this problem, a PZT device with a smaller dielectric loss and more layers will be required. One possibility is PMN-PT (lead magnesium niobite-lead titanate), which has a piezoelectric constant (d\({}_{.33}\)) of more than twice that of PZT, and a dielectric loss (tan \(\delta\)) that is only 27% of that of PZT PZT [18,19]. We expect that a multilayer element with a piezoelectric constant greater than that of PZT and generating less heat will be commercialized. In parallel with the power consumption problem, the voltage applied to the PZT device must be reduced to 5 V or less for practical use in hearing aids because the battery voltage used in a hearing aid must be considered. Another advantage of middle-ear implants or light-driven hearing aids is that the feedback sound is very minimal, so ringing can be avoided even if the ear canal is opened. The PU receiver is expected to yield this advantage because it can reduce the sound leakage compared to typical hearing aid receivers, due to the characteristics of its ultrasonic waves with small diffraction. Therefore, it will be necessary to design a cap for the PU receiver to take advantage of these characteristics and prevent the occlusion effect. The most important issue remaining is the biosafety of ultrasonic hearing aids, which requires further research. Most published guidelines about the maximum output of hearing aids range from 100 to 115 dB SPL, and no specific data are available beyond that [13,17,20].