Strange frequency

The HydroMoth is working fine. If you record at 384 kHz you can resolve frequencies in the FFT from 0 to 192 kHz. You shouldn't expect to see a peak in the FFT at the maximum frequency. There is a peak here at 24 kHz - most likely from a power supply - and then the normal decay in amplitude as the frequency increases. I'm not sure what you mean when you say 'I transmit signal from smartphone at 48Khz for example'. The speaker and sound hardware of the smartphone probably won't generate 48 kHz tones. You can normally only generate audible tones up to about 20 kHz from a small speaker.

Yes, the normal smartphone can generate the highest frequency at 44.1Khz at most cases. I can show you the files when a smartphone transmit ultrasound signal. And you can see the peak at 20Khz. That is what I expect to receive the signal with large amptitude. I am sorry I am not familiar with hardware. So I am not sure whether I can config the sensor like this.

The sample rate of the smartphone is typically 44.1 or 48 kHz for both output and input. That means it can typically generate sounds on 0 to 22.05 or 24 kHz, although there is likely to be some additional filtering to limit the very top frequency. In the AudioMoth Chine app, we make the smartphone produce tones at 18 kHz which works on all devices. I'm not sure what you are trying to acheive, but if you sample at 384 kHz on the HydroMoth you will be able to resolve all frequencies from 0 to 192 kHz.

You won't be able to generate sounds above 20 kHz without specialist playback equipment. The HydroMoth doesn't generate any sound.