So, what sort of dB would be generated at 3.3V and how many volts should the receiver see? Assuming I’m interpreting this correctly…
Datasheet Facts
Transmitting Sound Pressure Level at 40.0Khz; 108 dB min, 0dB re 0.0002Pbar per 10Vrms at 30cm
Receiving Sensitivity at 40.0Khz 0dB = 1 volt/μbar -75dB min.
From the
ref:
Determining SPL at the front end of Receiver
SPL loss for 3 Vrms driving voltage = 20 * log (3V / 10V) = -10.46 dB
SPL Gain at 10 cm = 20 * log (30 cm / 10 cm) = 9.54 dB
Wave absorption = ~ .1886 dB/m * .1 = .02 dB (
page 4)
The SPL at 10 centimeters becomes = 108 – 10.46 + 9.54 - .02 = 107 dB
Converting SPL to μbar: 107 dB = 20 * log ( X / 0.0002 μbar)
X = 45 μbar
Determining Receiver Sensitivity in Volts/μbar
Converting Sensitivity to Volt/μbar: -75 dB = 20 * log ( X / 1 Volt/μbar)
X = 0.178 mV/μbar
Voltage generated under 0.178 mV/μbar = 0.178 mV/μbar * 45 = 8 mV
At 5V it the receiver would be:
Determining SPL at the front end of Receiver
SPL loss for 3 Vrms driving voltage = 20 * log (5V / 10V) = -6.02 dB
The SPL at 10 centimeters becomes = 108 – 6.02 + 9.54 - .02 = 111.5 dB
Converting SPL to μbar: 107 dB = 20 * log ( X / 0.0002 μbar)
X = 75.16 μbar
Determining Receiver Sensitivity in Volts/μbar
Voltage generated under 0.178 mV/μbar = 0.178 mV/μbar * 75.16 = 13 mV
Update: The Cortex M7 ADC has 12 bits and covers -3.3 to 3.3, so steps of 1.6 mV? If so, wouldn't need a transistor to boost the receiver signal for a chirp, but to curve match we'd get a better fit with a transistor.