I played around with changing the value of C19 to 220pF. This has the same effect of a high pass filter and it attenuate frequency below 1kHz by another 20dB, still the hums remains. This tells me that the noise didn't come from the transducer.
Spectrum plot of slowed down audio when running off AA NiMH (boost converter)
Spectrum plot of slowed down audio running off Li-ion battery (buck converter)
The differences tells me that it is power supply related noise. Since the frequency is sub-harmonics, I would assume that the boost converter is running in PFM mode for high efficiency under low load. There is a 3V LDO for the ARM microcontroller, so that means that the noise is coming from the ultrasonic receiver. I guess the low pass filter filter for the power supply rail isn't low enough.
I'll need to look at alternate PWM based boost converter or at least one with pin selectable PWM. The other alternative is to feed the analog circuit with its own LDO.
Silly me didn't check that first. One quick fix is to use MCP1640B or MCP1640D parts which are variants to the one I am using, but has the PFM mode disabled.
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