While I agree that much of the time 40 kHz is representative of ultrasonics it could also represent VLF radio frequency. I know WWVB runs at 60 kHz and other radio stations run below that.
The following was taken from a VLF group site: "The Natural Radio VLF group is a discussion group dedicated to those who enjoy monitoring radio frequencies in the VLF radio spectrum and slightly below the VLF (very low frequency) spectrum (3-30 KHz). This is a form of radio astronomy. We listen to the amazing "radio" sounds created by planet earth such as whistlers, chorus, tweeks, risers, spherics and hiss on simple VLF receivers (whistler receivers)."
I presume that a 40 kHz oscillator intended for ultrasonic use would be coupled to a transducer - one for transmitting would be coupled to an antenna. I would expect that to drive significant differences in circuitry.