From Message #561: The 777 uses a 'new' acoustic based fuel volume measurement technique. Presumably a pure ultrasonic tone is far more likely to set up a standing wave than the white noise from a vibration source, unless of course the designers had already thought of the potential problem and use frequency agility based on prime numbers?
I guess such a standing wave would only occur at particular resonant path lengths dependent on air and/or fuel volume shapes.
Just to clarify this, the ultrasonic fuel measurement for each tank is calculated from sensors contained in individual hollow probes (a total of 20 in each wing tank and 12 in the centre tank). As such, the ultrasonics themselves would not be capable of creating any standing wave.