[Animats]

That's direct-contact transmission at very short range: "an acoustic energy emitting device is disposed on coupling medium which in turn is disposed against skin surface. The coupling medium may comprise carageenan, xanthum gum, alginic acid, silicon gel, or other acoustic impedance matching material that is suitable for long-term skin contact." With a viscous gel, as used for ultrasonic medical scanning, efficiency is much better than through air.