Kind of related. This week in Medellín, people got fed up of car dealerships parking in sidewalks and made fun interventions like practicing yoga and setting up beach furniture (article in spanish).
How is it that they receive and transmit ultrasonic frequencies with a computer sound card? Is there not a low-pass filter at around 22KHz on inputs and outputs of all sound cards?
http://www.semana.com/nacion/articulo/en-medellin-se-cansaro...