Besides the rotating cup anemometer, there is the sonic anemometer, swinging-plate (or pressure-plate) anemometer, pressure-tube anemometer, bridled(or windmill) anemometer, and the aerovane. Weather stations often use sonic anemometers, which calculate both wind speed and direction. Four ultrasound transducers are set up in a circle, evenly spaced apart, in two pairs placed across from each other. A transducer will send out an ultrasonic signal to the one directly across from it. Winds blowing across this path will cause the signal to travel faster, slower, or change direction, thus indicating wind conditions. Pressure-plate and pressure-tube anemometers work by the fact that wind blowing against a plate or through a tube will exert a measurable pressure. Aerovanes and windmill anemometers can measure both speed and direction. As the blades on these devices spin, it is possible to calculate wind speed, and both will turn into the oncoming wind, which indicates direction.