The ultrasonic generator is active whenever any bit is nonzero.
It looks like the generator's controller is sampling the bits periodically. You probably need to change all the bits "at once".
Look at: http://www.arduino.cc/en/Reference/PortManipulation
Another possibility is to use some other chip in between, to get the same effect. Like an HC373 or a shift register like HC595.
Some chip info on the ArduinoInfo WIKI here - http://arduinoinfo.info http://arduino-info.wikispaces.com/Popular-ICs