Final answer:
In pipe resonators, longitudinal standing waves are formed due to the back and forth vibrations of air that create pressure variations, reflecting and interacting within the pipe to produce resonant frequencies. Therefore, the correct answer is a) Standing waves.
Step-by-step explanation:
The kind of waves that form in pipe resonators are standing waves, specifically longitudinal waves. In pipe resonators, such as those found in musical instruments like pipe organs or flutes, the air inside the pipes vibrates back and forth along the length of the pipe.
These vibrations create pressure variations that travel down the pipe, reflect off the ends, and interact with new waves being produced to form standing waves.
With a pipe closed at one end, such as in a clarinet, the standing wave will have a node at the closed end and an antinode at the open end.
For a pipe that is open at both ends, like a flute, there will be an antinode at both ends. The resonant frequencies in these pipes depend on the length of the pipe and the speed of sound, with different harmonic frequencies - or overtones - being possible within the same pipe.