Waveguide
There are different types of waveguides for different types of waves. The most common meaning is a hollow metal pipe used to carry high frequency radio waves. These are usually microwaves. Transparent dielectric waveguides and optical fibers are used as waveguides for light.   In acoustics, air ducts and horns are used as waveguides for sound. They are used in musical instruments and loudspeakers. Metal rods with special shapes are used to conduct ultrasonic waves in ultrasonic machining.