What type of pipe allows both ends to be open for sound waves to propagate?

An open pipe is characterized by having both ends open, which allows sound waves to propagate freely in and out of the pipe. This condition permits the formation of standing waves that have anti-nodes (points of maximum displacement) at both ends. This standing wave pattern supports a fundamental frequency and its harmonics that are integral multiples of the fundamental frequency, allowing for greater sound propagation and resonance.

In contrast, a closed pipe has one end closed and restricts the movement of air at that end, creating a different wave pattern with a node at the closed end and an anti-node at the open end. A half-open pipe has one end closed and one end open, leading to a distinct standing wave pattern that only supports certain frequencies. The term "echo pipe" is not a standard term in wave physics and does not accurately describe a specific type of pipe related to sound wave propagation.

Thus, the correct answer is the open pipe, as it facilitates the optimal conditions for sound waves to travel through both ends, allowing for a fuller resonance and sound quality.