Open tube ends preferentially transmit high frequencies and reflect low ones, smearing echoes.

Evidence

The half-open tube presents another interesting scenario. A hammer tap from behind moves the closed end in suddenly (we suppose it is a disk that moves in a bit and stays in its new position), producing a positive pressure pulse traveling down the tube. The sharp disturbance consists of many frequencies all present at once. If there is no reflection and no echo, the power spectrum is therefore flat. If there is an open end, the high frequencies (short wavelengths) will preferentially escape on their first encounter with it. The lower frequencies, especially with wavelengths longer than the diameter of the tube, will preferentially tend to reflect. Therefore, the returning pulse cannot be as sharp as it was when it left the source, since it has lost some of the high frequencies required to form a short pulse. The pulse returns to the source as an echo, somewhat spread out in time, and inverted. This return pulse then reflects from the closed end, and later again from the open end, where more losses take place and now a positive reflection occurs, followed by a positive echo, and so on. After a long time, only the lowest mode remains, the slowest to escape of all those contained in the original pulse, since it has the longest wavelength (four times the length of the tube) and the highest reflection at the open end. — Eric J. Heller, Why You Hear What You Hear- An…, 1073

Categorization

• XREF: Connects to acoustic filtering at open pipe ends — the boundary condition that lets only wavelength-discrete modes persist, related to how brass/woodwind toneholes behave.
• DEFINE: Explains why the persistent mode has wavelength four times the tube length — the open-closed pipe fundamental.