Any pipe diameter change causes partial sound reflection at the junction

Evidence

If the pipe changes diameter, the pulse will meet a change in impedance. Cells on the other side of the diameter change will push back too hard (if the impedance it meets is higher than its own), or too little (if the impedance it meets is lower). This will cause partial reflections of the sound at such junctions. — Eric J. Heller, Why You Hear What You Hear- An…, 119