Pipe diameter changes reflect pressure pulses with opposite sign

Evidence

If a pipe suddenly becomes narrower, or wider, there is a corresponding abrupt impedance change (mismatch) at the junction of the two sections of pipe. If a positive pressure pulse is traveling from a wider to a narrower pipe, a positive pressure pulse returns from the junction, reflecting part of the energy. If instead it encounters a wider pipe, a negative pressure pulse reflects part of the energy (figure 1.9). — Eric J. Heller, Why You Hear What You Hear- An…, 125