Later echoes refine the spectrum into sharper peaks limited by the echo delay

Evidence

Any echoes arriving even later, at times T′ > T, will refine the spectrum by building narrower features out of the coarser shape resulting from earlier echoes. Clay can now be moved around only by even smaller distances, say, δf = 1/2T′. — Eric J. Heller, Why You Hear What You Hear- An…, 1099