Echoes reshape but never change the total energy in a source spectrum

Evidence

S(f) and Sr(f) are plotted in figure 11.4 for b = 0.75, along with a flat spectrum for no echo. The area under all three curves averages to the same value. Sharper peaks are sculpted from blunter surrounding material, but the total amount of clay remains the same. The amount of clay is set almost instantaneously and corresponds to the total energy supplied by the hammer tap. — Eric J. Heller, Why You Hear What You Hear- An…, 1107