Though an echo is a fairly simple acoustic phenomenon, it occurs only under very specific circumstances. The object reflecting sound must be more than 11.3 meters away from the sound source, or the echo will return too soon to be distinguishable from the original sound. A object reflecting sound more than about 170 meters, on the other hand, will rarely produce an audible echo, since sound dissipates with distance. Further, multiple surfaces each reflecting the same original sound to the same listener will likely not produce an echo, but a reverberation. Common though echoes are then, it is unsurprising that some sounds seem to produce no echo.

A centuries-old tradition holds that a duck's quack does not echo. Scientists from the University of Salford set out to test and explain this claim. They recorded a duck, first in an anechoic chamber filled with sound-absorbing fiberglass wedges, then in an echo chamber with the acoustical properties of a small cathedral. While the recordings differed, researchers admitted that noticing an echo in the latter without recently hearing the former would be challenging. Partly this is because a quack isn't a single burst of sound, but fades in and out, so that the beginning of the echo might blend with the end of the original sound. The Salford researchers also speculate that most people may simply not encounter ducks in proximity to reflectors such as buildings or mountains.

According to the passage, all of the following make an audible echo unlikely EXCEPT

(A) a reflecting surface too close to the original sound
(B) a reflecting surface too far from the original sound
(C) multiple reflecting surfaces
(D) multiple listeners
(E) sound-absorbing materials



Consider each of the answer choices separately and indicate all that apply.

The passage suggests that which of the following would propagate echoes?

An anechoic chamber
A cathedral
A mountain