Questions 69–70 are based on the following reading passage.
Though an echo is a fairly simple acoustic phenomenon—a reflection
of sound waves
off some hard surface—it occurs only under very specific circumstances.
Imagine a listener
standing at the sound source. The reflecting object 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
5 sound. A reflecting object 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,
a persistent sound gradually decreasing in amplitude until the listener
can no longer hear it.
Common though echoes are then, it is unsurprising that some sounds
seem to produce no echo.
10 A centuries-old tradition holds that a duck’s quack does not echo.
Scientists in the Acoustics
Department of the University of Salford set out to test and explain this
claim. They recorded a
duck, Daisy, first in an anechoic chamber filled with sound-absorbing
fiberglass wedges, then
in an echo chamber with the acoustical properties of a small cathedral.
The sound of the duck
quacking in the anechoic chamber was clearly different from the sound
of the duck quacking in
15 the echo chamber, but the researchers acknowledged that it would be
very hard to recognize an
echo in the latter recording without having very recently heard the
former. 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. Partly it is because a quack is
just not very loud. The
Salford researchers also speculate that most people may simply not
encounter ducks in proximity
20 to reflectors such as buildings or mountains. A further complication,
though one the researchers
leave unremarked, is that people generally hear ducks in flocks, where
one quack might be
indistinguishable from the echo of another.
69. 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.
70. The passage suggests that which of the following would propagate
echoes?
An anechoic chamber
A cathedral
A mountain
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