Origin and history of echo

from mid-19c. in U.S. in reference to natural features in popular caverns that created echoes; see echo (n.) + chamber (n.). Also used of legislative halls under domed ceilings that made whispers travel in surprising ways.

It is attested by 1931 in reference to sound engineering:

For instance, in England and Germany an echo chamber is used in broadcasting certain programs to give the effect of a large auditorium. [Associated Press article on broadcasting reprinted in various U.S. newspapers Aug.-Sept. 1931]

Figurative use is attested by 1924 in the older sense ("Ellis Island, that echo chamber of Europe's tragedies ..."), by 1934 in reference to artificial amplification or exaggeration of voices in politics, governance, etc., probably from the broadcasting use.

In an age whose vaunt is liberty of thought and liberty of action, men are but walking through an echo chamber where their own voices are drowned in the platitudinous sentiments of their fellows. ["Social Conscience," in The St. Bernard (La.) Voice, April 7, 1934]

"oral instruction, catechism," 1753, from Latinized form of Greek katēkhesis "instruction by word of mouth," from katēkhein "to instruct orally," originally "to resound" (with sense evolution via "to sound (something) in someone's ear" to "to teach by word of mouth"). This is from kata "down" (in this case, "thoroughly;" see cata-) + ēkhein "to sound, ring," from ēkhē "sound" (see echo (n.)).