'Silence' could be used for the purpose, but human language does not really have a word for the opposite of 'echo', just as silence is not the opposite of sound (merely its absence).
However, if one considers an echo to have periodicity, we could consider something incessant, such as "din" to be its opposite. However, it is only opposite in a shade of meaning.
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The opposite of an echo would be silence or quietness, where sound is not reflected back.
The repetition of sound caused by the reflection of sound waves is called an echo. It occurs when sound waves bounce off surfaces and return to the listener after a delay, creating a distinct repetition of the original sound.
reflected sound is either reverberation or echo
The bouncing back or change in direction of a wave after it strikes a barrier or object is called reflection. Reflection occurs when the wave encounters a boundary and is sent back in the opposite direction, akin to an echo in sound waves.
Pseudo Echo was formed in 1982 in Melbourne, Australia.
It is called an echo.