Noel Hanna is a member of the Australian Acoustical Society, Acoustical Society of America, Australian Institute of Physics and the Teachers' Guild of New South Wales. He holds the role of Leading Education Professional (Physics) with UNSW Global, a wholly owned subsidiary of UNSW Sydney. That’s a tricky question. The simplest way to answer is to say that an echo is a sound that later comes back to where it came from. Before we get into what makes an echo, we need to have a think about sound. What is a sound? What we call “sound” is really just the air in our ears moving back and forth. The air can move fast or slow. We can hear air moving back and forth between 20 and 20,000 times per secon
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