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If I go camping and shout anywhere, in the forest , or on a cliff, I usually hear the echo of my voice.

Why when I shout in my room I do not hear any echos?

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3 Answers

up vote 9 down vote accepted

You just haven't tried a big enough room...try a large, empty gymnasium or something similar (but not a concert hall as they are usually designed to suppress echos.).


The speed of sound is roughly $v_s = 340\text{ m/s}$ (1100 feet per second), and hearing an echo requires at least

  1. a perceptible time between the end of your shouting and the onset of the returning wave (not sure how long, but lets say $t_c = 0.1\text{ s}$ as a guess (a lot of human perception works on time scales not to far from that))
  2. that the returning wave be sufficiently loud to be distinguished above the background noise
  3. that the returning wave be sufficiently distinct from the returning waves from other surfaces
  4. a surface that gives a good reflection

So lets think about how a indoor situation might fail:

  • The room is too small. If the longest dimension is much less than $v_s*t_c \approx 34\text{ m}$ then the maximum delay between the end of a loud shout and the onset of the echo may be too short for you to distinguish the echo

  • The multiple returns from the several walls are overlapping and preventing you from picking out one echo.

  • The ventilation system and other ambient noises are comparable in volume to the echo.

  • The walls may be non-flat or made of materials that absorb much of the sound energy (this is often a design goal for large spaces)

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Small rooms are ok too. There are blind people who echolocate, in small rooms as well. youtube.com/watch?v=uobuBc2GO0o –  MBN Jan 6 '12 at 20:11
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Presumably they have trained their perceptions to improve the limits. The values above are very rough guess based on my own experience (and therefor the limits of my untrained perception). –  dmckee Jan 6 '12 at 20:13
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Some data supporting dmckee's response is available in this document. –  twistor59 Jan 6 '12 at 20:19
    
It takes time for them to be able to tell if there is a round object or flat, or other details, but to hear echo you need to make the 'click' and pay attention. You should go to the nearest anechoic chamber and see (sorry hear) what it is to have a room with no echo. –  MBN Jan 7 '12 at 11:57
    
It is worth pointing out that even if the longest dimension is 17 meters, i.e, half of 34 meters, echoes can be heard. Sound will travel 17 meters forth and 17 meters back, thus covering 34 meters. May be this is the reason for the point of @MBN... –  Awal Garg Mar 24 at 10:53
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Sound does echo inside a room but you might not notice it much for two reasons.

Firstly the time for the echo to return is very small so you will not hear a long sound repeated as an echo, instead you will get a resonance type echo, like when you sing in the bathroom.

Secondly most rooms are full of soft furnishings that quickly absorb the sound and damp the resonance.

If you are in a large room with no furnishings and you clap once loudly you will certainly hear the echo rebounding rapidly off the walls.

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If you happen to move all furniture and curtains out of a room the difference in sound resonance is very apparent. It is the wave phenomenon that is responsible for both echo and resonance, except in an empty room it is multiple reflections one can hear, which do not carry the signal any longer. I remember returning home after a summer in camp mainly outdoors. The house seemed to resonate, until I got used to the background. –  anna v Jan 7 '12 at 5:16
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This is because of furniture. Try in an empty room.

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