What is minimum distance for an echo to be produced?

As the sensation of sound persists in our brain for about 0.1 s, to hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s. If we take the speed of sound to be 344 m/s at a given temperature, say at 22 ºC in air, sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1s. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344 m/s) ×0.1 s = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance. I.e., 34.4/2 = 17.2 m [given speed of sound 344m/s] read less

When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/15 of a second. Thus, since the velocity of sound is approximately 343 m/s at a normal room temperature of about 25 °C, the reflecting object must be more than 11.3 m from the sound source at this temperature for an echo to be heard by a person at the source. Sound travels approximately 343 metres/s (1100 ft/s). If a sound produces an echo in 2 seconds, the object producing the echo would be precisely that distance away (the sound takes half the time to get to the object and half the time to return). The distance for an object with a 2-second echo return would be 1 sec X 343 metres/s or 343 metres (1100 ft). In most situations with human hearing, echoes are about one-half second or about half this distance, since sounds grow fainter with distance. read less

Depends on the quality of the sound. The minimum distance to hear an echo will be partly based upon the observers ability to resolve different sounds (i.e. so that they can distinguish between the actual noise, and the arrival of the reflected sound - a little 100 year old lady may not be as good as a 15year old!). This will vary between observers, as their auditory reaction times vary In the perfect Physics world though: Assuming a sharp sound (like a gunshot) the minimum distance (d=vt) to hear distinct echo is 17.2m considering the speed of sound to be 344ms. In general the "minimum echo distance" is always equal to half a wavelength from the reflector. read less

Depends on the quality of the sound. The minimum distance to hear an echo will be partly based upon the observers ability to resolve different sounds (i.e. so that they can distinguish between the actual noise, and the arrival of the reflected sound - a little 100 year old lady may not be as good as a 15year old!). This will vary between observers, as their auditory reaction times vary In the perfect Physics world though: Assuming a sharp sound (like a gunshot) the minimum distance (d=vt) to hear distinct echo is 17.2m considering the speed of sound to be 344ms. In general the "minimum echo distance" is always equal to half a wavelength from the reflector. read less