17.17:
Echo
Sound waves get reflected when they hit the interface between two mediums.
Imagine standing on one end of a long hallway of length L and emitting a sound wave. The reflected wave takes some time to return to the source, given by the total distance traveled and the speed of sound.
Human ears can distinguish between two sources of sound if they are separated by a time gap greater than one-tenth of a second.
The emitted and reflected waves are perceived as distinct sound waves if the time difference exceeds the distinction time. This phenomenon is called an echo.
By equating the two sides and substituting the speed of sound in ambient air at twenty degrees Celsius, the minimum distance to hear an echo is obtained.
Conversely, if the time difference between the emitted and reflected waves is known, the distance between the source and the reflector can be calculated. Ships use this technique to measure the distance of various objects on the ocean floor.
17.17:
Echo
The human ear cannot distinguish between two sources of sound if they happen to reach within a specific time interval, typically 0.1 seconds apart. More than this, and they are perceived as separate sources.
Imagine the sound is reflected back to the ears. Assuming that the source is very close to the human, the difference between hearing the two sounds—the emitted sound and the reflected sound—may be more than the minimum time for perceiving distinct sounds. If this is the case, then the phenomenon is called echo.
As it turns out, the minimum distance to hear an echo is around 16.5 meters. As such, long hallways or distant cliffs are required to hear an echo.
The calculation can be reversed to calculate the distance between the emitter and the reflector. If the time difference between the emitted and reflected sound waves can be measured precisely, the distance between the emitter and the reflector can also be calculated. In the animal world, bats use this technique to locate prey.
Suggested Reading
- Young, H.D and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson: section 15.6; page 489.
- OpenStax. (2019). University Physics Vol. 1. [Web version]. Retrieved from https://openstax.org/books/university-physics-volume-1/pages/1-introduction: section 17.2; page 853.
- Wölfel, Matthias; McDonough, John (2009). Distant Speech Recognition. Chichester: John Wiley & Sons: section 2.4.2; page 48.