An Echo is a Sound Reflection
In acoustics, an echo is a sound reflection, arriving at the listener some time after the direct sound. And, it is said that echo has the last word.
We often see a cartoon with a person overlooking a canyon, with a word or phrase yelled and the reflections being returned from the distant canyon walls, with each successive reflection reduced in loudness. One is often disappointed in that often the conditions for an echo do not exist. However, there are certain locals/structures that are famous for their echoes, with one of the most famous being the Hamilton Mausoleum in Scotland, where an echo record of 15 seconds was recorded.
The Mythology of Echo
In Greek mythology, Echo was the fairest of the nymphs. Hera, suspected that her husband Zeus was frolicking with one of the nymphs, and when investigating them, could not discover which. Instead, not finding which nymph, but diverted by Echo’s gay chatter, in her vindictive and usual injustice, turned against Echo. So Echo became another unhappy girl punished by Hera, even though she had eyes and desires only for a beautiful lad named Narcissus, not Zeus. Unfortunately, as his name describes, Narcissus loved only himself. Hera condemned Echo never to use her tongue again except to repeat what was said to her. “You will always have the last word,” Hera said, “but no power to speak first.”{{1}}[[1]]Hamilton, E. Mythology: Timeless Tales of Gods and Heroes, 1940, p.87, A Mentor Book[[1]]. So, this unhappy girl could only follow Narcissus, but could not speak to him.
The above story of the curse of Echo (by the Roman poet Ovid) is only one version. Another version written by Longus, a Greek author, was based around Echo’s rejection of the love of the god Pan, which ends with her body being torn into little pieces by shepherds, until all that remained was her voice echoing through the forest. Other stories exist as well, but I prefer the first story because it ties into the title of this post better, and coming from a house of women……
The Science of Echo
A true echo is a single reflection of a sound source by a discontinuity in the propagating medium, and returns with sufficient magnitude and delay to be perceived. For example, as a sound wave reaches the end of its medium, it undergoes certain behaviors and depending on the boundary (wall, canyon cliff, water, etc.), some transmission/refraction, or diffraction can occur. In the case of a sound wave in air hitting a solid wall, most of the sound is reflected. If the wall is relatively flat, perpendicular to the sound source, and far enough away (but not too far), the reflected waveform can be heard as an echo.
How much delay? The delay to distinguish an echo from the original sound must be greater than 1/15 of a second, or about 66 msec. after the original sound dies off.
How far away should the reflecting surface be? If the speed of sound is 1100 ft/sec (343 m/sec) at normal room temperature (25 Celsius or 77 Fahrenheit), the reflecting object must be no less than about 66 feet (17.2 m) from the sound source for an echo to be heard by a person at the source. The time it takes for the reflected sound to be returned (delay), is the extra distance divided by the speed of sound. Different temperatures will affect the speed of sound.
With sound traveling at 1100 f/sec, an echo that returns in 2 seconds, is precisely that distance away (the sound takes half the time (1 second) to get there and half the time (1 second) to return, or 1100 feet distant).
In summary, for an echo to exist, the following conditions could be listed for the formation of an echo:
- The size of the obstacle/reflector must be large compared to the wavelength of the incident sound (for reflection of sound to take place).
- The distance between the source of sound and the reflector should be at least 17 m (so that the echo is heard distinctly after the original sound is over).
- The intensity or loudness of the sound should be sufficient for the reflected sound reaching the ear to be audible. The original sound should be of short duration.
Echo or Reverberation?
Ordinarily, echo is not heard as the reflected sound gets merged with the original sound. When so many reflections arrive at a listener that they are not able to distinguish between them, the proper term is reverberation.
Singing in a shower often produces a reverberation. Your Pavarotti-like sound is the result of reflections of sounds combined with the original sounds. And, because the shower walls are typically less than 17 meters away, the combined sound waves create a prolonged sound – a reverberation.
Uses of Echo
Besides the novelty of hearing your words repeated, echoes can be used for many other purposes. As examples, distance to, size, and shape of objects can be estimated. The velocity of the sound itself can be determined. Special effects can be created with echoes reflecting off certain surfaces and used in sonar and medicine (ultrasound). Bats produce a sharp click or chirping sound and then listen for and processes any echoes off objects in the area to avoid or find, including gourmet moths. Some bats have very large ears that are very sensitive to sounds in certain wavelengths. Dolphins also use echo for locating objects. When used for finding a way around or to find prey, this is called echo location.
Echoes may be desirable (as in sonar), or undesirable (as in telephone systems), or even in hearing aids. In a concert hall, echoes can ruin a performance if the walls and ceiling are not properly designed. If the walls are too hard, or too flat, they make good reflecting surfaces for the sound waves. Special effects can be created with echoes reflecting off certain types of surfaces.
Music has used electric echo effects since the 1950s, both for vocals and instruments. Early approaches used a tape delay effect (Echoplex system) and were used heavily by guitar players, but gained favor also with recording studios. Today, most echo effect units use electronic or digital circuitry to recreate the echo effect.
