=327. Echoes.=--_An echo is the repetition of a sound caused by its reflection from some distant surface_ such as that of a building, cliff, clouds, trees, etc. The interval of time between the production of a sound and the perception of its echo is the time that the sound takes to travel from its source to the reflecting body and back to the listener. Experiments have shown that the sensation of a sound persists about one-tenth of a second. Since the velocity of sound at 20°C. is about 1130 ft. per second, during one-tenth of a second the sound wave will travel some 113 ft. If the reflecting surface is about 56 ft. distant a _short_ sound will be followed immediately by its echo as it is heard one-tenth of a second after the original sound. The reflected sound tends to strengthen the original one if the reflecting surface is less than 56 ft. away. If the distance of the reflecting surface is much more than 56 ft. however, the reflected sound does not blend with the original one but forms a distinct echo. The echoes in large halls especially those with large smooth walls may very seriously affect the clear perception of the sound. Such rooms are said to have poor _acoustic_ properties. Furniture, drapery, and carpets help to deaden the echo because of diffused reflection. The Mormon Tabernacle at Salt Lake City, Utah, is a fine example of a building in which the reflecting surfaces of the walls and ceiling are of such shape and material that its acoustic properties are remarkable, a pin dropped at one end being plainly heard at the other end about 200 ft. away. Important Topics 1. Waves: transverse, longitudinal; wave length, condensation, rarefaction. 2. Wave motion: in coiled spring, in air, on water. 3. Reflection of waves: law, echoes. Exercises 1. A hunter hears an echo in 8 seconds after firing his gun. How far is the reflecting surface if the temperature is 20°C.? 2. How far is the reflecting surface of a building if the echo of one's footsteps returns in 1 second at 10°C.? 3. Why is it easier to speak or sing in a room than out of doors? 4. Draw a curve that represents wave motion. Make it exactly three full wave lengths, and state why your curve shows this length. Indicate the parts of the curve that correspond to a condensation and to a rarefaction. 5. How long does it take the sound of the "pin drop" to reach a person at the farther end of the building mentioned at the end of Art. 327? 6. An echo is heard after 6 seconds. How far away is the reflecting surface, the temperature being 70°F.? 7. Why are outdoor band-stands generally made with the back curving over the band? 8. A man near a forest calls to a friend. In 4 seconds the echo comes back. How far away is he from the forest? 9. Would it be possible for us ever to hear a great explosion upon the moon? Explain. 10. If a sunset gun was fired exactly at 6:00 P.M. at a fort, at what time was the report heard by a man 25 miles away, if the temperature was 10°C.?