Musical sounds and noise. Sound and health. Sound in architecture
Pleasant, rhythmic sounds are called musical sounds, irregular, unpleasant ones noise; the boundary is relative (music to one person can be noise to another, and even a pleasant tune becomes noise when too loud). In musical instruments sound arises from vibration of an air column (flute, karnay, surnay, clarinet), of a string (rubab, dutar, tanbur, g‘ijjak, violin), of a stretched skin or membrane (doira, nog‘ora, drum) and from electronic oscillations. Noise, especially loud and long-lasting, tires the nerves, weakens attention and can damage hearing (long exposure above about 85 dB is dangerous), so we should not play very loud sound in headphones. To reduce noise, sound-absorbing materials are used. The sound absorption coefficient α is the ratio of absorbed sound energy to incident energy: wall 0.02–0.03, carpet 0.21, a 9 cm layer of glass wool 0.51 (at 500 Hz). Architectural acoustics studies how sound is heard well in halls and buildings: theatre ceilings are shaped to send sound evenly to all parts of the hall, and walls and seats are covered with materials that absorb unwanted echo.
“Quiet corner”: listen in class to high and low sounds (a spoon on a pot, clapping, an instrument) and judge which are musical and which noise; then block the sound with a carpet or cotton wool and see how loudness changes. Safety: do not play loud music, do not make loud sounds near ears, and keep headphone volume moderate.