The term 'SOUND, UNIT OF' can be broken down into four crossword solutions: MACH, DECIBEL, PHONEME, and PHON. MACH is a unit of speed equal to the speed of sound in air at sea level, which is approximately 340.3 meters per second. DECIBEL is a unit used to measure the intensity of a sound, with 0 decibels being the threshold of human hearing. PHONEME is the smallest unit of sound in a language that can distinguish one word from another. Finally, PHON is a unit of loudness, equal to the loudness of a 1, 000-hertz tone at 40 decibels.