The term 'SOUND NUMBER' can be broken down into two crossword solutions, MACH (4 letters) and MECCAS (6 letters). MACH stands for the speed of sound in a particular medium, usually expressed in terms of the number of feet or meters per second. MECCAS is an acronym for the Measurement of Environmental Conditions and Sound Acoustics System, which is a system used to measure sound levels in various environments.