What do troughs represent in a sound wave?
Sound is a pressure wave in a fluid medium. That medium has a &quot;static&quot;or average pressure and sound is simply a small change of that pressure (as a function of time). Negative sound pressure just means that the current pressure is a little lower than the average pressure.
For example, static air pressure is around 100,000 Pascals (Pa) . Normal audible sound is quite a bit smaller, maybe 0.1 Pa (for 76 dB SPL). That means that the pressure are the microphone (or your ear) varies from 99,999.1 Pa to 100,000.1 Pa