This is a specific type of low frequency noise that can easily compromise even the finest recordings. Rumble normally occurs at frequencies below 50Hz, with some as low as 18-32Hz. The most common sources of rumble are air conditioning and heating systems, as well as traffic noise (18-wheelers, buses, cement mixers and so forth) or perhaps a nearby train. Top professional studios, such as those here at Sweetwater, are specifically engineered to be acoustically isolated and thus free of such low-frequency noise, but for most home studios this is a pricey solution. Rumble can be reduced or even eliminated in some cases by the use of specific software such as BIAS SoundSoap Pro, Waves Audio Restoration Bundle, or Digidesign DINR. Other problems with rumble may be minimized or eliminated through the use of a graphic or parametric EQ or a highpass/low-cut filter.