Cooling Fans come in several shapes and sizes, offering a wide range of modes, amongst which some include heating operations. In the case of propeller fans it is ideal to provide a smooth rotation without obstacles, so that the sound is consistent, and an engine that, if tonal, is surrounded by insulating material to reduce any overly present tonalities. The latter is also applicable to non-propeller fans. Inconsistencies, tonalities and overall high levels can easily disturb rest and cause annoyance amongst users.