All electronics must dissipate some heat, particularly power supplies and electronic loads, but they don't have to be noisy to do so.
Silent Passive / Convection cooledFor many test instruments, the amount of heat to be dissipated is very low and just the conduction through the casing is sufficient, even when mounted in a rack. For other devices that need to dissipate more heat such as power supplies, completely silent cooling is possible by using heatsinks with a vertical air path that still allows the devices to be cooled by convection alone. These will usually have either external heatsinks with vertical vanes, or similar internal heatsinks with vents on the top and bottom panels. Such devices require space above and below when rack mounting to allow the air to move through the instrument. Likewise, the vents on the top and bottom of the units must not be obstructed. When used on a bench, loose objects must not be placed on the top of the power supply if there is a risk of them blocking or falling into the vents. All Aim-TTi EL-R and TSX series power supplies are silent passively cooled, as are all but the most powerful EX-R series. All Aim-TTi power supplies use internal heatsinks and ventilated cases.
Fan assisted convectionAt slightly higher power dissipation levels, fan assisted convection is used. This retains the vertical air path, but adds a low noise fan to increase the air flow going up through the case. This can use a variable speed fan whose speed (and therefore noise) only increases when there is excess heat to dissipate. Keeping the fan speed down as much as possible not only keeps the noise down, but increases its life and reduces the amount of dust that is drawn into the unit. With Aim-TTi power supplies, the PL, PLH and the more powerful models in the EX-R series use intelligent variable-speed low-noise fans to assist convection.
Forced air coolingForced air cooling uses one or more fan, usually at the back of the chassis to draw air through. This arrangement allows units to be stacked and rack mounted without concerns about blocking the air flow. For some Aim-TTi signal generators a small quiet 40mm fan on the rear panel is sufficient to remove the heat from the casing. For power supplies, one or more larger low speed fan is typically used as they provide quieter operation and longer life than small high speed fans for the required air flow. High power density 1U power supplies (whose side panels cannot accommodate an axial fan larger than 40mm) are almost invariably noisier than equivalent 2U power supplies for this reason. The CPX, MX, QL and QPX series power supplies and the LD and LDH series electronic loads all use low-noise variable-speed fan cooling.