In residential ventilation, is the supply air or the exhaust air noisier outside?

Hello,

can you tell me if you hear the supply air or the exhaust air louder outside at the ventilation grilles? There must be differences.

Best regards
Sabine

At the same distance from the ventilation unit and with ventilation grilles of the same size, the exhaust air outside is usually louder to hear, partly caused by flow separation at the edges of the ventilation grilles. With very short ventilation ducts, the humming of the fans can also be heard outside on the supply air side because the silencers are only installed on the inside of the house. The noise level can, depending on the installed pipe material and the method of pipe fastening, possibly be transmitted to and amplified by the building structure.

With very short ventilation ducts, you can also hear the humming of the fans on the outside air supply because the silencers are only installed inside the house.

Is there a recommendation on how long the ventilation duct should be at least?

Best regards
Sabine

It depends very much on the design of the system... my supply air duct is 50cm long and practically silent outside even at the highest setting... Exhaust air, on the other hand, is always audible... but here it is due to the aforementioned flow noise caused by the grilles.