Why are MacPro's so quiet?

They use large heatsinks on the CPUs and memory so they don't need fast airflow for cooling, and they use large fans turning at low speeds to move the air. The case has lots of holes on the front and back, and little obstruction to the air flow inside, so the airflow doesn't make a lot of noise. I have mine on a desk in front of a window, and can see straight through the case to the window. There are many temperature sensors inside, so the fans can be run just fast enough to keep everything at a safe temperature.

Apple started paying a lot of attention to airflow, starting with the G5 models, after the last G4s got the nickname Wind Tunnel.

Hi,
my MacPro is louder then my old pc. the ramstack has a temp of 80°C/176F and the northbridge 65°C. room temp has increased dramatically, it's like a little a sauna.
lg

How old is your old PC? And I also hope you do realize that, inside every MP, we have two 2.8GHz+ Quad-Core Intel Xeon processors, 1600MHz FSB, 800MHz DDR2 ....

BTW, my northbridge is 24°C and memory 40°C. The fans are spinning so slow and I can't hear them at all. It just shows that the SMC fan control is working 😉

Hi,
whats your fan rpm?
rgds
robert

Malcolm covered many of the design decisions; Apple went pretty deep with the thought process. For example, why do you have to get RAM with special heat sinks? Because Apple decided that if you could get the RAM to throw off heat more efficiently, the fans would not have to work as hard, meaning they could be quieter.

When the current case design was first released several years ago, Apple had videos and other material explaining all of the little things they did to optimize cooling and airflow in a way that would allow the use of quiet, low-speed fans.