Noise is, by far, the biggest factor that drives most creators to consider a spindle upgrade. Getting straight to the point, routers are load! Spindles not so much. Why?
Router and spindle noise depends on bearing precision and cooling methods. Let’s discuss bearings first.
When it comes to bearings, there is a distinct difference in how much noise is generated depending on the precision of the bearings being used. Routers and spindles feel tight and precise, but spindles are built with much higher precision bearings which greatly reduces “rattling” in the ball bearings.
“Rattle” is not the best term to use but is easier to visualize. When it comes to bearings precision vs noise, spin a router and spindle up to 20,000 rpm’s and the sound difference becomes abundantly clear. Less precise bearings have more play between the ball bearings and the ‘raceway’ they travel in. This translates to rattling within the bearing.
Now, add in the air cooling and you have a lot of noise! You might not think cooling and noise are related, but they are. Let me explain.
Both spindles and routers generate heat. This heat must be controlled to prevent overheating. There are two methods of cooling when it comes to routers and spindles. Air-cooled and water-cooled.
Routers are always cooled by a fan built right onto the spinning part (spindles will have the same thing if they are air cooled). A fan blade whirring around that fast simply makes a lot of noise. Think of the sound a helicopter makes and you will get it.
Routers make a lot of noise because of air cooling fans and lower-precision bearings. Short bursts of noise is tolerable, but when the machine runs for a couple hours, it can be overwhelming. The biggest drawback is that the sound can be loud enough to penetrate walls, sending high-pitched sound throughout your home. It can even be loud enough to irritate neighbors. Routers are so loud, they require hearing protection. Of course, you should wear hearing protection regardless since the carving process from the router bit can be noisy.
Air cooled spindles are a bit quieter because of the tighter bearings and air cooling is managed better through the housing. But what really tones them down is when you have a water-cooled spindle. This type makes almost no sound at all except the sound of the bit cutting the material.
When it comes to the sound, routers are the worst! Air cooled spindles are a bit better. Water-cooled spindles are almost dead silent! If noise is an issue, you absolutely want a water-cooled spindle.
As an added note, if you plan on cutting dense materials (like aluminum), water-cooled is also highly recommended. These materials require a lot more force to cut/carve and will quickly heat up air-cooled spindles (which are only as efficient at cooling based on ambient temperature of the air being moved through them).