Occupationally, an individual cannot be exposed to a noise source that exceeds 85dBA based on an 8-hour time weighted average without some sort of protective measures (engineering or administrative controls, and/or the use of personal protective equipment such as ear plugs, muffs, or a combination of the two). Many companies are creating industrial-type equipment that keeps sound pressure levels under that 85dBA criteria. Manufacturers may rate their products at a particular noise level, but variability in location, use, setup, etc. may cause a large swing in actual noise values experienced.
Interestingly, there are a lot of variables in determining how noisy (noise being defined as unwanted sound) a fan/booth setup is going to be. One of the posters on this thread has mentioned bearings, and that is true--more on that in just a moment (See f. below). A few of the other variables include the following:
a. How large is the area where the booth is going to be located? Smaller areas typically result in more (louder) noise.
b. Construction materials of the room. A concrete block basement is going to cause noise (particularly the higer frequencies) to ricochet, thereby amplifying the original noise source. Wallboard will have a more absorptive effect.
c. Booth size. A booth is a big box. Vibration (which is sound) can cause the booth to become a speaker depending on the sound frequency and booth construction (sheet metal or wood, for example)
d. Fan mounting. If the fan is mounted directly to the booth, vibration can be transmitted directly to the booth. A simple neoprene rubber gasket between the motor and booth can contribute substantially to reduced vibration.
e. Duct size. A 4" duct moving air at 5,000fpm is a veritable wind tunnel. The smaller the area, the higher the velocity, the noisier it gets. Increasing duct size can reduce noise.
f. Motor/fan maintenance. Using a fan/motor that has been used religiously for the last 20 years is probably going to be more noisy than a new one of the same model simply because the bearings may be wearing out. In addition, a fan blade is aerodynamic in design. Accumulation of paint residue reduces aerodynamic blade efficiency, but more importantly, throws the blades out of balance, which can cause a real racket--and cause bearing failure. Use of good paint filters can prolong the longevity of the fan.
g. The obvious: Fan size, rpm, rated airflow, and direct drive versus belt drive.
These are just a few of the variables that can cause noisy booths.
A good rule of thumb for those who don't have a sound level meter at their disposal is called the "3-foot rule", or "normal human conversation rule": If you're carrying on a conversation with another individual at a normal distance of about 3 feet, and you have to raise your voice to be heard over your paint booth, then the paint booth is probably exceeding the 85dBA noise criteria (this assumes reasonably good hearing in both parties). A good set of ear plugs is in order, or some sort of modification to your booth.
By the way, a good friend of mine owns an Artograph booth. It produces great airflow, and is very quiet.
Hope this helps some.
Gip