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Re: Low Noise Fans

What dB level, in your opinion, qualifies as a "low noise fan?" Also, I am soon going to be trying my hand at a marine setup with soft corals (and eventually stony corals). I am building a hood to house anywhere from 440 to 660 watts (depending on how much I can fit in it) of VHO lighting. Eventually, I will add some MH bulbs, probably 2 x 250 watts. What size, number, and flow rate of fans would you recommend for this endeavor? It is for a 4 foot long x 18 inch wide tank.

The noise level you find acceptable is going to be something you'll need to find out yourself. I typically find 30-35 dB fans fairly quiet, with the <30 dB fans nearly silent. The 20-25dB fans are very difficult to hear running. Anything over about 40 dB is going to sound loud. FYI, 90 dB is considered the "pain threshold" for humans, although I doubt you'll find any normal fans that even approach that.

The more tricky part is what kind of sound you will find to be annoying. Many fans have the same noise level in dB (usually dBA actually), but will have a lower or higher pitch sound to them. I personally can tolerate a louder fan that is lower in pitch much more than a fan of the same noise level in dB that generates a higher-pitched whine. Typically the faster the fan spins in RPM, the higher the pitch of the fan noise will be. Also, a higher speed small fan will blow the same amount of air as a larger fan running a slower speed (over simplified a bit, but the general idea is the same). For low noise you are much better off using a LARGER, SLOWER SPEED fan over a SMALLER, HIGHER SPEED fan.

Just for example, from the Digi-Key catalog, page 1034 (ebm brand fans), an 80mm (common computer size fan) moving 33 CFM of air will produce 35 dBA, while a 92mm fan moving 36 CFM of air produces only 27 dBA. 10dB is a 10x reduction is sound energy, 3 dB is a 2x reduction. The actual results you will "hear" will sound smaller than that, and I can't remember at the moment exactly what the conversion is. Unfortunately they do not list rotational speeds so I can't make the comparison there. The larger fan should generate noise of a lower pitch, and will sound *significantly* quieter than the smaller fan, even though it is moving slightly *more* air. The larger fan also uses about 1/2 watt *less* power to move *more* air. Unless you need the higher air pressure that a higher speed fan can generally generate, for common air-circulation-for-cooling applications a larger, slower-speed fan will generally be a better choice for low noise. The slower-speed fans will also generally last longer before their bearings die.

For your hood, you could take the rated temperature rise of your ballasts, the exposed surface area of the ballasts, and the ambient air temperature, and then calculate the minimum airflow required to keep the temperature of the ballast below it's rated maximum. It's far easier to just put enough fannage to turn the air volume of your hood over a few times an hour and position the fans near the heat sources in your hood. And remember that cooler parts will only last longer, so a few extra fans won't hurt anything.

BTW, you can further reduce the sound levels the fans will generate by isolating their vibration from the enclosure. The easiest way to do this is to mount the fans on rubber washers and adhesive weather stripping. Also, the less obstructions to the fan's airflow there are, the lower the noise due to air turbulence will be.

One last thing -- if you are planning on moving to MH lighting in the future, you should probably consider starting with MH instead of VHO. It sounds like you are going to be building your hood, and MH parts aren't terribly expensive when purchased through electrical supply houses instead of fish places. You then won't have the expense of redoing everything later, and I think you'll find that the MH bulbs will be cheaper in terms of parts replacement over time than the VHO bulbs will be. The newer PCF lights are also longer-lasting than the older VHO lights and will be cheaper to run over time. Something to think about...


Thanks, all!


Waveform Technology
UNIX Systems Administrator