There are many fan controllers on the market.  The price can range $20.00 up to $300. They all have pros and cons.  

 

 

 

The first type of controller are the most common ones I would say.  Most major hydroponic manufactures have their own version.  Speedster is probably the most well know.  R&M Supply/Cap Control, Oracle Garden Supply, and many others use the same exact one just re-branded   These are the cheapest ones you can buy averaging $25.00. How these controllers work is what is known as pulse-width modulation.  The fan speed is controlled by pulsing the power going to fan.  Switching the power on and off lets the fan coast so the speed slows down.  Most ceiling fans work along the same lines.  If you are looking to reduce noise this method is not for you.  I wanted to reduce the noise outside my window by slowing the air down.  I was able to reduce that noise, but the hum was so loud that it was more noticeable at a farther distance. 

 

These units are cheap and do have some drawbacks.  The first being that they create a hum anytime the speed is slowed down.  For a lot of people this doesn't bother them, but for me it was very annoying.  If you share walls with a neighbor I would not suggest using one of these as the hum can easily go through walls.  Other than the hum you can also damage your fan if you slow it down to much. The Fan has to work hard to keep the fan at that slower speed.  The fan is technically going on and off a good number of times a second.  Due to this reason the fan is having to work to speed up the fan constantly making it work harder than if it just has to maintain the speed.  This causes excess heat putting wear on the motor, and the fact that the fan uses the airflow to cool the motor this will greatly reduce the life of the fan.  I know Can-Fan has a fan that they warn not to use these on.

 

For myself the humming and possible damage was not worth the savings the cheaper options offered.  Googling "fan speed controller hum" produces a huge number of results, and the common, and only solution for a long time was the Can-Fan Can-trol model.  The Can-Trol gets rid of all the negative issues.  You will still have to make sure you don't run it to slow, but can safely run it slower than with the previous versions.  These benefits do come at a cost.  With a price tag around $250 it does hit the wallet a bit.  I have been seeing new companies offer the same method for around the same price or a little bit cheaper.  Unlike the cheaper models method of switching the power on and off these will lower the voltage going into the fan.  If one of your goals in lowering the speed is to reduce noise this is the way to go.  without the hum when you lower the air flow you greatly reduce the noise.

 

The third option out there is the same insides as The Can-Trol and other comparable controllers.  At the core is what is called a variac.  Variac Stand for variable alternating current.  The variac uses a transformer to drop down the voltage going to the fan.  I recently bought a variac on ebay for $44.00.  There where a few around that price for a minute but they seem to have sold out and the prices have jumped back up.  I found one for $69.99 from Frys electronics http://www.frys.com/product/4638758.  That is still a pretty good deal in my opinion.  

As I said the variac lowers the voltage going to the fan so do not have to worry about damaging your fan, or dealing with the hum.  When choosing what unit to by make sure it is rated to handle amperage your fan uses.  The one linked to is rated for 500va.  That comes to just a little over 4 amps.  I would say that that is enough power to handle most fans on the market.  

 Looking back I wish I would have bought one of these rather than the cheap pulse with modulation one I used first.  When you think about it $40 is worth not having to replace your fan prematurely.  If you decide to pick up a variac I would suggest first checking ebay and google to see if there happens to be any good deal posted.  You can also use the search terms auto transformer or voltage regulator.  When it comes down to it I think they are without a doubt worth the money.  

I will be posting pics soon, and hopefully get a youtube video showing the difference in sound.