# Does halving the Voltage on a fan, also give half the noise??

#### strokeside

##### Member
If I have a fan(12V) which gives 100cfm, at 2000rpm and a noise level of 40dB, will it go to 50cfm at 1000rpm at 20dB if I cut the voltage to 6V???
Ignoring the fact that i will actually be changing the voltage to 7V and not 6, is the above assumption correct? especially concerning the noise levels.

And ideas??

#### Roger24

##### Member
You're probably right about the rpm and cfm. I'm not sure about the noise though, since the db scale is logarithmic. I'm wondering the same thing myself.

#### Shroomer

##### member with no avatar
BTW... 10db =1 bell = 1/2 of the noise...so

20db reference
30db twice the noise
40db 4x the noise
50db 8x the noise...

see why those deltas are so loud!

#### stool

##### Senior Member
The only real way to tell is to pick up a sound level meter, and measure the dBA from the same distance for each setting. I picked one up in RS on sale for \$19.99, but I believe the regular price is around \$30-\$40.

#### Soloithz

##### Registered
Halving the power doesnt halve the noise... Like what was mentioned above because sound is logarithmic. If you decrease the power by half then your sound output in theory will drop by 3db. Note that 3db is not a big difference, you could tell the difference but it wont sound much quieter.
If you end up getting that radio shack meter, use the A-weighted setting not the C-weighted, its more accurate and is the industry standard scale.

Last edited:

#### Ugmore Baggage

##### Member
Some pitches will amplify better than others. Due to that, an adjustable rheostat (just one of those that you can set with a screwdriver) is the best solution. Only a tiny amount of noise is generated by the motor bearings but that noise is amplified by interaction with the fans blades and the mounts like the strings of a guitar -- keep in mind that an electric guitar and acoustic gutar's strings are basically the same and you can see how vibration can ampify as it transfers though surfaces. Also a lot of noise is generated by air turbulence which can also be amplifiable.

Quick recap: Certain pitches are more prone to amplification, which is what musical octaves are based on. Put your fan on a pot and slowly turn it down until it reaches its minimum volume, it should stay at that minumum for a while, as amplification increases and base noise decreases so you can ignore noise levels that are equal or slightly lower than the low point.

If you took physics lab you'll probably know what I'm talking about.

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