Cfm and noise at 7v

[Egrimm]

Is there any way to calculate cfm and noise of fans when they run at 7v?
I've got two 46.5cfm / 37db Papst fans for example, and would like to know how noisy they are now when they run at 7v instead of the normal 12v.

 

[klein297]

Best bet would be to just wire them up and listen to them.
Use one of those power splitter 4 pin connectors.
Red on fans to Yellow, Black on fans to Red.

 

[Egrimm]

I've already done that and they're quite a bit more silent, I just want to know if there's any way to calculate the exact values

 

[L00PY]

It really depends on the fan. But if you can monitor RPMs it becomes a lot easier. Two formulas you can use for that are as follows:

Airflow: CFM2=CFM1(RPM2/RPM1)
Noise: N2=N1 + 50 Log10(RPM2/RPM1)


If you want to read more, Google pointed to this guy who claims you can model it linearly using the Airflow formula with volts in place of RPMs.

 

[Egrimm]

Thanks. No, I cannot monitor the rpm as it is older Papst fans without with a molex connector without rpm monitoring.
Thanks for the link, it appears to be what I was searching for.