Fan cooler for JLH 10W a-class

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Fan cooler for JLH 10W a-class

Please, Can anybody help me?
I bought for my JLH amp (under construction) 4 coolers Cooler Master TP2-5020B2. It's a pentium 2 cooler, Rth=0.56C°/w if fan turns at 4700 rpm.
I intend to use them at half speed in order to reduce the fan noise (32 dB at 4700 rpm).

Now my problem is: how can I calculate the new Rth when speed is only 2300 rpm?
I mean must I think to add passive heatsinks? (The dissipated power on each MJ15003 will be 40W.)

many thanks,
AR:confused:
 
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