According to Noctua it is "...built on the basis of the legendary NH-D14 and carrying on its quest for ultimate quiet cooling performance, Noctua’s flagship model NH-D15 is an elite-class dual tower cooler for the highest demands. Its expanded heatpipe layout and two premium grade NF-A15 140mm fans with PWM support for automatic speed control allow it to further improve the NH-D14’s award-winning efficiency...". The NF-A15s are the 1500rpm version* that Noctua supplies with coolers and they come with low-noise adapters, to reduce the PWM range from 300-1500 rpm to 300-1200 rpm. A PWM Y adapter cable and 30cm/1 foot PWM extension cable is also supplied. (*The retail NF-A15 has a top speed of 1200 rpm).Further details here.