We have found Nexus quiet fans to be the best mix of true silence, airflow, and price to be found in a quiet case fan.
Many quiet fan manufacturers give decibel ratings less than 15 dB. However, background noise levels in certified sound chambers bottom out at about 15 dB. Many of these silent case fan decibel ratings are extrapolated based on subjective measurement, rather than being actually measured. These manufacturer decibel ratings are only really helpful in comparing with another product by the same manufacturer. Nexus tests the noise level of their case fans in an ISO-certified chamber.
Also, in our extensive experience building quiet computers. Nexus case fans are extremely quiet. This is why we use them in our Quiet PCs.