I can’t clarify, but would trust Noctua over what 45HL puts on the store page, noting a few points;
a) dB and dBA are not the same, if they truly mean just dB. dBA adjusts for low frequencies not sounding as loud to the human ear.
b) Because dB/dBA scales logarithmically, six 24.6 dBA fans would have a combined max noise level of about 32.4 dBA. Over an ambient of 36 dB that is a total of about 37.6 dBA.
c) Total noise for the system will also include the PSU fan, active CPU cooling if present, and the spinning HDDs. I don’t know if the store specs are trying to describe the noise level of one fan, or a full system set to max cooling.
For reference, using a sound meter app on my phone, not professional or calibrated equipment, and an HL15 v1 with the CoolerGuys 120mm fans connected to PWM headers, not doing extensive fan curve tweaking (think they’re spinning around 1600 RPM), and with 15 12 TB Seagate SATA consumer HDDs … With an ambient of about 30 dBA the total noise is in the low 40s, maybe around 42 dBA, 1 ft directly in front of the server.
I’m sure I could get it quieter, but it’s not in the same room as me, so I just need to not hear it with the closet door closed.