Question about be quiet! Dark Power Pro 12

I have 4 case fans, a graphics card with 3 fans and a Noctua NH-D15S installed. The enclosure is not insulated. If I hold my sound level meter up to the Dark Power Pro 1500 power supply from outside at full load, I get values between 50 and 58 dBA.

Quiet is always relative. With a device that requires 1,200W or more, I also wonder whether the power supply unit is not the quietest part of the device...

You can check the dB with the manufacturer, he even gives them with different loads.