Add components, pick a typical noise profile, and get an estimated combined dBA level at 1 meter — plus a "can it live in your bedroom?" rating.
Decibels are logarithmic, so two 30 dBA fans don't make 60 dBA — they make about 33 dBA. We use the standard 10·log₁₀(Σ 10^(dBᵢ/10)) formula to combine sources, which is the same approach used in HVAC and datacenter noise modeling. Distance is assumed at 1 meter; double the distance and you'll subtract roughly 6 dBA from the result.
The single most common reason a homelab gets dismantled isn't cost or complexity — it's a partner who can't sleep. A 1U rackmount server with 40mm fans can hit 65 dBA at idle, which is louder than most living room conversations. A tuned mini PC build with a passive or near-silent case can come in under 25 dBA, which is below the noise floor of most rooms. The difference is the difference between "homelab" and "former homelab."