How does Synology measure NAS noise?
With the system fully installed with Synology SATA HDDs and idle, and two GRAS 40AE microphones 1 m from the front and rear. Here is the footnote in full, from the DS225+ page:
"Noise testing was conducted with the Synology system fully installed with Synology SATA HDDs and in an idle state. Two GRAS 40AE microphones are placed at a 1 m distance from the front and rear of the device."
Background noise level, temperature range and humidity range follow. That makes a named instrument, a named position, a distance and the conditions of the room. No appliance maker on this site publishes any of those, and no other NAS maker here names a microphone type.
The gap is real, though. Synology ships three fan modes, Full-Speed, Cool and Quiet, and names none of them in the footnote, so a figure taken on Quiet and one taken on Full-Speed would read exactly alike on the page.
Hunting for these numbers yourself? Use the specification tab on the product page. Seven Synology datasheet PDFs, downloaded and searched, held no noise figure between them.
The drives make most of the noise
Synology's own range shows it. The DS223 publishes 14.6 dB(A), measured "with Synology SATA SSDs". The DS223j publishes 18.2 dB(A), measured "Fully loaded with Seagate 2TB ST2000VN000 hard drive(s)". Same maker, same 92 mm fan, similar chassis. The 3.6 dB gap comes from the bays.
The first question about any figure in this guide is therefore what was fitted when it was taken, ahead of how low it is.
A 14.6 dB(A) box with four spinning drives in it is not a 14.6 dB(A) box.
Our hard drive guide ranks the drives themselves, and for a NAS it points to 5400 RPM CMR models. The same logic makes the Asustor FLASHSTOR interesting despite its lack of conditions: it has no drive bays, only M.2 slots, and nothing inside it spins.
A device cannot be measured directly at a level below the room it sits in. The DS223's footnote states a background of 16.49 to 17.51 dB(A), and its published figure is 14.6 dB(A), so the number must have been derived some other way rather than read off a meter.
Synology does not explain how. We will not guess. The figure is recorded as published, flagged, and kept out of the top pick, because a figure you cannot interpret is not one to buy on.
TerraMaster, Asustor and UGREEN say less
Their conditions are thinner than Synology's, and QNAP gives none at all.
TerraMaster publishes a background level of 17.3 dB(A) and a 1 m distance, better than most. Its condition reads "Using 2 SATA HDDs/SSDs in standby mode", though, which leaves the drive type unresolved, and the same two drive condition covers the four bay F4-424. A four bay machine measured half empty is not the machine you will own. Oddly, TerraMaster names an exact drive for its power consumption test and no drive at all for its noise test.
Asustor puts its decibels in marketing sentences, not specification rows, and quotes the same 19 decibels for a two bay and a four bay in one series.
UGREEN is the one NAS brand to state a fan mode: "In quiet mode, noise levels stay between 29 and 34dB". Those are the highest numbers in this guide. Either UGREEN is louder, or it is measuring something the others are not, and nothing published settles which.
QNAP publishes nothing
We checked sixteen current models. Not one specification page carries a decibel figure. QNAP's downloadable datasheet is generated from that same table, so there is no fallback.
Is Synology's NAS noise disclosure getting worse?
Yes. The older Synology footnote names the drive, "Fully loaded with Seagate 2TB ST2000VN000 hard drive(s) in the idle state", while the newer one says only "Synology SATA HDDs".
Both generations are live on Synology's site right now. That is how the DS223j can tell you exactly what was in it while the newer DS225+ cannot, and it means the older models are the ones that let you reproduce a figure.
Running the NAS from a battery backup? Our UPS guide notes which units suit one and how to silence their alarms.
