Comparing suppliers, one thing stands out: everyone promises good insulation, but the figures differ — 25 dB, 30 dB, “up to 40 dB”. The problem is that the number can be measured in several ways, and without a standard the figures are not comparable.

What ISO 11957 measures

ISO 11957 is an international standard describing how to measure an enclosure's insulation: with what signal, across which frequencies, and from where. When a manufacturer refers to it, the number came from a repeatable method rather than from the most flattering spot.

The certificate is not a formality. It is the difference between “we measured 35 dB” and “an independent laboratory measured to the standard and recorded 35 dB”.

ISO 23351-1: the standard written for this product

ISO 11957 came from the world of industrial control rooms. ISO 23351-1 was written specifically for furniture and enclosures used in offices, and it measures something closer to what a buyer actually cares about: speech level reduction, the drop in the level of a voice generated inside the enclosure as measured outside it. The direction matters and is frequently stated backwards — the source is the person in the cabin, not the office around it.

Because it is expressed as a single speech-weighted value, ISO 23351-1 sorts products into classes rather than leaving you to interpret a raw curve. Where a supplier quotes it, you can compare two catalogues line for line. Where a supplier quotes neither standard, you are comparing marketing copy.

Why “up to 40 dB” is a warning sign

The words “up to” usually mean the best result at the most favourable frequency. Human speech mostly sits between 250 and 4000 Hz, and that is where insulation matters. A single figure with no frequency curve and no standard tells you nothing about whether it holds where speech lives.

The give-away is the shape of the curve, not the peak. Insulation tends to be easiest to achieve at high frequencies and hardest around 250–500 Hz, where the vowel energy of a raised voice sits. A product that posts a strong headline figure but sags in the lower half of the speech band will let through exactly the part of a conversation that carries the emotional register — you will not hear the words, but you will hear that an argument is happening.

Three places sound escapes

The door. The weakest link in any enclosure. Insulation is only as good as the seal: if it is not even around the full perimeter, part of the measured result is gone.

The glass. A single pane resonates at frequencies that coincide with human speech. Three-layer laminated glass with panes of different thickness solves it.

The ventilation ducts. Air has to move, but the same opening carries sound, which is why the ducts hold acoustic silencers.

How to translate decibels

The decibel is logarithmic, so a 10 dB difference is not “slightly more”. Every 10 dB is roughly a halving of perceived loudness. The gap between 35 dB and 25 dB is therefore much larger than the numbers suggest at first glance.

Concretely: in a unit certified at 35 dB a conversation is inaudible from a metre away. At around 25 dB you can hear that someone is talking, but not what they are saying.

What to ask for in writing

Procurement teams that get this right ask for three documents rather than a number: the test report naming the standard and the laboratory, the frequency-band results rather than the single weighted figure, and confirmation that the tested configuration matches the one being quoted — same glazing, same door, same ventilation. A certificate for a different build is not a certificate for yours. Certificates and test reports for the range are published by the Silentbox manufacturer.

Other standards you will see mentioned

ISO 9001 concerns manufacturing quality management, not insulation. The CE mark means conformity with EU safety requirements. All useful — only the acoustic standards say anything specific about sound. See also the Duet specification.