When you rent or refurbish an office, the acoustics are usually described in one sentence: “there will be an acoustic ceiling”. That says what is being bought, but not what the result will be. In an open-plan office the result can be measured and written into the brief as a number.
Why “echo” is not the main problem
Reverberation time shows how long sound stays in a room. In most modern offices with a suspended acoustic ceiling it is already good – around 0.5 s.
And yet colleagues from the neighbouring team can be understood. The reason is that speech reaches you directly, over the desks, not via reflections. The ceiling absorbs reflected sound but does not stop direct sound.
In the Concentrix call centre the reverberation time was 0.5–0.6 s – within the norm. Even so, conversations were intelligible 14 m away.
The three numbers to ask for
EN ISO 3382-3 describes an open-plan office with three quantities measured along a line of desks:
- D2,S – spatial decay rate of speech. How many dB the speech level drops with each doubling of distance. Good value: 7 dB or more. At 4–5 dB, sound “flows” through the office.
- Lp,A,S,4m – speech level at 4 m. How loudly you hear a colleague speaking normally 4 m away. Good value: 48 dB or less.
- rD – distraction distance. The distance beyond which a neighbouring conversation is no longer intelligible (STI below 0.5). Good value: up to 5 m. Over 10 m means the whole team hears every conversation.
ISO 22955:2021 “Acoustics — Acoustic quality of open office spaces” goes one step further. It groups offices by type of work and gives target values for each type. A call centre and a space with mixed teams – designers, lawyers, programmers – have different needs.
What helps and what does not
- Screens of the right height, with absorption. This is the measure that moves rD and Lp,A,S,4m. In the Concentrix model, 1.5 m screens with 50 mm absorbing material brought rD down from about 14 m to under 8 m.
- Layout. Noisy teams (phones, sales) should not sit next to teams that need to concentrate.
- Wall and ceiling panels. They help when the reverberation time is high. If it is already about 0.5 s, more panels hardly change intelligibility. If the room should also carry an image, the wall can be an acoustic wall with print design – example: the meeting rooms of Telelink Business Services.
- Sound masking. Loudspeakers with an even background sound reduce the intelligibility of distant conversations. It works, but it raises the overall noise level and must be designed carefully.
Measuring before and after
A measurement to EN ISO 3382-3 takes about a working day on site. Done before the design, it shows whether the problem lies in absorption, direct speech or background noise. Done after installation, it shows whether you got what you paid for.
We calibrate a computer model of the office with the measured values and compare the options – height, layout, material – before any materials are paid for. For EGT Digital , the control measurement after installation differed from the model forecast by 1–5%.
What to write in the brief
“Open-plan zones shall achieve D2,S ≥ 7 dB and Lp,A,S,4m ≤ 48 dB according to EN ISO 3382-3, verified by measurement after completion.”
One sentence that turns “acoustic ceiling” into a result that can be checked. If you would like us to measure your office, see office acoustics or write to us at [email protected].
