You added sound-absorbing material to the office, yet the meeting room still hums and rings — a common complaint. The cause, more often than not, is that the required absorption area was never actually quantified.
How a Noisy Environment Affects Work
As work-from-home, hybrid work, and solo-creator setups have diversified where people work, 'sound quality' has become a critical variable for productivity. ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends a background noise level of NC 40–50 — equivalent to 49–58 dBA — for open-plan offices. ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends a background noise level of NC 40–50 — equivalent to 49–58 dBA — for open-plan offices.
Above 55 dBA, degraded concentration, increased stress, and reduced speech intelligibility are consistently reported.
The problem is that dBA (A-weighted sound pressure level, in decibels) measures only 'how loud' the space is.
At the same 53 dBA, the perceived quality of a reverberant meeting room (speech smears) and a well-damped office (speech is clear) are entirely different. Addressing both the 'quantity' and the 'quality' of sound simultaneously is the essence of interior acoustic design.
The Three Axes of Indoor Acoustic Management
Noise management breaks down into three axes. Each one differs in purpose, physical principle, material, and installation location — confuse them and you can spend the budget without seeing results.
Absorption · Insulation · Masking — at a glance
These three approaches are complementary. Thick walls alone do not stop internal echo.
| 기준 | 흡음 · Absorption | 차음 · Insulation | 음 마스킹 · Masking |
|---|---|---|---|
| Purpose | Reduce indoor reflections | Block sound between rooms | Mask speech / privacy |
| Metric | NRC · α | STC · Rw | Speech Privacy Index |
| Materials | PET · melamine foam · wood wool | Concrete · gypsum · MLV | Masking speaker |
| Install location | Ceiling · walls · partition surface | Inside walls · under floors | Ceiling-distributed nodes |
| Standard | ISO 354 · ISO 11654 · KS F 2805 | ISO 717-1 · DIN 4109 | ASTM E1130 |
This guide focuses on Absorption — the approach of reducing the 'ringing' caused by sound generated inside the room reflecting off walls and ceilings back to the listener. The majority of office noise complaints stem not from external intrusion but from internal reflected sound.
Ringing and Reverberation — The Principle of Absorption
Shout 'hello' in the mountains and an echo comes back. The same thing happens indoors.
The time taken for emitted sound to bounce off walls, ceilings, and floors and return to the listener is called Reverberation Time (RT60 or T60) — the time for sound pressure to decay 60 dB after the source stops.
What absorptive materials do is straightforward. When sound waves (vibrations in air) pass through the micro-pores inside a porous material (PET fibre, melamine foam, wood wool, etc.), friction converts acoustic energy into heat so the sound is not reflected back. The thicker and more porous the material, the lower the frequencies it can treat.
How Absorption Is Measured — ISO 354 and KS F 2805
The absorption performance of a material is measured in a laboratory using the reverberation chamber method.
The difference in reverberation time before and after installing a test sample is back-calculated using the Sabine formula to obtain the absorption coefficient α. The measurement standards ISO 354 (international) and KS F 2805 (Korea) are equivalent.
The frequently cited NRC (Noise Reduction Coefficient) is the average absorption coefficient across four frequencies — 250, 500, 1,000, and 2,000 Hz — representing the everyday speech range.
Example: NRC 0.85 means an average of 85% of incident energy is absorbed and only 15% is reflected. However, to evaluate performance at low frequencies (125 Hz) and high frequencies (4,000 Hz), the αw rating or the full-frequency curve must also be reviewed.However, to evaluate performance at low frequencies (125 Hz) and high frequencies (4,000 Hz), the αw rating or the full-frequency curve must also be reviewed.
Calculating the Required Absorption Area Yourself — The Sabine Formula
"How much absorptive material do I need?" should never be decided by gut feel. The starting point is the formula that Wallace C. Sabine derived from his Harvard auditorium acoustic experiments in 1898.
Worked example. A meeting room of 150 m³ (5 m wide × 10 m long × 3 m ceiling height), with a target reverberation time of 0.6 s:
A = 0.161 × 150 ÷ 0.6 = approximately 40 m² (sabins) of equivalent absorption area required. If NRC 0.85 PET panels are installed on the ceiling: 40 ÷ 0.85 ≈ 47 m² — the conclusion is that approximately 94% of the 50 m² ceiling area must be covered.If NRC 0.85 PET panels are installed on the ceiling: 40 ÷ 0.85 ≈ 47 m² — the conclusion is that approximately 94% of the 50 m² ceiling area must be covered.
Reverberation Targets and Absorption Strategies by Space Type
"What reverberation target should we set for our space?" The variables requiring a decision are room volume, intended use, and occupancy. Use the following matrix to identify the starting point for each scenario.
Space type → target RT → strategy
Small meeting room · 50–150 m³
"4–8 people, presentation/call focused"
Open office · 200–800 m³
"Hard floor, dense seating, frequent calls"
Studio · call center · 50–200 m³
"Recording, broadcasting, deep focus"
Cafe · lobby · 100–500 m³
"Visible finish + absorption together"
Home studio · creator · 30–80 m³
"YouTube recording, lecture filming, streaming"
Auditorium · venue · 500 m³+
"Lecture, concert, worship — clarity + reverberance"
Frequently Asked Questions
Absorption design — FAQ
Q1I covered the entire ceiling but the room still echoes. Why?
Q2Is NRC 0.85 the same as αw 0.85?
Q3If I must pick only one — absorption or insulation?
Q4Why does Sabine-calculated area often differ from real install quantity?
Q5What Korean absorption standards exist? Is a KCL test report enough?
Glossary
NRC (Noise Reduction Coefficient) — Absorption coefficient. Average of four measurement points at 250, 500, 1k, and 2k Hz. Range: 0–1.
αw (Weighted Sound Absorption Coefficient) — Weighted absorption coefficient. Single-number rating per ISO 11654, weighted across the full 125–4k Hz range.
RT60 · T60 (Reverberation Time) — Time (in seconds) for sound pressure to decay 60 dB after the source stops.
STC (Sound Transmission Class) — Sound isolation rating. A single-number value indicating how much sound a partition attenuates (ASTM E413, USA).
dBA (A-weighted Decibel) — Sound pressure level unit weighted to reflect human hearing characteristics.
Sabin — Unit of equivalent absorption area. 1 sabin = 1 m² of perfectly absorptive surface.
References
[1] ISO 354:2003 — Acoustics — Measurement of sound absorption in a reverberation room. https://www.iso.org/standard/34545.html
[2] ISO 11654:1997 — Acoustics — Sound absorbers for use in buildings — Rating of sound absorption.
[3] KS F 2805 — Method of measuring sound absorption in a reverberation chamber (Korean Industrial Standard; equivalent to ISO 354). https://www.kssn.net/search/stddetail.do?itemNo=K001010104081
[4] DIN 18041:2016 — Hörsamkeit in Räumen — Anforderungen, Empfehlungen und Hinweise für die Planung.
[5] Sabine, W. C. (1898) — Reverberation time formula. https://www.acousticlab.com/en/reverberation-time-and-sabines-formula/
[6] ASHRAE / WELL Standard — Internally generated noise (office 40–55 dBA recommendation). https://standard.wellcertified.com/comfort/internally-generated-noise