Why Does the Office "Echo"? Understanding RT60 Physics
Imagine an office as a musical instrument. Every hard surface — a glass facade, concrete ceiling or smooth flooring — acts as a mirror for sound waves. This phenomenon is called reflection, and its cumulative effect is precisely reverberation. The RT60 (Reverberation Time) indicator is the technical measure of how quickly acoustic energy "dies out" in a given space.
At Offis, designing acoustic systems for a decade, we have observed a recurring pattern: investors prioritise design (glass and concrete), and then struggle with "acoustic smog" that makes it impossible to understand a colleague sitting two metres away. When RT60 is too high, sound waves overlap, creating a fatiguing acoustic background.
Standard PN-B-02151-4:2015-06: Law, Not a Suggestion
Many property managers still consider acoustics a decorative element. This is a costly mistake. The Polish acoustic standard has been legally binding for all new and renovated public-use buildings since June 2015. This stems from its direct reference in the Regulation of the Minister of Infrastructure on the technical conditions of buildings.
What are the consequences of exceeding the RT60 standard?
- Occupancy refusal: The State Sanitary Inspectorate (Sanepid) has the authority to refuse to grant occupancy permits if inspection tests reveal excessively long reverberation time.
- Compensation risk: Employees working in conditions of excessive noise may demand that the employer improve working conditions or seek compensation for building defects.
- Post-completion retrofit costs: Introducing sound-absorbing materials into a finished, furnished office is 3 to 5 times more expensive than including them at the design stage.
How Long Should Reverberation Last? Target Values for Your Team
According to the standard's requirements, the permissible reverberation time depends on the volume and function of the room:
- Open space (over 50 people): RT60 ≤ 0.5 s. In such large gatherings, the margin for error is zero.
- Open space (up to 50 people): RT60 ≤ 0.6 s. A standard allowing comfortable collaboration without needing to raise one's voice.
- Conference rooms: 0.4–0.6 s. Here, speech intelligibility during video conferences is key.
- Offices and Call Centres: ≤ 0.4 s. The highest acoustic rigour, essential for continuous voice work.
It is worth noting that a typical, unfinished office achieves a result of 1.0–1.4 s, making the space an "acoustic smog" even before the first desk is installed.
How Do We Measure RT60? T20, T30 and Speech Bands
For a sound engineer, RT60 is not a single number but a graph. Sound is measured separately for each octave band. From an office perspective, the most important frequencies are 500 Hz and 1000 Hz, as they contain the majority of human speech.
In laboratory practice, noise of sufficient intensity to allow direct measurement of a full 60 dB decay is rarely generated. Instead, T20 or T30 methods are used (measuring a 20 or 30 dB decay) and extrapolated to 60 dB. This gives a precise result even in noisy offices.
DIY Measurement: The Clap and Balloon Method
You do not need to hire a team with certified equipment to make a preliminary assessment of your office. You can perform a so-called perceptual audit, using a smartphone and simple physical tests.
Step 1: The Clap Test
Stand in the middle of an empty office and clap your hands firmly.
- If the sound cuts off immediately, RT60 is probably within the norm (below 0.6 s).
- If you hear a distinctive metallic resonance or a sound "tail" lasting a noticeable moment, your reverberation exceeds 1 second. This is a signal for immediate intervention.
Step 2: Mobile App Measurements
Microphones in modern phones allow for RT60 estimation with an error margin of around 10–20%.
- Recommended tools: iOS (BuzziSpace RT60), Android (Room Acoustics: RT60 Meter).
- Procedure: Turn off the air conditioning, close the windows, start the measurement and burst an inflated balloon with a pin. The balloon generates an ideal sound wave in all directions.
Reduction Strategy: How Offis Shortens Reverberation Time
Effective acoustic adaptation consists of converting reflections into absorption. Our strategy is based on three pillars:
1. Suppression at Source
Desktop panels SEAL eliminate conversation noise before it has a chance to reflect off the ceiling. We use open-cell foam in them, which drastically reduces acoustic background directly at workstations.
2. Diffusion and Absorption
Freestanding partitions System 20 and System 32 reduce noise annoyance by 11–15 dB. Upholstered partitions with additional acoustic foam absorb sound waves, preventing echo formation in high halls and large open spaces.
3. Total Isolation
For tasks requiring the highest concentration — video conferences, deep focus work — we use booths such as DOT Plus or the S model. They offer isolation at the level of 32–33 dB and are completely "echo-free" inside thanks to 5 m² of bass traps.
Frequently Asked Questions About RT60 and Office Acoustics (FAQ)
Does the RT60 standard apply to buildings constructed before 2015?
Regulations do not apply retroactively. If a building was constructed before June 2015 and is not undergoing major renovation or change of use, meeting the standard is treated as best practice, not a legal obligation. Nevertheless, improving RT60 parameters drastically increases the market rental value of such a space.
Are ordinary curtains or carpets enough to reduce reverberation?
Home solutions rarely cope with speech bands (500–1000 Hz). Thin fabrics absorb only high frequencies. Effective reduction requires certified materials with a minimum thickness of 20–50 mm, such as those used in System 20.
How quickly can acoustic rearrangement be carried out without renovation?
Thanks to Offis modular systems, this process typically takes 1–2 working days. Installation of SEAL or System 20 partitions is done "dry", without dust or noise, allowing the team to work without downtime.
Do glass office partitions improve RT60?
Quite the opposite. Glass is a hard material that reflects almost 100% of sound energy. If your office is dominated by glass, you must introduce compensatory surfaces, such as desktop panels SEAL or System 32.
What is the sound absorption coefficient (alpha w)?
This is a material parameter that determines what fraction of sound energy a given surface absorbs. A value of 1.0 means full absorption, 0.0 — full reflection. In our projects, we use materials with a high alpha index, which allows us to guarantee reverberation reduction by specific values in seconds.