How the RT60 measurement planner works
This tool helps you plan an in-room reverberation time measurement following the ISO 3382 standard. Enter your room dimensions and an estimated T60, and the planner calculates the Schroeder frequency (the boundary between modal and statistical behaviour), recommends how many source–receiver pairs to use, tells you the minimum impulse response capture length, and suggests a grid spacing for systematic position coverage. Planning these parameters before you start saves time and ensures your measurements are statistically meaningful.
Estimated RT60 vs measured RT60
An estimated RT60 calculator predicts reverberation time from room volume, materials and absorption assumptions before treatment is installed. A measured RT60 uses an impulse response from the real room, so it captures actual leakage, furniture, boundary conditions, placement and construction details. Use the RT60 Reverb Time Calculator for planning, then use this measurement planner to check the finished room.
Step-by-step measurement process
A complete room acoustic measurement typically follows these steps. First, ensure the room is in its normal operating condition — furniture in place, HVAC off, doors closed. Then choose an excitation method: an omnidirectional balloon burst or starting pistol is simple; a sine sweep played through a dodecahedral loudspeaker gives better signal-to-noise ratio and is the method specified in ISO 3382-2 for general rooms. Record the impulse response (IR) using a measurement-grade omnidirectional microphone at each source–receiver position. Post-process each IR in software such as Room EQ Wizard (REW) to extract T20, T30, and EDT values per octave band. Average multiple positions to get reliable room-level figures.
Understanding your results
The Schroeder frequency (fs) divides the frequency spectrum into two regions. Below fs, individual room modes dominate and decay at different rates — you will typically see high variation between microphone positions. Above fs, modes overlap densely enough to behave statistically, and T60 becomes more consistent across positions. The more positions you average, the more reliable your result below fs. In small rooms, fs can be as high as 300–400 Hz, meaning modal effects extend well into the speech frequency range.
T20, T30, and EDT explained
In practice, a clean 60 dB decay is rarely measurable in a room with background noise. Instead, measurement software extracts T20 and T30 — the time to decay by 20 dB or 30 dB respectively, extrapolated to 60 dB. Early Decay Time (EDT) uses only the first 10 dB of decay and correlates more closely with the subjective liveness a listener perceives. A large difference between EDT and T30 often indicates strong early reflections from nearby surfaces, which can be addressed with absorption at first reflection points.
Key terms
- Impulse response (IR) — a complete record of how a room responds to an instantaneous sound. Length must capture the full decay, typically at least 3 × T60.
- Schroeder frequency (fs) — transition point between sparse low-frequency modes and statistically dense modal behaviour; approximately 2000 × √(T/V) Hz.
- ISO 3382 — international standard family for measuring room acoustic parameters including T20, T30, EDT, clarity (C80), and definition (D50).
- T20 / T30 — practical substitutes for T60 derived by measuring the 20 dB or 30 dB decay slope and extrapolating; more reliable in noisy environments.
- EDT (Early Decay Time) — decay time extrapolated from the first 10 dB of decay; correlates with perceived room liveliness more closely than T60.
- Source–receiver pair — one loudspeaker position plus one microphone position; ISO 3382 recommends a minimum of 6 combinations in most room types.
Frequently asked questions
- How long should my impulse response be? Capture at least three times the expected T60 so the full decay tail is included. If you expect a T60 of 0.5 s, record at least 1.5 s of IR — 2 s gives comfortable margin. Shorter recordings clip the decay and cause software to report artificially low RT values.
- What equipment do I need to measure RT60? A calibrated omnidirectional measurement microphone, an audio interface, a computer running free software such as Room EQ Wizard (REW), and a noise source. A balloon burst or starting pistol works for rough checks. For ISO 3382-compliant results, use a dodecahedral loudspeaker emitting a sine sweep — this gives much better signal-to-noise ratio and repeatable results.
- What is the Schroeder frequency and why does it matter? Below the Schroeder frequency, individual room modes are spaced far apart and each decays at a different rate. This means T60 varies significantly depending on exactly where you place the microphone. Above it, modes overlap so densely that you get smooth, consistent decay. The Schroeder frequency sets the practical lower limit for meaningful T60 averaging.
- How many measurement positions do I need? ISO 3382-2 specifies a minimum of 6 source–receiver combinations for general rooms, and more for larger or more complex spaces. For home studio checks, 4–6 positions spread around the listening area give useful results even if not ISO-compliant.
- What causes uneven RT60 across frequencies? Surface absorption varies strongly with frequency — most common materials absorb mid and high frequencies far better than bass. It is normal for the 125 Hz RT60 to be 2–3 times longer than the 1 kHz value in an untreated room. The goal of acoustic treatment is to even out this frequency response of decay time.
- What is the difference between T20, T30, and EDT? T20 extrapolates from the 5–25 dB decay range and is most reliable in quiet rooms. T30 uses the 5–35 dB range and is more commonly used in practice. EDT uses only the 0–10 dB range and reflects perceived liveliness rather than total energy decay. A room with a long EDT relative to T30 will feel reverberant even if the bulk of the decay is short.