ISO 3382-1 compliant -- T20, T30, EDT with Schroeder backward integration
Plays a 3s log sine sweep (20 Hz - 20 kHz) through speakers and records the room response.
Education & training rooms
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ISO 3382-1 requires r² ≥ 0.995 for a valid measurement. Lower values indicate insufficient signal-to-noise ratio or non-exponential decay.
RT60, also known as reverberation time, is the single most important acoustic parameter for characterizing enclosed spaces. Defined as the time required for sound energy to decay by 60 dB after the excitation source stops, RT60 was first described by Wallace Clement Sabine in 1898 and remains the cornerstone of architectural acoustics. ISO 3382-1:2009 (Measurement of room acoustic parameters -- Part 1: Performance spaces) and ISO 3382-2:2008 (Part 2: Reverberation time in ordinary rooms) standardize the measurement procedures used by acousticians worldwide.
Because achieving a full 60 dB dynamic range in field measurements is often impractical due to background noise, ISO 3382 defines two extrapolated metrics. T20 evaluates the decay slope between -5 dB and -25 dB (a 20 dB evaluation range), then extrapolates to 60 dB. T30 uses the range from -5 dB to -35 dB, offering a more robust estimate when the signal-to-noise ratio permits. Both metrics require a linear regression fit to the Schroeder energy decay curve, with the correlation coefficient r-squared (r²) serving as the quality indicator. ISO 3382-1 mandates r² ≥ 0.995 for T20 and r² ≥ 0.997 for T30 to consider a measurement valid.
Early Decay Time (EDT) is evaluated over the initial 10 dB of decay (0 to -10 dB), then extrapolated to 60 dB. Unlike T20 and T30, which describe the statistical reverberation of the room, EDT captures the subjective impression of reverberance perceived by listeners. In rooms with non-uniform sound fields -- such as those with strong early reflections from nearby surfaces -- EDT can differ significantly from T30. The EDT/T30 ratio is a useful diagnostic: values close to 1.0 indicate a diffuse, uniform decay, while values below 0.8 suggest that early energy dominates the subjective experience.
The Schroeder backward integration, introduced by Manfred Schroeder in 1965, converts a noisy impulse response into a smooth, monotonically decreasing energy decay curve (EDC). Instead of averaging multiple noise decays (as in the interrupted noise method), the Schroeder method integrates the squared impulse response from the end of the signal backward in time, providing an ensemble-averaged decay from a single measurement. This approach is mathematically equivalent to averaging an infinite number of interrupted noise decays and is the foundation of all modern RT60 measurement systems.
ISO 3382 supports multiple excitation methods. The logarithmic sine sweep (also called exponential swept sine or ESS) offers the highest signal-to-noise ratio and is the preferred method for precision measurements. The sweep is played through a loudspeaker and the recorded response is deconvolved using the corresponding inverse filter, yielding a high-quality impulse response. Impulsive sources such as balloon pops, starter pistols, and hand claps provide a simpler alternative when a loudspeaker is unavailable, though they typically offer lower dynamic range and less uniform spectral coverage. SonaVyx supports both approaches: automated sweep measurement with built-in deconvolution, and direct upload of impulse response WAV files captured by any method.
Optimal reverberation time depends on room function and volume. Speech-oriented spaces like classrooms (0.4 - 0.8 s per ANSI/ASA S12.60) and offices (0.4 - 0.6 s) require short RT60 for intelligibility, quantified by the Speech Transmission Index (STI). Recording studios and control rooms target 0.2 - 0.5 s for accurate monitoring. Concert halls for orchestral music typically aim for 1.5 - 2.2 s to provide the envelopment and warmth that musicians and audiences prefer. Houses of worship balance speech clarity and musical richness at 1.2 - 1.8 s. These targets generally apply at mid-frequencies (500 - 1000 Hz); a rising low-frequency RT60 (bass ratio) often enhances warmth in music venues.
T20 and T30 estimate RT60 by fitting a straight line to the energy decay curve over different evaluation ranges (-5 to -25 dB and -5 to -35 dB, respectively) and extrapolating. T30 is generally preferred because the wider evaluation range provides a more robust estimate. EDT (Early Decay Time) evaluates only the first 10 dB of decay, reflecting the subjective perception of reverberance rather than the statistical room reverberation. In a well-diffused room, all three should agree closely.
ISO 3382-1 requires the impulse response to have a decay range at least 35 dB above the noise floor for T20, and 45 dB for T30. This is typically assessed by the impulse-to-noise ratio (INR). Using a logarithmic sine sweep provides 20-40 dB more dynamic range than impulsive sources, making it the preferred method in noisy environments. If the INR is insufficient, the r-squared value of the linear regression will drop below the 0.995 threshold.
ISO 3382-2 specifies a minimum of 2 source positions and 3 microphone positions for ordinary rooms, yielding at least 6 source-receiver combinations. For performance spaces (ISO 3382-1), the standard recommends more positions to adequately sample the sound field. SonaVyx allows you to perform individual measurements and export the results; spatial averaging should be calculated from multiple measurement positions.
Yes. A balloon pop or loud clap generates a broadband impulse that can be recorded and analyzed. However, the achievable dynamic range is typically 30-45 dB, which may be insufficient for T30 measurements (which require 45 dB). The frequency spectrum of impulsive sources is also less uniform than a log sine sweep, particularly at low frequencies. For best results, upload the recorded WAV file to SonaVyx and check the r-squared values: if T30 shows poor validity, use T20 instead.
Low-frequency sound waves have longer wavelengths and interact differently with room boundaries and absorptive materials. Most acoustic treatments (porous absorbers, panels) are less effective below 250 Hz, resulting in longer low-frequency RT60. This frequency dependence is normal and expected. In music venues, a rising bass ratio (RT60 at 125-250 Hz relative to 500-1000 Hz) of 1.0-1.3 is often desirable for warmth. In speech venues, a flat or slightly falling RT60 spectrum is preferred.