Impulse Response
Definition
Impulse Response
An impulse response is the time-domain output of an acoustic system when excited by a theoretically perfect impulse. It captures every reflection, resonance, and absorption characteristic of a room or device. ISO 3382-1:2009 specifies measurement methods using sweep or MLS excitation. SonaVyx captures and analyzes impulse responses directly in your browser.
How Impulse Responses Are Measured
Modern impulse response measurement uses an excitation signal — either a logarithmic sine sweep (Farina method, AES-2id:2023) or Maximum Length Sequence (MLS). The room response is captured by a microphone, then deconvolved against the original excitation to extract the impulse response. SonaVyx performs deconvolution in Rust WASM using frequency-domain division for sweeps and circular cross-correlation for MLS signals.
Practical Example
An acoustician measures the impulse response of a 500-seat theater using a 5-second log sine sweep from 20 Hz to 20 kHz. The resulting IR reveals the direct sound at 12 ms, a strong first reflection from the stage floor at 18 ms, and dense reverberant decay. From this single measurement, SonaVyx automatically extracts RT60 (T30 = 1.4s), C80 (2.1 dB), D50 (0.52), and Ts (88 ms).
Anatomy of an Impulse Response
An impulse response contains three distinct temporal regions. The direct sound arrives first as a sharp spike, representing the shortest path from source to receiver. Early reflections arrive within the first 50 to 80 milliseconds from nearby surfaces (floor, ceiling, walls), reinforcing speech clarity when they arrive within the fusion time. Late reverberation consists of dense, diffuse reflections that decay exponentially, determining the perceived size and warmth of the space.
Log Sine Sweep vs MLS
The logarithmic sine sweep (Farina method) offers superior signal-to-noise ratio because all excitation energy concentrates at one frequency at a time. Sweep lengths of 5 to 20 seconds provide 30 to 50 dB SNR improvement over impulsive sources. MLS excitation distributes energy across all frequencies simultaneously, achieving faster measurement but with lower immunity to time-variant noise. SonaVyx defaults to sweep excitation for maximum measurement quality.
Derived Parameters
A calibrated impulse response contains all information needed to compute room acoustic parameters defined by ISO 3382-1. Reverberation time (RT60) comes from the energy decay curve via Schroeder backward integration. Clarity C50 and C80 compare early-to-late energy ratios at 50 ms and 80 ms boundaries. Definition D50 measures the fraction of energy arriving within 50 ms. Centre time Ts indicates the temporal center of gravity of the impulse response energy.
Convolution and Auralization
Because the impulse response fully characterizes a linear system, convolving any dry audio signal with a measured IR produces what that signal would sound like in the measured space. This process, called auralization, enables A/B listening comparisons between rooms without physically being present. SonaVyx uses the Web Audio API ConvolverNode to apply measured impulse responses to audio in real time for before/after comparison.
Quality Validation (INR)
The Impulse-to-Noise Ratio (INR) determines the usable dynamic range of the measurement. ISO 3382-1 requires at least 35 dB INR for T20 calculation and 45 dB for T30. SonaVyx automatically computes INR and warns when the measurement environment is too noisy for reliable parameter extraction, prompting the user to increase sweep level or duration.
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Capture room impulse response — free measurement in your browser