If you could capture a room's entire acoustic personality in a single recording, you would have its impulse response. Every echo, every reflection, every frequency-dependent decay, every spatial characteristic — all encoded in one time-domain signal. The impulse response is the Rosetta Stone of room acoustics: from it, you can extract RT60, clarity, definition, speech intelligibility, and even recreate what music would sound like in that space. It is the foundation of modern acoustic measurement.
TLDR
An impulse response (IR) is the time-domain record of how a room responds to an ideal, infinitely short burst of sound (a Dirac delta function). In practice, it is measured by exciting a room with a known signal (swept sine, MLS, or balloon pop) and mathematically extracting the room's transfer function. The resulting IR contains every acoustic event in sequence: the direct sound arrival, early reflections from walls and ceiling, and the late reverberant tail. From the IR, all standard room acoustic parameters can be calculated — RT60 (ISO 3382-2), EDT, C80, C50, D50 (ISO 3382-1), STI (IEC 60268-16), and IACC (inter-aural cross-correlation). IRs are also used for convolution reverb in audio production, auralization in acoustic design, and forensic acoustics. The impulse response is the most information-dense measurement in room acoustics.
Real-World Analogy
Imagine dropping a pebble into a perfectly still pond. The splash creates a single, clean wavefront that radiates outward. As the wave hits the pond's edges, rocks, and islands, it reflects, splits, and interferes with itself. If you could photograph the entire pond surface every millisecond and stack those photos into a movie, you would have the pond's impulse response — a complete record of how the pond processes a single disturbance. An acoustic impulse response is the audio equivalent: a complete record of how a room processes a single burst of sound.
Technical Definition
The Ideal Impulse
Mathematically, a perfect impulse is a Dirac delta function: infinite amplitude, infinitesimal duration, unit energy. It contains all frequencies at equal amplitude. When this signal excites a room, the room's response is purely the room itself — no colouration from the source signal.
In practice, a true Dirac delta cannot be generated. Instead, engineers use signals that approximate or can be deconvolved to recover the impulse response:
Measurement Methods
Exponential Swept Sine (ESS): A sine wave that sweeps logarithmically from 20 Hz to 20 kHz over 5 to 30 seconds. The room's response is recorded, then deconvolved (cross-correlated with the inverse of the sweep) to extract the IR. ESS provides excellent signal-to-noise ratio and separates harmonic distortion from the linear response. This is the method specified in ISO 18233:2006 and is the current professional standard.
Maximum Length Sequence (MLS): A pseudo-random binary sequence used as the excitation signal. Cross-correlation with the known sequence extracts the IR. MLS was the standard method before ESS but is more sensitive to time-variance (e.g., air movement) and non-linear distortion.
Impulsive sources: Balloon pops, starter pistols, or clapper boards provide a physical approximation of an impulse. Quick and practical for field surveys, but with limited low-frequency energy, poor repeatability, and reduced signal-to-noise ratio compared to electronic methods.
Anatomy of an Impulse Response
A typical room IR contains three distinct regions:
- Direct sound (0 to 5 ms): The first arrival from source to receiver via the shortest path. It arrives as a single sharp spike.
- Early reflections (5 to 80 ms): Discrete reflections from the nearest surfaces (floor, ceiling, side walls). These are individually distinguishable as separate spikes. Per ISO 3382-1, early energy (0 to 80 ms for music, 0 to 50 ms for speech) is perceived as reinforcing the direct signal.
- Late reverberation (80 ms onward): A dense, statistically diffuse decay of overlapping reflections. The transition point where individual reflections merge into a smooth tail is called the mixing time. The slope of this tail determines the RT60.
Extracting Parameters from the IR
- RT60: Derived from the backward-integrated energy decay curve (Schroeder method, see ISO 3382-2 Annex B)
- EDT (Early Decay Time): The slope of the decay curve over the first 10 dB of decay, extrapolated to 60 dB
- C80 (Clarity): The ratio of energy in the first 80 ms to the energy after 80 ms, in dB
- D50 (Definition): The ratio of energy in the first 50 ms to the total energy
- STI: Derived from the modulation transfer function, which is computed from the IR
Why It Matters for Design
The impulse response is the bridge between measurement and design. When an acoustic consultant measures an existing room, the IR tells them everything: where reflections come from, how long the decay takes at each frequency, whether flutter echoes exist, and how intelligible speech will be at any position.
In design, simulated IRs from ray-tracing or image-source models let consultants predict a room's acoustics before construction. These simulated IRs can be convolved with dry audio recordings (speech, music) to auralize the space — letting clients hear what the finished room will sound like. This is the basis of AcousPlan's auralization feature.
How AcousPlan Uses This
AcousPlan's measurement import feature accepts impulse response data from field measurements. The platform extracts RT60, EDT, C80, D50, and STI from the uploaded IR and compares measured values against the simulated predictions from the room model. The auralization engine uses simulated impulse responses to generate binaural audio previews, and the "measured baseline overlay" feature shows measured IR-derived parameters alongside calculated values.
Related Concepts
- What is a Decay Curve? — Derived from the impulse response
- What is the Schroeder Method? — How RT60 is extracted from the IR
- What is RT60? — The most common parameter derived from IRs
- What is Clarity (C80/D50)? — Early/late energy metrics from IRs
- How is STI Measured? — Speech intelligibility from IRs
Calculate Now
Upload measured impulse response data or model your room to generate simulated parameters. AcousPlan extracts RT60, clarity, and STI from the acoustic fingerprint.