Impulse Response
Canonical Definition
An impulse response is the time-domain output of a system in response to an ideal impulse. In practical audio measurement it is commonly derived from a known sweep or other test signal and describes how the system distributes energy over time.
The trace must be interpreted with its timing reference, bandwidth, polarity, windowing, microphone position, and driver type.
How REW Produces It
Room EQ Wizard normally measures with a logarithmic swept sine. It compares the captured output with the known stimulus and derives the system's impulse response. The software is not relying on a single physical click to reveal the full response.
What The Trace Can Show
| Feature | Possible meaning | Required caution |
|---|---|---|
| Direct arrival | First meaningful energy from the measured source | Marker depends on bandwidth and timing reference |
| Initial sign | Polarity of the measurement chain and source arrival | Filters and bandwidth can shape the first lobes |
| Early reflections | Glass, dash, seats, floor, or nearby boundaries | Can overlap the direct sound in a vehicle |
| Long decay or ringing | Resonance, enclosure behavior, cabin mode, or filter response | Confirm with decay, magnitude, and repeat measurements |
| Relative timing | Channel or driver arrival difference | Requires a shared timing reference |
Timing Reference Is Mandatory For Comparisons
Two impulse traces can look similar but have arbitrary time-zero positions if each measurement was captured without a common reference. To compare channel delay, use:
- a wired loopback timing reference, or
- an acoustic timing reference routed through a repeatable high-frequency-capable reference speaker.
Keep the microphone fixed and use the same reference for every driver or channel that will be compared, subtracted, or summed.
Tweeter, Midbass, And Subwoofer Peaks Are Not Interchangeable
A tweeter has broad bandwidth and often produces a sharp timing feature. A subwoofer is band-limited and its impulse response is broad and delayed by its limited bandwidth and filters. Aligning the largest peak of every driver to the same sample can create a bad result.
REW itself uses the impulse peak for many loudspeaker delay estimates but treats subwoofer delay differently, using the start of the response because the subwoofer peak is broad. The start is harder to locate precisely, which is one reason subwoofer alignment should be verified through the overlap region rather than by one peak alone.
Windowing And Time Zero
The impulse window selects which portion of the time response contributes to derived magnitude and phase data.
- A short window can remove reflections but reduce low-frequency resolution.
- A long window includes more low-frequency information but also more cabin reflections.
- Moving time zero changes the displayed phase slope and group delay.
- Minimum-phase and excess-phase calculations depend on the impulse window.
Record the window and time-zero treatment whenever phase or group-delay results will be compared.
Alignment Workflow
- Fix the microphone at the defined listening position.
- Choose and verify the timing-reference method.
- Mute all but one driver or channel.
- Measure at a safe level with the intended crossover state.
- Repeat for every driver without moving the microphone or reference.
- Use impulse timing for a starting delay.
- Verify adjacent drivers through magnitude, phase, normal sum, and reverse-polarity null.
- Verify subwoofer integration through its acoustic overlap with the next band.
Common Mistakes
- Comparing measurements that do not share a timing reference.
- Calling the largest peak the acoustic center for every driver.
- Mistaking a reflection for direct arrival.
- Ignoring crossover and polarity state.
- Changing the impulse window between traces without recording it.
- Using clipped or low-signal measurements for precise timing.