Sweep Measurements

LinFIR features a built-in exponential sine sweep generator for impulse response capture. This system provides high-quality measurements with automatic quality validation.

Overview

The sweep measurement system generates an exponential sine sweep that excites the system across a specified frequency range. After playback and recording, deconvolution with the inverse sweep extracts the impulse response.

Accessing Sweep Controls

Sweep controls are located in the IR Management window:

To open the window: Driver → Manage IR
Section: Measurement Settings
Start button: 🎤 Measure

Sweep Parameters

Duration

Length of the sweep signal in seconds.

Range: 1 to 10 seconds
Shorter (1-3s): Faster measurements, lower frequency resolution
Longer (5-10s): Better low-frequency resolution, improved SNR
Recommended: 5-7 seconds for full-range measurements

Longer sweeps provide better time-domain resolution at low frequencies, improving measurement accuracy in the bass region.

Amplitude

Output level of the sweep signal.

Range: -50 to 0 dB
Purpose: Control the acoustic level of the measurement
Typical: -12 to -6 dB for moderate levels
Maximum (0.0): Full digital scale output

Adjust amplitude based on driver sensitivity and desired measurement level. Too low may result in poor SNR; too high may cause clipping, distortion or damage.

Start Frequency

Lower frequency limit of the sweep.

Default: 0.1 Hz (captures near-DC response)
Range: 0.1 Hz to 20 kHz
Full-range: 20 Hz (typical for full-bandwidth measurements)
Subwoofers: 10-20 Hz for low-frequency drivers
Tweeters: >1000 Hz for high-frequency analysis only

Setting the start frequency too high will miss low-frequency information. For full-range measurements or subwoofers, use 20 Hz or lower.

End Frequency

Upper frequency limit of the sweep.

Default: Nyquist frequency (sample rate / 2)
Range: Start frequency to Nyquist
Auto-clamped: Cannot exceed sample rate / 2
Full-range: 20,000 Hz for complete audible spectrum
Subwoofers: 200-500 Hz for focused low-frequency measurements
Limited bandwidth: Useful for measuring specific frequency ranges

Reset Range Button

Quickly restore the frequency range to full bandwidth:

Action: Sets Start Freq to 0.1 Hz and End Freq to Nyquist frequency
Use: Return to default full-range measurement after bandwidth-limited sweeps

Starting a Measurement

Measure Button (🎤 Measure)

Clicking the 🎤 Measure button initiates the sweep measurement process.

Default behavior (confirmation enabled):

Click 🎤 Measure
Confirmation dialog appears with warnings:
- Check microphone and speaker positioning
- Set appropriate gain levels to avoid clipping
- The sweep will be audible
- Windows-specific timing warning (if applicable)
Click Start Measurement to proceed or Cancel to abort
Sweep plays and records automatically
IR is processed and validated

Direct mode (confirmation disabled):

Measurement starts immediately when clicking 🎤 Measure
No confirmation dialog shown
Useful for quick repeated measurements

Disabling Confirmation Dialog

To disable the confirmation dialog:

Open Settings (Cmd/Ctrl + ,)
Navigate to Audio Settings
Enable “Skip confirmation dialog” option
Measurements will now start immediately

Warning: Disabling confirmation means sweeps start instantly. Ensure your setup is ready before clicking the button to avoid unexpected loud sweeps.

Level Recommendations

Target levels for best measurement quality:

Optimal: -6 dB to -12 dB
Acceptable: -12 dB to -20 dB
Too low: Below -25 dB (poor SNR, noisy measurements)
Too high: Above -3 dB (risk of clipping)

Quality Validation

LinFIR automatically validates each capture and rejects poor-quality measurements.

Clipping Detection

Criteria: Any sample reaching digital full-scale (±1.0)
Result: Capture rejected immediately
Reason: Clipping introduces harmonic distortion that corrupts the impulse response

Solution: Reduce input gain or lower acoustic level (move microphone farther or reduce driver volume).

Level Check

Criteria: Peak level must be ≥ -25 dB
Result: Capture rejected if too low
Reason: Low levels result in poor signal-to-noise ratio

Solution: Increase input gain, raise driver volume, or move microphone closer.

Best Practices

Microphone Positioning

On-axis measurements: Position microphone directly in front of driver at typical listening distance
Listening position: For room measurements, place microphone at primary listening position
Height: Maintain consistent height, typically at seated ear level (90-150 cm)
Distance:
- Farfield recommended: 0.5-1.5 meters with windowing (quasi-anechoic technique)
- Avoid nearfield: Nearfield measurements (<20cm) capture reactive near-field effects (evanescent waves) that don’t propagate to listening position
- Consistency critical: Use the same microphone position for all drivers to maintain correct relative phase and amplitude relationships

Why Avoid Nearfield for Multi-Way Systems?

Physical limitations:

At different distances, the reactive impedance field varies between drivers
Nearfield captures evanescent waves (reactive air-driver coupling) that decay exponentially and never reach the listener
These reactive effects create phase rotations and magnitude errors that don’t represent real acoustic radiation

Practical consequences:

Impossible to maintain consistent distance across drivers of different sizes (tweeter vs woofer require different nearfield distances)
Phase relationships corrupted: Crossover alignment becomes unreliable when drivers measured at different distances
Baffle effects missed: Nearfield ignores diffraction and baffle step that dominate the actual system response

Recommended approach:

Measure at 0.5-1m distance (same position for all drivers)
Use time windowing to eliminate room reflections (quasi-anechoic)
Captures correct far-field radiation including baffle interactions
Maintains phase coherence between drivers for accurate crossover design

Acoustic Environment

Minimize noise: Turn off HVAC, fans and noisy appliances during measurement
Reduce reflections: For anechoic-like measurements, gate reflections using time windowing
Room measurements: Accept reflections, use time windowing to exclude late reflections only
Outdoor measurements: Recommended to avoid room gain and obtain better results at low frequencies

Measurement Microphone

Type: Use calibrated measurement microphone (e.g., UMIK-1, Earthworks M23, Beyerdynamic MM-1)
Calibration: Load manufacturer calibration file before measurements
Quality: Consumer microphones lack flat frequency response and will give inaccurate results

Gain Staging

Start with moderate input gain
Run a test sweep
Check peak level indicator
Adjust gain to achieve -6 dB to -12 dB peaks
Re-measure if clipping or too low

Sweep Duration

Full-range (20 Hz - 20 kHz): 5-7 seconds recommended
Limited bandwidth: Shorter sweeps acceptable (2-3 seconds)
Very low frequencies (< 20 Hz): Use longer sweeps (7-10 seconds)

Capture Rejection Messages

“Capture Rejected - Clipping Detected”

Cause: Input signal exceeded digital full-scale.

Solutions:

Reduce audio interface input gain
Lower driver volume
Move microphone farther from driver
Check gain staging in signal chain

“Capture Rejected - Signal Too Low”

Cause: Peak level below -25 dB threshold.