Microphone Calibration

Manage calibration files for measurement microphones and define default calibrations for new drivers.

Overview

Microphone calibration compensates for the frequency response and sensitivity of your measurement microphone. LinFIR supports:

• Multiple calibration files stored in application settings
• Per-driver calibration selection in the IR Management window (see IR Management)
• Default calibration for newly created drivers
• Automatic application to new projects when a default is set
• Minimum-phase correction computed in real-time from calibration data

Calibration scope:

• Calibration data is embedded in project files (.lnf) for portability
• Calibrations from projects are also stored in application settings for reuse
• Each driver can use a different calibration file or none at all

Calibration File List

The main section displays all imported calibration files in a scrollable grid:

Automatic driver updates:

• When you delete a calibration file that’s currently assigned to one or more drivers, those drivers are automatically reset to “None”
• A warning toast will notify you which drivers were affected
• This prevents broken references and ensures project stability

Default Calibration

Select a calibration file to automatically apply to newly created drivers.

Dropdown selector:

• Shows “None” when no default is set
• Lists all imported calibration files by name
• Changes apply immediately to new drivers and projects

When the default is applied:

• Manual driver creation: When you add a new driver via the Drivers toolbar
• New Loudspeaker Design project: All initial drivers receive the default calibration
• New Room Calibration project: The initial measurement receives the default calibration

Note: Changing the default does not affect existing drivers in the current project or previously saved projects. Only new drivers created after the change receive the new default.

Import Calibration File

Click “Import Calibration File” to load a microphone calibration file.

Supported file format:

• Plain text files with two columns: Frequency (Hz) and Magnitude (dB)
• Whitespace or comma-separated
• Lines starting with non-numeric characters are treated as comments/headers
• Optional header line with sensitivity factor and serial number

Example file format:

# Sens Factor = -2.5dB, SERNO: 41486651
# Frequency (Hz)  Magnitude (dB)
20.0              -0.5
50.0              -0.3
100.0             -0.1
200.0              0.0
500.0              0.1
1000.0             0.0
2000.0            -0.2
5000.0            -0.5
10000.0           -1.0
20000.0           -2.0

Sensitivity factor and serial number:

• Sensitivity factor (in dB) is automatically extracted from the file header if present (line containing “Sens Factor”)
• Serial number is automatically extracted if present (SERNO: field in header)
• If no sensitivity factor is found in the file, it defaults to 0 dB
• Serial number is used to differentiate calibrations with identical filenames when sharing projects

Storage: Calibration data (frequency response, sensitivity, and serial number) is embedded in project files (.lnf) to ensure portability:

• Projects can be shared without requiring recipients to have the calibration files
• When loading a project, calibrations are automatically added to application settings if not already present
• If a calibration with the same name exists:
  • With serial number: differentiated using serial number (e.g., filename (SN:7016078))
  • Without serial number: differentiated using numeric suffix (e.g., filename (1), filename (2))
• You can also manually import calibrations to your settings library for reuse across projects

How Calibration is Applied

Minimum-phase correction: LinFIR converts the imported frequency response correction into a minimum-phase FIR filter applied during IR processing.

Processing steps:

1. Frequency filtering: Bins below 5 Hz are removed to prevent phase distortion at very low frequencies
2. Interpolation: Calibration data is interpolated to match your IR’s sample rate and FFT size
3. Minimum-phase conversion: Magnitude correction is converted to minimum-phase using Hilbert transform
4. Sensitivity compensation: The sensitivity factor (dB) is applied as a gain adjustment
5. IR application: The resulting correction filter is convolved with the driver’s impulse response

Phase behavior: Because the correction uses minimum-phase reconstruction, the calibration affects both magnitude and phase of the measurement. This is physically accurate for measurement microphones, which exhibit mainly minimum-phase behavior in their frequency response deviations.

Per-Driver Calibration Selection

Each driver can use a different calibration file (or none). To assign a calibration to a specific driver, use the microphone calibration dropdown in the IR Management window.

See: IR Management → Microphone Calibration

Best Practices

File organization:

• Use descriptive filenames that include microphone model and serial number
• Example: UMIK-1_SN123456.txt or Earthworks_M30_SN789.txt

Calibration maintenance:

• Import calibration files for all your measurement microphones
• Set your most frequently used microphone as the default

Project workflow:

• Assign calibrations in the IR Management window after importing measurements
• Use “None” for measurements taken with already-flat reference microphones
• Different drivers in the same project can use different calibrations (useful when combining measurements from multiple sessions)

Frequency range:

• Ensure calibration files cover the full measurement bandwidth
• LinFIR interpolates between provided points, but extrapolation beyond the calibration range may produce inaccurate results
• Most measurement microphones provide calibration data from 20 Hz to 20 kHz

Sensitivity factor:

• Use the value from your microphone’s calibration certificate
• This is typically a negative dB value (e.g., -0.5 dB, -2.4 dB)
• If unsure, use 0 dB and rely solely on the frequency response correction