Spectrum Calibration in Raven Workbench

Calibration in Raven allows for the conversion of raw power levels (dB Full Scale) into calibrated pressure levels (e.g., dB re 1 μPa). This document provides a technical overview of how calibration works and a guide on how to use the feature in the application.

Calibration feature is available in Raven Expedition and Raven Annotate.

How it Works

Technical Implementation

Raven uses a SpectrumCalibration model to apply corrections to spectrum and spectrogram data. Calibration consists of three primary components that are combined to determine the final power value:

1. Transfer Function (Offsets): A list of frequency-offset pairs that define how the sensitivity of the recording system varies with frequency.
2. Gain (Fixed Offset): A constant offset (in dB) that is added to the transfer function to account for fixed gain adjustments.
3. Reference Pressure: A baseline reference that shifts the entire power scale.

Calibration Calculation

The calibrated power value is calculated as follows:
Calibrated Power = Raw Power - (Transfer Function + Fixed Offset) + Reference Correction

Note: The reference correction is added to the calculated power. Internally, Raven calculates the reference pressure in dB using the formula 10 * Log10(1 / pRef²). For a standard reference of 20 μPa in air, this results in a value of approximately -26.02 dB.

Interpolation

When a specific frequency does not have an explicitly defined offset, Raven performs linear interpolation. To ensure physical accuracy, the interpolation is performed on the linear power scale:

1. The dB values of the neighboring frequencies are converted to linear values.
2. Linear interpolation is performed between those values.
3. The result is converted back to the dB scale.

Supported Reference Pressures

• Air: dB re 20 μPa² Hz⁻¹ (standard for terrestrial recordings)
• Water: dB re 1 μPa² Hz⁻¹ (standard for underwater recordings)
• None: dB FS (Decibels relative to Full Scale, no pressure correction)

How to Use Calibration

1. Opening the Calibration Dialog

To open the Calibration menu:

1. Go to the Edit menu.
2. Select Calibration….

2. Loading Calibration Data

You can define a transfer function by loading a file containing frequency-offset data.

Loading a File

• Click the Select File button (represented by a folder icon) to browse for a file.
• Alternatively, you can drag and drop a valid file directly onto the table in the Calibration dialog.

Valid File Types

• CSV (.csv)
• Text (.txt)

File Format

The file must contain two columns of numbers. Each row represents a single data point:

1. Column 1: Frequency (in Hz)
2. Column 2: Offset (in dB)

Columns can be separated by a comma, tab, or space.

Example format:

1000, -12.5
2000, -10.2
5000, -5.0

3. Applying Settings

• Fixed Offset (dB): Enter a value to shift all measurements by a constant amount.
• Reference: Select the appropriate medium (Air, Water, or None) from the dropdown menu.
• Presets: You can save your current calibration settings as a preset for later use by using the Preset panel at the top of the dialog.

Once configured, click Apply to update all open charts with the new calibration settings.

Sidecar Files

Raven automatically saves calibration data in a JSON-formatted sidecar file with the extension .ravencalibration located in the same directory as the source audio file. This allows Raven to automatically re-load the calibration whenever you open that specific recording.