Starship Calibration¶
A starship is CFTS's probe assembly: an integrated probe-tube microphone plus two speaker drivers (primary and secondary), used to present sound very close to the eardrum and record what's happening there. This calibrates a starship against a physical coupler, using a calibrated measurement microphone as the reference.
What you'll need¶
- A calibrated measurement microphone (see Measurement Microphone Calibration) and its preamp.
- The starship you're calibrating.
- A calibration coupler.
- An analog to digital converter and a digital to analog converter.
Opening the workspace¶
Launch CFTSCal and select the Starship Calibration workspace.
Settings¶
| Field | What it means |
|---|---|
| Cal. Mic. (first drop-down) | Which sound-card input the reference measurement microphone is plugged into. |
| Cal. Mic. (second drop-down) | Which calibration of the measurement microphone to use as the reference (made in Measurement Microphone Calibration). |
| Cal. Mic. → dB gain | The gain, in dB, set on the measurement microphone's amplifier. |
| Coupler | Which physical coupler you're calibrating into. Recorded in the calibration's metadata; cftscal doesn't otherwise act on it. A free-form, user-managed list — starts empty, so click + to add your coupler labels (e.g. tube-2mm, tube-0mm, 3D-basic) before first use. − removes the selected label from the list. |
| Starship (first drop-down) | Which starship connection you're calibrating, if your system has more than one (e.g. Connection A/B). |
| Starship (second drop-down) | Which starship is plugged into that connection. Click + to add a new one to the drop-down list, or − to remove the selected name from the list (no calibrations are deleted). |
| Starship → dB gain | The gain, in dB, set on the amplifier for the starship's own microphone (20 or 40). |
| Target folder | Which folder the new calibration is saved in. Leave it on (auto create from starship) to have a folder named after the selected starship — Golay and Chirp both name it this way. To create a new folder, right-click in the Calibrations list. |
Entries suffixed \"(EPL)\"
Some starship names in the drop-down end in (EPL) — these are calibrations imported from the legacy EPL CFTS program. They're read-only reference entries; you can't run a new calibration into one directly. With one selected, Golay and Chirp stay disabled.
Running the calibration¶
To run the calibration, click Golay or Chirp below the starship you want to calibrate. Both buttons stay disabled until a reference microphone calibration and a starship have both been selected.
- Golay plays a pair of complementary Golay-code sequences, several times each, and cross-correlates the recorded response against them. More robust to background noise, at the cost of taking longer.
- Chirp plays a single frequency sweep. Much faster, but somewhat more sensitive to noise.
Both runs expose a Smoothing window parameter (default 10) — the width, in frequency bins, of a Hamming-weighted moving average applied to the computed sensitivity curve. Set it to 0 to see the raw, unsmoothed measurement, which is worth doing before you conclude that a notch in the curve is or isn't real. See Speaker Calibration for the other run parameters, which are the same.
How the calibration is computed¶
Two things are being established at once: the sensitivity of the starship's own probe-tube microphone (against the reference microphone), and the transfer function of its speaker drivers (measured with that probe-tube microphone).
Given a probe-tube microphone of known sensitivity \(S_{PT}(f)\) — a function of frequency \(f\) in Hz, meaning it takes a different value at each frequency — the sound pressure in the coupler is \(O(f) = V_{PT}(f) / S_{PT}(f)\), and the speaker transfer function follows:
Calibration Math works through this in both linear and dB form.
A coupler calibration is not an in-ear calibration
This workspace calibrates the starship against a coupler on the bench. Inserting the probe into an ear changes the acoustics of the system: the ear canal presents a different acoustic load (largely a compliance, set by the enclosed volume), which shifts the system's resonances. So this calibration does not describe what the starship is doing once it's in an ear. An in-ear calibration has to be redone every time the probe is repositioned while it's in the ear; Starship Check is the workspace for verifying the starship in the ear it's actually sitting in.
Reviewing the results¶
Starship Sensitivity plots the frequency response (in dB re 1 Vrms) of every calibration currently selected in the list below. As with the speaker workspace, the plotted value is the dB SPL produced at a 1 Vrms drive, so you can read the voltage needed for a target level straight off the curve — see Reading cftscal's reported numbers.
Calibrations (the list) shows every calibration ever run for this workspace, with the following columns:
| Column | Meaning |
|---|---|
| Name | Which folder the calibration is filed under — by default, one folder per starship (see Device below if you've filed calibrations into folders that don't match the device). |
| Device | The starship label recorded at calibration time, independent of which folder the calibration is filed under. Usually matches Name — compare the two if you've reorganized calibrations into folders. |
| Microphone | Which reference measurement microphone was used. |
| Mic. Channel | Which input channel the reference microphone was wired to. |
| Starship Channel | Which physical connection the starship was plugged into (e.g. Connection A/B). |
| Gain | The preamp gain, in dB, applied to the starship's own microphone. |
| Mic. Gain | The preamp gain, in dB, applied to the reference microphone's channel. |
| Coupler | Which coupler was selected at the time. |
| Stimulus | Whether Golay or Chirp was used. |
Working with the list¶
- Tick a calibration to plot it; tick a starship to plot all of its calibrations.
- Right-click a calibration for more:
- Edit note… attaches a free-text note (e.g. "probe tube replaced"). A calibration with a note shows a sticky-note icon after its name; hover over the row to read the note. Clear the text to remove it.
- Set as current chooses which of the starship's calibrations is used from now on, marked ★; without one, the most recent is used. Clear current goes back to the most recent.
- Export as WAV… and Delete.
- Double-click a calibration to rename it, and drag it onto a folder to move it. Right-click a folder (or an empty part of the list) to create, rename or delete folders.
- A calibration whose files can't be read is shown in red with a warning icon after its name. Hover over it, or right-click → Show problem…, to see what is wrong.
Sanity-checking a calibration¶
- Does the response look like previous calibrations of the same starship? A sudden change usually means the probe tube shifted, got clogged with debris, or the coupler seal broke.
- Is the curve reasonably smooth, without unexpected notches? That usually points to a leak or obstruction rather than a real change in the starship. Bear in mind that a probe tube has genuine acoustic resonances inside the measurement range — a 20 mm tube resonates at roughly 4 kHz with further modes above that (see Acoustic tube resonance) — so some structure is expected. What matters is whether it looks like last time's structure. A calibration measures and compensates for those resonances correctly, but only as long as the geometry doesn't change afterwards.
- Was the correct coupler selected? Calibrating with the wrong coupler produces a response that won't match how the starship is actually used.
Troubleshooting¶
Common pitfalls
- Probe tube not fully seated in the coupler, or partially blocked by debris, is the most common cause of a bad calibration.
- Gain mismatch between the Settings panel and the physical starship preamp shifts the whole curve by a fixed, predictable amount.
- Wrong reference microphone selected, or that microphone's own calibration is stale, propagates straight into the starship's measured response.