Sunship Audio

Building a Loaded Compression Driver Test Jig

Compression drivers are often judged on sensitivity and extension, yet electrical behaviour matters when designing a passive crossover. Mounting a driver on a horn changes the acoustic load seen by its diaphragm, so an open-air terminal reading can conceal the behaviour that matters. This article explains how to build a test jig for measuring compression driver impedance under load with repeatable mechanical and electrical methods.

Under load means the driver is coupled to its intended horn, with the rear termination correctly fitted, rather than merely screwed to a plate. The horn transforms acoustic impedance at the throat, while cable resistance, crossover parts and amplifier damping can add error. The jig should reproduce the intended conditions, isolate the driver from the crossover, and allow changes without rebuilding the bench.

A practical jig comprises a rigid carrier, an adapter plate or cradle, a rear sealing arrangement and a protected location for the measurement electronics. It should resist movement yet remain portable. Its purpose is to preserve the same horn alignment and load every time a reading is taken.

Australian details matter: test gear runs on 230 V, plugs and leads normally follow AS/NZS 3112, and metric fixings suit imported drawings. Many Aussie hi-fi enthusiasts enjoy DIY work, while grey-import drivers can complicate warranty and GST calculations at the border. Melbourne’s changeable temperatures, Sydney’s humidity and Brisbane’s heat also make it sensible to record room conditions.

Define what the measurement represents

Measure the compression driver with its crossover disconnected. A passive crossover contains inductors, capacitors and sometimes resistors; leaving it connected turns the result into a network measurement rather than a driver measurement. Fit the intended rear cap or termination, use the final lead length, and record whether the horn was fitted.

Treat the manufacturer’s nominal impedance as a reference, not gospel. Horn loading, wire resistance and test level can shift the curve. In an active or bi-amplified system, repeat the test with the prescribed input network fitted so the result matches the finished amplifier architecture.

Build a rigid and reversible horn fixture

Use 18–21 mm birch plywood or aluminium for the carrier, bracing it at the throat rather than across the mouth. A hardwood adapter ring, located by dowel pins, prevents rotation. Bolt it with compliant washers; never clamp a cast flange hard enough to twist it.

Reproduce the intended rear termination without trapping the driver. Leave the throat unobstructed and the mouth away from walls and ceilings. Mark the stand and microphone positions so every run returns to the same geometry.

Bi-radial wooden horns need rigid, symmetrical mounting. Sunship Audio’s gallery of finished systems provides visual references for driver placement, horn geometry and bracing. Put the guarded measurement electronics in a separate box beside the stand.

Select measurement electronics that suit low impedance

Use a low-impedance analyzer with true four-terminal measurement, or measure voltage and current separately and calculate magnitude and phase. A non-inductive shunt of roughly 0.05–0.1 ohm is a practical starting point for passive drivers; check power and peak current before sizing it.

Drive the driver with a stable voltage amplifier offering adequate current and a defined damping factor. Disable bass boost, loudness and limiting. Prevent ground loops, and do not rely on a basic bridge-tester meter for subtle resonance or phase features.

A microphone confirms acoustic output but cannot replace the electrical measurement. For acoustic checks, Cialistas measurement systems can provide suitable test equipment. Position an omnidirectional microphone on-axis at a documented distance, with its diaphragm aligned to the horn centreline.

Calibrate cables, shunt and microphone

Check the shunt against a calibrated meter at low frequency and confirm it again through the complete test lead. Lead length, solid connections and repeatable plug orientation matter at high frequencies. Record analyzer gain, cable length and any inline impedance adapters before taking data.

Calibrate the microphone with an acoustic calibrator if available, recording serial number, sensitivity and calibration date. Warm up the amplifier and analyzer, then photograph the jig, horn, wiring and microphone position. A saved setup makes comparisons between driver batches dependable.

Run the sweep without changing the load

Run a baseline with the horn removed only if a safe rear termination permits it, then fit the horn without changing cabling or support. Sweep logarithmically from 20 Hz to 30 kHz, with enough points to resolve the resonance region rather than relying on a coarse display.

Repeat at low, medium and high levels, such as 1, 5 and 10 volts RMS where the driver permits. Stay within thermal limits and stop if coil temperature changes materially. Save magnitude and phase, and log ambient noise; a quiet early-morning bench often gives cleaner traces.

Measure the driver alone, then optionally include crossover parts to obtain the complete loudspeaker-system curve. Recheck the baseline after the run to detect drift. Keep the two datasets separate when calculating passive crossover values.

Use the results to shape the passive crossover

A healthy passive compression driver usually approaches nominal impedance through the useful midband. Inductive phase rotation and rising impedance at the extremes are expected, while sharp resonant peaks can reveal a mechanical or electrical fault. Read the whole curve, not just the point nearest the advertised 4- or 8-ohm value.

Compare multiple drivers on the same jig. Consistent curves indicate a repeatable fixture; scattered traces can reveal a loose connector, inconsistent rear sealing or unit differences. This is useful when matching a stereo pair or drivers for a larger bi-radial horn installation.

Use the results to calculate passive crossover values, then verify sensitivity, impedance and phase together. Account for coil resistance, inductor resistance, capacitor tolerances and horn loss. A time-aligned passive crossover can behave very differently from a textbook divider in a generic test enclosure.

A practical workshop checklist

A disciplined checklist is useful in Australian workshops, where mains-powered test equipment often shares the bench with timber tools. Confirm the rig is stable and the horn cannot strike the floor before energising it.

Before applying power

Record enough context to reproduce the result and inspect the driver after demanding runs. Compression drivers can be damaged by sustained high level, incorrect phasing or an unsuitable horn.

During and after each run