Measuring Crossover Phase at the Binding Posts
A loudspeaker crossover can be assessed from several points in the signal path, but the binding posts provide the most practical electrical reference. They show the voltage that actually reaches the completed network after the amplifier cable, terminal hardware and internal wiring have been included. For a custom horn loudspeaker, this measurement helps verify crossover frequency, polarity, phase rotation and the intended relationship between compression driver and woofer.
The result is different from an acoustic measurement taken with a microphone. Electrical phase describes the behaviour of the crossover and its connected load, while acoustic phase also includes driver offsets, horn geometry, cabinet placement and the listening position. Keeping those two forms of phase response separate is essential when evaluating a time-aligned system in a real room, whether that room is in Melbourne, Sydney or a smaller Australian home.
Why measure at the binding posts
A binding-post measurement begins with the loudspeaker disconnected from the amplifier and the test instrument connected across its input terminals. A second connection provides the reference signal, usually from the same generator or audio interface. The software then compares the reference voltage with the voltage appearing at the speaker terminals, calculating gain and phase through the crossover’s complete input path.
This approach captures the electrical behaviour of the finished loudspeaker rather than relying only on a schematic. It can reveal a reversed connection, an unexpected impedance interaction or a wiring error that may be missed when individual components are measured on a workbench. In a passive network feeding a TAD-Pioneer compression driver and woofer, the result reflects the drivers’ changing impedance as well as the inductors, capacitors and resistors.
A useful distinction is that the binding posts measure terminal voltage, not the sound arriving at the listener. They cannot prove that two acoustic sources sum correctly at the chosen listening height. They can, however, establish whether the electrical network is behaving as designed before microphone measurements are used to assess the horn and driver integration.
| Measurement approach | Main information | Important limitation |
|---|---|---|
| Across binding posts | Electrical transfer function, crossover slope and phase rotation | Does not include acoustic spacing or room effects |
| Across an individual driver | Voltage delivered to that driver | Requires safe access to internal wiring |
| Near-field microphone | Local acoustic output from a driver or horn | Phase depends on microphone position and timing reference |
| Far-field microphone | Combined loudspeaker response at the listening position | Includes room reflections and placement variables |
Equipment and safe connections
A two-channel audio interface, measurement program and suitable attenuator are usually sufficient for low-power testing. One output channel feeds the crossover through a known series resistor or protective network, while an input channel monitors the voltage at the binding posts. Programs such as REW, ARTA and professional transfer-function systems can display magnitude, excess phase and group delay.
The load matters. A passive crossover does not operate into a fixed resistance unless it has been designed that way. Measuring only with a resistor may produce a neat graph that differs from the response obtained with the real compression driver and woofer attached. For a meaningful result, measure the assembled loudspeaker at modest level, with the intended drivers connected and the cabinet placed in a stable position.
Electrical safety deserves particular attention. Never connect an earth-referenced oscilloscope directly across the output of a bridged or floating amplifier, since the probe ground may short one output leg. A differential probe, isolated interface or a properly designed low-voltage test source avoids this risk. Australian mains power is nominally 230–240 volts, but the safer practice is to keep the loudspeaker test entirely at low voltage and remove the power amplifier from the first stage of diagnostic work.
Capturing a reliable phase trace
Phase is meaningful only relative to a reference. The reference channel should travel through the same interface and remain time-stable while the second channel measures the speaker terminals. Check polarity before making a sweep: a reversed input lead produces a 180-degree change that can be mistaken for crossover behaviour. Use a short, repeatable cable arrangement and record the interface latency or loopback delay.
A logarithmic sine sweep gives a broad view of the crossover’s amplitude and phase response. A stepped-sine or transfer-function measurement can provide higher control around the crossover region, where rapid phase rotation and deep cancellation often occur. Set the test level high enough to remain above interface noise, but low enough to avoid compression, protection circuits or driver heating. Horn compression drivers can reach useful acoustic output with very little electrical power.
Unwrapped phase is often easier to interpret than a graph restricted to plus or minus 180 degrees. The unwrapped trace shows continuous rotation through the low-pass and high-pass sections, while excess phase removes the dominant delay and makes network behaviour clearer. Group delay, calculated from the rate of phase change, can highlight resonances or unusually steep transitions, though it should not be treated as a substitute for acoustic time alignment.
Interpreting crossover behaviour
At the crossover region, the phase curves associated with the two drivers are more informative than the total terminal response alone. A passive filter may show substantial phase rotation while still producing a well-behaved acoustic sum once the driver positions and horn mouth geometry are included. Conversely, a smooth-looking electrical voltage trace cannot guarantee good acoustic addition.
The load impedance should be recorded alongside phase. A sharp impedance minimum can make the amplifier deliver more current, while a rising impedance can alter the voltage distribution between crossover components. This is particularly relevant in high-efficiency horn systems, where a small component-value change may shift the balance between a low-frequency woofer and a sensitive compression driver.
Polarity tests are valuable at the posts and at the drivers. If one section is electrically inverted, the expected phase relationship may produce a pronounced cancellation near the crossover frequency when measured acoustically. A terminal test can identify the wiring condition; a microphone measurement then confirms whether the physical driver arrangement requires normal or inverted electrical polarity for the intended acoustic sum.
From electrical data to a finished system
A well-documented post measurement forms part of a larger design record. Component values, driver impedance curves, cabinet wiring and terminal polarity should be retained with the phase plot. This is especially useful for custom loudspeakers, where a replacement driver, a changed horn throat or a different internal cable route can alter the result from an original prototype.
The distinction between electrical and acoustic alignment is central to a system using bi-radial wooden horns and heavily braced birch plywood cabinets. The crossover may be engineered for a particular acoustic centre offset, and the final confirmation must be made with the cabinet assembled. Photographs of construction details and completed systems in the Sunship Audio gallery provide useful context for why terminal measurements are only one stage of the verification process.
Australian listening rooms add practical variables. Open-plan homes in Brisbane or Perth can create long reflection paths, while compact rooms in inner Sydney and Melbourne may place the listening seat close to a rear wall. Long domestic cable runs and limited local stock can also encourage buyers to order custom systems rather than adapt an off-the-shelf crossover. Measuring at the binding posts gives a stable baseline before room placement, microphone position and local supply-chain substitutions enter the assessment.
For an Australian demonstration, the most useful sequence is therefore electrical transfer-function measurement at the posts, individual driver checks where accessible, and gated or time-windowed acoustic measurements in the listening room. The first stage verifies the crossover network; the second confirms wiring and driver response; the third shows whether the complete loudspeaker achieves the intended phase relationship in space.