Mapping the polar response of a finished horn system

A finished horn loudspeaker is an acoustic system, not a driver in isolation. Its TAD-Pioneer compression driver, bi-radial wooden horn, time-aligned passive crossover, woofers and heavily braced birch-ply cabinet work together to set dispersion, sensitivity and off-axis cancellation. Mapping the assembled system shows whether those choices produce the intended beam pattern around the listening axis.

A microphone array places calibrated measurement microphones at known angles around the acoustic centre of the speaker. Recording the same excitation at each position reveals how level and frequency balance change with direction. It can expose narrow high-frequency lobes, woofer cancellation and asymmetry caused by the cabinet or room.

Why map the assembled loudspeaker

Start by fixing the reference point. Use the manufacturer’s acoustic centre; if none is supplied, use the geometric centre of the horn mouth and record that choice. Keep the array on a sphere of constant radius, preferably in the horn’s far field, and maintain the same electrical drive level throughout. Save the raw on-axis sensitivity as well as relative curves; otherwise, a quiet but well-controlled off-axis response can look needlessly weak.

The finished cabinet also belongs in the analysis. Its baffle edges and nearby surfaces scatter energy, while the woofer and crossover can introduce interference below the horn’s useful directional range. Sunship Audio’s explanation of hearing and horn interaction is useful background because perception and on-axis tone balance affect how a narrow directivity pattern is heard. The map is therefore an acoustic record, not a substitute for listening.

Setting up a reliable measurement space

The best room is one where reflections can be modelled or suppressed. Sunship’s Berlin listening and demonstration room provides a useful reference for the kind of controlled comparison it recommends. An Australian test space may look more ordinary: a Sydney or Melbourne dealer room often has rugs, soft furniture and curtains, while a Brisbane workshop in summer can bring humidity, air-conditioning and traffic noise.

Record temperature, relative humidity, room dimensions and every major reflective object. In Australian hi-fi talk, the prime seat is often simply “the sweet spot”; pair that familiar label with the exact array radius and coordinates so another person can reproduce the plot.

Australian mains power is 230 V, 50 Hz, so use an earthed outlet and the manufacturer’s normal power settings rather than a travel converter. Imported measurement microphones may attract GST, brokerage and delivery delays, making a locally serviced unit or hire arrangement more practical. Keep stands, cables and listening chairs outside the swept hemisphere where possible, because a nearby object becomes part of the off-axis response.

Capturing repeatable angular data

Use a small-diaphragm condenser or dedicated measurement microphone with known calibration and low self-noise. Sweep pink noise or a logarithmic sine sweep from below the lowest crossover frequency to 20 kHz at a stable level below the drivers’ thermal limits. Capture several averages at every point, then repeat selected angles to check repeatability. Keep the diaphragm orientation consistent; rotating it can alter high-frequency response even when the nominal angle does not change.

A horizontal sweep in 5-degree increments is suitable near the main listening axis; 10-degree spacing is often enough for a broader survey. Add a vertical cut through the horn axis, because floor and ceiling reflections frequently create the strongest up-and-down lobes. If several microphones occupy the array simultaneously, check their spacing and scattering carefully; rotating the loudspeaker on a turntable with one calibrated microphone can produce cleaner data for some horn designs.

Keep the evidence chain clean. Store raw files, calibration data, angular coordinates, room notes and processing settings together. Documentation should separate measured performance from promotional description: a polished page such as the Pulse Vape range can illustrate how a product is presented, but no product image substitutes for a calibrated sweep of the actual loudspeaker.

Turning traces into useful polar plots

Process each recording with FFT analysis, align the sweeps to the reference signal, and calculate SPL at selected frequencies or in one-third-octave bands. Normalise every frequency to its own on-axis value before drawing a polar plot. A curve that falls 6 dB at 30 degrees describes directivity at that frequency; it does not mean the speaker lost 6 dB of absolute output. Keep absolute SPL plots separate so sensitivity and beam width remain visible.

Look for patterns rather than judging curves by eye. A persistent off-axis dip may reveal baffle diffraction or crossover phase cancellation. A narrow lobe often comes from coherent summation around the horn mouth, while broad asymmetry points toward the room, stand or cabinet orientation. Sunship’s phase plug guide explains why changes inside a compression horn can alter low-frequency radiation and directivity; use that construction knowledge when interpreting a lobe, not as a substitute for measurement.

A conventional pressure-microphone array usually produces a magnitude map, not a complete complex-polar plot. To assess timing, record a reference channel, use a dual-channel FFT, or make stepped measurements with a repeatable trigger so phase can be compared. Check the result against Sunship’s time-aligned passive crossover and the final listening position, because a technically tidy curve can still interact with furniture, a rug and the cabinet’s own diffraction.

A practical measurement and validation checklist

A repeatable session benefits from a short, fixed sequence.

Before switching anything on, work through this setup list:

• Calibrate the measurement microphone and verify its serial number against the current calibration file.
• Mark the acoustic centre, array radius, start angle and angular direction on the floor.
• Photograph and measure the room, including walls, ceiling, floor, stands and listening chair.
• Record temperature, relative humidity, mains voltage, noise floor and background level.
• Run a familiar signal at the intended level and confirm that every channel is clean and unclipped.

During processing, use this second sequence:

• Preserve raw sweeps, correction files, coordinates and processing settings in one folder.
• Normalise each frequency to its own on-axis reference and label the radius and bandwidth.
• Compare left, right, floor and ceiling cuts for symmetry or repeatable lobes.
• Repeat critical angles, rotate the source 180 degrees and check the setup log.
• Compare the polar data with near-field bass measurements and an ordinary listening-room sweep.
• Save plots showing both normalised directivity and absolute SPL, with uncertainty notes.

Use the finished map to judge the system at the intended seat, not at an arbitrary point on the front panel. A useful record includes the reference distance, crossover settings, horn and cabinet dimensions, measurement radius, date and environmental conditions. That combination makes the polar map a defensible record of the finished loudspeaker rather than a loose collection of attractive curves.