Measuring Horn Directivity With A Turntable And Microphone
Horn directivity describes how evenly a loudspeaker spreads sound as frequency changes. A horn may project high frequencies in a controlled pattern while allowing lower frequencies to widen into the room. Measuring that transition reveals whether the coverage is smooth, useful and consistent with the intended listening position.
A turntable and calibrated microphone provide a practical way to map this behaviour without a large laboratory. By rotating the loudspeaker through a series of angles and recording frequency responses, you can build horizontal or vertical polar data. The result shows where energy is concentrated, where it falls away, and whether the crossover introduces an abrupt change in coverage.
This method suits the kind of custom work carried out by Sunship Audio, where compression drivers, woofers, wooden horns and time-aligned networks are treated as one acoustic system. It is equally useful for a serious home project in Sydney, Melbourne or Brisbane, provided the measurement space is quiet and the setup is repeatable.
Why Directivity Matters In A Horn System
Frequency response measured directly in front of a loudspeaker does not tell the whole story. Two systems can produce almost identical on-axis curves while sounding very different across a room. The speaker with smoother off-axis behaviour usually creates a more stable reverberant field, clearer images and a more predictable balance at different seats.
A horn’s mouth size, flare profile and throat geometry govern its dispersion. A bi-radial horn may control the horizontal and vertical planes differently, which is often deliberate. Horizontal coverage can be broad enough for several listeners, while narrower vertical coverage limits floor and ceiling reflections. The goal is not maximum spread; it is controlled, even radiation.
Directivity also matters in Australian homes, where polished floors, plaster walls and open-plan living areas can add plenty of reflected energy. In a compact Melbourne listening room, a narrowing high-frequency beam may reduce side-wall splash. In a larger Brisbane room, consistent coverage can help the far seat retain tonal balance without simply increasing amplifier power.
Preparing The Turntable And Microphone
Place the loudspeaker on a rigid turntable whose rotation axis passes as close as possible to the acoustic centre of the horn. If the axis sits well behind or below that point, rotating the cabinet changes the microphone distance and shifts the apparent response. A simple laser, plumb line and marked platform can improve alignment considerably.
Use a calibrated measurement microphone on a stand, with its capsule at the selected reference height. For a horn system, that reference is commonly the acoustic axis of the compression driver. Measure at a distance that gives a useful balance between near-field detail and room reflections; two to four metres is often practical indoors, while a longer outdoor distance may work if the site is quiet.
Run a level check before the sweep. Keep the gain low enough to protect the driver but high enough to stay above computer and environmental noise. In Australia, outdoor testing near a suburban road can be spoiled by traffic, cicadas or wind moving through nearby gum trees, so early morning conditions are often more suitable than a busy afternoon.
Essential Setup Checks
- Lock the microphone stand so its height cannot drift during rotation.
- Mark the turntable’s zero-degree position and the loudspeaker’s centreline.
- Disable room correction, equalisation and automatic gain controls.
- Use the same sweep length, level and microphone orientation for every angle.
- Record temperature, distance, equipment settings and the measurement software used.
Rotating The Loudspeaker Through Angles
Start with an on-axis measurement at 0 degrees. Rotate the loudspeaker in consistent increments, such as 5 or 10 degrees, while keeping the microphone fixed. A complete horizontal sweep might cover -90 to +90 degrees. For a detailed horn study, smaller steps around the crossover region can reveal changes that wider intervals would hide.
Repeat the process vertically when the horn’s height or flare suggests that elevation is important. A vertical sweep can expose floor and ceiling interaction, driver-to-driver misalignment or a crossover region that behaves differently above and below the reference axis. The cabinet should rotate around the same acoustic centre in both planes.
Use a gated or time-windowed measurement where possible. The gate removes later reflections from walls and furniture, though it also limits low-frequency resolution. A turntable measurement therefore works best as a frequency-dependent directivity study: use the cleanest available data above the gate limit and treat bass radiation separately through near-field or ground-plane methods.
A Reliable Rotation Sequence
- Measure 0 degrees twice to confirm the system has not changed.
- Capture 10, 20, 30 and 40 degrees before moving toward the side.
- Continue to 60 and 90 degrees when the turntable and room allow it.
- Repeat suspicious points rather than averaging away an alignment error.
- Save each response with its exact angle in the filename.
The microphone should remain stationary throughout a horizontal sweep. Moving it to follow the cabinet changes the measurement from angular directivity to a mixture of angle and distance effects. If the speaker is physically large, note the distance from the turntable axis to the microphone and keep the mic far enough away for the intended spatial resolution.
Reading Polar Data And Directivity Index
Convert the angle-by-angle frequency responses into a polar plot or colour map. A smooth horn typically shows gradual narrowing as frequency rises, with no sudden lobes or holes near the crossover. The plot may be normalised to the on-axis response, allowing coverage changes to stand out without confusing them with the speaker’s overall tonal character.
Directivity index is another useful summary. It compares on-axis output with the average sound radiated into a defined angle. A rising directivity index means the speaker is concentrating more energy forward. Sudden steps in that curve may indicate a mismatch between woofer and horn coverage, an acoustic-centre offset or a crossover slope that does not suit the drivers.
A passive network has a strong influence on this result. A first-order filter allows both drivers to remain acoustically active over a broad region, so their polar behaviour must blend smoothly. Sunship Audio explains this relationship in its guide to first-order crossover design, where electrical simplicity is considered alongside horn geometry and driver integration.
Look for a stable power response rather than chasing a perfectly flat curve at every angle. A narrow, controlled horn can measure lower off-axis by design. Trouble appears when the change is abrupt, uneven from side to side, or accompanied by deep cancellations that will be audible as tonal shifts across the room.
Avoiding Common Measurement Errors
Room reflections are the most common source of misleading polar data. A wall behind the loudspeaker, a nearby desk or a hard floor can create comb filtering that changes as the cabinet rotates. Absorbent panels placed beside and behind the speaker help, but they do not replace a suitable time window and careful interpretation.
The turntable itself must be quiet and mechanically stable. A wobbly platform can tilt the horn by a fraction of a degree at each position, while a small change in microphone height may look like a vertical directivity feature. Check the physical setup before changing the crossover or adding equalisation.
Signs That A Result Needs Checking
- A narrow notch appears at one angle but not at neighbouring angles.
- Left and right rotations show different behaviour in a symmetrical horn.
- The response shifts substantially below the expected gate limit.
- A polar plot shows sharp spikes rather than smooth transitions.
- Repeating 0 degrees produces different curves.
For builders working with custom systems, measurements should be compared with listening rather than treated as an isolated verdict. The speaker may be designed for a particular distance, toe-in angle and room width. The custom horn range reflects that approach, using horn profiles and cabinet construction to control radiation as part of the complete loudspeaker.
Connecting Measurements With Listening
Once the directivity map is trustworthy, position the loudspeaker in the intended listening room and assess several seats. Listen for changes in vocal presence, cymbal energy and image focus as you move sideways. In a wide Australian lounge, the most useful result may be even tonal balance across the sofa rather than the highest possible on-axis level.
Toe-in should be tested alongside the measured coverage. A horn with controlled dispersion may need less toe-in than a conventional dome tweeter, while a narrow pattern may benefit from aiming directly at the main seat. Keep the microphone and listening reference consistent so that subjective impressions can be related to measured angles.
| Measurement Feature | Likely Meaning | Practical Response |
|---|---|---|
| Smooth narrowing with frequency | Controlled horn radiation | Check whether coverage suits the room |
| Abrupt change near crossover | Driver or filter mismatch | Review acoustic slopes and alignment |
| Deep off-axis cancellation | Interference or lobe formation | Inspect polarity, spacing and crossover |
| Uneven left-right pattern | Setup or cabinet asymmetry | Repeat alignment and rotation checks |
| Rising directivity index | Increasing forward concentration | Assess room reflections and seat coverage |
In a specialist system using TAD-Pioneer compression drivers, substantial woofers and braced birch plywood cabinets, directivity measurement helps connect construction decisions with the listening experience. The finished result should show purposeful control: a coherent main axis, predictable off-axis behaviour and a radiation pattern that works in the room where the loudspeaker will actually live.