How To Measure Horn Directivity At Home
Horn directivity describes how evenly a loudspeaker distributes sound as frequency and listening angle change. A horn may produce strong, controlled output directly ahead while becoming quieter or tonally different off axis. Measuring this behavior at home helps reveal whether the horn maintains a smooth coverage pattern or develops abrupt transitions, narrow beams, and uneven room excitation.
A useful measurement does not require a laboratory anechoic chamber. With a calibrated USB microphone, measurement software, a careful setup, and repeatable angles, you can create a practical directivity map. The result will be most valuable when you focus on relative changes between angles rather than claiming absolute acoustic performance from a living room.
For custom horn-loaded systems, this process can also clarify how horn geometry, compression-driver crossover points, woofer spacing, and cabinet alignment work together. It is especially helpful when comparing different configurations, such as those discussed in two-way or three-way systems.
What Directivity Reveals
A directivity measurement shows how sound pressure changes as the microphone moves away from the central listening axis. The usual reference is the on-axis response at zero degrees. Measurements at 10, 20, 30, 40, and 50 degrees can then be compared with that reference across the audible frequency range.
A well-controlled horn often maintains relatively consistent output across a useful horizontal and vertical window. The curves may gradually fall as the angle increases, but their shape should remain similar. A sudden dip on one off-axis trace can indicate diffraction, horn-mouth resonance, driver interference, or a crossover problem rather than simple narrowing.
Directivity is also related to room sound. A speaker with controlled dispersion sends less energy toward side walls and ceilings, which can improve clarity in a reflective room. This is one reason horn-loaded midranges can preserve articulation and harmonic detail; vocal texture and spatial separation are explored further in vocal harmony reproduction.
Prepare The Room And Equipment
The simplest useful setup includes a measurement microphone, an audio interface or USB microphone, a computer running Room EQ Wizard or similar software, and a stable stand. Use a microphone with a calibration file if possible. A phone microphone can demonstrate broad trends, but its frequency response and automatic processing make it unsuitable for dependable directivity work.
Place the loudspeaker away from large nearby surfaces. If practical, measure outdoors, in a garage with the door open, or in a room with the cabinet several feet from walls. Indoor reflections arrive after the direct sound and create comb filtering. A time-windowed impulse response can reduce this problem, but low frequencies require a longer window than a typical room may allow.
Use a moderate test level that gives a strong signal without stressing the compression driver or woofer. Disable automatic gain control, equalization, loudness compensation, and other signal processing. Keep the same amplifier gain, microphone height, sweep level, and speaker position for every measurement.
Establish A Repeatable Geometry
Begin with the microphone on the acoustic centerline of the horn, usually aligned with the compression-driver axis or the manufacturer’s stated listening axis. Set a practical distance, such as two or three meters, and record it. The microphone should point according to the calibration file’s instructions; some files assume a 0-degree orientation, while others assume 90 degrees.
Mark the center position on the floor and use a protractor, angle markings, or a rotating platform to create repeatable horizontal positions. Rotate the loudspeaker if its cabinet can be moved safely, or move the microphone along an arc whose center is the acoustic origin. Rotating the speaker is often easier for close-range tests, but it changes the relationship between the cabinet and room boundaries.
Measure both horizontal and vertical directivity. A horn may have a broad horizontal pattern but a much narrower vertical pattern, or the reverse. Keep the microphone at the same distance from the acoustic center for every angle. Small distance changes can alter the measured level and confuse the pattern.
Choose A Measurement Method
There are two practical approaches. A gated frequency-response sweep uses an impulse response and a short time window to reject reflections. A near-field or close-range sweep reduces the delay between direct sound and reflections, though it may not represent far-field behavior accurately at all frequencies.
| Method | Strengths | Limitations | Best Use |
|---|---|---|---|
| Outdoor far-field sweep | Cleanest direct sound and useful angular data | Weather, space, and background noise can interfere | Detailed horn coverage mapping |
| Indoor gated sweep | Convenient and repeatable | Low-frequency resolution is restricted by room reflections | Midrange and high-frequency directivity |
| Close-range sweep | Minimizes early reflections | Geometry and driver integration may differ from listening distance | Checking horn behavior and crossover regions |
| Phone-based measurement | Fast and inexpensive | Uncalibrated microphone and processing reduce accuracy | Rough visual comparison only |
For a home listening room, begin with a gated sweep from approximately 300 Hz upward, depending on the room and available time window. Inspect the impulse response and choose a window that ends before the first strong reflection. Do not apply heavy smoothing: one-sixth octave or psychoacoustic smoothing can make plots easier to read, but excessive smoothing may hide narrow anomalies.
Below the reliable gated range, use near-field measurements cautiously or treat the result as qualitative. A large woofer and a horn cannot be measured identically at very close range because their acoustic centers and radiation behavior differ. The transition region should be interpreted alongside the crossover design and physical driver spacing.
Capture And Normalize The Angles
Take an on-axis sweep first and save it as the reference. Then repeat the sweep at symmetrical horizontal angles, for example 10, 20, 30, 40, and 50 degrees to the left and right. Matching left and right traces can reveal cabinet asymmetry, nearby objects, or an inconsistent microphone position.
Afterward, subtract the on-axis response from each off-axis response. This produces a normalized directivity plot in decibels. If the 30-degree curve sits 4 dB below the reference at 2 kHz and 8 dB below it at 10 kHz, the horn is narrowing progressively with frequency. The exact level is less important than the shape and continuity of the change.
Repeat the same process vertically, using upward and downward angles. Pay particular attention to the crossover region. If the off-axis curves spread apart dramatically around the crossover frequency, the woofer and horn may have different coverage widths or a phase relationship that changes with angle. A smooth power transition generally produces more predictable room response.
Read The Pattern Without Overinterpreting It
Look for gradual attenuation, consistent left-right behavior, and similar curve shapes at neighboring angles. A directivity plot with evenly spaced traces usually indicates controlled radiation. A stack of curves that bunch together at low frequencies and separate at high frequencies is also normal: wavelengths become shorter as frequency rises, so the horn begins to beam more strongly.
Watch for deep, narrow notches that move substantially with angle. These can result from interference between drivers, reflections, or microphone-position errors. Broad, repeatable depressions are more likely to represent real acoustic behavior. Measure suspicious features again before changing crossover settings or applying equalization.
Directivity cannot be judged from an on-axis graph alone. A speaker may have a flat central response but send an uneven tonal balance into the room. Conversely, a gentle on-axis tilt can accompany smooth, controlled off-axis behavior and produce a more natural listening result. Combine the measurements with listening tests, especially for voice, percussion, and recordings with dense reverberation.
Build A Reliable Home Measurement Routine
A consistent routine matters more than expensive equipment. Use the same microphone calibration, distance, angle increments, gate settings, and output level each time. Save project files with clear names such as “Horizontal minus 30” or “Vertical plus 20,” and photograph the setup so it can be rebuilt later.
Useful practices include:
- Calibrate the microphone and audio interface before measuring.
- Keep the speaker and microphone away from reflective furniture and walls.
- Measure both sides of the horizontal axis and both vertical directions.
- Use normalized curves to compare angles rather than raw SPL alone.
- Repeat any unusual feature before treating it as a loudspeaker fault.
Once the basic map is complete, compare it with the listening position and room layout. A narrow vertical pattern may reduce floor and ceiling reflections, while a broad horizontal pattern can support several seats. These observations help determine whether placement, toe-in, crossover adjustment, or acoustic treatment is the most appropriate next step.
Use the measurements as an engineering aid rather than a substitute for a complete design review. Horn profile, throat matching, driver selection, cabinet diffraction, passive crossover components, and time alignment all influence the final result. The most useful home measurement is one that connects a visible acoustic pattern with an audible and repeatable change.
Set aside a quiet session, establish a marked microphone arc, and create your first horizontal and vertical directivity plots. With disciplined positioning and careful interpretation, a home setup can show how a horn behaves across the room and provide valuable evidence for refining a high-performance loudspeaker system.