Measuring A Bi-Radial Horn In A Real Room

A bi-radial horn does not have one fixed frequency limit that can be read from a brochure. Its practical bandwidth depends on the horn profile, mouth size, compression driver, crossover network, cabinet geometry and the room in which it is measured. Reflections, boundary gain and listening distance can make the same loudspeaker appear to reach further at both ends of its range.

For a useful result, measurement needs to separate the horn’s acoustic behaviour from the room’s contribution. A gated sweep can show direct sound and early reflections, while a longer in-room measurement reveals what a listener actually hears in a Sydney living room, a Melbourne terrace or a treated demonstration space in Berlin. The two views answer different questions and should be read together.

What The Horn Is Designed To Do

A bi-radial horn controls dispersion in two dimensions, usually offering a defined horizontal pattern and a different vertical pattern. This controlled directivity helps a compression driver maintain output across a broad band while reducing energy sent towards the ceiling, floor and side walls. The usable lower limit is often associated with the horn’s acoustic cutoff, but the transition is gradual rather than an abrupt on-off point.

The mouth dimensions matter because low frequencies require a larger radiating area to maintain loading and pattern control. As frequency falls below the horn’s practical operating region, directivity widens, efficiency changes and the driver may no longer be working into the intended acoustic impedance. At the upper end, diaphragm behaviour, phase response, throat geometry and material damping become more important than the nominal horn size.

A custom system based around TAD-Pioneer compression drivers and woofers may therefore be voiced around a carefully chosen crossover rather than the widest possible specification. A passive, time-aligned network can preserve coherent arrival at the listening position, although its slopes and impedance interaction must be included in any assessment of effective bandwidth.

Why Room Measurements Change The Answer

A real room adds strong low-frequency reinforcement and cancellation. Placing a loudspeaker near a wall or corner can increase bass output, while a room mode may create a deep null at the microphone. Above the Schroeder region, reflections create comb filtering, so a single trace may show sharp dips that belong to the room rather than the horn or compression driver.

Listening distance also changes the balance between direct and reflected sound. In a compact Australian apartment, a microphone two metres from the horn may capture substantial wall energy. In a larger detached house in Brisbane or Perth, the same loudspeaker may produce a longer direct-sound window before the first strong reflection arrives.

The effective frequency range should therefore be described with a tolerance, such as the band remaining within ±3 or ±6 dB after suitable smoothing. It is also useful to report whether the limit refers to on-axis response, listening-window response, early reflections or spatially averaged in-room sound.

Choosing A Reliable Measurement Method

Use a calibrated measurement microphone at ear height, aimed according to the microphone manufacturer’s calibration file. A software package such as REW can generate logarithmic sweeps, but the sweep level must remain safe for the compression driver and appropriate for the room. Record the microphone distance, crossover setting, amplifier gain, smoothing and any equalisation, because undocumented processing can disguise the natural response.

A time window is valuable for examining the horn itself. If the first reflection arrives 5 milliseconds after the direct sound, gating can isolate approximately the first 5 milliseconds, although the resulting low-frequency resolution will be limited. This method is most reliable through the midrange and treble; it cannot accurately show deep bass behaviour in a normal domestic room.

For research and comparison, keep web sources separate from measured evidence. Search results may mix serious acoustic information with unrelated commercial pages, such as this unrelated online material, so every graph should retain its original test conditions and file name.

Practical Measurement Checklist

• Calibrate the interface, microphone and input level before each session
• Measure on-axis, then across the intended horizontal listening window
• Save gated, ungated and spatially averaged traces separately
• Repeat sweeps at moderate and realistic listening levels

Reading The Directivity Transition

A bi-radial horn’s coverage pattern can be more revealing than its axial frequency response. Measure at several horizontal angles, such as 0, 15 and 30 degrees, then repeat vertically if space allows. A smooth narrowing or widening of the curves suggests controlled behaviour; sudden lobes and deep off-axis notches may indicate interference around the throat, mouth or crossover region.

The transition between woofer and horn deserves special attention. If the woofer’s dispersion is already narrowing when the horn remains wide, the power response can develop an imbalance. Conversely, a horn that becomes too wide near crossover may send extra energy into nearby surfaces. A well-integrated design aims for a predictable directivity match rather than an impressive single-axis peak.

Edge diffraction can also alter the measured shape, particularly around cabinet edges and the horn mouth. The discussion in this edge diffraction guide explains why cabinet proportions, rounded edges and horn mounting details influence ripples that may otherwise be blamed on the driver.

Accounting For Australian Listening Rooms

Australian homes present varied acoustic conditions. Lightweight plasterboard, brick veneer, tiled floors and open-plan kitchens can produce a livelier response than a heavily furnished European listening room. In Melbourne, a narrow room with a sofa close to the rear wall may need a different microphone layout from a spacious Adelaide room with substantial distance behind the listener.

Everyday listening habits matter as well. Many owners listen at moderate levels in the evening, especially in apartments around Sydney or Melbourne, where neighbours and strata by-laws limit high-level playback. A horn may sound dynamically effortless at low amplifier power, but the measurement should still include a realistic listening level because compression, room excitation and cabinet vibration can change with level.

The local market also rewards serviceability and accurate documentation. Custom loudspeakers may be ordered for a specific Australian room, imported as completed systems or built to suit existing electronics. Australian Consumer Law applies to consumer purchases, while electrical installation and supplied equipment should meet relevant Australian safety requirements. These considerations do not define the acoustic range, but they influence how a system is installed, powered and supported.

Turning Measurements Into A Useful Specification

A credible specification should distinguish between the horn-loaded section and the complete loudspeaker. State the nominal crossover point, the recommended listening distance, the response tolerance, the measurement axis and whether the result is gated or room averaged. A phrase such as “effective from 800 Hz to 18 kHz in the listening window” is more informative when accompanied by directivity plots and the conditions behind it.

Low-frequency extension belongs mainly to the woofer, enclosure and room, while the horn’s lower working limit is governed by loading and crossover protection. At the top end, the compression driver and horn may continue producing output beyond the most audible region, but smoothness, dispersion and distortion are more meaningful than an isolated 20 kHz figure.

A listening room remains the final reference. Sunship Audio’s Berlin listening room provides a controlled setting for hearing how cabinet bracing, wooden horn geometry, driver integration and time alignment work together. Measurements can identify the boundaries; listening confirms whether those boundaries translate into natural voices, stable imaging and effortless dynamics.

Room Variables To Record

• Room length, width, height and major reflective surfaces
• Loudspeaker distance from walls, corners and the listening seat
• Microphone height, angle, distance and averaging positions
• Doors, curtains, furniture and other changes made between tests

Making The Result Repeatable

Repeatability is more valuable than a single attractive graph. Mark the loudspeaker position on the floor, use the same microphone stand and keep the measurement volume consistent. If the room is altered with rugs, curtains or open doors, record that change rather than treating it as background detail.

Compare several traces instead of relying on one microphone position. A narrow listening-window average can reveal tonal balance, while a wider spatial average shows how the system behaves across a sofa. When directivity, distortion, impulse response and in-room response point in the same direction, the stated operating range is much more trustworthy.

The most useful description of a bi-radial horn is therefore practical: where it maintains controlled coverage, low distortion and smooth integration with the woofer in the intended room. That approach turns a nominal bandwidth figure into a measurement of how the loudspeaker actually performs in an Australian home.