Directivity Match: Horns, Waveguides, and Woofer Harmony

Demystifying Directivity Match

In the world of loudspeaker design, achieving a seamless transition between different driver components is crucial for optimal sound quality.

This article delves into the intricacies of directivity matching between horns or waveguides, and woofers, a process that ensures a cohesive and natural listening experience.

The Importance of Directivity Match

Consider a loudspeaker with a woofer and a guided high-frequency driver.

Each driver naturally radiates sound with a specific directivity pattern, which describes how sound propagates from the driver in different directions.

Ideally, at the crossover frequency – the point where the sound output transitions between woofer and high-frequency section – the directivity patterns of both drivers should be similar.

This ensures a smooth and consistent soundstage without any dips or peaks in certain frequencies off-axis, as the sound we perceived at critical distance is a more or less 50/50 balance beetween reverberated field (rebound from walls) and direct field from the speaker as seen in our critical distance article.

Another factor to consider is the physical distance between the acoustic centers of the woofer and the high-frequency section.

Ideally, this distance should be less than or equal to roughly 66% of the wavelength at the crossover frequency as see in vertical lobing article.

Physically bringing the woofer as close as possible to the horn/waveguide further enhances the transition.

The Perfect Match: A Myth?

The pursuit of a perfect directivity match at all frequencies, can be a misguided approach. Measurements taken in anechoic chambers (highly sound-absorbent environments) often reveal that achieving this perfect match is practically impossible. Here’s where the crossover filter design comes into play.

The crossover slope, along with meticulously adjusted time delays, plays a vital role in ensuring a smooth transition between the woofer’s coverage and the horn/waveguide’s coverage. This transition should be free of abrupt changes (on-axis or off-axis) to avoid unwanted coloration or distortions in the sound.

Moreover, a common misconception in modern loudspeaker design is prioritizing an exact 120-degree directivity match at crossover and up to 20 kHz on the depends of more important aspects like coverage adapted to listening distance.

This “one-120°-coverage-fits-all” approach leads to completely ignores psyckoacoustics principles and coverage adapted to distance as see in critical distance article.

Improved Directivity with Round-Over Returns

Our horns incorporate a design element called a “round-over return.” This feature enhances directivity control by mitigating the narrowing effect call midrange narrowing/beaming that occur in the midrange frequencies.

The round-over return is a smooth, curved transition that seamlessly follows the horn’s profile until it meets the side of the enclosure. This design minimizes disruptions to the wavefront, preventing unwanted narrowing of the sound dispersion pattern in the midrange.

Benefits of Round-Over Returns:

• Improved Directivity: By eliminating midrange narrowing, the round-over return ensures a more consistent and predictable directivity pattern across the entire frequency spectrum.

• Smoother Sound Transition: The smooth curvature of the return minimizes potential phase cancellations and contributes to a more natural and seamless transition between the horn and the enclosure.

• Enhanced Off-Axis Response: The improved directivity control translates to a more consistent tonal balance thanks to coherent power response (the fusion of direct and reverberated field as seen upper), for listeners positioned both on and off-axis from the speaker.

Directivity Matching Between Horns: A Different Approach

When dealing with multiple horns or waveguides in a loudspeaker design, the crossover approach differs slightly. Here, the ideal crossover point is not where each component loses its directivity, but rather where both maintain a constant directivity pattern. This constant directivity should be consistent not only at the crossover frequency but also one octave below and one octave above the crossover point.

Following these principles of directivity matching ensures a natural and cohesive sonic experience for the listener, free from unwanted artifacts and with a smooth transition between different driver components within a loudspeaker.