Cross wavefront syndrome: Diagonal matter

Wavefront Basics and Challenges:

Sound travels in waves, and the shape of the wavefront determines how the sound propagates in horns. Ideally, the wavefront should smoothly follow the horn’s curvature for optimal constant directivity (controlled sound dispersion).

However, horns with diagonal sections pose a challenge. A wavefront, while traveling at the speed of sound, inherently wants to maintain a 90-degree angle to its edges.

This tendency is crucial when the wavefront encounters sudden geometry changes, like those found in diagonal horn sections.

These rapid transitions can “pull” and deform the wavefront in some points, causing a phenomenon called diffraction (bending and spread out).

The Problem: Cross Syndrome in Horns or Waveguides

When a round waveguide transitions too quickly to a square or rectangular shape, bypassing the elliptical stage, it creates a sudden change in the wavefront’s edges expansion rate.

This rapid change causes diffraction primarily at the diagonals, disrupting the wavefront in high frequencies. This phenomenon is known as “cross syndrome.”

Why We Hear It: Audibility

Even subtle changes in the direct sound caused by diffraction will be audible due to how we listen in rooms.

Most listening happens within the “critical distance” where both direct sound from the speaker and reflected sound bouncing off walls contribute significantly in a ratio close to 50/50.

This means even diffraction effects off axis become noticeable even when listening directly in front of the speakers.

These compromised wavefront are audible and present in your listening experience. This will lead to a:

The X-Shape Horn Solution

X-Shape horns address this problem by focusing on the diagonal transitions.

Using advanced Finite Element Analysis (FEA), a specific diagonal profile is designed to ensure the wavefront maintains its integrity, even along these critical axes.

This allows the wavefront to smoothly follow the curvature without encountering drastic changes in geometry, removing diffraction as we can see here with a polar of the X-Shape diagonal (the worst place to measure a horn) :

X-Shape Horn 25 with RCF 350 ND
The accident after 14kHz is due to compression diaphragm breakup, as usual on big compression driver.

The ReShape Waveguide Solution

Thanks to an elliptical transition applied from the round throat to the rectangular mouth, our tweeter waveguide using ReShape haven’t this problem either.