Those interested in the technical aspects of sound reproduction may have heard or read, especially in the past decade or so, much about constant directivity speakers.
In this article, we take a look at what constant directivity speakers are, and why the evidence from psychoacoustics (the science of how we hear) suggests that constant directivity is a highly desirable goal in speaker design, contributing to both tonal and spatial sound quality.
An understanding of constant directivity begins with the concept of tonal balance (which more technically inclined readers will already know corresponds to the speaker’s frequency response or amplitude response). Any good speaker should excel at reproducing whatever tonal balance is captured in the recording, and not colouring it with the speaker’s own idiosyncratic tonal imbalance.
For many decades, high-end speakers have been capable of producing a natural, accurate tonal balance directly in front of the speaker (the on-axis amplitude response). However, in any real room, the majority of the sound arriving at the listener does not come directly from the speaker but rather comes from reflections off room surfaces (particularly walls, ceiling, and floor). These reflected sounds, which emanate from the speaker from a slew of different angles before bouncing back and recombining with the direct sound, are critically important to the listener’s perception of sound quality. Thus, it is essential that not only the speaker’s on-axis tonal balance but also its off-axis tonal balance is natural and accurate.
This is precisely where constant directivity speakers come in. A constant directivity speaker produces a dispersion pattern (the speaker’s tonal balance in all three dimensions of space) that is similar to that of the direct, on-axis tonal balance. This tonally balanced wavefront may be projected in all directions (an omnidirectional speaker) or through only a specific range of angles (for example, only to the front and not to the rear of the speaker). The optimal beamwidth (range of angles through which sound is projected) is a question of room acoustics (e.g. strength, angle, and spectrum of reflections), setup (e.g. distance from nearby walls), and listener preference.
A well-engineered constant directivity that is well-matched to its acoustic environment will do two important things. Firstly, it will ensure that the tonal balance of reflections arriving at the listener’s ears is similar to that of the direct sound. And, secondly, it will not project sound toward parts of the room where it will create unwanted reflections (for example, nearby walls). For the listener, this results in a natural perceived tonal balance and a thrilling reproduction of the spatial effects present in the recording.
