What is Constant Directivity aka CD Behavior ?
The goal of a horn or wave-guide is to control the sound radiation of a device (tweeter or compression driver).
When a directivity is not constant, the device loose more and more coverage when the frequency goes up, the polar aka the energy in dB relative to the angle of radiation show us the problem :
On this polar map, that represent how the sound energy radiate, we can see it become tightened after 5Khz. The listening experience will be a speaker that sound “tiny” because there is a radiation incoherence according to frequency.
The ideal response is :
It’s completely straight and the orange color (-6db) define the coverage angle in degrees, of course a speaker cannot be straight on all this bandwidth :
- On bottom frequency it will be the end of control of the horn directly dictated by horn width.
- On top frequency it will be stopped according to the throat size (2"/1.5"/1"), tinier the throat, upper the directivity control will be push.
So a good horn will look like this :
The game is to push the CD behavior upper we can and lower we can thanks to the allowed width and throat size. We will have to reduce the midrange narrowing and the midrange beaming as good as we can, see the “What is mid-range beaming or narrowing?” Q/A.
The listening experience will be a lot better in constant directivity, in a room around half of the sound you listen is a reflection from wall, so even if you listen alone in front of the speaker, a constant directivity horn will change everything and will bring a more natural and fidelity listening experience.
One things that it’s important to not forget, we always choose a frequency cross-over between two devices where the radiation pattern is similar between these two elements, see “What frequency I should use for my crossover?” Q/A.