Let me explain where I got those angles from.
When tweeters are tested, and occassionally when speaker systems are tested, they are test on the center line and at 15° and 30° off the center line. Most tweeter beam or, as the frequency rises, sends out an ever narrower beam of sound.
When tweeters are tested at 15° off the center line, it determine dispersion across the working frequency range, typically off the center line, as the frequency increase, the output decrease, meaning the beam of sound is getting narrower.
Typically at 15° (or ±7.5°) there is a very slight loss. At a wider ±30°, there can be a significant loss, especially as the frequencies go higher.
In my example, I used ±7.5° and ±15°, both should be well within the working range of the tweeter with only the slightest beaming, and only the slightest loss in output. If you are in this spread of arc, then you are for all intend and purpose, sitting on the center line.
So, even the wider ±15° is still in the relatively good range. This represents a trace of a drop, but not significantly so. In fact, many people prefer to be off the center line when listening to a given speaker.
±7.5° = 15°
±15° = 30°
If you go outside the 30° arc or cone, likely, the sound is going to drop off quickly and this is going to be less than ideal. But again, anything within the ±15° arc is going to be fine, not perfect, but pretty much fine.
Just to illustrate the point, here is a frequency response graph of a VIFA tweeter showing ±30° and ±60° -
Likely at 10khz, the 15° off center response would be barely noticeable.
Now, this only relates to the current thread as it tells us something about the dispersion of sound from a tweeter. If you are sitting in the ±7.5 or 15° arc, you are in the best working range of the speaker. If you are sitting in the ±15° or 30° arc you are still in the good working range of the speaker. But it drops fast outside the 30° arc. Keeping in mind for a vast majority of speakers, the sound fan out in a cone.
From midrange and mid-bass and bass driver, the beaming characteristic is pretty small. Most designer make sure the crossovers are away from the point where the speaker starts to bean. So, you can generally count on very wide dispersion of sound from Mid and Bass drivers.
So, the cone of sound from a speaker in a full 15° arc (±7.5°) is 0.3 times the distance.
The cone of sound from a speaker at in a full 30° arc (±15°) is 0.6 times the distance.
This is the narrowest
arc or cone of sound coming from the speaker. Mid and Bass are substantially wider.
Both of these are very workable. Anything outside of a full 30° arc is probably going to drop off pretty quickly.
Again, this merely helps you estimate if your seating position is with in the best arc of sound coming from the tweeter.