Directionality is a normally an almost completely unwanted quality in speakers. You want speakers to be heard equally well from all directions (that are frontward of the speaker's baffle). That's one reason why speaker boxes use multiple speakers, and these get smaller for the higher frequency ranges. "Dome tweeters" are shaped as such in order to radiate all around.
A "full range" speaker will send the lows in all directions but the highs mainly in the direction of its axis. A listener caught in the beam will hear a shrill sound, whereas someone off axis hears it muffled. Guitar speakers are like this; particularly the 12" ones and particularly in the 4x12 cabinet arrangement. Sometimes musicians use dispersing devices mounted on the speakers, like "beam blockers". Or the speaker is picked up by a microphone close to it, so that the audience hears it fro the PA system (which solves the sound dispersion problems in its own way).
There are situations in which it is desirable for a speaker box to "beam", like when it is mounted far away or high above a target listening area that is relatively small, calling for the speaker to be a kind of spotlight.
Speaker directionality gets interesting once you get to arrangements for festivals and concerts. Ideally the audience should all be able to hear well, which is difficult to achieve over such a large area and with multiple sources.
But you also don't want the neighbours to complain about the thumping bass. A number of subwoofers pointing backwards, with the right phase delay, destructively interferes with the most offensive frequencies and reduces the sound intensity behind the stage. It's called a 'cardiod' arrangement, because of the shape of the resulting sound intensity distribution.
Yes. And then you have the vexing problem of the stage sound where you want the musicians to hear themselves and each other. Without feeding back into their microphones, and not too loud please.
Modern in-ear monitoring has solved some of this, but it’s still tricky (and expensive).
> Sometimes musicians use dispersing devices mounted on the speakers, like "beam blockers".
Beam blockers seem to be the most common thing employed, though to my knowledge a much more effective measure is a "Mitchell donut". Basically, a soft foam disc with a soup can diameter hole cut in the center. The disc is sized to completely obstruct the speaker when mounted in front of it on the grill(cloth). With properly chosen foam of the right thickness, frequencies above about 1khz will be attenuated except when traveling through the center hole, meaning they'll behave more like they're coming from a point source and the constructive interference that causes the beam will be much reduced. Equalization may need adjusting since some highs are lost, and because if the player adjusted initially for on-axis sound they are now hearing the off-axis sound.
From a stroll through the audio cyclopedia, I think "mitchell donuts" are more properly called "acoustic lenses". A key difference between them and most beam blockers I see is that the blockers assume high frequencies come from the center of the speaker (cause that's where the beam is, and when close micing that's usually where you get the most treble), whereas donuts/lenses assume the entire speaker emits the highs, and that it is their constructive interference that causes both the beam and the trebliness of close micing a speaker on axis. I have not fully verified all the physics yet, but so far my understanding is that this later explanation is correct, at least in the context of guitar frequency ranges.
The consequence of that is that beam blockers usually/likely add a comb filtering effect from what I've read, and will still develop some beaminess in the far field despite removing it in the near field. The donuts/lenses should be effective in both the near and far fields. Anecdotally, my experience confirms the donut behavior. A previously painful-on-axis 4x12 was almost completely evened out by the addition of donuts.
If anyone has good pointers for honest to goodness physics books on speaker drivers and speaker cabinets, it would be much appreciated.
A "full range" speaker will send the lows in all directions but the highs mainly in the direction of its axis. A listener caught in the beam will hear a shrill sound, whereas someone off axis hears it muffled. Guitar speakers are like this; particularly the 12" ones and particularly in the 4x12 cabinet arrangement. Sometimes musicians use dispersing devices mounted on the speakers, like "beam blockers". Or the speaker is picked up by a microphone close to it, so that the audience hears it fro the PA system (which solves the sound dispersion problems in its own way).
There are situations in which it is desirable for a speaker box to "beam", like when it is mounted far away or high above a target listening area that is relatively small, calling for the speaker to be a kind of spotlight.