So why does this happen? Part of the reason is the room but the larges, single-most contributing factor is the ability (or lack thereof) of a speaker to widely disperse the sound it emits. There are a few physical issues that designers typically compromise upon to create a loudspeaker.
The first issue is asking a driver to reproduce a frequency that is higher than is should. It is not the fact that the speaker driver CAN produce uniform and accurate sounds above a certain point, it is the fact that it SHOULD NOT to maintain wide dispersion. Let us take into consideration a two inch extended-range driver, common to many systems today and see what I am talking about.
The physics involved are not that complicated: as the frequency rises, the pistons (whatever material the speaker driver uses to push against the air and make sound) start misbehaving (dispersion goes down and distortion goes up). When the physical length of the note (wavelength) begins to approach the physical diameter of the piston, dispersion diminishes and beaming occurs.
|As the Wavelength Gets Smaller, Dispersion Gets Smaller|
So a 2" diameter piston can produce frequencies up to 4,230Hz and deliver wide dispersion (not beam). As the frequency rises, the wavelength gets shorter so when the wavelength of the sound equals the physical diameter of the piston, severe beaming occurs. This means that a wavelength of 2" or 6,768Hz, beaming is pretty severe. Sounds may still be reproduced at a quality, uniform level, but the wide dispersion of sound at lower frequencies is all but gone, hence the sweet spot.
The crossover frequency chosen by the manufacturer determines how well the sound from the speaker will or will not disperse and how small of a sweet spot there will be.
There are some tricks that can improve the dispersion pattern, the most widely used is the dome-shaped piston (aka dome tweeter or dome midrange). Ring tweeters attempt to overcome this same issue by adding phase plugs that normalize the phase distortions produced from big pistons trying to produce high frequencies.
So what does this sound like? Good dispersing speakers sound very good at places other than the sweet spot and poor dispersing speakers only sound their best at the sweet spot. This phenomenon has nothing to do with how accurate a speaker is, just how good it sounds in other places.
So if you want to widen your sweet spot, you must find speakers that disperse well and sound accurate enough to your personal tastes.