Novice question: What are musical instruments - dipole or monopole? Does it vary by instrument type? (wind / string / percussion)
There is an ASR thread somewhere containing a link to a site featuring recordings of various instruments using mikes at different locations (heights and distances) around the instruments. Worth hunting down, probably.
Different instruments are going to have different radiation patterns, and the same instrument will show different patterns depending upon the location of the listener. In one study (also linked in an ASR thread somewhere) a kettle drum is shown to sound subjectively omnidirectional in the farfield, however up close the drum's perceived sound pattern is more directional.
That said, how any single instrument projects sound is not necessarily important to a loudspeaker in a home environment. I may have interpreted him wrong, but in the above post linking to his Website, S. Linkwitz starts with the proposition that the ideal reproducer is a single sphere, radiating omni throughout the listening environment, which is, of course, unrealized in practice, and other factors come in to play that necessarily delimit this sort of 'ideal' radiation pattern:
"The ideal monopole is omni-directional at all frequencies. Very few speaker designs on the consumer market approach this behavior. This type of speaker illuminates the listening room uniformly and the perceived sound is strongly influenced by the room's acoustic signature. The result can be quite pleasing, though, because a great deal of acoustic averaging of the sound radiated into every direction takes place. The speakers tend to disappear completely in the wide sound field. Unfortunately, the direct sound is maximally masked by the room sound and precise imaging is lost, unless the listening position is close to the speakers."