The best summary of drone sounds I’ve ever heard (in words/type) was given by Shawn Husk in this thread on the bobdunsire.com forum:
http://forums.bobdunsire.com/forums/showthread.php?t=122859
Henderson type – generally a big sounding pipe; big bold bass, bright ringing tenors – the two sounds do not blend seamlessly but are typically stand alone, although they do compliment each other well. Slightly to mostly a bass dominant sound.
Robertson type – another big sounding pipe; this is very much like the Henderson type in every way save for the bass is less dominant and the bass and tenors blend a bit more seamlessly.
Lawrie type – big sounding pipe; very warm round bass sound, tenors are mild and very integrated. Overall a seamless bass dominant sound.
Naill type – generally a big sounding pipe with big bright tenors and a balanced bass. Overall a slightly tenor dominant sound. This type has a very big harmonic presence coming from the tenors.
Glen type – generally a more mellow sounding pipe; milder bass, bright ringing tenors – the two sounds tend to blend very well with the end result being a tenor dominant sound.
MacDougall type – generally a mellower sounding pipe; warm bass and very well integrated tenors – this type of sound is completely seamless, you can not hear the bass from the tenors but rather they blend completely together to form one solid tone. The harmonic character of this type of pipe tends to give the impression that the sound is coming from everywhere at once, it’s a much less focused sound than say the Glen or Henderson types; this results in some antitypical harmonics which are quite pleasing.