An instrument’s timbral category is not fixed but depends on playing technique

Evidence

Howard and Tyrrell note, though, “that the grouping of instruments into categories is somewhat generalized, since it may well be possible to produce sounds on these instruments where the acoustic output would fall into another timbral category based on the frequency domain [hearing modeling] descrip­ tion, for example by the use of an extended playing technique.” — David Howard, Jamie Angus, coustics and Psychoacoustics, 1541