Noted computer music researcher Perry Cook points out that there are almost no examples of actual musical instruments which change their resonance during the course of a note, one of the fundamental ways in which voices differ from instruments.

In other words, the shifting formant regions of a voice are quite unusual in the musical world. some of the few examples we can think of are the electric guitar wah-wah pedal, the trumpet with a plunger mute, the famous Leslie organ speaker (which rotates mechanically) and a certain kind of North Sumatran open-backed lute called a hasapi, which the player rotates against her stomach to produce different timbres. Naturally, the only picture we've got is of the obscure North Sumatran lute. This is the back of the instrument, with the whole for changing the resonance against the belly.

Can you think of some other instruments where the pitch is constant but where the resonance changes in time?

Figure .x Picture of back of a hasapi, a two-stringed open backed lute from the Batak of North Sumatra, Indonesia which is often used for trance music.