Role of Resonator in Instrument Families

Percussion:

The vibrator determines all or almost all of the properties of the sound and is responsible for coupling the sound to air. Any resonator plays a relatively small role.

String:

Most of the properties of the sound are determined by the vibrator (the string) and the way that the string is excited (plucked, hammered, bowed). The resonator couples the sound energy to the air. It is designed to reproduce the sound faithfully, not to play a major role in the sound quality.

Woodwinds:

The vibrator supplies energy but no pitch and plays only a moderate role in determining the sound quality. The resonator determines the pitch and plays a crucial role in determining the sound color as well as coupling the sound energy to the air.

Air Jets and Edges

When air flows slowly through a narrow opening (a jet) then it makes no sound. We say that the flow is Laminar (in thin sheets).

When air flows somewhat faster the flow becomes Turbulent and starts to make a hissing sound. It starts to create white noise.

If the air jet is allowed to fall on a narrow edge then the turbulence increases. Under many conditions of jet speed and distance from the edge you hear a tone emerging. It is usually somewhat noisy.

Edge Tones

The pitch of this edge tone depends on the separation between the jet and the edge and on the air speed in the jet. Larger separations generally producing lower pitches and higher air speeds producing higher pitches.

The edge tone is believed to arise from a feedback mechanism whereby the air striking the edge generates pressure variations back at the jet that make the air stream wobble up and down.

The pitch of an edge tone is high and not well controlled. Typical jets produce kHz frequencies. The pitch varies under the influence of small external forces and is not musically useful.

Resonator Coupled Edge Tones

If we place a resonant chamber near the edge/jet assembly then the jet and the chamber will interact.

As the jet wobbles around the pressure at the opening of the chamber will vary. If that variation has a component at a resonant frequency of the chamber then the pressure variation will be magnified by the resonance.

The higher pressure variations at the chamber resonant frequency will couple to the jet and induce it to wobble more at that frequency.

The increase in jet wobble will increase the strength of the resonance which will increase the jet wobble which will…….

The result is to lock the jet frequency to the resonant frequencies of the chamber.