Source Filter Theory

Sources of sound exist all-around us. Sound sources are often accompanied with a filter, and changes in the shape of the filter, like changes in the slide of a trombone, has a strong impact on the emitted sound. We first describe two archetypes of filters, the Helmholtz resonator and the pipe resonator. The Helmholtz resonator consists of a larger cavity with a narrow opening, and the pipe resonator consists of a uniform tube.

We also describe an experimental approach that allows researchers to estimate the resonance characteristic of both types of filters. Three types of sound sources are used to test a resonator: a swept sine wave, a broadband noise, and an impulse. They can be played as an input to “excite” the resonator, and the output can be recorded. The ratio of the output over the input sound provides an image of the filter’s resonance characteristics. A computational approach permits researchers to numerically predict the resonance properties of the filter based on the geometrical dimensions of the filter. The computational approach provides a reasonably accurate prediction of the resonance characteristics of both types of filters. Finally, we apply these concepts to biological systems focusing on human speech production.

Keywords: / attenuate, Filter, Formant, Mode of vibration, Resonance, Source.