Knowles FED-23732-I34

In audio design, it is important to have as much control over the response as possible. The transducer environment provides several methods for achieving this control. Other Knowles application notes discuss the effects of coupling tubes and source impedance on receiver response, demonstrating how changing the acoustic termination alters frequency response characteristics. This application note describes similar results produced by modifying the microphone inlet, where changes to the inlet path alter the microphone response. The test setup cannot fully replicate real-world conditions, but the results provide insight into what occurs when the dimensions of the sound inlet path are varied. Knowles offers simulation models for microphones (and receivers) to customers who wish to predict the impact of the acoustic path on acoustic performance. As the inlet path becomes longer, the frequency of the response peak shifts downward; changes in inlet tube length not only cause the resonant frequency to shift, but also produce amplitude variations. The outer diameter of the inlet tube generally affects the amplitude of the high-frequency response. When both tube length and inner diameter are changed simultaneously, an increase in the ratio of tube length to cross-sectional area shifts the response peak frequency downward and reduces high-frequency sensitivity; a decrease in this ratio progressively diminishes the effect on the response. Inserting a filter or other acoustic resistance into the microphone inlet path reduces high-frequency sensitivity, and the farther the filter is from the microphone, the greater the damping effect. By modifying the sound inlet path and employing filters, tubes, and similar elements, the response characteristics of a transducer can be altered.