While in most cases Knowles microphones are used for receiving audible signals, several models may also be used in ultrasonic applications. Knowles production line testing typically is performed at several frequencies up to 10 kHz. Therefore the microphone specifications are written with performance data up to 10 kHz, but not beyond. While this is sufficient for ensuring the quality of the outgoing microphone, it does not mean that the microphone is limited to 10 kHz in practice. In this presentation typical performance data up to 100 kHz is given for the microphone models most commonly used in ultrasonic devices, different microphones are recommended for specific applications.