A narrow-band interference filter is used as optical filter, which only transmits a specific electromagnetic
wavelength. The pulse repetition rate is usually of the order of 10-20 Hz. This is
a compromise between the stronger signal with increasing u and the decrease of Q for
most microphones below 10 Hz. In contrast to other methods, for the PAD a closed
chamber is used to measure the acoustic pressure which can be realised with a gas
permeable membrane at low cost.