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In acoustics, there are three different special definitions of impedance - as resistances that counteract the propagation of vibrations in a specific environment. The properties of the propagation medium , obstacles, transitions to other propagation media as well as objects, surfaces or areas with certain acoustic properties have an influence on the impedance .
The greater the difference between two materials (e.g. air, water) in their field impedance, the greater the proportion of sound energy that is reflected when sound waves hit an interface (e.g. from air on water) ; the other part is let through . For the example mentioned, the impedance of water is about 3000 times higher than that of air, which means that most of the sound energy is reflected. (This is why we can hear all the noises that arise in the water well underwater , but we hardly perceive any noises that come from the air.)
Acoustic flow impedance
The acoustic flow impedance Z A , also simply referred to as acoustic impedance, describes the resistance that is opposed to the propagation of sound in pipes . It results from the quotient of sound pressure and sound flow q :
Acoustic flow impedance, sound pressure and sound flow are described here as complex quantities that depend on the frequency and the phase angle φ:
If the sound pressure and sound flow are in phase, the acoustic flow impedance is a real quantity.