Mechanical impedance

Underimpedance all resistors are combined to counteract the propagation of vibrations in a given environment. Influences on the impedance of the propagation medium have the features, obstacles, transitions to other propagation media or other articles, surfaces or areas with certain acoustic characteristics.

In the field of acoustics 3 impedance definitions with different content are used:

  • The acoustic field impedance
  • The acoustic impedance flow
  • The mechanical impedance

Acoustic impedance field

Also called acoustic impedance, this impedance definition is used to describe the ( free ) sound field the resistance, which is opposite to the propagation of sound. The acoustic field impedance ZF is obtained from the ratio of sound pressure p and sound velocity v.

Acoustic field impedance, sound pressure and particle velocity are hereby described as complex quantities that depend on the frequency f. There are sound pressure and particle velocity are in phase, the acoustic field impedance is a real quantity.

In the free field acoustic impedance field is determined by the properties of the propagation medium:

( Zf: Sound wave resistance, ρ: density, c: speed of sound) See also: characteristic acoustic impedance.

Acoustic flow impedance

Often referred to as acoustic impedance. This definition impedance is used to describe for sound propagation in tubes the resistance, which is opposite to the sound propagation. The acoustic flow impedance Z is obtained from the ratio of sound pressure p and volume velocity q.

Flow acoustic impedance sound pressure and flow in this case are described as complex magnitudes φ depends on the frequency f and the phase angle. There are sound pressure and flow are in phase, the acoustic flow impedance is a real quantity.

Mechanical impedance

This impedance definition is used to describe mechanical vibrations such as speaker diaphragms, microphones, ossicles or mechanical filter resistance, which is opposite to the vibration propagation. The mechanical impedance ZM is obtained from the quotient of the force F and velocity v.

Mechanical impedance, force and velocity are in this case described as complex quantities which depend on the frequency f, and phase angle φ. There are power and speed in phase, the mechanical impedance is a real quantity.

General information about impedances

The impedance is a complex quantity and is composed of the Resistance (real part ) and reactance ( imaginary part) together.

The reciprocal of the impedance is referred to as admittance.

Materials having a large impedance difference, the most part of transmitted acoustic energy at the interface is reflected. With low impedance difference it comes to the transmission of the sound waves.

The field impedance is depending on the medium ( eg water, air ) are different, this will in impact on a boundary surface reflected ( eg from air to water), depending on the difference in impedance of the media is part of the sound energy and left a part.

For the above example, the impedance of water is about 3000 times higher than that of air, whereby most of the sound energy is reflected. ( Therefore, although we can hear all sounds good under water, resulting in water, but there practically hear outside sounds that come from the air. )

  • Acoustics
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