Heinz Franke

Reconstructing two-dimensional acoustic object fields by use of digital phase conjugation of scanning laser vibrometry recordings

A scanning laser Doppler vibrometer is used to record two-dimensional ultrasound fields in air. The laser light of the vibrometer traverses the sound field to and from a rigid reflector and determines the velocity field, a quantity proportional to the sound pressure rate, in each scanned point relative to the sound source. The object sound is the scattered field from objects outside the recording area. Digital reconstruction by use of phase conjugation (time reversal) of the object sound field is then performed, and the original object field intensity and phase is reconstructed.

5E-4 Refracto-Vibrometry for Visualizing Ultrasound in Gases, Fluids and Condensed Matter

A novel non-contact method for measuring and visualizing the propagation of the in fact invisible ultrasound in transparent gases, liquids and condensed matter is presented. This method is based on a scanning laser-Doppler vibrometer.

Acoustic sensors for analyzing binary gas mixtures

This paper concerns the development of robust, directly acting, acoustic gas sensors, which can be used for analyzing two-component gas mixtures even under harsh measuring conditions, such as high temperatures up to 300degC; aggressive,explosive, or toxic pollutions in the gas mixture; dust; electromagnetic disturbances; nuclear radiation; or very fast concentration changes of the mixture components. These characteristics result from the predominantly mechanical structure of the sensors. An important application of the robust sensor is the in situ measurement of humidity in the hot and contaminated exhaust air of industrial driers

Laser-scanning vibrometry for the investigation of sound generation and propagation

The paper describes how with one identical method the surface of a vibrating structure as well as the emitted acoustic sound field in air as well as the internal mechanical stress σ(t) in the vibrating structure can be investigated and visualized. This unique capability opens new possibilities for improving and optimizing the development of effective long-term stable sound transducers with optimized emission characteristics. Furthermore, the method can be used for investigating the internal stress of models of dynamically stressed mechanical structures. The simultaneous visualization of vibrating surfaces, internal stress σ(t) and emitted sound contributes to a better understanding of the behavior of complex dynamically excited structures.

Scattered ultrasound fields measured by scanning laser vibrometry

A scanning laser Doppler vibrometer is used to make quantitative measurements of 2D ultrasound fields in air. The laser light traverses the measurement volume to and from a rigid reflector and determines the velocity of the change in optical path length, which with constant geometry only depends on the changes in index of refraction. Assuming adiabatic conditions, the refractive index rate is proportional to the sound pressure rate and quantitative measures of the sound field are possible to achieve. The emitted or scattered ultrasound being measured origins from a source or object outside the recording area. Using phase conjugation the sound field is then digitally reconstructed outside the recording area, and the reconstructed phase and intensity reveals the location of the source or object. The combination of several such reconstructions of ultrasound fields of different wavelengths, so called wavelength scanning, provides an intensity map that very accurately gives the position of the source. This opens many new possibilities to study hidden or unknown sound sources or scattering objects.

Vibrations of thin-walled tubes and organ flue pipes

Sound radiation from vibrating thin walled tubes and from organ flue pipes is investigated. Measurements with a laser scanning vibrometer on this object show some new interesting results, e.g. the basic azimuthal vibration mode of thin walled tubes and organ pipes is of a stable quadrupole type. Typical vibrometer scans of thin walled tubes, metallic and wooden organ pipes are presented and interpreted with respect to sound emission and design of organ pipes. The application of the results to the domains of organ building and noise reduction seems to be useful.

Robust acoustic humidity sensor for industrial drying

Most conventional humidity sensors are unfortunately sensitive to extreme process and environmental conditions. This paper concerns the development of a robust acoustic humidity sensor which can be used even under extreme conditions, such as high temperature up to 300/spl deg/C, aggressive, explosive or toxic pollutions in the gas to be analysed, dust, electromagnetic disturbances, nuclear radiation or very fast changes of the humidity concentration. An important application of the sensor is the in-situ measurement of humidity in the exhaust air of industrial driers for paperboard production.

Ultrasound object fields in air reconstructed using digital phase conjugation

A scanning laser Doppler vibrometer is used to record 2D ultrasound fields in air. The laser light of the vibrometer traverses the sound field to and from a rigid reflector and determines the velocity field, a quantity proportional to the sound pressure rate, in each scanned point relative to the sound source. The object sound is the scattered field from objects outside the recording area. Digital reconstruction using phase conjugation (time reversal) of the object sound field is then performed and the original object field intensity and phase is reconstructed.