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ACOUSTICS

Sound and vibration are perceived by humans, sometimes with pleasure sometimes with anger. Music, speech or noise are audible sounds, carrying information and generating emotions. The ear listens to acoustic waves in a frequency range of central importance for human communication. But sound is not restricted to this small spectral window: Low frequent sound, e.g. generated by atmospheric turbulence, is not heard but obviously "felt" in some way, often causing indisposition. Sound of high frequency, ultrasound, penetrates into solids and human organs, and can be used as a tomographic tool for visualisation of internal structures.

Which physical laws govern the observed acoustic phenomena? Which physical and psychophysical models are capable of describing the observations?

 

SUBJECTS

The ear’s perception of sound is investigated in the scientific field of psychoacoustics. Disturbing sound, noise, is registered and analysed in environmental acoustics, and noise awareness increases the quality of life. The sense of sound is optimised by sound design. Acoustic virtual reality improves not only the aesthetics of music reproduction and enhances the acoustics of virtual events, but is also an important tool in the laboratory research in order to examine audible sound psychoacoustically.

Ultrasound propagates in solids and indicates the internal structure of matter after appropriate signal processing. Ultrasound serves as a remote-sensing tool for non-destructive testing in medical diagnosis and material science. The propagation of sound waves and the mutual interaction with matter are investigated in physical acoustics.

The field of acoustics governs more than the acceptable acoustics of a concert hall. Speech is sound, music is sound, vibrations in solids are sound, earthquakes are structure sounds of long wavelength. A large variety of medical, technical, physical, geophysical applications are covered by acoustics.

The subject of acoustics is found in many different scientific and technical disciplines, a considerable part of which are objectives of research in the acoustics group of Oldenburg University.

 

PSYCHOACOUSTIC

Physical properties of sound are linked to audible features in order to develop and improve ear-related measuring devices, e.g. for determining the loudness or tonality of a sound. These entities are used – among others – to estimate the annoyance of noise.

The relation of the audible event with the signal of the acoustic source gives important impacts for improving electro acoustics, audio coding (data reduction), sound design (of inevitable technical noise), room acoustics, and noise reduction.

Acoustic nightmare

About 80% of the German population complain about noise pollution from various sources, mainly transportation noise. Annoyance is in particular increased by combined effects from different sources. Noise has become a health problem and contributes as an additional stress factor to cardiac infarcts. The correlation of noise and annoyance is a subject of environmental acoustics in laboratory and field studies. The record of persons’ subjective evaluations plays an important role. Why and when is a noise annoying? What is the meaning of "annoyance"? Which acoustic and subjective parameters act together? It is necessary to collaborate with psychology and social science in order to answer such questions and to achieve sustainable community noise abatement.

 

PHYSICAL ACOUSTICS

Research of the onset and structure of turbulence is of basic importance for numerous technical and physical systems. The advantage of probing the fluid with sound waves lies in the practically impact-free remote sensing without disturbance of the motion of the fluid: The structure of movement of the atmospheric boundary layer is inferred from properties of sound propagation, allowing for remote measurement of profiles of wind, turbulence, and temperature. The findings help to optimize wind-energy turbines, support micrometeorological measurements, improve models of distribution of pollution. Sound rays are measured and evaluated in a similar way as is done with signals in computer tomography. Another method is to measure the scattering of sound at turbulent structures with an acoustic radar (so-called SODAR). A continuous profile of wind or turbulence with height can be assessed by SODAR, and a costly and clumsy meteorological mast is avoided (apart from the difficulties of erecting such a mast e.g. in the landing zone of an airport…)

Physicist, leaning against an air flow of 20 m/s in the
acoustic wind tunnel (at the left side an array of
directional micrphones).

A large anechoic chamber (cut-off frequency of 50 Hz, structure-born vibration of 3 Hz) is used for the development of acoustic measuring techniques. Flow noise phenomena, e.g. noise generated at turbine blades, are investigated in an extremely laminar and noiseless wind tunnel (50 m/s).

Noise abatement is carried out with "acoustic material". The micro-mechanical properties of such porous material determine the capability of sound absorption. A new device allows sound absorption to be calculated from the measured reflection coefficient of sound at grazing incidence. In particular it is possible to determine the acoustic properties of an unknown material by in situ measurements without taking probes from a wall or façade.

The same principle is used in a new device for underwater acoustics for the discovery of drowned liquid pollutants covering the sediment of the sea bottom. The acoustic sensor is combined with chemical and optical sensors for monitoring the marine environment. The sensors are mounted on a remote-operated underwater vehicle.

 

 

Physicist with dummy head in the anechoic chamber

VIBRATIONS

Research of the perception of vibration is of increasing importance for the transportation industry (automotive, airplane etc.). The perceived structure-born sound has a large impact on subjective comfort and the handling of machines. A vibration pad was developed together with the Institute for Technical and Applied Physics (ITAP). The pad allows for the simulation of real vibration signals, e.g. picked up from a vehicle in motion. Investigations are carried out on the threshold and just-noticeable-differences of the sensation of vibration.

Reproduction of multi-channel vibration and audio signals.

 

Contact:

Prof. Dr. Volker Mellert

Phone : 0441 - 798 3572

Fax:      0441 - 798 3698

E-mail: mellert@aku.physik.uni-oldenburg.de

URL:    http://www.physik.uni-oldenburg.de/Docs/aku

 

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Suggestions for improvements to:

hans.j.pade@uni-oldenburg.de

[an error occurred while processing this directive] 31 May. 2000