Oscillations, Waves, and Interactions - GWDG

Applied physics at the “Dritte” 9
Sound absorption and sound velocity in water, aqueous solutions of electrolytes,
and other liquids were studied in wide frequency and temperature ranges (see Section
10). Also, the influence of gas bubbles on the acoustic properties of liquids
was investigated [58,59], further leading to intensive research on single bubble oscillations,
cavitation, and luminescence (see Section 7 of this article and the pertinent
contributions to this book [60,61]).
Erwin Meyer was always interested in the improvement of measurement techniques,
also in the field of underwater sound. In close analogy to the anechoic chambers for
air-borne sound, for which Meyer had invented the lining by wedges of sound absorbing
porous material [62], previous experiments [63] eventually culminated in the
installation of a 100-m 3 water tank with rib-type absorbers [64], providing near freefield
conditions from 7 kHz through 70 kHz. Less costly impedance measurements
in resiliently lined water-filled tubes [65] and sound absorption or sound power measurements
in a resiliently lined reverberation tank [66] were also performed. As an
interesting result it was shown that the edge effect is much stronger for absorbers
on sound-soft than on rigid walls [67]. Impedance measurements at low frequencies
were performed with thick-walled “pressure chambers” [68]. A shallow water basin
with sound-soft bottom and absorbing perimeter allowed studies of two-dimensional
sound fields [69].
The scattering of underwater sound was investigated in our institute for more
than 30 years, starting 1965 with sound-soft objects [70], and since 1989 resonance
scattering with possible application to the detection and classification of objects
buried in marine sediments [71]. Motivated by analogous microwave systems, Peter
Wille constructed a slim directional hydrophone with low flow resistance (in analogy
to the microwave “rod radiator”) [72], and an aspect-independent sonar reflector,
based on inhomogeneous lenses [73].
Upon Erwin Meyer’s initiative, the international Journal Acustica was founded
in Rome 1950, to succeed the German Akustische Zeitschrift (1936–1944). Since
much unpublished work had accumulated in the meantime, the editors of Acustica
published Akustische Beihefte as supplements in which these papers were collected,
presenting, among others, the major results of the war-time work of Meyer’s group
on hydroacoustics. One such paper [74] describes the thin-layer two-circuit resonance
absorbers – combinations of a parallel and a series resonance circuit – originally made
of rubber with air holes in a central layer. With the availability of a great variety
of modern high polymers, it was attempted to improve the resonance absorbers by
searching for better suited materials. Since a material with a high loss factor of the
bulk modulus could not be found, polymers with lossy Young’s and shear moduli
were applied [75,76]. It was, however, soon recognised that the desired high loss
factor was inevitably related to a strong frequency and temperature dispersion of the
modulus because they are coupled by the Kramers-Kronig relations (e. g., [77]), which
made practical applications unrealistic. As an alternative, the absorption of sound by
constrained liquid flow was investigated, and various structures were designed [78,79].
With electrorheological fluids (ERF), absorbers have been developed the properties
of which can be electrically adjusted to account for a changing environment such as
hydrostatic pressure (i. e., depth of water) or temperature [80].
Inspired by the success of these absorbers, a two-circuit resonance absorber of