Oscillations, Waves, and Interactions - GWDG

Applied physics at the “Dritte” 5
Sound diffusion in small and large rooms, radio studios as well as concert halls, was
investigated by Rolf Thiele, whose results were made visible by a “Schalligel” (“sound
hedgehog”) giving the different sound intensities in different spatial directions [7].
Meyer and Thiele also introduced the concept of “Deutlichkeit” (definition), the
sound energy in the first 50 milliseconds of the impulse response [8].
In general, there was a world-wide interest to supplement reverberation time by
other physical parameters to characterise the acoustic quality of performance spaces.
One of these criteria, favoured in the early 1950s, was the so-called “frequencyirregularity”
of the sound transmission between source (on the stage) and a listener’s
ears. Experimental results obtained by Heinrich Kuttruff and Rolf Thiele [9] in the
Herkulessaal in Munich (and other concert halls) showed that the number of maxima
of the frequency response did not correspond to the number of normal modes
(resonances) – as posited by a faulty theory by Bolt – but was actually more than a
thousand times less. This astonishing result was explained by a statistical theory by
Manfred Schroeder, then a post-doc at the institute [10]. This theory showed that
the frequency response (sound pressure and phase as functions of frequency) is, approximately,
a complex Gaussian process. As a result, the average distance between
maxima above a critical frequency (“Schroeder frequency”) is fully determined by
the (reciprocal) reverberation time – thus not affording the much sought-after new
quality parameter.
Beside measurements in actual rooms, scale models were also studied. Following
Meyer’s inclination to consider sound waves and electromagnetic waves simultaneously,
microwave models were also included. Schroeder could show that the distribution
of the frequencies and excitations of the normal electromagnetic modes
in metallic cavities were highly irregular – even for very small deviations from the
symmetry of a perfectly rectangular space, such as a cube [11]. Thus, for all practical
purposes in room acoustics, the normal modes (resonances) can be considered
completely random.
Fundamental investigations, such as on the perception of “echoes”, were also conducted.
For this purpose, the attic of the “white house” on Bürgerstraße was converted
into a makeshift “anechoic” space. Later a large Reflexionsfreier Raum (“freespace
room”) was constructed, which – unique in the world – was also designed to be
nearly free from reflections for microwaves to facilitate free-space measurements with
electromagnetic waves [12]. Also a large reverberation room (“Hallraum”) was built
which, again, was reverberant for both sound waves and electromagnetic waves [13].
In 1963, Meyer and Kuttruff studied the reflective properties of the ceiling of
Philharmonic Hall in New York by means of a scale model leading to an explanation
of the observed low-frequency deficiency in the actual hall [14]. In the early 1970s, in
a large study of concert hall quality, Dieter Gottlob, Manfred Schroeder, and Karl-
Friedrich Siebrasse – on the basis of measurements in 22 concert halls in Europe –
showed that the lack of early lateral reflections in many modern halls with low ceilings
and wide (fan-shaped) ground plan was the main culprit [15]. To counteract this
deficiency, Schroeder conceived, after 1975, sound-diffusing structures (“reflection
phase gratings”) based on number-theoretic principles [16], which have found broad
acceptance in room acoustics. At the same time, Schroeder proposed a new method of
accurately measuring reverberation times by “backward integration” of the (squared)