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DisasterMan
estimate by Waetzmann that for a parabola having an opening diameter
of 3.2 meters and a depth of 0.8 meter the magnifi cation was about fi ve
times compared with unaided ears for whispers and less for lower notes.
The fi rst quantitative work published on sound concentrators was a
report by Olson and Wolff
3
in 1929 of their development of a combination
horn and refl ector. The theory behind this was that the amplifi cation
of a refl ector-type sound concentrator depends on wavelength of the
impinging sound being less than the dimensions of the refl ector. Hence
the low frequencies whose wavelengths are larger than the dimensions are
amplifi ed very little. But by building a horn on the parabolic refl ector, the
amplifi cation of the horn raised the low-frequency response. This design
worked fairly well and microphone concentrators of this type have been
used in Hollywood for recording motion pictures.
Obata and Yosida,
4
engineers of the Tokyo Imperial University's
Aeronautical Research Institute, made a study of acoustical properties
of some sound collectors for the aircraft sound locator in 1930. They
made measurements of the amplifi cation and directivity of two different
horns and two 200-centimeter diameter open-bowl parabolic refl ectors of
different focal distance.
Dreher
5
reported on the use of microphone concentrators in motion
picture production in this journal in 1931. Military searchlights with a
microphone at the focus were used in outdoor recording, and other types
of parabolic bowls were also used.
The developments of the National Broadcasting Company were
announced in 1931by Hanson,
6
chief engineer. Measurements of the
amplifi cation, directivity, and effect of microphone position on the axis
were reported on a design of an open-bowl parabolic refl ector built by NBC
engineers.
Engineers of the Aeronautical Research Institute, Sato, Sasao, Kubo,
and Nisiyama published several papers
7, 8
on the sound fi elds of parabolas
in 1932. Their measurements were performed on deep parabolic refl ectors
and hence the results are rather complicated looking. The measurements
were taken in the region beyond the focus, for the most part. These writers
did not explain these results but merely said, `The experiment was very
laborious and troublesome and therefore was carried out with only two
pitches of sound; C
2
and C
4
... For C
4
, the sound fi eld becomes very complex