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Fig. 6.14 Internal process of the Bathcratch system
rubbing directions, and pressure values. However, these sounds have a fundamental
frequency (F0) and specific harmonic structures, as shown in Fig. 6.15 . This spectro-
gram shows the harmonic structure and its continuous characteristic. We confirmed
that the same characteristic exist for various bathtub squeaking sounds. The range of
F0 is 100-600 Hz.
In addition to squeaks, taps and knocks on a bathtub also produce solid vibra-
tions, although they do not have the same characteristics as squeaks; they have short
durations and do not have a distinct harmonic component, as shown in Fig. 6.16 . The
other sounds with harmonic components in a bathroom are human voices. However,
we confirmed that a contact microphone attached to a bathtub filled with water will
not detect a human voice. Thus, in order to isolate squeaks, the system must identify
signals with a certain continuous harmonic structure and amplitude. However, the
current system does not detect a continuous harmonic structure accurately. There-
fore, the external object sigmund in the Max/MSP software environment is used to
estimate F0 instead.
6.4.2.2
Using TubTouch and Projection Display on Bathtub
Bathcratch partially utilizes the TubTouch system, which enables switching of
rhythm tracks, scratching phrases, and sounds while playing. The embedded contact
microphone and capacitive touch sensors are invisible and no changes are made to
the surface of the bathtub, as mentioned above. Instead, a video projector installed
above the tub projects virtual interactive objects over the touch sensors and indicates
a designated rubbing area near the microphone (see Fig. 6.12 ). Note that the contact
microphone (piezo sensor) can be installed anywhere on the edge of the bathtub as
the solid vibrations are conducted quite well through the bathtub. The designated
rubbing area is only intended as a visual aid to prompt the user to rub the bathtub.
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