Franzis HT7550 Bedienungsanleitung Seite 23

e.g. the scratching of a fingernail on a sheet of paper, will also
be clearly audible.
If you carry around the device in your flat, you will be amazed
at the many places where ultrasound is emitted. Energy saving
lamps are powerful ultrasonic sources. The built­in coil of the
electronic ballast usually generates ultrasound within the 30 to
50 kHz range. The noise is often modulated by a buzzing that
equals the supply frequency.
Many other mains operated devices contain switching power
supplies with frequencies in the reception range of the ultra­
sound detector. The built­in coils or transformers often gen­
erate ultrasonic sounds that the device can turn into audible
sounds. When tuning across the range, there is one point on
the scale where you will hear a deep sound that will get higher
in both directions. The frequency of this superposition tone
equals the difference between the signal frequency and the set
oscillator frequency.
Beat tones
CRT monitors and TV sets emit particularly stable ultrasonic
sounds. You can hear the line frequency or their overtones. A
traditional CRT TV set works at 15.625 kHz and a high­resolu­
tion PC monitor at approximately 60 kHz. Using the frequency
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