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# The relative change in length is within a maximum

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Unformatted text preview: thin 1/4 to 1/6 of the sound wavelength, the waves are reflected diffusely, whereas smooth surfaces have a maximum angle of approx. ±5° to the sound cone, roughly structured substances, e.g. bulk goods, can be detected up to an angle of approx. ±45°. The wavelength λ equals: λ= v ν λ = Wavelength ν = Frequency v = Speed of sound Example At a frequency of 200 kHz and a speed of sound in air of approx. 340 m/s, the following value is obtained in respect of the wavelength m 340 v s λ= = = 1.7 ⋅ 10 −3 m = 1.7 mm ν 200 ⋅ 10 3 Hz 178 © Festo Didactic GmbH & Co. KG • FP 1110 11. Physical fundamentals 11.3.1 Generation of ultrasound There are three different methods of generating ultrasound: mechanical, magnetic and electrical. In this context, mechanical generation of ultrasound is only of minor importance. Magnetic generation With the help of magnetorestriction, it is possible to generate ultrasound of up to approx. 50 kHz. Ferromagnetic substances change their length in a magnetic field. The relative change in length is within a maximum range of 4 · 10-5. -6 x 10 40 l/l 1 2 20 10 0 8...
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