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FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 8 4 9 depends on the diameter, wall thickness, material, and other factors. When the electrical frequency of the induction coil is sufficiently close to the tube’s own resonant frequency, then high amplitude vibration and excessive audible noise may occur. In other words, as a radio receiver transforms electromagnetic waves into an audible sound, somewhat similar mechanisms can occur that cause a tube or pipe to act as an amplifier when dealing with certain frequencies. • A decision as to whether noise could be reduced by choosing a different (higher or lower) frequency de- pends on a combination of the structural self-resonant frequency (SRF) of a particular tubular workpiece and the applied electrical frequency. Therefore, in cases like this it is beneficial to measure the workpiece SRF by simply hitting it with a hammer and measuring the resonant audible noise with some kind of audible re- ceiver with the ability to detect the frequency and am- plitude of the measured signal. As a result, more intel- ligent decisions can be made in determining whether a certain frequency would improve the noise level or not compared to a frequency that produces unacceptable noise. For example, if the SRF of a certain workpiece is 300 Hz, then the further away the selected frequency is, the lower the noise that will be produced. In this case, 6 kHz would produce a noticeably lower noise compared to 1 kHz. In contrast, if the SRF is 5 kHz, then the use of 6 kHz will make it worse compared to 500 Hz. Many times, a protective or reducing atmo- sphere is used while heating metallic materials. There- fore, if your induction system will be located inside a gas-tight chamber/enclosure, this will substantially reduce the noise level. • Noise generated by vibration of power cables, buses, and fixtures . There is a very small probability of using the wrong design of fixtures, power cables, and bus bars. However, any concerns may be addressed by an induction heating expert. ~HTPro Note: More answers to commonly raised questions can be found in Ref. 1. Formore information: All arewelcome to send questions to Dr. Rudnev at rudnev@inductoheat.com. Selected questions will be answered in his columnwithout identifying the writer unless specific permission is granted. Reference 1. V. Rudnev, D. Loveless, and R. Cook, Handbook of In- duction Heating, 2nd Edition, CRC Press, 2017. 17