According to the Japanese dictionary, AC (Alternating Current) is "an electric current whose direction changes at regular intervals," with household AC 100V power being a typical example. DC (Direct Current) is "an electric current that flows in only one direction," with dry cell batteries being a typical example. As you all know, if we roughly classify AC signals as constantly changing signals and DC signals as constant signals that do not change, then when observing the waveform of a sensor that extracts physical quantities as voltage signals, paying attention to whether the signal is AC or DC, and which aspect to focus on during measurement, will allow for appropriate measurements. So, in vibration and noise countermeasures, frequency analysis is performed on acceleration signals and microphone signals, but are those signals AC or DC? Sound is a pressure fluctuation that humans can perceive, and in air, it is a compression wave (longitudinal wave). The audible frequency range is approximately 20Hz to 20kHz, and when considered as a signal, it is a constantly changing signal. Also, if the train's speed is constant, the passengers' bodies won't sway, but if the train sways or starts and stops, acceleration occurs, causing the body to sway and stumble. A change in speed is acceleration. Since both acceleration and sound are constantly changing physical phenomena, they can be called AC signals. What about speedometer and pressure signals? These are voltage signals proportional to a constant speed or static pressure. If there is no change in speed or pressure, it is a constant signal, so it is a DC signal; if it is constantly changing, it is an AC signal. So what about signals when there is a slight change even if the speed or pressure is constant? It is a signal that is a mix of the constant component that does not change and the minute component that is changing, that is, a mix of DC and AC. Generally, measuring instruments can switch the input coupling to AC or DC depending on the signal. In other words, if you want to see a state where DC and AC are mixed, you use the DC setting, and if you want to focus only on the changing part, you use the AC setting.
For the reason why, please see the next column.
(Excerpt from the email newsletter issued on November 28, 2001)
