April 2024 Newsletter: Current Affairs

A buzzing florescent bulb. Crackling power lines. A humming computer graphics card. We can all recognize the distinctive sounds of electricity that surround us, but where does this noise actually come from? Read on to discover some of the ways electricity and acoustics intersect.

Electricity comes in two varieties – direct current and alternating current. Direct current always flows in the same direction, while alternating current oscillates between flow in opposite directions. The power generated at a power plant and transported over power lines, for example, utilizes alternating current. How many times a second it oscillates depends on where exactly you live, but in North America, the standard is 60 times a second, or 60 hertz (Hz).

A moving electric charge, like current in a wire, generates a magnetic field. In AC, the current is constantly changing, thus the magnetic field it generates is also constantly fluctuating. This fluctuating field can cause nearby metal objects to vibrate. The fundamental frequency of this vibration is twice the frequency of the alternating current, so for a 60 Hz current, the fundamental frequency is 120 Hz. Because the entire power grid is powered by 60 Hz alternating current, this pervasive vibration is often referred to as the mains hum.

Varying magnetic fields can cause objects to expand and contract by a phenomenon known as magnetostriction. Take the iron core of a transformer, for example; as the magnetic field of the coil increases, it pulls on the iron in all directions, causing it to expand. As it drops, the core returns to its normal size, only to expand again as current increases. These changes in dimension may be imperceptible to the human eye, but not the ear. They are particularly problematic in devices with large cores, like a transformer, but can also be heard in smaller devices like the magnetic ballasts used in older fluorescent light fixtures.

The windings of the coil itself can also expand and contract, and alternating current isn’t required either: a changing direct current can cause it. In computers, for example, rapidly changing computational demands can lead to rapid changes in current. And because the frequency of these changes isn’t tied to the 60 Hz alternating current of the main power grid, if the right frequency occurs it can resonate with the coil’s structure and amplify the vibrations. Because the coils in personal devices are small, this resonant frequency is often much higher than the mains hum, which results in a distinct noise called coil whine.

Another source of electrical sound is known as corona discharge. If a conductor is carrying a high enough voltage, it can cause a normally non-conductive fluid surrounding it, like air, to ionize and breakdown electrically, becoming conductive and allowing the electric charge to flow in a localized area around the conductor. Corona discharge often occurs around high voltage power lines, and results in a more broadband crackle or hiss in comparison to the mains hum or coil whine.

With all this in mind, hopefully next time a friend stops by to ask, “What’s the buzz?”, you can answer with confidence.

The New T. M. Pierce Elementary School
Eclipse Party!