What causes the audible hum from some electronics and wires? How is the actual sound being generated?
In: Physics
The electricity that comes out of a battery is called direct current. That means electricity comes out of one end of the battery and goes into the other end of the battery. It constantly moves in one directions.
The electricity that travels over the power lines in your city and comes out of your wall sockets is alternating current. The electricity sort of “sloshes” back and forth inside the wires. Depending on where you live, it’s going to “slosh” 50 or 60 times per second, or 50 or 60 hertz.
As the electricity wiggles back and forth at whatever frequency, it may make a part of a device physically move. The most obvious example of this would be a speaker or guitar amp. As you’re moving wires or cables around, you’ll sometimes get a characteristic low buzz out of a speaker. This is the speaker turning the frequency of the mains electricity into sound. Of course, a speaker is really good at making sound, because that’s its job, but sometimes other gadgets physically wiggle in response to the electricity running through them. They’re going to physically wiggle at the same frequency as that electricity, so you’re going to hear that same characteristic hum.
The metal core of transformers shrink and expand slightly under magnetic fields and therefore generate sound like tiny, kinda crappy, speakers at the same frequency AC power is fed into them. Additionally magnetic fields leaking from the transformers also tugs on metal parts around it like the case and turns those into little speakers as well. For non switching power supplies this gets you the hum you hear from some household electronics and big power line transformers.
Switching power supplies like the ones that run laptops use electronics to run the transformer at higher frequency, usually higher than the pitch you can hear, though sometimes (usually under low load with lesser quality power supplies) the switching frequency drops low enough that you hear it as a high pitched sound.
The frequency of the electric field causes magnetism, which in turn causes metal to vibrate, creating the sound.
I may have missed it in the other replies, but the phenomenon causing hum is referred to as magnetostriction.
The electricity supplied to your home switches direction 50 or 60 times a second, usually depending on your country. It’s as if you had a battery as a power supply but you could switch the positive and negative connections back and forth that fast. It’s called “alternating current,” or AC, and the rate is the “mains frequency.” In contrast, the battery alone supplies “direct current,” or DC.
The reason for using alternating current is that a simple magnetic device called a transformer can be used to create higher or lower voltages, depending on how it is wired. You cannot use a transformer to change voltage with DC.
It turns out that if you want to send electricity through wires over long distances, it is more efficient to do so at very high voltages. This is why power lines are generally strung high across tall towers, to keep the wires far from one another and from the ground or towers. On the sending end, the voltage from the generator in a power plant is boosted by a transformer, sometimes up to hundreds of thousands of volts for very long distance runs. On the receiving end, the voltage is lowered using another transformer effectively wired in reverse. The voltage will be lowered even more, in stages along the way, by more transformers until it gets to your home. Search on “electric power distribution” for more on that.
So why the hum? If you’re talking about a hum from an appliance like a microwave, it’s generally due to mechanical vibration from a transformer, because the magnetic field in the transformer is alternating and can be quite strong. The metal core or coils of wire inside it can vibrate or it might be the field interacting with other metal around it.
If it’s from a motor, that’s because most household appliance motors also exploit the AC, and their RPM is a multiple of the mains frequency.
If it’s from an audio device like an amplifier, there can be any number of sources. A common one is the electric and magnetic fields that permeate our homes due to appliances and house wiring. These fields radiate across instruments, cords, cabling and create tiny but annoying AC voltages, which are amplified along with the music.
Sorry this got kind of long, but I hope it helps.
P.S. Spooky thing here. I’m in the US, where the mains frequency is 60 Hz, and when I hear 50 Hz hum, such as in Europe, it sounds kind of growly and scary. I was thinking of posing the question of how “the other” frequency sounds to people and coincidentally found this thread–totally stumbled on it, not as a result of a search.
Experienced power systems engineer here– these answers crack me up. Without more specifics, what you’re likely referring to is the low pitched hum of transformers. The voltage on the power system alternates at 50 or 60 Hz depending on the region. What this translates to is magnetic flux causing the core of the transformer to expand and contract at double the power system frequency, so 100 or 120 Hz.
There are also other types of hums you may be referring to, including grounding issues with speakers or the high pitched squeal of certain types of power supplies, and I can address those if you have more specifics.
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