science

Mains hum

Mains hum is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mains hum rather than just read about it. In short: Mains hum, electric hum, cycle hum, or power line hum is a sound associated with alternating current which is twice the frequency of the mains electricity. The fundamental frequency of this sound is usually double that of the local power-line frequency: that is to say, 100 Hz in areas with 50 Hz power, and 120 Hz in areas with 60 Hz power.

Key takeaways

Mains hum belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mains hum to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mains hum from memory before moving on to harder problems.

Reference excerpt

Mains hum, electric hum, cycle hum, or power line hum is a sound associated with alternating current which is twice the frequency of the mains electricity. The fundamental frequency of this sound is usually double that of the local power-line frequency: that is to say, 100 Hz in areas with 50 Hz power, and 120 Hz in areas with 60 Hz power. The sound often has heavy harmonic content above 50/60 Hz. Due to the presence of mains current in mains-powered audio equipment as well as ubiquitous AC electromagnetic fields from nearby appliances and wiring, 50/60 Hz electrical noise can get into audio systems, and is heard as mains hum from their speakers. Mains hum may also be heard coming from powerful electric power grid equipment such as utility transformers, caused by mechanical vibrations induced by magnetostriction in magnetic cores. Onboard aircraft (or spacecraft) the frequency heard is often higher pitched, due to the use of 400 Hz AC power in these settings because 400 Hz transformers are much smaller and lighter.

Causes Electric hum around transformers is caused by stray magnetic fields causing the enclosure and accessories to vibrate. Magnetostriction is a second source of vibration, in which the core iron changes shape minutely when exposed to magnetic fields. The intensity of the fields, and thus the "hum" intensity, is a function of the applied voltage. Due to the magnetic flux density being strongest twice every electrical cycle, the fundamental "hum" frequency will be twice the electrical frequency. Additional harmonics above 100/120 Hz will be caused by the non-linear behavior of most common magnetic materials. Around high-voltage power lines, hum may be produced by corona discharge. In the realm of sound reinforcement (as in public address systems and loudspeakers), electric hum is often caused by induction. This hum is generated by oscillating electric currents induced in sensitive (high gain or high impedance) audio circuitry by the alternating electromagnetic fields emanating from nearby mains-powered devices like power transformers. The audible aspect of this sort of electric hum is produced by amplifiers and loudspeakers (note that this is not to be confused with acoustic feedback). The other major source of hum in audio equipment is shared impedances; when a heavy current is flowing through a conductor (a ground trace) that a small-signal device is also connected to. All practical conductors will have a finite, if small, resistance, and the small resistance present means that devices using different points on the conductor as a ground reference will be at slightly different potentials. This hum is usually at the second harmonic of the power line frequency (100 Hz or 120 Hz), since the heavy ground currents are from AC to DC power supplies that rectify the mains waveform. (See also ground loop.) In vacuum tube equipment, one potential source of hum is current leakage between the heaters and cathodes of the tubes. Another source is direct emission of electrons from the heater, or magnetic fields produced by the heater. Tubes for critical applications may have the heater circuit powered by direct current to prevent this source of hum. Leakage of analogue video signals can give rise to hum sounding very similar to mains hum.

Prevention It is often the case that electric hum at a venue is picked up via a ground loop. In this situation, an amplifier and a mixing desk are typically at some distance from one another. The chassis of each item is grounded via the mains earth pin, and is also connected along a different pathway via the conductor of a shielded cable. As these two pathways do not run alongside each other, an electrical circuit in the shape of a loop is formed. The same situation occurs between musical instrument amplifiers on stage and the mixing desk. To fix this, stage equipment often has a "ground lift" switch which breaks the loop. Another solution is to connect the source and destination through a 1:1 isolation transformer, called variously audio humbucker or iso coil. An extremely deadly option is to break contact with the ground wire by using an AC ground lift adapter or by breaking the earth pin off the power plug used at the mixing deck. Depending on the design and layout of the audio equipment, lethal voltages between the (now isolated) ground at the mixing desk and earth ground can then develop. Any contact between the AC line live terminals and the equipment chassis will energize all the cable shields and interconnected equipment.

Humbucking Humbucking is a technique of introducing a small amount of line-frequency signal so as to cancel any hum introduced, or otherwise arrange to electrically cancel the effect of induced line frequency hum. Humbucking is a process in which "hum" that is causing objectionable artifacts, generally in audio or video systems, is reduced. In a humbucker electric guitar pickup or microphone, two coils are used instead of one; they are arranged in opposing polarity so that AC hum induced in the two coils will cancel, while still giving a signal for the movement of the guitar strings or diaphragm. In certain vacuum-tube radio receivers, a winding on the dynamic speaker field coil was connected in series with the power supply to help cancel any residual hum. Some other common applications of this process are:

Humbucking transformers or coils used in video systems. Telephone (and other audio) system and computer communications wiring.

Consequences

In music In musical instruments, hum is usually treated as a nuisance, and various electrical modifications are made to eliminate it. For instance, humbucker pickups on electric guitars are designed to reduce the hum. Sometimes hum is used creatively, for example in dub and glitch music.

John Lennon demos

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Mains hum

Start with the simplest possible case. Write down what Mains hum claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mains hum before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mains hum ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mains hum

In research: Mains hum appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mains hum in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mains hum is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical phenomena, Noise, Sounds by type, so understanding it makes those chapters shorter.
In everyday life: Look for Mains hum outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Mains hum” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Mains hum in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mains hum means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mains hum out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mains hum in simple terms?

Why does Mains hum matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mains hum?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mains hum.