You are not hearing the electricity itself, but the physical vibration of materials caused by the electrical current passing through them. This sound, often described as a hum or “coil whine,” is common near many electronic and electrical devices. Understanding the origin of this noise requires looking closely at how electrical energy translates into mechanical movement within specific components.
The Physical Mechanism Generating the Hum
The primary physical process responsible for the low-frequency hum is called magnetostriction. This is the property of certain ferromagnetic materials, like the iron cores in transformers and inductors, to change shape when exposed to a magnetic field. Since household electricity is Alternating Current (AC), the magnetic field it generates constantly fluctuates and reverses direction.
In a power system, the AC voltage creates a magnetic field that cyclically changes around the core material. This causes the core to physically expand and contract rapidly. The resulting mechanical movement is transferred to the surrounding air, creating audible sound waves perceived as a persistent hum.
The magnetic field’s intensity peaks twice during every cycle of the AC waveform. Because the core material changes shape at both the positive and negative peaks, the resulting mechanical vibration frequency is double the electrical frequency. This transformation from electrical energy to mechanical vibration allows the presence of power in a device to be heard.
Common Electrical Sources and Their Frequency Signature
The pitch of the electrical hum is a direct consequence of the frequency of the power grid it is connected to. In North America, the standard power frequency is 60 Hertz (Hz). Since the physical vibration occurs twice per electrical cycle, the fundamental audible hum frequency is 120 Hz.
In regions using a 50 Hz power standard, the resulting hum is lower pitched, vibrating at a fundamental frequency of 100 Hz. This frequency signature is characteristic of devices containing coils and magnetic cores, such as large utility transformers and power bricks used for charging electronics. Older electronics, including fluorescent light ballasts and CRT monitors, are also common culprits because they rely on magnetic fields to operate.
Some modern devices, like computer power supplies and compact power adapters, produce a higher-pitched sound called “coil whine.” This sound is caused by small inductors vibrating at much higher frequencies than the main power line. This higher frequency noise is still a form of magnetostriction, but it is generated by high-frequency switching circuits rather than the main power line frequency.
Factors Influencing Auditory Perception
The ability to hear the electrical hum relates to both the source’s loudness and the listener’s hearing sensitivity and environment. Young people often possess a broader hearing range, allowing them to perceive high-frequency coil whine that older adults cannot hear due to age-related hearing loss. This sound falls outside the typical audible range for many adults.
Environmental factors significantly impact perception. A quiet room removes competing sounds that would otherwise mask the hum, making the low-level mechanical vibrations from a power adapter much more noticeable. The sound is not amplified, but the signal-to-noise ratio shifts dramatically.
Some individuals experience heightened sensitivity to sound, known as hyperacusis, which can make normal environmental noise feel uncomfortably loud or annoying. This involves an abnormal increase in sensitivity within the central auditory system, causing external sounds to be perceived more intensely. Consequently, a quiet 120 Hz hum can become a source of significant distress.
When the Hum Signals a Potential Problem
While a soft, steady hum is normal for many devices that contain transformers, a sudden change in the sound can signal a developing safety issue. A louder or sharper buzzing, a crackling sound, or a noticeable change in pitch suggests that something is no longer operating as intended. This altered noise is not the typical magnetostriction hum but rather a sign of physical or electrical stress.
A loud buzz or sizzling often indicates loose connections or faulty wiring, which can cause electricity to arc or spark across a small gap. This arcing generates intense heat that can melt wire insulation and is a leading cause of electrical fires. A buzzing sound from an electrical panel or circuit breaker box is particularly concerning, as it may signal an overloaded circuit or malfunctioning equipment.
Overheating is another cause of problematic noise; when components are under excessive load, the increased electrical resistance generates heat. This heat can cause materials to expand or warp, leading to different, louder vibrations or a burning odor. If the hum is accompanied by a burning smell, scorch marks, or a device that is hot to the touch, the device should be immediately unplugged and a licensed electrician should be contacted for an inspection.