question: What makes the audible hum in electric boxes?
comment: The high current passing through the conductors causes them to vibrate. In large buildings they use metal plates as conductors rather than wires and they are taped together with plastic in between to absorb/dampen the vibrations.
passages: There are many sources of electromagnetic noise which cause the characteristic display patterns of static. Atmospheric sources of noise are the most ubiquitous, and include electromagnetic signals prompted by cosmic microwave background radiation, or more localized radio wave noise from nearby electronic devices.
  
  The display device itself is also a source of noise, due in part to thermal noise produced by the inner electronics. Most of this noise comes from the first transistor the antenna is attached to.
   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.
  
  Section::::Prevention.
   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.
   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:
  
  BULLET::::- Humbucking transformers or coils used in video systems.
  
  BULLET::::- Telephone (and other audio) system and computer communications wiring.
  
  Section::::Tone.
   Being magnetic devices, audio transformers are susceptible to external magnetic fields such as those generated by AC current-carrying conductors. "Hum" is a term commonly used to describe unwanted signals originating from the "mains" power supply (typically 50 or 60 Hz). Audio transformers used for low-level signals, such as those from microphones, often include magnetic shielding to protect against extraneous magnetically coupled signals.
   The sound can be that of any musical instrument, but the effect is most commonly associated with the guitar. The rich harmonics of an electric guitar are shaped by the mouth, producing a sound very similar to voice, effectively allowing the guitar to appear to "speak".
   BULLET::::5. Twisted pair wiring – Twisting wires in a circuit will reduce electromagnetic noise. Twisting the wires decreases the loop size in which a magnetic field can run through to produce a current between the wires. Small loops may exist between wires twisted together, but the magnetic field going through these loops induces a current flowing in opposite directions in alternate loops on each wire and so there is no net noise current.
   Console noise comes from magnetic components on or within the console. These include ferrite in cores in inductors and transformers, steel frames around LCDs, legs on IC chips and steel cases in disposable batteries. Some popular MIL spec connectors also have steel springs.
   A third application of baluns in audio systems is in the provision of balanced mains power to the equipment. The common-mode rejection of interference characteristic of balanced mains power, eliminates a wide range of noise coming from the wall plug, e.g. mains-borne interference from air conditioner/furnace/refrigerator motors, switching noise produced by fluorescent lighting and dimmer switches, digital noise from personal computers, and radio frequency signals picked up by the power lines/cords acting as antennae. This noise infiltrates the audio/video system through the power supplies and raises the noise floor of the entire system.
   The design of most of the products used sealed cases (no ventilation) and linear power supplies. This resulted in very long life, particularly when used in dirty environments. The sealed cases meant that the electronics were protected from dirt and corrosion. Products such as the exciters, audio failure detectors and radio links remain in regular use around the world, despite being 20–30 years old.
  
  Section::::Locations.
   Explosion-proof enclosure
  
  Explosion-proof enclosures are solid cabinets that contain different electrical components like switches, plugs, sockets, transformers, controls, and knobs to keep the surroundings safe from electrical hazards. Integrated with sound technology, these boxes are resistant to spark and shock and have a high tolerance to extreme temperatures. An ideal solution for hazardous locations; these explosion-proof enclosures keep any interior explosion from spreading to the external environment and damaging life and property.
  
  Section::::Types of explosion-proof enclosures.
   The frequency of the noise depends on the nature of electromagnetic forces (quadratic or linear function of electrical field or magnetic field) and on the frequency content of the electromagnetic field (in particular if a DC component is present or not).
  
  Section::::Electromagnetic noise and vibrations in electric machines.
   Section::::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. Because the magnetic flux density is strongest twice every electrical cycle, the fundamental "hum" frequency will be twice the electrical frequency. Additional harmonics above 100 Hz or 120 Hz will be caused by the non-linear behavior of most common magnetic materials.
   The Hum has been reported worldwide.
  
  Section::::Possible explanations.
  
  Section::::Possible explanations.:Mechanical devices.
  
  Although an obvious candidate, given the common description of the hum as sounding like a diesel engine, the majority of reported hums have not been traced to a specific mechanical source.
   Section::::Mechanical Description.:On the OLEV.
  
  Attached beneath the vehicle, are "pick-up" modules, or the secondary coils, that consist of wide W-shaped ferrite cores with wires wrapped around the center. When the pick-ups "pick up" the flux from the primary coils, each pick-up gains about 17 kW of power from the induced current. This power is sent to the electric motor and battery through a regulator (a managing device that can distribute power based on need), thereby charging the OLEV wirelessly.
  
  Section::::Models.
   BULLET::::- coils: in magnetic resonance imaging, "coil noise" is that part of total system noise attributed to the receiving coil, due to its non-zero temperature.
  
  Section::::Examples of device subject to electromagnetic noise and vibrations.:Rotating devices.
  
  Rotating devices include radial and axial flux rotating electric machines used for electrical to mechanical power conversion such as
  
  BULLET::::- induction motors
  
  BULLET::::- synchronous motors with permanent magnets or DC wound rotor
  
  BULLET::::- switched reluctance motors
   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 hum. Sometimes hum is used creatively, for example in dub and glitch music.
  
  Section::::Consequences.:In audio systems.
  
  Power line hum can be alleviated using a band-stop filter.
  
  Section::::Consequences.:In video systems.
   Post office boxes were common pieces of scientific apparatus in the UK O-Level and A-Level schools public examination physics syllabus in the 1960s.
  
  Section::::Construction.
  
  A typical post office box is in a wooden box with a hinged lid and a metal or bakelite panel showing circuit connections. Coils of wire are wound non-inductively, mounted in the body of the box, and have a negligible temperature coefficient.
  
  Pairs of ratio arms are each 10, 100, 1000 ohms. Resistance arms contains a number of coils from 1 to 5000 ohms with a plug for infinite resistance.
   Electric strikes are sometimes equipped with buzzers that allow someone outside the door to hear when the door is open. The buzzing noise is typically made by applying alternating current (AC) to the strike instead of direct current (DC). When using a DC powered strike, a buzzer accessory can be added to create the buzzing noise, if desired.
   Piezoelectric transducers are often used as contact microphones to amplify sound from acoustic musical instruments, to sense drum hits, for triggering electronic samples, and to record sound in challenging environments, such as underwater under high pressure. Saddle-mounted pickups on acoustic guitars are generally piezoelectric devices that contact the strings passing over the saddle. This type of microphone is different from magnetic coil pickups commonly visible on typical electric guitars, which use magnetic induction, rather than mechanical coupling, to pick up vibration.
  
  Section::::Varieties.:Fiber optic.
   Passengers who have flown on the Airbus A320 will frequently hear the PTU 'barking dog', generally when only one engine is running, or when the Yellow system electric motorpump is the only active hydraulic power source, the PTU is mechanically activated. Consequently, normally the PTU is only heard on start-up or shut down. Very rarely is it heard in flight unless a momentary power deficit is present when retracting the gear, or a hydraulic fault has occurred.
   where "v" is the speed of sound, "L" and "L" and "L" are the dimensions of the box. formula_20, formula_21, and formula_22 are nonnegative integers that cannot all be zero.
  
  If the small loudspeaker box is airtight the frequency low enough and the compression 
  
  is high enough, the sound pressure (decibel level) inside the box will be the same anywhere inside the box, this is hydraulic pressure.
  
  Section::::Resonance of a sphere of air (vented).
   Connections are made by pushing the cable's plug into the female jack on the device. The signal-carrying pin protrudes from the plug, and often comes into contact with the socket before the grounded rings meet, resulting in loud hum or buzz if the audio components do not share a common ground and are powered while making connections. Continuous noise can occur if the plug partially falls out of the jack, breaking the ground connection but not the signal. Some variants of the plug, especially cheaper versions, also give very poor grip and contact between the ground sheaths due to their lack of spring action.
   The signal passes through the tone and volume circuits to the output jack, and through a cable to an amplifier. The current induced is proportional to such factors as string density and the amount of movement over the pickups.
  
  Because of their natural inductive qualities, magnetic pickups tend to pick up ambient, usually unwanted electromagnetic interference or EMI. This mains hum results in a tone of 50 or 60 cycles per second depending on the powerline frequency of the local alternating current supply.
   In inductors, also called reactors or chokes, magnetic energy is stored in the airgap of the magnetic circuit, where large Maxwell forces apply. Resulting noise and vibrations depend on airgap material and magnetic circuit geometry .
  
  Section::::Electromagnetic noise and vibrations in passive components.:Transformers.
  
  In transformers magnetic noise and vibrations are generated by several phenomena depending on the load case which include Laplace force on the windings, Maxwell forces in the joints of the laminations, and magnetostriction inside the laminated core.
  
  Section::::Electromagnetic noise and vibrations in passive components.:Capacitors.
  