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  1. Acoustics
  2. AKG Acoustics
  3. Audio feedback
  4. Audio level compression
  5. Audio quality measurement
  6. Audio-Technica
  7. Balanced audio connector
  8. Beyerdynamic
  9. Blumlein Pair
  10. Capacitor
  11. Carbon microphone
  12. Clipping
  13. Contact microphone
  14. Crosstalk measurement
  15. DB
  16. Decibel
  17. Directional microphone
  18. Dynamic range
  19. Earthworks
  20. Electret microphone
  21. Electrical impedance
  22. Electro-Voice
  23. Equal-loudness contour
  24. Frequency response
  25. Georg Neumann
  26. Harmonic distortion
  27. Headroom
  28. ITU-R 468 noise weighting
  29. Jecklin Disk
  30. Laser microphone
  31. Lavalier microphone
  32. Loudspeaker
  33. M-Audio
  34. Microphone
  35. Microphone array
  36. Microphone practice
  37. Microphone stand
  38. Microphonics
  39. Nevaton
  40. Noise
  41. Noise health effects
  42. Nominal impedance
  43. NOS stereo technique
  44. ORTF stereo technique
  45. Parabolic microphone
  46. Peak signal-to-noise ratio
  47. Phantom power
  48. Pop filter
  49. Positive feedback
  50. Rode
  51. Ribbon microphone
  52. Schoeps
  53. Sennheiser
  54. Shock mount
  55. Shure
  56. Shure SM58
  57. Signal-to-noise ratio
  58. Soundfield microphone
  59. Sound level meter
  60. Sound pressure
  61. Sound pressure level
  62. Total harmonic distortion
  63. U 47
  64. Wireless microphone
  65. XLR connector



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Nominal impedance

From Wikipedia, the free encyclopedia


The term nominal impedance in electrical engineering or audio has a number of different meanings. It refers to the approximate designed impedance under certain conditions. The actual impedance may vary from this nominal figure. The term is used when the exact value of impedance is not important to the operation of the circuit or system.

Nominal impedance and matching

Output impedances

Most signal-processing equipment is designed to operate with the internal impedance of a signal source much smaller than the load impedance of the input to which it is connected. This provides the most efficient voltage transfer, and is best in most, but not all, situations. The nominal output impedance is the assumed impedance for the purpose of deciding whether the equipment can drive another piece of equipment with a known input impedance.

If impedances are mismatched between inputs and outputs that are connected together:

  • Power is wasted or signal level is reduced
  • Frequency response is no longer as linear
  • The signal is more subject to crosstalk and electromagnetic interference
  • Distortion may result due to excessive loading
  • Damage to both units is a possibility

Professional audio system impedances

The nominal line impedance is 600 Ω. Both of these, and the name "line", come from the standards of the earliest telephone networks. It is unlikely that the cable will have an exact 600 ohm characteristic impedance, but this is generally unimportant for the relatively short runs of cable compared to the highest audio frequencies (20 kHz) in use. The main reason for standardisation is for the source and loads to be matched to each other for maximum power transfer.

This is a common standard for connections between pieces of electronic equipment, but 100 mV at 600 Ω is considerably more level than is common for a low impedance input, and represents a greater signal strength than is common for high impedance connections as well, so connecting a high or low impedance signal into a line input normally requires a pre-amplifier, and going the other way may require a dummy load.

Loudspeaker impedance

Nominal impedance here refers to the impedance of the speaker at a defined low frequency in free air. The impedance will change with loading and frequency. In the days of valves, most loudspeakers had a nominal impedance of 16 Ω. Valve outputs require an output transformer to match the very high output impedance and voltage of the output valves to this lower impedance. These transformers were commonly tapped to allow matching of the output to a multiple loudspeaker setup. For example, two 16 Ω loudspeakers in parallel will give an impedance of 8 Ω.

Since the advent of solid-state transformerless outputs, these multiple-impedance outputs have become rare, and lower impedance loudspeakers more common. The most common nominal impedance for a single loudspeaker is now 8 Ω. Most solid-state amplifiers are designed to work with loudspeaker combinations of anything from 4 Ω to 8 Ω.

See also: Loudspeaker

RF cables

Cables for transmission of high frequency signals usually come in one of two nominal impedances: Either 50 ohm or 75 ohm. This means that the characteristic impedance of the cable is close to one of these figures. The figure may vary slightly with frequency.

Comparison with effective impedance

The effective impedance of input or output is the equivalent impedance of all of the output or input circuitry of a device lumped into one (imaginary) component. It should not be confused with nominal impedance. An impedance is a combination of resistance, capacitance, and inductance.

See impedance for a more technical discussion.

See also

  • Voltage divider
  • Source impedance
  • Load impedance
  • Impedance matching
  • Cable impedance
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