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ARTICLES IN THE BOOK

  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

 

 



MICROPHONES
This article is from:
http://en.wikipedia.org/wiki/Equal-loudness_contour

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License 

Equal-loudness contour

From Wikipedia, the free encyclopedia

 

An equal-loudness contour is a measure of sound pressure (dB SPL), over the frequency spectrum, for which a listener perceives a constant loudness. The unit of measurement for loudness levels is the phon, and by definition two sine waves that have equal phons are equally loud.

The human auditory system is sensitive to frequencies from 20 Hz to a maximum of around 20,000 Hz, although the hearing range decreases with age. Within this range, the human ear is most sensitive between 1 and 5 kHz, largely due to the resonance of the ear canal and the transfer function of the ossicles of the middle ear.

Equal-loudness contours were first measured by Fletcher and Munson using headphones (1933). In their study, listeners were presented with pure tones at various frequencies and over 10 dB increments in stimulus intensity. For each frequency and intensity, the listener was also presented with a reference tone at 1000 Hz. The reference tone was adjusted until it was perceived to be of the same loudness as the test tone. Loudness, being a psychological quantity, is difficult to measure, so Fletcher and Munson averaged their results over many test subjects to derive reasonable averages.

A new experimental determination was made by Robinson and Dadson (1956) which was believed to be more accurate, and this became the basis for a standard (ISO 226) which was considered definitive until 2003.

Because of perceived discrepancies between early and more recent determinations, the International Organization for Standardization (ISO) recently set out to revise its standard curves as defined in ISO 226, referring to the results of several studies, by researchers in Japan, Germany, Denmark, UK, and USA. (Japan was the greatest contributor with about 40% of the data.) This has resulted in the recent acceptance of new curves standardized as ISO 226:2003. The committee responsible for this revision has commented on the surprisingly large differences, and the fact that the original Fletcher-Munson contours are in better agreement with recent results than the Robinson-Dadson, which appear to differ by as much as 1015 dB especially in the low-frequency region, for reasons that are not explained.[1]

Equal loudness curves derived using headphones are valid only for side-presentation. Frontal presentation, from a single central loudspeaker, can be expected to show reduced sensitivity to high frequencies, which are partially masked by the head, and presentation using two loudspeakers, as for stereo will reveal more complicated differences related the HRTF (head related transfer function) which is also dependent on elevation of the sources and plays a major role in our ability to locate sounds. The Robinson-Dadson determination used loudspeakers, and for a long time the difference from the Fletcher-Munson curves was explained on this basis. However, the ISO report actually lists the latter as using 'compensated' headphones.

Good headphones, well sealed to the ear, can provide a very flat low-frequency pressure response measured at the ear canal, and at low frequencies the ear is purely pressure sensitive and the cavity formed between headphones and ear is too small to introduce any modifying resonances. It is at high frequencies that things get dubious, and the various resonances of pinnae (outer ear) and ear canal are severely affected by proximity to the headphone cavity. With speakers, exactly the opposite is true, a flat low-frequency response being very hard to obtain except in free space high above ground or in a very large and anechoic chamber free from reflections down to 20 Hz. A flat free-field response up to 20 kHz though is comparatively easy to achieve with modern speakers on-axis. These facts have to be borne in mind when comparing results of various attempts to measure equal loudness contours.

The lowest equal loudness contour represents the quietest audible tone and is also known as the absolute threshold of hearing. The highest contour is the threshold of pain.

Although the A-weighting curve, in widespread use for noise measurement, is said to have been based on the 40-phon Fletcher-Munson curve, it should be noted that determinations of equal loudness made using pure tones are not directly relevant to our perception of noise. This is because the cochlea in our inner ear analyses sounds in terms of spectral content, each 'hair-cell' responding to a narrow band of frequencies. The high-frequency bands are wider in absolute terms than the low frequency bands, and therefore 'collect' proportionately more power from a noise source. Equal loudness curves derived using noise bands therefore show an upwards tilt above 1 kHz and a downward tilt below 1 kHz.

The 468-weighting curve, originally proposed in CCIR recommendation 468, but later adopted by numerous standards bodies (IEC, BSI, JIS) was based on BBC Research using a variety of noise sources ranging from clicks to tone bursts to pink noise, and incorporates a special Quasi-peak rectifier to account for our reduced sensitivity to short bursts and clicks. It is widely used by Broadcasters and is by far the preferred weighting to use for all forms of noise measurement, enabling subjectively valid comparisons of different equipment types to be made even though they have different noise spectra and characteristics.

See also

  • Fletcher-Munson curves
  • Robinson-Dadson curves
  • A-weighting
  • Sound level meter
  • Audiometry
  • Audio quality measurement
  • CCIR (ITU) 468 Noise Weighting
  • Noise
  • Noise measurement
  • dB(A)

External links

  • ISO Standard
  • Interesting comparisons of ISO with R-D and Fletcher Munson (PDF)
  • Fletcher-Munson is not Robinson-Dadson (PDF)
  • Full Revision of International Standards for Equal-Loudness Level Contours (ISO 226)
  • Test your hearing - A tool for measuring your equal-loudness contours
  • Equal-loudness contour measurements in detail
Retrieved from "http://en.wikipedia.org/wiki/Equal-loudness_contour"