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Intelligibility (communication)

In phonetics, Intelligibility is a measure of how comprehensible speech is, or the degree to which speech can be understood. Intelligibility is affected by spoken clarity, explicitness, lucidity, comprehensibility, perspicuity, and precision.

Noise levels

For satiable communication, the average speech level should exceed that of an interfering noise by 6dB; lower sound:noise ratios are rarely acceptable (Moore, 1997). Manifesting in a wide frequency range, speech is quite resistant to many types of masking frequency cut-off—Moore reports, for example, that a band of frequencies from 1000 Hz to 2000 Hz is sufficient (sentence articulation score of about 90%).

Quantity to be measured Unit of measurement Good values
STI[1] Intelligibility (international known) > 0.6
CIS Intelligibility (international known) > 0.78
%Alcons Articulation loss (popular in USA) < 10%
C50 Clarity index (widespread in Germany) > 3 dB
RASTI (obsolete) Intelligibility (international known) > 0.6

Word articulation remains high even when only 1–2% of the wave is unaffected by distortion:[2]

Intelligibility with different types of speech

Lombard speech

The human brain automatically changes speech made in noise through a process called the Lombard effect. Such speech has increased intelligibility compared to normal speech. It is not only louder but the frequencies of its phonetic fundamental are increased and the durations of its vowels are prolonged. People also tend to make more noticeable facial movements.[3][4]


Shouted speech is less intelligible than Lombard speech because increased vocal energy produces decreased phonetic information.[5]

Clear speech

Clear speech is used when talking to a person with a hearing impairment. It is characterized by a slower speaking rate, more and longer pauses, elevated speech intensity, increased word duration, "targeted" vowel formants, increased consonant intensity compared to adjacent vowels, and a number of phonological changes (including fewer reduced vowels and more released stop bursts).[6][7]

Infant-directed speech

Infant-directed speech—or Baby talk—uses a simplified syntax and a small and easier-to-understand vocabulary than speech directed to adults[8] Compared to adult directed speech, it has a higher fundamental frequency, exaggerated pitch range, and slower rate.[9]

Citation speech

Citation speech occurs when people engage self-consciously in spoken language research. It has a slower tempo and fewer connected speech processes (e.g., shortening of nuclear vowels, devoicing of word-final consonants) than normal speech.[10]

Hyperspace speech

Hyperspace speech, also known as the hyperspace effect, occurs when people are misled about the presence of environment noise. It involves modifying the F1 and F2 of phonetic vowel targets to ease perceived difficulties on the part of the listener in recovering information from the acoustic signal.[10]


  1. ^ Speech Intelligibility Measurement Methods
  2. ^ Moore, C.J. (1997). An introduction to the psychology of hearing. Academic Press. 4th ed. Academic Press. London. ISBN 978-0-12-505628-1
  3. ^ Junqua, J. C. (1993). "The Lombard reflex and its role on human listeners and automatic speech recognizers". The Journal of the Acoustical Society of America 93 (1): 510–524. PMID 8423266. doi:10.1121/1.405631.  edit
  4. ^ Summers, W. V.; Pisoni, D. B.; Bernacki, R. H.; Pedlow, R. I.; Stokes, M. A. (1988). "Effects of noise on speech production: Acoustic and perceptual analyses". The Journal of the Acoustical Society of America 84 (3): 917–928. PMC 3507387. PMID 3183209. doi:10.1121/1.396660.  edit PDF
  5. ^ Pickett, J. M. (1956). "Effects of Vocal Force on the Intelligibility of Speech Sounds". The Journal of the Acoustical Society of America 28 (5): 902–901. doi:10.1121/1.1908510.  edit
  6. ^ Picheny, M. A.; Durlach, N. I.; Braida, L. D. (1985). "Speaking clearly for the hard of hearing I: Intelligibility differences between clear and conversational speech". Journal of speech and hearing research 28 (1): 96–103. PMID 3982003.  edit
  7. ^ Picheny, M. A.; Durlach, N. I.; Braida, L. D. (1986). "Speaking clearly for the hard of hearing. II: Acoustic characteristics of clear and conversational speech". Journal of speech and hearing research 29 (4): 434–446. PMID 3795886. doi:10.1044/jshr.2904.434.  edit
  8. ^ Snow CE. Ferguson CA. (1977). Talking to Children: Language Input and Acquisition, Cambridge University Press. ISBN 978-0-521-29513-0
  9. ^ Kuhl, P. K.; Andruski, J. E.; Chistovich, I. A.; Chistovich, L. A.; Kozhevnikova, E. V.; Ryskina, V. L.; Stolyarova, E. I.; Sundberg, U.; Lacerda, F. (1997). "Cross-language analysis of phonetic units in language addressed to infants". Science 277 (5326): 684–686. PMID 9235890. doi:10.1126/science.277.5326.684.  edit
  10. ^ a b Johnson K, Flemming E, Wright R. (1993). "The hyperspace effect: Phonetic targets are hyperarticulated". Language 69 (3): 505–28. JSTOR 416697. doi:10.2307/416697. 

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