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In speech communication, intelligibility is a measure of how comprehensible speech is in given conditions. Intelligibility is affected by the level (loud but not too loud) and quality of the speech signal, the type and level of background noise, reverberation (some reflections but not too many), and, for speech over communication devices, the properties of the communication system. A common standard measurement for the quality of the intelligibility of speech is the Speech Transmission Index (STI). The concept of speech intelligibility is relevant to several fields, including
phonetics Phonetics is a branch of linguistics that studies how humans produce and perceive sounds, or in the case of sign languages, the equivalent aspects of sign. Linguists who specialize in studying the physical properties of speech are phoneticians. ...
,
human factors Human factors and ergonomics (commonly referred to as human factors) is the application of psychological and physiological principles to the engineering and design of products, processes, and systems. Four primary goals of human factors learnin ...
,
acoustical engineering Acoustical engineering (also known as acoustic engineering) is the branch of engineering dealing with sound and vibration. It includes the application of acoustics, the science of sound and vibration, in technology. Acoustical engineers are typical ...
, and
audiometry Audiometry () is a branch of audiology and the science of measuring hearing acuity for variations in sound intensity and pitch and for tonal purity, involving thresholds and differing frequencies. Typically, audiometric tests determine a subje ...
.


Important Influences

Speech is considered to be the major method of communication between humans. Humans alter the way they speak and hear according to many factors, like the age, gender, native language and social relationship between talker and listener. Speech intelligibility may also be affected by pathologies such as speech and hearing disorders. Finally, speech intelligibility is influenced by the environment or limitations on the communication channel. How well a spoken message can be understood in a room is influenced by the *
background noise Background noise or ambient noise is any sound other than the sound being monitored (primary sound). Background noise is a form of noise pollution or interference. Background noise is an important concept in setting noise levels. Background n ...
, *
reverberation Reverberation (also known as reverb), in acoustics, is a persistence of sound, after a sound is produced. Reverberation is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is abso ...
, and *
frequency response In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and phase of the output as a function of input frequency. The frequency response is widely used in the design and analysis of s ...
of the room, as well as the *
sound pressure level Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydropho ...
and *
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signa ...
of the
sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sou ...


Noise levels and reverberation

Intelligibility is negatively impacted by background noise and too much reverberation. The relationship between sound and noise levels is generally described in terms of a signal-to-noise ratio. With a background noise level between 35 and 100 dB, the threshold for 100% intelligibility is usually a signal-to-noise ratio of 12 dB.Robinson, G. S., and Casali, J. G. (2003). Speech communication and signal detection in noise. In E. H. Berger, L. H. Royster, J. D. Royster, D. P. Driscoll, and M. Layne (Eds.), ''The noise manual'' (5th ed.) (pp. 567-600). Fairfax, VA: American Industrial Hygiene Association. 12 dB means that the signal should be roughly 4 times louder than the background noise. The speech signal ranges from about 200–8000 Hz, while human hearing ranges from about 20-20,000 Hz, so the effects of masking depend on the frequency range of the masking noise. Additionally, different speech sounds make use of different parts of the speech frequency spectrum, so a continuous background noise such as
white White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White ...
or
pink noise Pink noise or noise is a signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequency of the signal. In pink noise, each octave interval (halving ...
will have a different effect on intelligibility than a variable or modulated background noise such as competing speech, multi-talker or "cocktail party" babble, or industrial machinery. Reverberation also affects the speech signal by blurring speech sounds over time. This has the effect of enhancing vowels with steady states, while masking stops, glides and vowel transitions, and prosodic cues such as pitch and duration. The fact that background noise compromises intelligibility is exploited in audiometric testing involving spoken speech and some
linguistic Linguistics is the scientific study of human language. It is called a scientific study because it entails a comprehensive, systematic, objective, and precise analysis of all aspects of language, particularly its nature and structure. Linguis ...
perception experiments as a way to compensate for the ceiling effect by making listening tasks more difficult.


Intelligibility standards

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


Intelligibility with different types of speech


Lombard speech

The
human brain The human brain is the central organ of the human nervous system, and with the spinal cord makes up the central nervous system. The brain consists of the cerebrum, the brainstem and the cerebellum. It controls most of the activities of ...
automatically changes speech made in noise through a process called the
Lombard effect 250 px, noise_pollution.html"_;"title="Great_tits_sing_at_a_higher_frequency_in_noise_pollution">noise_polluted_urban_surroundings_than_quieter_ones_to_help_overcome_the_auditory_masking_that_would_otherwise_impair_other_birds_hearing_their_bird_ ...
. 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.PDF


Screaming

Shouted speech is less intelligible than Lombard speech because increased vocal energy produces decreased phonetic information. However, "infinite peak clipping of shouted speech makes it almost as intelligible as normal speech."


Clear speech

Clear speech Clear may refer to: *Transparency and translucency, the physical property of allowing light to pass through Arts and entertainment Music Groups * Clear (Christian band), an American CCM group from Cambridge, Minnesota * Clear (hardcore band), a ...
is used when talking to a person with a
hearing impairment Hearing loss is a partial or total inability to hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to acquire spoken la ...
. 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).


Infant-directed speech

Infant-directed speech—or baby talk—uses a simplified
syntax In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituenc ...
and a small and easier-to-understand
vocabulary A vocabulary is a set of familiar words within a person's language. A vocabulary, usually developed with age, serves as a useful and fundamental tool for communication and acquiring knowledge. Acquiring an extensive vocabulary is one of the ...
than speech directed to adults Compared to adult directed speech, it has a higher fundamental frequency, exaggerated pitch range, and slower rate.


Citation speech

Citation speech occurs when people engage
self-conscious Self-consciousness is a heightened sense of awareness of oneself. It is not to be confused with consciousness in the sense of qualia. Historically, "self-consciousness" was synonymous with "self-awareness", referring to a state of awareness that ...
ly 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.


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.


Notes


External links

{{wiktionary, intelligibility
Intelligibility conversion ALcons to STI and vice versa
Sound Hearing Interpersonal communication