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, human factors, acoustical engineering, and audiometry. 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, reverberation, and frequency response of the room, as well as the sound pressure level and distortion of the sound reinforcement system 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. 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 or pink noise 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