Speech intelligibility
Speech intelligibility is adversely affected by noise. Most of the acoustical energy of speech is in the frequency range of 100-6000 Hz, with the most important cue-bearing energy being between 300 and 3000 Hz
Speech interference is basically a masking process, in which simultaneous interfering noise renders speech incapable of being understood. Environmental noise may also mask other acoustical signals that are important for daily life, such as door bells, telephone signals, warning signals and music.
Speech intelligibility in everyday conditions is influenced by speech level, speech pronunciation, talker to listener distance, sound level and other characteristics of the interfering noise, hearing acuity and the level of attention. Indoors, speech communication is also affected by the reverberation characteristics of the room.
For full sentence intelligibility in listeners for normal hearing, the signal to noise ratio (the difference between be at the speech level and the sound level of the interfering noise) should be at least 15 dB(A).
Noise
| Noise level (dB) | Type of noise | Conversation | Effect on human ear |
| 135 | Jet airplane | Impossible | Physical pain |
| 120 | Noisy workshop | Impossible | Threshold of pain |
| 100 | Pneumatic drill | Possible by shouting | Noise very disturbing |
| 90 | Underground train | Possible by shouting | Hearing damage may occur |
| 80 | Heavily trafficed street | Difficult -raised voices | Noise very disturbing |
| 70 | Lightly trafficed street | Difficult -raised voices | Noise - annoying |
| 60 | Bustle of large store | Slightly raised voices | Noise - acceptable |
| 50 | Day-to-day noise of normal urban household | Normal voice | Noise - acceptable in day time |
| 40-20 | Day-to-day noise of quite country house | Normal voice | Peaceful |
| 10 | Silence of recording studio | Hushed voices | Abnormally quite |
| 0 | Total silence | Threshold of hearing |
