Speech intelligibility is the ability to understand spoken words. The presence of noise interferes with the understanding of what other people say. This includes face-to-face talks, telephone conversations, and speech over a public address system.

In order to be intelligible the sound level of speech must be greater than the background noise at the ear of the listener. People with otherwise unnoticeable hearing loss find it difficult to understand spoken words in noisy surroundings.

In noisy work situations, people are able to converse with difficulty at a distance of one meter for a short time in the presence of noise as high as 78 dB(A). For prolonged conversations, the background noise level must be lower than 78 dB(A).

In social situations people often talk at distances of 2 to 4 meters. In such cases noise level should not exceed 55 to 60 dB(A). In outdoor play and recreational areas, people communicate at distances of 5 to 10 meters. In such cases background noise should not exceed 45 to 55 dB(A). (Also see SIL).

(Speech Communication Capability versus Level of Background Noise in dB(A) Source: C.M. Harris. Handbook of Noise Control, 2nd Ed. New York: McGraw-Hill, 1979.
Communication	Below 50 dB(A)	50-70 dB(A)	70-90 dB(A)	90-100 dB(A)	110-130 dB(A)
Face-to-face (unamplified speech)	Normal voice at distances up to 6 m	Raised voice level at distances up to 2 m	Very loud or shouted voice level at distances up to 50 cm	Maximum voice level at distances up to 25 cm	Very difficult to impossible, even at a distance of 1 cm
Telephone	Good	Satisfactory to slightly difficult	Difficult to unsatisfactory	Use press-to-talk switch and an acoustically treated booth	Use special equipment
Intercom system	Good	Satisfactory	Unsatisfactory using loudspeaker	Impossible using loudspeaker	Impossible using loudspeaker
Type of earphone to supplement loudspeaker	None	Any	Use any earphone	Use any in muff or helmet except bone conduction type	Use insert type or over-ear earphones in helmet or in muffs; good to 120 dB(A) on short-term basis
Public Address System	Good	Satisfactory	Satisfactory to difficult	Difficult	Very difficult
Type of microphone required	Any	Any	Any	Any noise-canceling microphone	Good noise-canceling microphone

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