Design guidelines which incorporate the visual contribution to speech are developed for maximum room size and speaker‐listener distance, and for minimum lighting level on the speaker, when unamplified speech level and background noise level are known. Room‐scale studies on speech intelligibility are conducted at different speech‐to‐noise ratios and different levels of visual acuity. Visual acuity has not been considered as a factor in earlier studies on the visual contribution to speech [W. H. Sumby and I. Pollack, J. Acoust. Soc. Am.26, 212–215 (1954)]. The articulation index is evaluated on the basis of the modified rhyme test in accordance with the newly proposed international standard on speech intelligibility testing. The studies confirm the striking effect of speech‐to‐noise ratio on intelligibility. Also, the contribution of visual cues is shown to be very considerable, especially when the level of audible speech at the receiver is near the background noise level. The amount of improvement in speech intelligibility is shown to depend upon the illumination level on the speaker and the distance from the speaker.