The effect of masking by unwanted background noise has been touched on only briefly in this section. Such unwanted noise may be produced by sound from the outside environment, by noisy air handling equipment, by noisy backstage mechanical equipment or by the audience itself. For good listening conditions, the level of ambient noise as measured on the “A” scale should be at least 10 dB below the desired signal. Since the optimum level for reproduced speech in the absence of strong background noise is 65 - 70 dB(A) this means that background noise with a full audience should not exceed 55 dB(A). In auditoriums and concert halls, acoustical designers normally attempt to reduce background noise in an empty house to a level not exceeding 25 dB(A). In a church or meeting hall, the maximum tolerable background noise for an empty room is about 40 dB(A).