This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
269. It is a matter of great importance in all public buildings, such as churches, theaters, lecture rooms, etc., that a person speaking from the stage or platform should be distinctly heard at any point on the floor or in the galleries.
The transmission of sound is affected to a considerable degree by currents in the air, and by variations in its density, or temperature. If a sound projected by the speaker encounters an air-current, it will be deflected from its original course, and will appear to the hearer to be somewhat weaker than it otherwise would. If a number of currents are encountered, the weakening effect will be very noticeable.
The greatest obstruction, however, is caused by inequalities in the temperature of the air through which the sound passes. Sound is always retarded by passing from a denser medium into a lighter one, and vice versa. When an audience room is full of streaks of warm air, either ascending or descending, the voice of the speaker will be so retarded in passing through them successively, that persons in the remoter parts of the room will have great difficulty in hearing plainly.
For these reasons the practice of introducing the warm air through large openings in the vertical front of the stage, or through large floor registers between the audience and the speaker's platform, is a bad one, and should be carefully avoided.