Jaffe Holden Scarbrough Acoustics, Inc., 114A Washington St., Norwalk, CT 06854
Appl. Res. Lab., State College, PA 16804
The ``seat-dip phenomenon'' is so called because an anomalous attenuation (dip) in the frequency response of auditoriums is found to be caused by the presence of rows of seats. As sound from the stage grazes over seats, it is subject to broad attenuation of the low frequencies from 75 to 300 Hz. This frequency dip can be as much as 20 dB below the sound level without the presence of seats. Experiments were performed with a simple 1:10 scale model of an auditorium floor. It is found that the dip is caused by a coincidence of effects rather than the single effect dominant in the literature. The effects are considered. (1) A standing wave between the seat and source with nulls relating to the quarter-wavelength distance, (2) half-wave cancellations where reflected and/or diffracted waves interfere with the on-coming wave, and (3) the propagation of the sound of interest at nearly zero degrees over the seats. It is shown that the standing waves and sound pressure cancellations associated with each successive row of seats occur at frequencies that are very close to each other. The effect of frequency spreading of (1) and (2) are presented.