Phase

Even if there is one sound source only, acoustic waves reflect from walls, ceiling, and floor of the room. Some of the concert halls have baffles strategically placed to reflect sound from the stage toward audience.In the figure given below three sound paths X, Y, and Z are likely to posses different lengths. Thus, the 3 reflected waves will not arrive in same phase at the listener’s ears. The direct path (D), a straight line from speaker to listener, is always the shortest path. In this particular situation, there are at least 4 different paths via which sound gets from speaker to the listener. In several practical situations there are more.

Assume that, at a frequency of 700 Hz, the acoustic waves for all 4 paths arrive in exactly the same phase in listener’s ears. Then sounds at that frequency will be exaggerated in volume. The same phase coincidence may also occur at harmonics of 700 Hz: which means, 1.4, 2.1, 2.8 kHz, and so on. But it is not probable to happen at any other frequencies. This is an undesirable effect as it causes acoustic peaks, called as anti nodes, at specific frequencies, distorting original sound. At some of the frequencies, the waves may mix in phase opposition, such that the energy is canceled out. This produces dead zones, called as nodes, a phenomenon which the acoustics engineers dread.

If listener moves a few feet, the antinodes will no longer be observed at 700 Hz and the harmonics of. The frequency may change; the effect may diminish or even seem to disappear. As produced by set of baffles, the antinode for 700 Hz and its harmonics may well be confined to a single, small spot in room.One of the most significant considerations in acoustical design is avoidance, to the greatest extent possible, of important nodes and antinodes at any frequency within range of human hearing. The perfect acoustic chamber delivers the sound effectively to all members of listening audience, without favoring particular regions of room or frequencies of sound