Harman Kardon AVP-1A Owners Manual - Page 29

These frequencies are often referred to as "room resonances" or "room modes"-that is, the frequencies at which the room tends to vibrate of its own accord. These resonances lead to uneven frequency response, the greatest problems being in the 60-150 Hz range for a typical domestic living room. (At lower frequencies in larger room.) Unfortunately, there is no way to eliminate the effects of standing waves completely. The best that can be done is to minimize their effect through a variety of strategies. STANDING WAVE SOLUTIONS: ROOM RATIOS In new construction, the best way to minimize the audibility of standing waves is to plan for an even distribution of them, so that their effects do not "pile up" on top of each other. In this regard the ratios of room dimensions are the critical factor. Rooms having equal dimensions are the worst, since the standing waves in all directions reinforce one another. Room dimensions which are even multiples of one another are also to be avoided where possible. STANDING WAVE SOLUTIONS: SPEAKER PLACEMENT Speaker placement also has an effect on standing waves and their audibility. In particular, placement of any speaker (including subwoofers) where the wall and the floor meet will tend to stimulate all of the available standing waves, causing the most irregular response. The displacement required to minimize a particular standing wave depends on its frequency, with lower frequencies requiring more movement owing to their longer wavelengths. As a result, minimizing colorations due to standing waves often requires significant adjustment of subwoofer placement. Leave yourself some latitude with regard to subwoofer placement when planning your system-the final adjustment will probably have to be done on something of a trial-and-error basis. STANDING WAVE SOLUTIONS: ABSORPTIONS In theory, it is possible to damp standing waves with absorptive material. The difficulty is that the thickness of the absorptive material would have to be approximately one-half the wavelength of the lowest frequency requiring damping. This means a five-foot thickness of fiberglass would be required in order to damp everything down tg 100 Hz-not very practical. Standing wave energy tends to be concentrated in the corners of rooms,which is why these are the worst places for subwoofers. Because of this fact, it may be possible to break them up somewhat by "breaking up" the corner. This can be accomplished by placing a column of thick, absorptive materials in the corners (covered by acoustically transparent cloth, of course). The column ought to be at least a foot on a side, and run from floor to ceiling. A variation on this theme is to run an absorptive panel diagonally across the corner, leaving open air space behind it. Neither of these techniques is more than a partial solution, at best, but they are easily tried and sometimes quite effective. STANDING WAVE SOLUTIONS: ROOM EQUALIZATION If a room exhibits severe standing wave problems, the best solution is to know your own limitations- hire a trained acoustician. These professionals have the necessary background to analyze the various room modes and recommend appropriate action. This will sometimes take the form of a custom-designed bass trap, which may be easily constructed. But it takes specific skills to determine its optimal design. On other occasions, careful third-octave analysis and equalization may be appropriate. Optimize everything else that you can, then use EQ, if necessary, to "touch up" the room. This is its best use. If everyone used EQ this way, it wouldn't have the bad reputation that it has in home consumer electronics circles. Professionals use it all the time, with excellent results-because they know its limitations and how to apply it. Never equalize a room by ear. Room analysis is not as straightforward as it may seem. The analysis should be done using equipment with at least one-third octave resolution, using both spatial and temporal averaging. The final electronic equalization should be applied by means of a one-third octave graphic equalizer at a minimum. In any event, rooms requiring this level of treatment are relatively rare, and the skills necessary to handle them properly are highly specialized. Do not hesitate to use the professional services of an acoustician when you need them.