Audibility of Acoustical Interference & Loudspeaker Design Implications

For very high frequencies, the loudspeaker designer must pay extra care in keeping multiple tweeters positioned as closely together as driver size and cabinet configuration will allow.  If the speakers are allowed to operate over a range above where the physical distance between the sources is large compared to the wavelengths of sound, then acoustical interference starts becoming a problem (ie placing tweeters on opposite ends of the cabinet as pictured here).  Serious designers making top shelf products will avoid making this mistake.  Luckily you won't find these type of speakers on the market anymore but in the 70's and early 80's they were pretty commonplace. 

It's important to note that multiple tweeters in a loudspeaker is NOT necessarily a bad thing. Some designers employ multiple woofers and tweeters in such a way to average the lobing response errors which can result in more consistent sound off axis and thus provide a wider listening area. (the argument is there are so many peaks and dips that they should average themselves away).  This can be seen in the example below for the RBH Sound SI-6100/R in-wall speaker.

The audible effects of comb filtering between different radiating sound sources playing the same program material vs acoustical interference between multiple drivers of the same sound source are dependent upon loudspeaker driver and cabinet design, room design, and listener position, distance from the sources and the program material being reproduced. Multi driver systems should NOT just be blindly discounted as invalid designs.  If properly executed, these systems can provide much greater dynamic range and a larger soundstage than a competing conventional 2 or 3 way design.

A lot has to do with the distance from the speakers at which one listens and the dispersion characteristics of the speakers. If the listener is close to the speakers—say less than about 3 feet—he will be in what many engineers call the “near field,” meaning the soundfield where the sound that reaches the listener’s ear is primarily direct sound from the speaker with relatively little room reflection.

Moving farther back away from the speakers, the listener will then be in what many refer to as the “critical distance,” which is that distance from the speakers where the sound reaching the listener’s ears is a good mix of direct sound from the speaker and reflected sound from the room’s surfaces.

Farther away from the speakers yet and the listener is in the “reverberant” or “far field,” where the sound reaching the listener’s ears is dominated by room reflections and contains a much smaller percentage of direct sound from the speaker.

Speakers with wide, uniform dispersion (uniform meaning that the designer has chosen the driver sizes, cabinet design, and crossover points such that the speaker’s dispersion remains relatively constant over a wide angle even as the frequency increases) will engage more room reflections than a speaker with narrower dispersion, like a horn speaker that intentionally limits its dispersion to a specified angle.

This very design aspect is a point of greatly differing opinion among well-respected speaker designers. Many designers like speakers to have wide dispersion—but not too wide—so that listeners seated 30 or 45 degrees off axis can still hear the direct first-arrival sound from the speaker clearly. Wide enough to create enough room reflections for a feeling of “spaciousness,” but narrow enough to be focused enough to give listeners a sense of immediacy and sharp imaging.

Allison:One Speaker System

Some famous designers have built speakers—like Roy Allison’s AR-3a, AR-LST and  Allison:One— which aimed for extremely wide dispersion and optimal far-field response (sometimes called Power Response), feeling that that was what listeners really heard in a normal listening room, not the so-called first-arrival on-axis anechoic response of the speaker.

Bose 901 Speaker System

Bose has taken this approach to a further extreme, by designing most of its speakers to produce the maximum amount of room reflections possible and limit the direct, localizable sound from the speaker.  With that said, we should discuss the loudspeaker power response and its implications.