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Audibility of Acoustical Interference & Loudspeaker Design Implications

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bad speakerFor 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.

RBH SI-6100The 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.

audibility of comb filtering depends on loudspeaker design and how it plays in the room, listener position, distance from the sources and the source material playing

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        

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 

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.

 

 

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Recent Forum Posts:

gotchaforce posts on September 09, 2012 21:55
Heh, no attention given to this article because it flies over most peoples heads.

It was explained VERY well most people just dont have the patience to take it in.

They rather talk about how pretty speaker with horrible headroom and power response measurements compares to the other pretty speaker with horrible headroom and power response measurements.
Beatmatcher247 posts on September 08, 2012 12:01
Sorry it didn't link in the post above.

Beatmatcher247 posts on September 08, 2012 11:58
A while back, I asked about adding another set of Synchrony One towers to the front would be and was advised against it because of comb filtering. I don't understand that much about it, but would the following setup sound like a disjointed mess because of comb filtering?

MinusTheBear posts on September 07, 2012 21:29
Is it a far assumption that the difference between acoustical interference occurring within the driver layout of a loudspeaker will ALWAYS share the same signal and amplitude while listening to a pair of speakers in stereo, the Left/Right speaker might not always share the same signal and/or amplitude. You talked about two sound sources sharing the same signal, say you change the amplitude of the one sound source, what implications does this have on comb filtering and/or acoustical interference?
agarwalro posts on September 07, 2012 01:52
Gene, from article
on the topic of comb filtering. Some engineers use this term interchangeably with acoustical interference implying the audible effects of comb filtering between a pair of speakers playing in a room is similar to multiple high frequency drivers spaced further apart than their common wavelengths of operation in the same cabinet.
Must be the same “engineers” who use speaker sensitivity/ efficiency or phase/time aligned as interchangeable terms.

Acoustic interference is a physical phenomenon resulting from interaction of sound waves moving in air, whereas comb filtering is an electrical phenomenon resulting from summing a signal with a delayed version of itself. Acoustical interference of single frequencies under controlled conditions can be made to graphically look similar to a comb filters' response. To be fair, in an ABX test the physical and electrical outcomes may be audibly indistinguishable.

In a real life listening scenario, the acoustic interference, like room modes and floor/ceiling/wall bounce will swamp out all but the most egregious electrical errors, like incorrect polarity between drivers and amp clipping.
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