Bose 901 loudspeaker Page 2

Now, let's consider for a moment some of the basic premises on which Dr. Bose based the design of the 901.

The major departure from conventional design here is the system's unique directional characteristics, with most of the sound coming from eight rear-facing speakers and the rest from a single front-facing speaker. According to Dr. Bose's literature, this was done to simulate the conditions in a concert hall, where measurements have shown that only about 11% of the sound reaching a listener is direct sound, coming straight from the instruments, while the rest is reverberant energy, due to reflections from the boundaries of the hall (walls, ceiling, etc.). The implication here is that the 901 turns your listening room into a mini-auditorium, and that the spaciousness of the 901's sound is due to reverberations in the listening room. It is our feeling, though, that Dr. Bose is either oversimplifying his explanation of what the 901 does or has drawn some dubious conclusions from his basic premise, because as far as we can see, the "reverberant" aspect of the listening room is not really pertinent to the operation of the system at all.

Reverberation is a slow decay of sound in a hall, due to reflections bouncing back and. forth between the hall boundaries (walls, ceiling, etc.) until they are absorbed. In a listening room of typical size, reverberation as such is too short in duration to contribute anything to the sound except some smearing of detail and the excitation of resonances within the audio range, so they are undesirable. And it is not reverberation that the 901 depends on anyway, it is reflection. And what the reflection does, we believe, is to produce the acoustical equivalent of an electrical comb filter (fig.1).

Fig.1 Frequency response (bandpass characteristic) of a comb filter.

We think of the spatial aspect in sound as being a function of the directions from which sounds are reaching our ears. This is only half of the story, though. With one ear stopped up, the other is virtually oblivious to the directions of incoming sounds. In a concert hall, the unblocked ear can readily hear the hall reverberations arriving from all directions, but the spatial sense is entirely absent, just as it is from a monophonic recording. With both ears functioning, neither is any more aware of direction than it is working alone, but now the two ears are hearing differences between the impinging reverberations, and it is the comparisons between each ear's "input" which our brain interprets as spatial information.

The sounds reaching us directly from the instruments are relatively simple in structure, consisting of a series of wavefronts like the walls of rapidly expanding balloons spreading out from each instrument. If we face an instrument head-on, its wavefronts pass each ear at the same instant and at the same angle—both ears hear exactly the same thing, and we localize the soundsource as dead ahead. Instruments located to one side reach the ears at slightly different moments, and the "shadow" of the head causes the more distant ear to hear a slight loss of volume and overtone content, and our brain tells us the sound is over there, to one side.

The reflected sounds which we hear as reverberation, though, are exceedingly complex. Since they reach us from different directions, it is obvious that they will have travelled different distances (from the source) before they reach us. The infinite number of distances involved means that many soundwaves reaching us will be out of phase with one another at certain frequencies from certain directions and at other frequencies from other directions, and the patterns of cancellation will be different from one side of the hall than from the other (footnote 1).

The effect, as far each ear is concerned, is a series of sharp dips in the frequency response of the reverberant sound, with one ear hearing the dips at one set of frequencies and the other hearing them at another set of frequencies. And, of course, the location of each instrument on the stage will cause it to produce dips at different frequencies in its reverberant sound.

The same principle has been used to produce pseudo-stereo recordings from monophonic ones, by means of a comb filter. This is a device which gives a frequency response that looks like a comb, and when two such devices are used, with the dips at different frequencies, the result is a pair of signals which simulate the spatial cues of reflective reverberation. Feed one to each speaker, and the mono sound will appear to spread across the space between them instead of appearing midway between them. (Other tricks are used in pseudo-stereo production to give the illusion of left-right positioning for certain instruments, but the main source of "spread" in these recordings comes from the comb filters.)



Footnote 1: You may argue that this would not be so if you sat exactly in the middle of the hall and listened to an instrument exactly on the middle of the stage, and we suppose it shouldn't, but it is anyway, which is probably one reason acoustical experts can't predict how a new hall is going to behave.—J. Gordon Holt
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COMMENTS
dalethorn's picture

So far I haven't been able to find the original Bose 901 review on the Internet - the review which contained the phrase "Bose is best, big or small, high or low."  I've always wanted the 901, just like I always wanted a set of Klipschorns.  But getting the best that either of those systems are capable of (and the only reason to have them) requires a listening room the likes of which I've never had.

GKearny's picture

I've had some BOSE 901s since 1999, and they ROCK when set up in my front yard. With the complete ABSENCE of room resonance, and connected to an ACURUS linear power amplifier, they SOUND LIKE WE ARE ATTENDING A LIVE CONCERT! I have rocked my entire neighborhood with Wagner's Ride of the Valkyrie, and the original BOSTON album! I think that the BOSE 901s are, with the addition of a LOT of horsepower ala a linear power amp, sound WONDERFUL. Our house is made of concrete blocks, and we have a solid rock veneer wall, and when we set up the BOSE 901s to reflect off of this heavy, rock-hard surface, the amount of sound that is projected is worth WHATEVER it costs! WE LOVE OUR BOSE 901s ! Gerry K in Arizona

drblank's picture

I know a lot of people want to get their audio system sound like they are at a concert. The problem is that concerts don't have really good room acoustics or sound systems.  Obviously, if the recording is a live recording, we hope that it sounds like we are at the concert, but having the speakers basically rely on bouncing audio from the back as the main source is actually bad.   Some people like omni directional speakers like MBL, or others that are simlar in design, but they don't rely mostly on bouncing the sound from the rear.   With the 901's, you are actually listening to what you want less of, reflected sound.

I had an acoustic engineer come to my home to take some measurements and give me his recommondations for room treatment as my room measurements were probably the worst you could have.  a square room with a pitched vaulted cieling with a drastic slope to it.  He explained to me that it's good to have reflected sound, but you don't want too much as it removes detail and coherancy from the music.

When companies like ASC and others work with a client to measure room acoustics, they work with Articulation measurements of the room and they measure the direct sound compared to the reflections and they want to create a natural room sound rather than having it too dead, too live where the reflected sound is as loud as the direct sound coming from the speakers.

The 901's just simply do it wrong.  If the front had a bunch of drivers and the rear sides had only one driver, that might be better.

One thing to note.  When you walk into a Bose dealership or Bose Store, don't let them use those CDs that are produced by BOSE.  I talked to a dealer that carried BOSE for a short period of time and they explained to me that BOSE produced demo CDs that were mastered to sound good on BOSE systems, but when you listened to the same CD on another, they sounded like crap so those demos they use to sell the speakers mislead the consumer because most of the recordings you listen to weren't mastered specifically for BOSE speakers.  The local dealership that told me this doesn't carry BOSE anymore and the only reason why they carried the line is they got a lot of people asking for BOSE because of BOSE marketing and they seccumbed under the pressure of customers, and in the process, they got ribbed by their higher end customers like myself, but in the end, they dropped the line as people would actually walk into a higher end store and once they started listening to companies like Paradigm and others, they became disinterested in BOSE and then sales started to wain.

Best advice I can give someone is use your own CDs, and go to a variety of audio stores (not the big box stores) and start REALLY listening to a variety of speakers, there are plenty of really good products on the market that walk all over BOSE.  Paradigm is just one of many, there are Martin Logan, and many other small companies that make nice affordable products these days.  If I wanted the omni directional sound, but couldn't afford MBLs or something in that price range, I would be looking at Martin Logan, Magnapan and a few others that sound so much better than 901's.

machelp's picture

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