Seldom does a consumer product attain the success or longevity to affect a collective consciousness, let alone a hi-fi product. But the Bose 901 speaker system is exactly one such product. With a nearly half-century-long history, the 901 has enjoyed incredible commercial success and has generated much conversation amongst audiophiles. The 901 is one of the most unconventional designs in the history of audio and one of the most recognizable.
The history and operational concept of the Bose 901 have often been discussed in the press and online, but, oddly, its design philosophy, its development, and its impact on loudspeaker design have never been given much attention. These untold pieces of the story behind the Bose 901 are important parts of a grander story, that of hi-fi, and are of much interest to audiophiles. To gain some insight into those parts, I spoke with the man who headed the design of the 901, Sherwin Greenblatt.
After earning his master’s degree in electrical engineering from the Massachusetts Institute of Technology (MIT), Greenblatt was hired by Amar Bose, his former professor and Bose founder, in 1964—the company’s first employee. Over the course of his career with Bose, Greenblatt has served variously as a project engineer, chief engineer, director of engineering, and executive vice president. He served the last 15 years of his career as the company’s president. Greenblatt is currently retired, but he retains a director emeritus status on Bose Corporation’s board of directors.
Before I delve into my conversation with Greenblatt, however, a brief introduction to the Bose 901 loudspeaker is in order for the benefit of readers unfamiliar with it. The Bose 901 is a compact standmount that is typically used with a matching tulip-style stand, although it can also be suspended. Its handsome and distinctive pentagonal cabinet is styled with attractive mid-century design cues reminiscent of the Eames lounge chair and the Chemex coffee brewer. It houses nine drivers: one facing the listener, eight facing the front wall. This unique arrangement combines direct and reflected sound to create a strong spatial presence. An active equalizer unit was included with the speakers to be placed either between a preamplifier and a power amplifier or in the tape loop of an integrated amplifier or a receiver.
The Bose 901 is possibly the longest-produced loudspeaker system. Several iterations have been released over the years, beginning with the Series I, introduced in 1968, and culminating with the Series VI, which was discontinued in 2017. The 901 is also one of the most commercially successful loudspeakers ever produced. Its success, however, did not extend to hardcore audiophiles, many of whom remained skeptical and dismissive of it.
A concert hall for the home
The story behind the 901 and its success is a fascinating one. In the early 1960s, Amar Bose was a professor at MIT; Greenblatt, as mentioned, was one of his students. They were both music lovers through and through and regularly frequented concerts at Symphony Hall in Boston. “We would go and listen,” Greenblatt told me, “and . . . come back enthused about the concert we’d heard, take the audio equipment—the hi-fi equipment of the day—and listen to that music at home.” That proved to be an exercise in frustration. “It sounded like shit compared to the live music.”
Bose and Greenblatt became obsessed with why music playback at home sounds so different from live music at a concert hall. They devised and carried out various experiments to investigate the matter, including fitting microphones into a model of a human head and placing it in the concert hall to measure the sound of a live acoustic concert as each ear experiences it. “The result of all this,” Greenblatt said, “was a laboratory model of a loudspeaker . . . that we thought would be an ideal sound reproducer.” When Greenblatt completed his studies and went to work with Bose, that ideal loudspeaker prototype would be refined to become Bose’s first home hi-fi product, the Bose 2201.
The 2201 comprised 22 identical drivers evenly arrayed on the surface of an octant-shaped cabinet. This configuration, Greenblatt explained, closely simulated a perfect sound source, “an ideal pulsating sphere.” It was acting as a spherical, omnidirectional loudspeaker would in a large room, radiating sound broadly and evenly. Ingenious as it was, the 2201 was not without drawbacks, Greenblatt admitted. “It was too big to put in someone’s house; you needed to put it into a corner, by design, so you needed two corners where you could have stereo speakers; it was very expensive because we made them by hand; and it looked weird. So, after about a year, we suddenly realized we had a failure.”
Bose and Greenblatt headed back to the drawing board with what they’d learned from their experiments and the 2201, and over the course of a summer, they refined the concept for the 901 into an optimal compromise between practicality and performance.
Cubing the sphere
By all accounts, including Greenblatt’s, Amar Bose was brilliant, which may explain how he managed to run his company and tend to its various exigencies while keeping his post at MIT. “He was a great problem solver,” Greenblatt recounted. “He could take [basic principles of acoustics] . . . and turn them into something practical; something you could work with.” The 901 began its life as a concept drawing Amar Bose dropped one day into Greenblatt’s lap—just another interesting idea in applied acoustics.
The devil is in the details, of course, and Greenblatt, overseeing the development of the 901 into a commercial product, was responsible for those details. He told me that the 901’s success was due to three design features that it shared with its precursor, the 2201.
The first was that both speakers radiated sound directly toward the listener as well as toward the walls of the listening room to be reflected for a more lifelike representation of a live performance. The 901’s driver arrangement yielded an approximation of the uniformly dispersed sound of the 2201 (“a good one,” according to Greenblatt); but more important, it resulted in a ratio of direct to reflected sound that approximated that in a concert hall. The 901’s driver arrangement was also much simpler to implement, Greenblatt noted, than the 2201’s.
The second design feature carried over from the 2201 was a multiplicity of drivers. Greenblatt and Bose identified two shortcomings of speakers of the time: a significant variation between individual drivers and nonlinear frequency response. “When you have a multiplicity of identical drivers,” Greenblatt explained, “they all work together and average [out], with the defect of one working into the strength of another. The result is an extremely smooth sound distribution.”
The third design feature taken from the 2201 was active equalization. Whereas conventional loudspeakers use a built-in passive crossover made up of capacitors, inductors, and resistors, the 901 (and the 2201 before it) employs an active equalizer module—a small external box, powered separately, by which the speakers’ frequency response is made to conform to that intended. Although uncommon in loudspeakers, Greenblatt told me that such technology has been used since the 1930s to help relay telephone signals over long distances. Indeed, it was that application which inspired its use in the 901. Electronic equalization, Greenblatt said, allowed them to tweak the sound of the speaker to perfection.
He let me in on an interesting fact regarding the 901’s equalization: it was never intended to yield a flat response. The object of the 901 system was to reproduce the sound of a live concert, Greenblatt said, but how do you turn a room into a concert hall? Bose and Greenblatt decided to first record live concerts at Symphony Hall and investigate the way sound moved through the expanse of a concert hall. They then carried out experiments to study sound propagation and perception in a living-room-size space. “We were able to characterize the kind of room you wanted to listen in,” he explained, “and then, using active equalization, we made the room sound as much like a concert hall as we could—not a flat frequency response, but whatever the concert hall would do, we could make the room do.”
The resulting frequency response curve became known around the lab as the Golden Curve. This combination of electronic manipulation and direct and reflected sound from a multiplicity of drivers, Greenblatt said, is “the secret sauce of the 901.”
We have liftoff
Unlike the 2201, the Bose 901, whose practical design made it easier to build and sell, achieved immediate commercial success. An experienced hi-fi seller in the Boston area undertook distribution duties, which helped cope with the growing demand for the 901, but perhaps the biggest factor in its success was exposure.
In the years since, Bose has become known for the success of its marketing, but in 1968 the nascent company had next to no budget for advertising. Then, in September of that year, a magazine called HiFi/Stereo Review (Sound & Vision now) featured a review of the 901 by Julian Hirsch. Hirsch was known for his neutral, measured, professional tone. He was not one given to sentimentality or hyperbole. But when his review of the 901 came out, “it was as close to a rave as anything he ever wrote,” Rob Sabin remarked in his introduction to Sound & Vision’s republished version of that review. Hirsch wrote, “I have never heard a speaker system in my own home which could surpass, or even equal, the Bose 901 for overall realism of sound,” concluding that “it [ranked] with a handful of the finest home speaker systems of all time.” These pronouncements instantly elevated the Bose 901’s stature and catapulted it to fame. “By 1970, it was one of the best-selling speakers on the market,” Greenblatt recalled. “And that’s in spite of it also being one of the most expensive . . . at that time.”
The 901 has been revised several times over the course of its life, but it has changed very little. According to Greenblatt, most changes were made to improve its appearance or the efficiency of its manufacturing. The 1976 release of the 901 Series III, however, introduced new Bose-branded drivers that incorporated a proprietary helical voice coil and were much more consistent in performance. They also required less amplifier power. “We didn’t mean to go into the speaker [driver] business, but we were kind of forced into it because we couldn’t buy what we wanted,” Greenblatt explained.
Along with the new drivers, the 901 acquired a ported enclosure in the Series III, which improved its efficiency. With these revisions, the basic design of the 901 reached its maturity. Changes in subsequent releases were mostly tweaks for improved aesthetics. Production of the 901 continued until its retirement in 2017, when the last of the Series VI speakers rolled off the line.
The Bose 901 was an interesting, successful loudspeaker, to be sure, but it also left a legacy of acoustic innovation that is still important today, though often overlooked. Why does this small, odd-looking speaker that’s based on a bizarre mode of operation, matter? Mainly, because it delivered as promised. It brought the experience of a symphony orchestra playing in a large concert hall into people’s living rooms. It could create the illusion of a musical performance taking place in a much larger acoustic space. Much of the criticism of the 901 stemmed from critics’ misunderstanding of its design intent. Viewing the design elements used to materialize that intent in the light of the state of the industry today reveals just how important and influential the 901 really was.
Consider first the 901’s multiplicity of drivers. While still a relatively unusual design choice, many of today’s speakers incorporate an arrangement of multiple identical drivers to achieve some desired acoustic effect—the line-array design of the T+A Solitaire S 530 loudspeaker and the tweeter clusters on many of Tekton’s loudspeakers are examples of good use of this technique. It’s not unusual for even conventional tower speakers to use two, three, or four identical woofers to improve frequency response and efficiency and ameliorate distortion. Bose didn’t originate this concept, but its implementation in the 901 was certainly early.
The influence of the 901’s avant-garde design is most evident in the widespread adoption of active equalization. Whether a part of an onboard room-correction suite, a separate active crossover or DSP module, or the electronics in active speakers, active EQ is everywhere. SoundStage! Simplifi contains examples aplenty of active speakers with built-in EQ, the recently reviewed Klipsch The Nines being one such example. The equalizer included with the 901 is rather quaint in comparison, but in shaping the sound to conform with a target curve, Bose’s Golden Curve ideal, it is not unlike modern-day DSP equalization, whether employed in a mass-market soundbar or in an ultra-high-end custom Atmos installation.
Dolby’s Atmos system implementation clearly echoes concepts pioneered by Bose in the 901. This technology is based on the idea of sound objects, spatially tracked individual sound sources in a scene. Positional metadata is processed by sophisticated software to allow Atmos to transcend channel-based audio and present sound objects with startling realism for a more immersive experience. The physical implementation includes height channels, usually two or four ceiling-mounted speakers, in addition to the more typical side and rear surround speakers.
Various manufacturers have come up with inventive ways to make Atmos systems more accessible and easier to set up. In lieu of ceiling- and wall-mounted speakers, Atmos-equipped soundbars and height speakers that sit atop the main front speakers are used, designed to reflect sound off the ceiling and walls to achieve the Atmos effect. These systems were developed independently, but the underlying concept of using reflected sound to simulate a larger acoustic space and present sound sources as they would appear in such a space is the one investigated by Bose and Greenblatt in the 1960s.
The directivity and off-axis response of a speaker, even in a conventional stereo setup, have been shown to have a significant effect on its sound. This is why, when we publish our own measurements of loudspeakers we’ve reviewed, we include listening-window and off-axis frequency-response graphs in addition to the on-axis frequency-response graph. Sound dispersion is an important aspect of a loudspeaker’s behavior.
While ordinary speakers project sound into a room, the 901 can be said to play the room itself, and therein lies its greatest contribution. The room is an essential part of the system. The 901 won’t work in an open space or an anechoic chamber. While noteworthy contemporaries such as Edgar Villchur and Henry Kloss also gave due consideration to loudspeaker-room interaction, the 901 was perhaps the first to make this its primary design criterion. In fact, the 901’s design was driven by the quest to optimize the interaction between the speaker and the room—a real-life room. Many loudspeaker designers have learned from this approach, which seems to be setting the course of future advancements in loudspeaker technology.
The legacy of an open door
For his part, Greenblatt is just surprised that it took so long for the ideas he pioneered with the 901 to catch on. “The thing that’s always amazed me is how slow the industry has been to do the things we were doing,” he said, adding that the increasing adoption of active equalization confirms his belief that the 901 was decades ahead of its time. Indeed, the Bose 901 is now more often seen as an influential classic than as an oddball novelty. The 901 may not appeal to every audiophile, but it is appreciated by many. I think that hearing it ought to be on everyone’s bucket list. Designer Sherwin Greenblatt agrees. “Have you ever heard a pair?” he asked. “They sound marvelous.”