Bose 901 series V Speaker System Review
The Bose 901 series speakers, which have been manufactured since 1968, probably qualify as having the longest tenure of any currently made hi-fi product. Although they have undergone a continuing process of design modification- and the new Series V differs in almost every detail from the original Model 901-the basic concept of the system remains unchanged to this day.
The design of the Bose "Direct/ Reflecting" speaker is based on the fact that most of the acoustic energy reaching a listener in a concert hall has been reflected at least once from the ceiling, floor, or walls. Dr. Amar Bose, president and founder of the Bose company, decided that in order to give a subjective effect more like that of a live performance, a similar proportion of a speaker's sound output should be reflected from a home listening room's walls.
To that end, every Series 901 speaker, from the first model to the new Series V, has used nine identical full-range drivers, with two groups of four on its angled rear panels and a single driver facing forward. Before reaching the listener, the output of the rear drivers is reflected from the wall a foot or so behind the speaker, and usually from a side wall as well. The front driver, which provides only 11 per cent of the system's output, supplies the early-arrival information needed for stereo localization.
Each of the 4-1/2-inch drivers can be viewed as a miniature woofer rather than a large tweeter. The total radiating area of the drivers in each Bose 901 is roughly equal to that of a 13-inch-diameter speaker, but the high-frequency performance of the small drivers is far superior to that of a large single-cone driver of equivalent area. The 901 uses no crossover network, and an electronic active equalizer is required to generate a flat response in the listening room. The equalizer, which is furnished with the speakers, normally operates in an amplifier's tape-monitor loop, whose functions are duplicated on the equalizer.
The Bose 901-V differs from the earliest versions, which had acoustically sealed wooden cabinets, in using an unusually sophisticated form of venting. The cabinet is a very complex plastic molding that Bose calls an "Acoustic Matrix" enclosure. Although the drivers are isolated from each other, their outputs are combined through a network of nine chambers, two couplers, and three reactive air columns.
The nominally 8-ohm Bose 901-V is efficient enough to be driven by a 10-watt amplifier. Because of its multiple drivers and flat-wire voice coils, the speaker can also handle very high power levels.
The front and much of the rear of the speaker cabinet are covered by a non-removable dark-brown cloth grille. The cabinet is 21 inches wide, 13 inches deep, and 12-5/8 inches high, and each speaker unit weighs 35 pounds. Black steel speaker stands, 18 inches high, are available as options.
The 901-V equalizer is 13 inches wide, 4-7/8 inches deep, and 2-3/4 inches high. Its exterior is gold satin-finish aluminum with a dark-brown plastic front panel and side plates. Price: $1,400 per pair (including equalizer). The optional speaker stands are $120 per pair.
Because of its multidriver configuration, the quasi-anechoic FFT frequency-response curves from the Bose 901-V are much less regular than the actual response of an individual driver. Actually, an averaged room-response measurement is probably a more meaningful indication of this speaker's performance in a real listening room. We still used our digital IQS FFT signal-analysis system, with close microphone spacing, to gain some insight into the performance of the drivers over limited frequency ranges. Measurements were made first with the equalizer controls centered and then repeated at their limit settings to evaluate the equalizer's effect on the system response. We also measured the equalizer's response with a swept sine wave, as we would with any amplifier or preamplifier.
Measured with the center equalizer settings, the in-room frequency response above 500 Hz was unusually flat. Although it had the usual small fluctuations due to standing-wave interference effects, its average value was a horizontal line from below 500 Hz up to our 20,000-Hz upper measurement limit. Close-miked measurements were made of one group of four speakers on a rear panel, with the microphone close to the midpoint of the cluster. The resulting bass-response curve was flat within ±3 dB from 38 to 620 Hz, and it spliced very nicely with the room curve to produce a ±3-dB response variation from just under 40 Hz all the way to 20.000 Hz.
The measured on-axis sensitivity actually indicates a much higher real sensitivity since most of the audible sound from the speaker is radiated from the rear drivers and contributes little to the measured output. Bass distortion was measured with the same microphone placement used for the bass-response measurement. The distortion level was very low down to 40 Hz, then rose rapidly below that. The amplifier power required at the lowest frequencies was rather high because of the equalizer boost, which reached 17.5 dB at 40 Hz. Around that frequency (and below), appreciable air noise could be heard near the rear vents, although the vents are designed to minimize air turbulence.
We measured the peak power capability of the Bose 901-V by driving it with one-cycle "single-shot" tone bursts. At 100 Hz, the amplifier clipped before the speaker displayed audible distortion or other signs of excessive cone excursion. The drive level (based on a sine-wave signal of the same amplitude as the single-cycle burst) was about 270 watts into the actual speaker impedance of 16 ohms at that frequency, or 540 watts referred to the speaker's nominal 8-ohm rating. At 1.000 Hz, we could see (and hear) clipping on the waveform from the speaker at 125 watts (into 8 ohms). At 10,000 Hz the speaker output no longer increased when the drive level was raised beyond 128 watts into its 25-ohm impedance at that frequency, or 400 watts into 8 ohms. Although the presence of the equalizer makes it difficult to relate these measurements to those of a conventional speaker, they leave no doubt that the Bose 901-V can absorb enormous power input without damage, significant distortion, or appreciable sound compression.