Bose 901: A Classic EQ Assisted, Extended Reflex Alignment

The venerable Bose 901 is an actively assisted bass reflex system with an abnormally low tuning frequency. The 901 and its pro-sound cousin, the 802, both have a reputation for respectable bass, especially considering their sizes. The 901 embodies several of the key ideas in this article, and carries them to an extreme.

The 901’s fb is 40 Hz (see US Patent 4146745). Its active equalizer sharply cuts off response below 35 Hz. Figure 1 shows the curve of the active EQ from the patent.

Rarely does one see a 4.5” woofer tuned as low as 40 Hz. If the 901 were an acoustic suspension system or if it didn’t have custom EQ, results would be poor. But nine 4.5” woofers are equivalent to a single 13” woofer. And tightly integrated such as it is, the 901 achieved solid bass and become one of the great classic speakers in the history of the audio business.

The 901 illustrates the power of using EQ to take full advantage of the reflex system’s 80/20 leverage. (In the 21st century, DSP is also a much better way to achieve this than analog EQ.)

The Bose 802 professional version illustrates a second principle, which we explore in the main article: The ability to trade bandwidth for output. The 802 ports are tuned to fb = 55 Hz and the EQ boost is at 50 Hz not 35, with sharp cutoff below that frequency. This increases the maximum output of the speaker by 5.5 dB. In pro sound reinforcement, the extra 6 dB at 55 Hz is more useful than extending bass to 40 Hz.