Sealed-Box Speakers or Vented: Which Are Better?
When talking about “better” in audio (and probably most other things), the real question is “better for what?” Sealed and vented speakers provide designers and listeners with different assortments of benefits and challenges.
A little history: Up until the mid-1950s speakers had to be highly efficient because amplifiers (tube of course) were very low powered. Two watts was typical; fifteen watts was big and sixty watts was about the biggest (and very expensive). How do you make a very efficient design? You make the driver have a light moving mass (basically cone and voice coil) that is large in diameter – 15-18” were popular in the “high-end” of the day. These were often horn-loaded to increase the efficiency. Many cabinets were sealed and so large that their only function was to absorb the back-waves; thus allowing the driver to respond down to its natural resonance. Some would have an open panel or a vent to add bass at the cabinet/vent tuning frequency (some guitar speakers still do this). The best speakers of the day were the size of large refrigerators. Altec-Lansing’s A7, Bozak’s Concert Grand, the Klipschorn, the James B. Lansing Paragon and EV’s Patrician (using a 30” driver) were among the best of the best. Luckily, this was in the very early days of stereo, so only one of these giant speakers was needed for your hi-fi music system.
But all was not Paradise. Stereo was on the way and having two of these monsters was beyond what most could afford in dollars and space. When you went to smaller boxes, you lost bass and/or efficiency.
So, in the 1950s, a new approach was developed and pioneered by Edgar Villchur, founder of Acoustic Research. It employed a sealed box and used the air captive within the enclosure to control driver excursion. This development has been called both “air suspension” and “acoustic suspension.” This design allowed bass as deep as the monsters in a small, two cubic foot cabinet. Henry Kloss, one of the original AR engineers, brought out acoustic suspension designs under KLH, Advent and Cambridge Soundworks brands. Boston Acoustics, an off-shoot of Advent, continues the tradition. They were much less efficient, requiring much more power for the same sound levels; but amplifiers were getting cheaper and solid-state amps were on the way.
With the introduction (during the 1970’s in the US) of a new mathematical model developed by A. N. Thiele and refined by Richard Small (two Australian engineers), designing vented enclosures went from trial and (mostly) error to being straightforward. The designer could now shape the response in a predictable manner. Designers were able to overcome many of the obstacles that limited the utility and practicality of vented designs and take advantage of their potential benefits: smaller boxes or deeper bass at the same efficiency. And the sealed boxes became harder to find.
Ohm was started in this transition period. Ohm’s initial speakers were of the acoustic suspension design. Our second generation of bookshelf speakers used the Thiele (this was before Small refined the concept) techniques to gain these benefits. The Ohm L was designed to have the bass response (and efficiency) of the big ARs and Advents in a much smaller box and went on to be an all-time favorite.
At the top we said the real question is “Better for what?” Today at Ohm, we employ both the sealed and vented designs, utilizing the strengths of each in particular applications. In the full-range, floor standing Walsh models, we use vented systems to get the deep bass from relatively small cabinets and to have reduced excursion as the frequency goes lower. We usually add a high-pass filter to reduce the wasted driver excursion below tuning frequency.
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In Walsh Satellites we use the same drivers in much smaller sealed boxes and built-inhigh-pass filters (usually set to 80 Hz) to make an easy transition to the subwoofer we expect will be used with these.
In most of Ohm’s powered subwoofers, we use vented systems to have a small cabinet and higher output, while the amplifier is equalized both to extend the deep bass and to eliminate the wasted driver excursion below the tuning frequency.
With the recent development of very powerful subwoofer amplifiers and very, very long excursion drivers, our most recent subwoofer (in the F0015) uses two advantages of sealed systems (slower roll-off below cabinet tuning and phase linearity) to achieve room-shaking bass –below 20 Hz from a cabinet too small to have the vent or passive radiator required for a ported system. We have moved back to an extreme version of the acoustic suspension design to get more deep bass out of a box too small to be vented. We call this an XtremeXcursion design.