Air suspension systems roll off at a rate of 12 dB/oct below their cutoff point, and are damped by the sealed cabinet’s ‘air spring’ at low frequencies, which prevents excessive cone excursion in the presence of low end noise sources, like record warp or rumble. Most modern vented designs are susceptible to these noises, as they are ‘unloaded’ below their tuned cutoff point. One of the first modern reflex systems sold, the ElectroVoice Interface:A, came with a sharp active high pass filter module you put in a tape loop to filter this stuff out. The arrival of CD made it practical to skip this precaution. Old reflex systems used mechanical means like stiff corrugated surrounds and spiders to avoid over-excursion, but in those days HiFi was not a commodity marketed to everyone…it was a hobby for enthusiasts who didn’t mind big boxes. Air suspension made sense in the stereo analog era, and vented made more sense in the CD era after record’s low end noise ceased to be a problem. You can get more bandwidth and sensitivity without penalty by applying Theil-Small filter theory…and computer modeling made it easier.
Why not acoustic suspension?
When I was young, late 60's early 70's, the sealed or acoustic suspension speakers were quite commonplace. Now bass reflex, ported speakers seem to be much more common. My understanding is that both speaker designs, assuming competent design will perform very well. Perhaps with the ported getting a slightly lower bass response and higher efficiency gives it the edge. Thinking efficiency though, amplifier watts don't cost that much more today so it doesn't seem like a deal maker. What I don't understand from a pragmatic corporate standpoint is why more acoustic suspension speakers aren't available. My understanding is that the sealed speaker box can be smaller which would be a plus from a cost standpoint, both in less material, lighter cabinet and more speakers could be shipped in a truck reducing shipping cost. Any thoughts as to why the industry has shifted so hard in the bass reflex direction?
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As with anything, there are pros and cons, and good and bad examples of each type. Bass from a ported box often tends to stand out with a bit more punch, which makes for a good first impression during a quick demo to a lot of folks. Not so different than having somewhat forward upper mids/treble. In the long run a lot of us might prefer the more refined approach of an acoustic suspension design, and more neutral upper mids, but that quick hook for a sale is king. |
Bass reflex (ported) relies on the port resonance to extend bass response. Done optimally, this results in an undamped 3 db boost at the system resonance and 24 db/octave roll-off below. The lack of damping results in the boominess mentioned by others here (and the common addition of "port plugs"). Acoustic suspension can be tuned as desired: critically damped (qT=.5), maximally flat (qT = .707), or other and rolls off below at 12 db/octave. The preferred speaker Q for the 2 alignments is different. I believe most woofers these days are designed for bass reflex, most likely because speaker designers chose that alignment. Like some others here, I prefer the closed box sound myself. |
@winoguy17 Very happy as you should be. :) If I ever upgrade from my old AR9's, it will be with these. The Legacy designs are very similar to old AR's. |
@yogiboy I have also always preferred sealed enclosures. Bass is tighter, seems much more accurate. Takes more power. So be it. |
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