This is an interesting thread resulting from Funnehaha’s request for NS-10 replacement suggestions. I am old enough that when I started my audiophile journey the market was evenly split between sealed and vented speakers and then observed as vented types came to dominate the market. I have also recently further optimized my speaker system that is presently based on a sealed box bass section. I am very pleased with the results and thus in a pro-sealed box mood; the reader should of course discount that statement by the usual amount due to pride of authorship I have for my system.
Like Funnehaha I was pleased to learn that I had not noticed NHT remains true to sealed box designs. Of course the big name using (or shall I say reviving) that technology is Magico up in the funny money call the piano movers for the install part of the market. Back in the real world if a clean well-kept pair of ADS speakers of the desired size is available I remain a big fan for that sadly defunct brand. I have my ADS 500s in storage awaiting a binding post and capacitor refresh.
I want to add to the discussion some more engineering details when comparing sealed vs. vented designs and my opinions on how those translate to our experiences in the listening room. Let’s begin quoting from this thread then go a little deeper.
Above dB Cooper wrote “While it is true that not all sealed boxes are acoustic suspension, all acoustic suspension speakers are sealed boxes, which is the point Carl V was making. Both approaches seem to have largely fallen out of favor in recent years- acoustic suspension because of advances in drivers and cabinet loading technique which make it possible to avoid the 'one-note' bass which was the trademark of many older bass reflex designs while preserving the sensitivity advantages, and because ports seem to have become something of a 'selling point' (I have seen computer speakers with 1" deep 'reflex' tubes... Useless.) Infinite baffle speakers fell out of favor at least partially because they are usually fairly large (the Bozak Concert Grand is the size of a refrigerator).”
And JLM wrote about a Jordan based system “But it's also available in a slightly larger ported design where F3 moves (again as I recall) from 90 Hz to 50 Hz. Which explains why sealed designs are less popular.”
I agree with both dB Cooper and JLM. I will add that most HiFi sales are a result of buying with our eyes over our ears. My thesis is vented has come to dominate because customer’s eyes like the extra feature of a port on the box and the ‘apparent’ deeper bass for a given box size on specification sheet. I quote apparent because while the F3 measured low end cutoff yields a lower frequency for the vented speaker in my experience the sealed design sounds deeper, cleaner, and more capable. Let’s dig into my somewhat contrary contention.
As Graham Bank and Julian Wright write in chapter 7 of John Borwick’s book ‘Loudspeaker and Headphone Handbook’ comparing sealed vs vented enclosures “Superficially, this configuration (vented) provides ‘more bass’ than the closed box, but the fourth-order (24 dB/octave) low-frequency roll-off generally results in greater output in the upper bass (circa 80 Hz) but less ‘deep bass’ (circa 30 Hz).”
In the previous paragraphs Bank and Wright review the dipole baffle, sealed box, and vented box options for turning woofers into bass systems. These systems act as high-pass filters passing frequencies above their cutoff (F3) frequencies and progressively attenuating output below F3. For dipole woofer on-a-baffle systems the filter is a 1st order type output dropping at -6 dB/octave below F3. For closed or sealed box types we get a 2nd order filter output dropping at -12 dB/octave below F3. And vented/ported systems are 4th order with output dropping at -24 dB/octave below F3.
Now back to Borwick’s book ‘Loudspeaker and Headphone Handbook’ in chapter 8 authored by Glyn Adams and revised by John Borwick part of the summary of a through derivation of a speaker interacting with its room is: “We can conclude from these examples that, when the loudspeaker system is positioned some distance away from the boundaries, the power output will be increased by 9 dB at low frequencies but will fall to the free-space value at high frequencies.”
So the room tends to add +9 dB of bass at low frequencies. My belief from experience is when a vented system rolls off at -24dB/octave the addition of +9 dB of boost courtesy of room gain is far less apparent that a sealed system rolling off at -12 dB/octave that gets +9 dB back from the room. In other words when we compare speaker+room systems rather than the speaker alone the vented case rolls off at (-24+9)=-15 dB/octave vs. sealed (-12+9)=-3 dB/octave. I believe this is why when listening to sealed box systems I more often hear the very low frequency information on a program.
Correlating the market vs. the engineering I believe Bank and Wrights observation “greater output in the upper bass (circa 80 Hz) but less ‘deep bass’ (circa 30 Hz)” observation shows vented systems are a better fit for most popular music. As has often been pointed out a great deal of what we call bass is in that second octave circa 80 Hz, to get something in the lowest first octave we need to pull out the organ, synthesizer, and movie effects sources.
Two more points as this is long enough already. First admitting the audibility of low frequency phase shift is a matter of great contention it is true that sealed systems exhibit a gradual phase shift of +90 to -90 degree shift across the LPF range vs. vented where at both the vent and woofer tuning frequencies the output does an abrupt -180 to +180 degree shift. Second (and finally) perhaps the biggest advantage I see for sealed systems (especially in smaller systems like the NS-10 size) is that below F3 the woofer remains loaded by the trapped air volume. In like sized vented systems below 50ish Hz the only thing keeping the woofer from pounding its mechanical stops is its mechanical suspension.