Axiom Home Page Forums General Discussion Advice from Axiom Owners Combining sealed (EP800) and ported (EP600) subs ?

By logical extension, you're really insulting me.

Hi Jason and all,

Here are some informed comments on group delay at low bass frequencies. I posed the question to Tom Cumberland, Axiom's chief R & D engineer, who answered me this way:

"The issue of group delay can best be explained by moving a sub one to two feet in a given direction. This being said, there is group delay in everything that relates to room placement. Whether a sub is ported or sealed only affects its overall frequency response. Of course, a smooth linear response is the optimal performance from any loudspeaker. Simply moving a subwoofer around the room creates group delay. This movement, of course, helps remove the nulls and peaks in the frequency response in the room whether it is one sub or multiple subs."

I think this is helpful. There may be updates to this as I learn more.

Regards
Alan

Aww Alan, you broke the silence, and I really had these guys going!

Seriously, thank you for reviewing my question and passing it along to Tom & others. It's nice to know it's not anything to be seriously concerned about, but if you hear from some of the other folks you mentioned you'd be asking, I'll look forward to hearing more.

Even though it sounds like it's not going to cause a problem in my room, it's something I hadn't ever heard of before this, so it's interesting to learn more about it.

Thanks again.

Jason

For anyone interested, I found this report on group delay. It's fairly complicated and admittedly lost me at certain points, but what I read in the report seems to confirm that the difference between a good vented sub and a good sealed sub aren't likely to be large cause for concern.

In this test, the difference of max delay between their vented and sealed sub was around 2 hundredths of a second.

If anyone is up for more Q&A (a dangerous proposition I've seen), I'd be interested in a brief explanation of what the author means when he says that one system is a "second order system" and another is "a fifth order system." I've heard those terms before but do not understand the distinction.

Jason

Jason, as Mike Drew suggested above, when I saw the question I frankly wasn't interested to either just comment that the difference was almost certainly insignificant or to research for an authority to lend some weight to that view. Since you've taken the time to find that good Audire explanation(which I hadn't seen before), I decided to try to retrieve some of the stuff that I'd seen in the past. One thing that stuck in my mind was this discussion by physicist and speaker designer John Murphy taken from some of his posts on an audio discussion group. Scrolling down to the June 2, 1999 post, his concept of relating the amount of group delay that might be audible to how many "cycles" it equated to at very low subwoofer frequencies seems quite powerful and the one cycle threshold for audibility that he proposed seems reasonable. As shown in his example there involving the ACI driver, the amount of group delay for even the vented system was well under one cycle. The difference between the vented and closed enclosures was still smaller of course, and even less likely to be audible. So, my view(not that I've spent much time pondering the question)has been that this isn't of real-world significance.

As to the "orders" of the various enclosure types, the significance lies in how rapidly the response rolls off below the tuned frequency, which is quite important(and how the group delay at the lower frequencies increases in proportion to the steepness of the rolloff, apparently not very important). The rolloff is 6dB per octave per "order", e.g., a closed enclosure said to be 2nd order rolls off at 12dB per octave, a vented 4th order enclosure rolls off at 24db per octave.The way the numbering of the orders arises is through the analogy to electrical(rather than acoustical)crossovers which also create rolloffs. A first order crossover contains basically just one element(a capacitor in series with the tweeter, an inductor with the woofer)per driver and rolls it off at 6db/octave above(woofer)or below(tweeter)the selected crossover frequency. As more elements are added to the crossover to increase the steepness of the rolloff, the number of elements determines the order of the crossover; e.g., in a second order crossover both an inductor and capacitor are used for each driver, series capacitor and parallel inductor to the tweeter, series inductor and parallel capacitor to the woofer, resulting in a 12dB/octave rolloff. As more elements are added there can be 3rd order, 4th order, etc. crossovers with proportionally steeper rolloffs but also higher power losses in the crossover.

I love when I have just the vaguest idea what someone might be talking about.

Glad to see I am not the only one.

Dems lots o purdy wurds!

John - thank you for the informative response. It only took us 4 pages as a group to get to this point , but it was worthwhile as I'm now learning something new.

Thank you for taking the time to research and respond.

Jason

Only 4 pages!?, We must be slipping...