• http://www.forsman.no/referans.htm#John Watkinson

All speakers have a resonant frequency and therefore can be excited into resonance at that frequency. A pulse sent to them will tend to cause overring, and the alignment will determine the amount of damping. This is what determines the amount of a "ringing" artifact is produced - Not whether it is sealed or ported, but rather the Q of the particular alignment.

The speaker diaphragm and suspension is a spring/mass system which is mechanically reactive. There's no way around it. A sealed cabinet will offer damping - like that of a shock absorber - but it is helpless to remove the "spring" action in this system from the diaphragm mass and suspension. So a sealed cabinet can assist the motor by damping it, but it cannot remove the reactive nature of the mass/spring action of the motor.

You will see a similar sort of "ringing" at resonance in a sealed cabinet as you will in a bass reflex cabinet. The more damped a system is, the less it will ring. But this is determined by alignment, and not by whether a cabinet is ported or not. As we examine this a little further, we actually find that a sealed box is incapable of providing as much damping as a optimally tuned bass reflex box, at least in the resonance region between fb and fh.

That's pretty deep for almost 3 a.m., I've been awake since not much later than. I'm going to have to go drink my Wheaties, eat my coffee and then reread it! :) Jerrod

What's more important is the amount of damping. If damping is sufficient, then the duration of output is short - maybe only an attenuated half cycle. But for systems that are poorly damped, the overring can be several cycles. Well damped speakers are able to generate a very short staccato bass note, like a bass drum impact, but poorly damped speakers cannot - Instead you head a note of much longer duration. One would expect a quick "whap" and instead, you hear "whoooooom." Sorry for the onomatopoeic words - "whap" and "whoooooom" - but I can't find a way to descibe the sounds any better.

I've always been particular about this characteristic of speakers. I love deep and powerful bass, but I also want it to be clean. I expect bass drum strikes to have quick impact along with that thump in my chest. I want to hear the roar of a low frequency bass organ, but I want these to be two different and clearly distinguishable sounds. Systems that have great low frequency resonse and well damped resonance can provide this, but poorly damped systems cannot.

One of my first, and easiest listening tests was to listen to the song "All good people" from the 70's British art rock band, "Yes." There are two bass notes that are repeatedly played as 16th notes, in rapid succession. If I can't hear two distinguishable notes in this song, then I think the speaker sucks. But it's truly amazing how many "reproduce" this passage as single bass tone. Elephant farts.

What's happening on such a system is that the speaker is continuing to "ring" at this frequency so long that it "fills in" the cycles between the two bursts. What should have been two short bursts are made into one long one. And this is just as common on acoustic suspension speakers as it is on bass reflex. It's a symptom of an out of control woofer at resonance, and a cabinet that cannot effectively damp it.

Luke Johnson
A lover of cleanbass
cleanbass2@home.com
www.basslinear.com

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